document_title: Chemical reaction

question_text: what names are given to the materials at the end of a chemical reaction

question_tokens: what names are given to the materials at the end of a chemical reaction

text: Chemical reaction

text: History

text: Chemical reactions such as combustion in fire, fermentation and the reduction of ores to metals were known since antiquity. Initial theories of transformation of materials were developed by Greek philosophers, such as the Four-Element Theory of Empedocles stating that any substance is composed of the four basic elements – fire, water, air and earth. In the Middle Ages, chemical transformations were studied by Alchemists. They attempted, in particular, to convert lead into gold, for which purpose they used reactions of lead and lead-copper alloys with sulfur.[2]

text: The production of chemical substances that do not normally occur in nature has long been tried, such as the synthesis of sulfuric and nitric acids attributed to the controversial alchemist Jābir ibn Hayyān. The process involved heating of sulfate and nitrate minerals such as copper sulfate, alum and saltpeter. In the 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride. With the development of the lead chamber process in 1746 and the Leblanc process, allowing large-scale production of sulfuric acid and sodium carbonate, respectively, chemical reactions became implemented into the industry. Further optimization of sulfuric acid technology resulted in the contact process in the 1880s,[3] and the Haber process was developed in 1909–1910 for ammonia synthesis.[4]

text: From the 16th century, researchers including Jan Baptist van Helmont, Robert Boyle and Isaac Newton tried to establish theories of the experimentally observed chemical transformations. The phlogiston theory was proposed in 1667 by Johann Joachim Becher. It postulated the existence of a fire-like element called "phlogiston", which was contained within combustible bodies and released during combustion. This proved to be false in 1785 by Antoine Lavoisier who found the correct explanation of the combustion as reaction with oxygen from the air.[5]

text: Joseph Louis Gay-Lussac recognized in 1808 that gases always react in a certain relationship with each other. Based on this idea and the atomic theory of John Dalton, Joseph Proust had developed the law of definite proportions, which later resulted in the concepts of stoichiometry and chemical equations.[6]

text: Regarding the organic chemistry, it was long believed that compounds obtained from living organisms were too complex to be obtained synthetically. According to the concept of vitalism, organic matter was endowed with a "vital force" and distinguished from inorganic materials. This separation was ended however by the synthesis of urea from inorganic precursors by Friedrich Wöhler in 1828. Other chemists who brought major contributions to organic chemistry include Alexander William Williamson with his synthesis of ethers and Christopher Kelk Ingold, who, among many discoveries, established the mechanisms of substitution reactions.

text: Equations

text:
Chemical equations are used to graphically illustrate chemical reactions. They consist of chemical or structural formulas of the reactants on the left and those of the products on the right. They are separated by an arrow (→) which indicates the direction and type of the reaction; the arrow is read as the word "yields".[7] The tip of the arrow points in the direction in which the reaction proceeds. A double arrow (⇌) pointing in opposite directions is used for equilibrium reactions. Equations should be balanced according to the stoichiometry, the number of atoms of each species should be the same on both sides of the equation. This is achieved by scaling the number of involved molecules (

A
,
B
,
C

{\displaystyle {\ce {A,B,C}}}

 and

D

{\displaystyle {\ce {D}}}

 in a schematic example below) by the appropriate integers a, b, c and d.[8]

text: More elaborate reactions are represented by reaction schemes, which in addition to starting materials and products show important intermediates or transition states. Also, some relatively minor additions to the reaction can be indicated above the reaction arrow; examples of such additions are water, heat, illumination, a catalyst, etc. Similarly, some minor products can be placed below the arrow, often with a minus sign.

text: Retrosynthetic analysis can be applied to design a complex synthesis reaction. Here the analysis starts from the products, for example by splitting selected chemical bonds, to arrive at plausible initial reagents. A special arrow (⇒) is used in retro reactions.[9]

text: Elementary reactions

text: The elementary reaction is the smallest division into which a chemical reaction can be decomposed, it has no intermediate products.[10] Most experimentally observed reactions are built up from many elementary reactions that occur in parallel or sequentially. The actual sequence of the individual elementary reactions is known as reaction mechanism. An elementary reaction involves a few molecules, usually one or two, because of the low probability for several molecules to meet at a certain time.[11]

text: The most important elementary reactions are unimolecular and bimolecular reactions. Only one molecule is involved in a unimolecular reaction; it is transformed by an isomerization or a dissociation into one or more other molecules. Such reactions require the addition of energy in the form of heat or light. A typical example of a unimolecular reaction is the cis–trans isomerization, in which the cis-form of a compound converts to the trans-form or vice versa.[12]

text: In a typical dissociation reaction, a bond in a molecule splits (ruptures) resulting in two molecular fragments. The splitting can be homolytic or heterolytic. In the first case, the bond is divided so that each product retains an electron and becomes a neutral radical. In the second case, both electrons of the chemical bond remain with one of the products, resulting in charged ions. Dissociation plays an important role in triggering chain reactions, such as hydrogen–oxygen or polymerization reactions.

text: For bimolecular reactions, two molecules collide and react with each other. Their merger is called chemical synthesis or an addition reaction.

text: Another possibility is that only a portion of one molecule is transferred to the other molecule. This type of reaction occurs, for example, in redox and acid-base reactions. In redox reactions, the transferred particle is an electron, whereas in acid-base reactions it is a proton. This type of reaction is also called metathesis.

text: for example

text: Chemical equilibrium

text: Most chemical reactions are reversible, that is they can and do run in both directions. The forward and reverse reactions are competing with each other and differ in reaction rates. These rates depend on the concentration and therefore change with time of the reaction: the reverse rate gradually increases and becomes equal to the rate of the forward reaction, establishing the so-called chemical equilibrium. The time to reach equilibrium depends on such parameters as temperature, pressure and the materials involved, and is determined by the minimum free energy. In equilibrium, the Gibbs free energy must be zero. The pressure dependence can be explained with the Le Chatelier's principle. For example, an increase in pressure due to decreasing volume causes the reaction to shift to the side with the fewer moles of gas.[13]

text: The reaction yield stabilizes at equilibrium, but can be increased by removing the product from the reaction mixture or changed by increasing the temperature or pressure. A change in the concentrations of the reactants does not affect the equilibrium constant, but does affect the equilibrium position.

text: Thermodynamics

text: Chemical reactions are determined by the laws of thermodynamics. Reactions can proceed by themselves if they are exergonic, that is if they release energy. The associated free energy of the reaction is composed of two different thermodynamic quantities, enthalpy and entropy:[14]

text: Reactions can be exothermic, where ΔH is negative and energy is released. Typical examples of exothermic reactions are precipitation and crystallization, in which ordered solids are formed from disordered gaseous or liquid phases. In contrast, in endothermic reactions, heat is consumed from the environment. This can occur by increasing the entropy of the system, often through the formation of gaseous reaction products, which have high entropy. Since the entropy increases with temperature, many endothermic reactions preferably take place at high temperatures. On the contrary, many exothermic reactions such as crystallization occur at low temperatures. Changes in temperature can sometimes reverse the sign of the enthalpy of a reaction, as for the carbon monoxide reduction of molybdenum dioxide:

text: This reaction to form carbon dioxide and molybdenum is endothermic at low temperatures, becoming less so with increasing temperature.[15] ΔH° is zero at 7003185500000000000♠1855 K, and the reaction becomes exothermic above that temperature.

text: Changes in temperature can also reverse the direction tendency of a reaction. For example, the water gas shift reaction

text: is favored by low temperatures, but its reverse is favored by high temperature. The shift in reaction direction tendency occurs at 7003110000000000000♠1100 K.[15]

text: Reactions can also be characterized by the internal energy which takes into account changes in the entropy, volume and chemical potential. The latter depends, among other things, on the activities of the involved substances.[16]

text: Kinetics

text: The speed at which reactions takes place is studied by reaction kinetics. The rate depends on various parameters, such as:

text:
Reactant concentrations, which usually make the reaction happen at a faster rate if raised through increased collisions per unit time. Some reactions, however, have rates that are independent of reactant concentrations. These are called zero order reactions.
Surface area available for contact between the reactants, in particular solid ones in heterogeneous systems. Larger surface areas lead to higher reaction rates.
Pressure – increasing the pressure decreases the volume between molecules and therefore increases the frequency of collisions between the molecules.
Activation energy, which is defined as the amount of energy required to make the reaction start and carry on spontaneously. Higher activation energy implies that the reactants need more energy to start than a reaction with a lower activation energy.
Temperature, which hastens reactions if raised, since higher temperature increases the energy of the molecules, creating more collisions per unit time,
The presence or absence of a catalyst. Catalysts are substances which change the pathway (mechanism) of a reaction which in turn increases the speed of a reaction by lowering the activation energy needed for the reaction to take place. A catalyst is not destroyed or changed during a reaction, so it can be used again.
For some reactions, the presence of electromagnetic radiation, most notably ultraviolet light, is needed to promote the breaking of bonds to start the reaction. This is particularly true for reactions involving radicals.

text: Several theories allow calculating the reaction rates at the molecular level. This field is referred to as reaction dynamics. The rate v of a first-order reaction, which could be disintegration of a substance A, is given by:

text: Its integration yields:

text: Here k is first-order rate constant having dimension 1/time, [A](t) is concentration at a time t and [A]0 is the initial concentration. The rate of a first-order reaction depends only on the concentration and the properties of the involved substance, and the reaction itself can be described with the characteristic half-life. More than one time constant is needed when describing reactions of higher order. The temperature dependence of the rate constant usually follows the Arrhenius equation:

text: where Ea is the activation energy and kB is the Boltzmann constant. One of the simplest models of reaction rate is the collision theory. More realistic models are tailored to a specific problem and include the transition state theory, the calculation of the potential energy surface, the Marcus theory and the Rice–Ramsperger–Kassel–Marcus (RRKM) theory.[17]

text: Reaction types

text: Four basic types

text: Synthesis

text: In a synthesis reaction, two or more simple substances combine to form a more complex substance. These reactions are in the general form:

text: Two or more reactants yielding one product is another way to identify a synthesis reaction. One example of a synthesis reaction is the combination of iron and sulfur to form iron(II) sulfide:

text: Another example is simple hydrogen gas combined with simple oxygen gas to produce a more complex substance, such as water.[18]

text: Decomposition

text: A decomposition reaction is when a more complex substance breaks down into its more simple parts. It is thus the opposite of a synthesis reaction, and can be written as[18][19]

text: One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:

text: Single replacement

text: In a single replacement reaction, a single uncombined element replaces another in a compound; in other words, one element trades places with another element in a compound[18] These reactions come in the general form of:

text: One example of a single displacement reaction is when magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas:

text: Double replacement

text: In a double replacement reaction, the anions and cations of two compounds switch places and form two entirely different compounds.[18] These reactions are in the general form:[19]

text: For example, when barium chloride (BaCl2) and magnesium sulfate (MgSO4) react, the SO42− anion switches places with the 2Cl− anion, giving the compounds BaSO4 and MgCl2.

text: Another example of a double displacement reaction is the reaction of lead(II) nitrate with potassium iodide to form lead(II) iodide and potassium nitrate:

text: Oxidation and reduction

text: Redox reactions can be understood in terms of transfer of electrons from one involved species (reducing agent) to another (oxidizing agent). In this process, the former species is oxidized and the latter is reduced. Though sufficient for many purposes, these descriptions are not precisely correct. Oxidation is better defined as an increase in oxidation state, and reduction as a decrease in oxidation state. In practice, the transfer of electrons will always change the oxidation state, but there are many reactions that are classed as "redox" even though no electron transfer occurs (such as those involving covalent bonds).[20][21]

text: In the following redox reaction, hazardous sodium metal reacts with toxic chlorine gas to form the ionic compound sodium chloride, or common table salt:

text: In the reaction, sodium metal goes from an oxidation state of 0 (as it is a pure element) to +1: in other words, the sodium lost one electron and is said to have been oxidized. On the other hand, the chlorine gas goes from an oxidation of 0 (it is also a pure element) to −1: the chlorine gains one electron and is said to have been reduced. Because the chlorine is the one reduced, it is considered the electron acceptor, or in other words, induces oxidation in the sodium – thus the chlorine gas is considered the oxidizing agent. Conversely, the sodium is oxidized or is the electron donor, and thus induces reduction in the other species and is considered the reducing agent.

text:
Which of the involved reactants would be reducing or oxidizing agent can be predicted from the electronegativity of their elements. Elements with low electronegativity, such as most metals, easily donate electrons and oxidize – they are reducing agents. On the contrary, many ions with high oxidation numbers, such as H
2O
2, MnO−
4, CrO
3, Cr
2O2−
7, OsO
4 can gain one or two extra electrons and are strong oxidizing agents.

text: The number of electrons donated or accepted in a redox reaction can be predicted from the electron configuration of the reactant element. Elements try to reach the low-energy noble gas configuration, and therefore alkali metals and halogens will donate and accept one electron respectively. Noble gases themselves are chemically inactive.[22]

text: An important class of redox reactions are the electrochemical reactions, where electrons from the power supply are used as the reducing agent. These reactions are particularly important for the production of chemical elements, such as chlorine[23] or aluminium. The reverse process in which electrons are released in redox reactions and can be used as electrical energy is possible and used in batteries.

text: Complexation

text: In complexation reactions, several ligands react with a metal atom to form a coordination complex. This is achieved by providing lone pairs of the ligand into empty orbitals of the metal atom and forming dipolar bonds. The ligands are Lewis bases, they can be both ions and neutral molecules, such as carbon monoxide, ammonia or water. The number of ligands that react with a central metal atom can be found using the 18-electron rule, saying that the valence shells of a transition metal will collectively accommodate 18 electrons, whereas the symmetry of the resulting complex can be predicted with the crystal field theory and ligand field theory. Complexation reactions also include ligand exchange, in which one or more ligands are replaced by another, and redox processes which change the oxidation state of the central metal atom.[24]

text: Acid-base reactions

text: In the Brønsted–Lowry acid–base theory, an acid-base reaction involves a transfer of protons (H+) from one species (the acid) to another (the base). When a proton is removed from an acid, the resulting species is termed that acid's conjugate base. When the proton is accepted by a base, the resulting species is termed that base's conjugate acid.[25] In other words, acids act as proton donors and bases act as proton acceptors according to the following equation:

text: The reverse reaction is possible, and thus the acid/base and conjugated base/acid are always in equilibrium. The equilibrium is determined by the acid and base dissociation constants (Ka and Kb) of the involved substances. A special case of the acid-base reaction is the neutralization where an acid and a base, taken at exactly same amounts, form a neutral salt.

text: Acid-base reactions can have different definitions depending on the acid-base concept employed. Some of the most common are:

text:
Arrhenius definition: Acids dissociate in water releasing H3O+ ions; bases dissociate in water releasing OH− ions.
Brønsted-Lowry definition: Acids are proton (H+) donors, bases are proton acceptors; this includes the Arrhenius definition.
Lewis definition: Acids are electron-pair acceptors, bases are electron-pair donors; this includes the Brønsted-Lowry definition.

text: Precipitation

text: Precipitation is the formation of a solid in a solution or inside another solid during a chemical reaction. It usually takes place when the concentration of dissolved ions exceeds the solubility limit[26] and forms an insoluble salt. This process can be assisted by adding a precipitating agent or by removal of the solvent. Rapid precipitation results in an amorphous or microcrystalline residue and slow process can yield single crystals. The latter can also be obtained by recrystallization from microcrystalline salts.[27]

text: Solid-state reactions

text: Reactions can take place between two solids. However, because of the relatively small diffusion rates in solids, the corresponding chemical reactions are very slow in comparison to liquid and gas phase reactions. They are accelerated by increasing the reaction temperature and finely dividing the reactant to increase the contacting surface area.[28]

text: Reactions at the solid|gas interface

text: Reaction can take place at the solid|gas interface, surfaces at very low pressure such as ultra-high vacuum. Via scanning tunneling microscopy, it is possible to observe reactions at the solid|gas interface in real space, if the time scale of the reaction is in the correct range.[29][30] Reactions at the solid|gas interface are in some cases related to catalysis.

text: Photochemical reactions

text: In photochemical reactions, atoms and molecules absorb energy (photons) of the illumination light and convert into an excited state. They can then release this energy by breaking chemical bonds, thereby producing radicals. Photochemical reactions include hydrogen–oxygen reactions, radical polymerization, chain reactions and rearrangement reactions.[31]

text: Many important processes involve photochemistry. The premier example is photosynthesis, in which most plants use solar energy to convert carbon dioxide and water into glucose, disposing of oxygen as a side-product. Humans rely on photochemistry for the formation of vitamin D, and vision is initiated by a photochemical reaction of rhodopsin.[12] In fireflies, an enzyme in the abdomen catalyzes a reaction that results in bioluminescence.[32] Many significant photochemical reactions, such as ozone formation, occur in the Earth atmosphere and constitute atmospheric chemistry.

text: Catalysis

text: In catalysis, the reaction does not proceed directly, but through reaction with a third substance known as catalyst. Although the catalyst takes part in the reaction, it is returned to its original state by the end of the reaction and so is not consumed. However, it can be inhibited, deactivated or destroyed by secondary processes. Catalysts can be used in a different phase (heterogeneous) or in the same phase (homogeneous) as the reactants. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid–liquid system or evaporate in a solid–gas system. Catalysts can only speed up the reaction – chemicals that slow down the reaction are called inhibitors.[33][34] Substances that increase the activity of catalysts are called promoters, and substances that deactivate catalysts are called catalytic poisons. With a catalyst, a reaction which is kinetically inhibited by a high activation energy can take place in circumvention of this activation energy.

text: Heterogeneous catalysts are usually solids, powdered in order to maximize their surface area. Of particular importance in heterogeneous catalysis are the platinum group metals and other transition metals, which are used in hydrogenations, catalytic reforming and in the synthesis of commodity chemicals such as nitric acid and ammonia. Acids are an example of a homogeneous catalyst, they increase the nucleophilicity of carbonyls, allowing a reaction that would not otherwise proceed with electrophiles. The advantage of homogeneous catalysts is the ease of mixing them with the reactants, but they may also be difficult to separate from the products. Therefore, heterogeneous catalysts are preferred in many industrial processes.[35]

text: Reactions in organic chemistry

text: In organic chemistry, in addition to oxidation, reduction or acid-base reactions, a number of other reactions can take place which involve covalent bonds between carbon atoms or carbon and heteroatoms (such as oxygen, nitrogen, halogens, etc.). Many specific reactions in organic chemistry are name reactions designated after their discoverers.

text: Substitution

text: In a substitution reaction, a functional group in a particular chemical compound is replaced by another group.[36] These reactions can be distinguished by the type of substituting species into a nucleophilic, electrophilic or radical substitution.

text: In the first type, a nucleophile, an atom or molecule with an excess of electrons and thus a negative charge or partial charge, replaces another atom or part of the "substrate" molecule. The electron pair from the nucleophile attacks the substrate forming a new bond, while the leaving group departs with an electron pair. The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged. Examples of nucleophiles are hydroxide ion, alkoxides, amines and halides. This type of reaction is found mainly in aliphatic hydrocarbons, and rarely in aromatic hydrocarbon. The latter have high electron density and enter nucleophilic aromatic substitution only with very strong electron withdrawing groups. Nucleophilic substitution can take place by two different mechanisms, SN1 and SN2. In their names, S stands for substitution, N for nucleophilic, and the number represents the kinetic order of the reaction, unimolecular or bimolecular.[37]

text: The SN1 reaction proceeds in two steps. First, the leaving group is eliminated creating a carbocation. This is followed by a rapid reaction with the nucleophile.[38]

text: In the SN2 mechanism, the nucleophile forms a transition state with the attacked molecule, and only then the leaving group is cleaved. These two mechanisms differ in the stereochemistry of the products. SN1 leads to the non-stereospecific addition and does not result in a chiral center, but rather in a set of geometric isomers (cis/trans). In contrast, a reversal (Walden inversion) of the previously existing stereochemistry is observed in the SN2 mechanism.[39]

text: Electrophilic substitution is the counterpart of the nucleophilic substitution in that the attacking atom or molecule, an electrophile, has low electron density and thus a positive charge. Typical electrophiles are the carbon atom of carbonyl groups, carbocations or sulfur or nitronium cations. This reaction takes place almost exclusively in aromatic hydrocarbons, where it is called electrophilic aromatic substitution. The electrophile attack results in the so-called σ-complex, a transition state in which the aromatic system is abolished. Then, the leaving group, usually a proton, is split off and the aromaticity is restored. An alternative to aromatic substitution is electrophilic aliphatic substitution. It is similar to the nucleophilic aliphatic substitution and also has two major types, SE1 and SE2[40]

text: In the third type of substitution reaction, radical substitution, the attacking particle is a radical.[36] This process usually takes the form of a chain reaction, for example in the reaction of alkanes with halogens. In the first step, light or heat disintegrates the halogen-containing molecules producing the radicals. Then the reaction proceeds as an avalanche until two radicals meet and recombine.[41]

text: Addition and elimination

text: The addition and its counterpart, the elimination, are reactions which change the number of substitutents on the carbon atom, and form or cleave multiple bonds. Double and triple bonds can be produced by eliminating a suitable leaving group. Similar to the nucleophilic substitution, there are several possible reaction mechanisms which are named after the respective reaction order. In the E1 mechanism, the leaving group is ejected first, forming a carbocation. The next step, formation of the double bond, takes place with elimination of a proton (deprotonation). The leaving order is reversed in the E1cb mechanism, that is the proton is split off first. This mechanism requires participation of a base.[42] Because of the similar conditions, both reactions in the E1 or E1cb elimination always compete with the SN1 substitution.[43]

text: The E2 mechanism also requires a base, but there the attack of the base and the elimination of the leaving group proceed simultaneously and produce no ionic intermediate. In contrast to the E1 eliminations, different stereochemical configurations are possible for the reaction product in the E2 mechanism, because the attack of the base preferentially occurs in the anti-position with respect to the leaving group. Because of the similar conditions and reagents, the E2 elimination is always in competition with the SN2-substitution.[44]

text: The counterpart of elimination is the addition where double or triple bonds are converted into single bonds. Similar to the substitution reactions, there are several types of additions distinguished by the type of the attacking particle. For example, in the electrophilic addition of hydrogen bromide, an electrophile (proton) attacks the double bond forming a carbocation, which then reacts with the nucleophile (bromine). The carbocation can be formed on either side of the double bond depending on the groups attached to its ends, and the preferred configuration can be predicted with the Markovnikov's rule.[45] This rule states that "In the heterolytic addition of a polar molecule to an alkene or alkyne, the more electronegative (nucleophilic) atom (or part) of the polar molecule becomes attached to the carbon atom bearing the smaller number of hydrogen atoms."[46]

text: If the addition of a functional group takes place at the less substituted carbon atom of the double bond, then the electrophilic substitution with acids is not possible. In this case, one has to use the hydroboration–oxidation reaction, where in the first step, the boron atom acts as electrophile and adds to the less substituted carbon atom. At the second step, the nucleophilic hydroperoxide or halogen anion attacks the boron atom.[47]

text: While the addition to the electron-rich alkenes and alkynes is mainly electrophilic, the nucleophilic addition plays an important role for the carbon-heteroatom multiple bonds, and especially its most important representative, the carbonyl group. This process is often associated with an elimination, so that after the reaction the carbonyl group is present again. It is therefore called addition-elimination reaction and may occur in carboxylic acid derivatives such as chlorides, esters or anhydrides. This reaction is often catalyzed by acids or bases, where the acids increase by the electrophilicity of the carbonyl group by binding to the oxygen atom, whereas the bases enhance the nucleophilicity of the attacking nucleophile.[48]

text: Nucleophilic addition of a carbanion or another nucleophile to the double bond of an alpha, beta unsaturated carbonyl compound can proceed via the Michael reaction, which belongs to the larger class of conjugate additions. This is one of the most useful methods for the mild formation of C–C bonds.[49][50][51]

text: Some additions which can not be executed with nucleophiles and electrophiles, can be succeeded with free radicals. As with the free-radical substitution, the radical addition proceeds as a chain reaction, and such reactions are the basis of the free-radical polymerization.[52]

text: Other organic reaction mechanisms

text: In a rearrangement reaction, the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. These include hydride shift reactions such as the Wagner-Meerwein rearrangement, where a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon. Most rearrangements are associated with the breaking and formation of new carbon-carbon bonds. Other examples are sigmatropic reaction such as the Cope rearrangement.[53]

text: Cyclic rearrangements include cycloadditions and, more generally, pericyclic reactions, wherein two or more double bond-containing molecules form a cyclic molecule. An important example of cycloaddition reaction is the Diels–Alder reaction (the so-called [4+2] cycloaddition) between a conjugated diene and a substituted alkene to form a substituted cyclohexene system.[54]

text: Whether a certain cycloaddition would proceed depends on the electronic orbitals of the participating species, as only orbitals with the same sign of wave function will overlap and interact constructively to form new bonds. Cycloaddition is usually assisted by light or heat. These perturbations result in different arrangement of electrons in the excited state of the involved molecules and therefore in different effects. For example, the [4+2] Diels-Alder reactions can be assisted by heat whereas the [2+2] cycloaddition is selectively induced by light.[55] Because of the orbital character, the potential for developing stereoisomeric products upon cycloaddition is limited, as described by the Woodward–Hoffmann rules.[56]

text: Biochemical reactions

text: Biochemical reactions are mainly controlled by enzymes. These proteins can specifically catalyze a single reaction, so that reactions can be controlled very precisely. The reaction takes place in the active site, a small part of the enzyme which is usually found in a cleft or pocket lined by amino acid residues, and the rest of the enzyme is used mainly for stabilization. The catalytic action of enzymes relies on several mechanisms including the molecular shape ("induced fit"), bond strain, proximity and orientation of molecules relative to the enzyme, proton donation or withdrawal (acid/base catalysis), electrostatic interactions and many others.[57]

text: The biochemical reactions that occur in living organisms are collectively known as metabolism. Among the most important of its mechanisms is the anabolism, in which different DNA and enzyme-controlled processes result in the production of large molecules such as proteins and carbohydrates from smaller units.[58] Bioenergetics studies the sources of energy for such reactions. An important energy source is glucose, which can be produced by plants via photosynthesis or assimilated from food. All organisms use this energy to produce adenosine triphosphate (ATP), which can then be used to energize other reactions.

text: Applications

text: Chemical reactions are central to chemical engineering where they are used for the synthesis of new compounds from natural raw materials such as petroleum and mineral ores. It is essential to make the reaction as efficient as possible, maximizing the yield and minimizing the amount of reagents, energy inputs and waste. Catalysts are especially helpful for reducing the energy required for the reaction and increasing its reaction rate.[59][60]

text: Some specific reactions have their niche applications. For example, the thermite reaction is used to generate light and heat in pyrotechnics and welding. Although it is less controllable than the more conventional oxy-fuel welding, arc welding and flash welding, it requires much less equipment and is still used to mend rails, especially in remote areas.[61]

text: Monitoring

text: Mechanisms of monitoring chemical reactions depend strongly on the reaction rate. Relatively slow processes can be analyzed in situ for the concentrations and identities of the individual ingredients. Important tools of real time analysis are the measurement of pH and analysis of optical absorption (color) and emission spectra. A less accessible but rather efficient method is introduction of a radioactive isotope into the reaction and monitoring how it changes over time and where it moves to; this method is often used to analyze redistribution of substances in the human body. Faster reactions are usually studied with ultrafast laser spectroscopy where utilization of femtosecond lasers allows short-lived transition states to be monitored at time scaled down to a few femtoseconds.[62]


document_title: Independence Day (United States)

question_text: when did fireworks on 4th of july start

question_tokens: when did fireworks on 4th of july start

root

text: Independence Day (United States)

text: Background

text: During the American Revolution, the legal separation of the Thirteen Colonies from Great Britain in 1776 actually occurred on July 2, when the Second Continental Congress voted to approve a resolution of independence that had been proposed in June by Richard Henry Lee of Virginia declaring the United States independent from Great Britain's rule.[5][6] After voting for independence, Congress turned its attention to the Declaration of Independence, a statement explaining this decision, which had been prepared by a Committee of Five, with Thomas Jefferson as its principal author. Congress debated and revised the wording of the Declaration, finally approving it two days later on July 4. A day earlier, John Adams had written to his wife Abigail:

tag: p

text: The second day of July, 1776, will be the most memorable epoch in the history of America. I am apt to believe that it will be celebrated by succeeding generations as the great anniversary festival. It ought to be commemorated as the day of deliverance, by solemn acts of devotion to God Almighty. It ought to be solemnized with pomp and parade, with shows, games, sports, guns, bells, bonfires, and illuminations, from one end of this continent to the other, from this time forward forever more.[7]

text: Adams's prediction was off by two days. From the outset, Americans celebrated independence on July 4, the date shown on the much-publicized Declaration of Independence, rather than on July 2, the date the resolution of independence was approved in a closed session of Congress.[8]

text: Historians have long disputed whether members of Congress signed the Declaration of Independence on July 4, even though Thomas Jefferson, John Adams, and Benjamin Franklin all later wrote that they had signed it on that day. Most historians have concluded that the Declaration was signed nearly a month after its adoption, on August 2, 1776, and not on July 4 as is commonly believed.[9][10][11][12][13]

text: Coincidentally, both John Adams and Thomas Jefferson, the only signers of the Declaration of Independence later to serve as Presidents of the United States, died on the same day: July 4, 1826, which was the 50th anniversary of the Declaration.[14] Although not a signer of the Declaration of Independence, James Monroe, another Founding Father who was elected as President, also died on July 4, 1831. He was the third President who died on the anniversary of independence.[15] Calvin Coolidge, the 30th President, was born on July 4, 1872; so far he is the only U.S. President to have been born on Independence Day.[16]

text: Observance

text:
In 1777, thirteen gunshots were fired in salute, once at morning and once again as evening fell, on July 4 in Bristol, Rhode Island. An article in July 18, 1777 issue of The Virginia Gazette noted a celebration in Philadelphia in a manner a modern American would find familiar: an official dinner for the Continental Congress, toasts, 13-gun salutes, speeches, prayers, music, parades, troop reviews, and fireworks. Ships in port were decked with red, white, and blue bunting.[17]
In 1778, from his headquarters at Ross Hall, near New Brunswick, New Jersey, General George Washington marked July 4 with a double ration of rum for his soldiers and an artillery salute (feu de joie). Across the Atlantic Ocean, ambassadors John Adams and Benjamin Franklin held a dinner for their fellow Americans in Paris, France.[18]

tag: ul

text:
In 1779, July 4 fell on a Sunday. The holiday was celebrated on Monday, July 5.[18]
In 1781, the Massachusetts General Court became the first state legislature to recognize July 4 as a state celebration.[18]
In 1783, Salem, North Carolina held a celebration with a challenging music program assembled by Johann Friedrich Peter entitled The Psalm of Joy. The town claims to be the first public July 4 event, as it was carefully documented by the Moravian Church, and there are no government records of any earlier celebrations.[19]
In 1870, the U.S. Congress made Independence Day an unpaid holiday for federal employees.[20]
In 1938, Congress changed Independence Day to a paid federal holiday.[21]

text: Customs

text: Independence Day is a national holiday marked by patriotic displays. Similar to other summer-themed events, Independence Day celebrations often take place outdoors. According to 5 U.S.C. § 6103, Independence Day is a federal holiday, so all non-essential federal institutions (such as the postal service and federal courts) are closed on that day. Many politicians make it a point on this day to appear at a public event to praise the nation's heritage, laws, history, society, and people.[citation needed]

text: Families often celebrate Independence Day by hosting or attending a picnic or barbecue; many take advantage of the day off and, in some years, a long weekend to gather with relatives or friends. Decorations (e.g., streamers, balloons, and clothing) are generally colored red, white, and blue, the colors of the American flag. Parades are often held in the morning, before family get-togethers, while fireworks displays occur in the evening after dark at such places as parks, fairgrounds, or town squares.[citation needed]

text: The night before the Fourth was once the focal point of celebrations, marked by raucous gatherings often incorporating bonfires as their centerpiece. In New England, towns competed to build towering pyramids, assembled from barrels and casks. They were lit at nightfall to usher in the celebration. The highest were in Salem, Massachusetts, with pyramids composed of as many as forty tiers of barrels. These made the tallest bonfires ever recorded. The custom flourished in the 19th and 20th centuries and is still practiced in some New England towns.[22]

text: Independence Day fireworks are often accompanied by patriotic songs such as the national anthem, "The Star-Spangled Banner"; "God Bless America"; "America the Beautiful"; "My Country, 'Tis of Thee"; "This Land Is Your Land"; "Stars and Stripes Forever"; and, regionally, "Yankee Doodle" in northeastern states and "Dixie" in southern states. Some of the lyrics recall images of the Revolutionary War or the War of 1812.[citation needed]

text: Firework shows are held in many states, and many fireworks are sold for personal use or as an alternative to a public show. Safety concerns have led some states to ban fireworks or limit the sizes and types allowed. In addition, local and regional weather conditions may dictate whether the sale or use of fireworks in an area will be allowed. Some local or regional firework sales are limited or prohibited because of dry weather or other specific concerns. On these occasions the public may be prohibited from purchasing or discharging fireworks, but professional displays (such as those at sports events) may still take place, if certain safety precautions have been taken.[citation needed]

text: A salute of one gun for each state in the United States, called a "salute to the union," is fired on Independence Day at noon by any capable military base.[23]

text: New York City has the largest fireworks display in the country, with more than 22 tons of pyrotechnics exploded in 2009.[24] It generally holds displays in the East River. Other major displays are in Seattle on Lake Union; in San Diego over Mission Bay; in Boston on the Charles River; in Philadelphia over the Philadelphia Museum of Art; in San Francisco over the San Francisco Bay; and on the National Mall in Washington, D.C.[25]

text: During the annual Windsor–Detroit International Freedom Festival, Detroit, Michigan hosts one of the largest fireworks displays in North America, over the Detroit River, to celebrate Independence Day in conjunction with Windsor, Ontario's celebration of Canada Day.[26]

text: The first week of July is typically one of the busiest United States travel periods of the year, as many people use what is often a three-day holiday weekend for extended vacation trips.[27]

text: Celebration gallery

text:
Play media

Patriotic trailer shown in theaters celebrating July 4, 1940

In addition to a fireworks show, Miami, Florida, lights one of its tallest buildings with the patriotic red, white and blue color scheme on Independence Day.

New York City's fireworks display, shown above over the East Village, is sponsored by Macy's and is the largest[24] in the country.

A festively decorated Independence Day cake

text: Notable celebrations

text:
Held since 1785, the Bristol Fourth of July Parade in Bristol, Rhode Island, is the oldest continuous Independence Day celebration in the United States.[28]
Since 1868, Seward, Nebraska, has held a celebration on the same town square. In 1979 Seward was designated "America's Official Fourth of July City-Small Town USA" by resolution of Congress. Seward has also been proclaimed "Nebraska's Official Fourth of July City" by Governor James Exon in proclamation. Seward is a town of 6,000 but swells to 40,000+ during the July 4 celebrations.[29]
Since 1912, the Rebild Society, a Danish-American friendship organization, has held a July 4 weekend festival that serves as a homecoming for Danish-Americans in the Rebild Hills of Denmark.[30]
Since 1959, the International Freedom Festival is jointly held in Detroit, Michigan and Windsor, Ontario during the last week of June each year as a mutual celebration of Independence Day and Canada Day (July 1). It culminates in a large fireworks display over the Detroit River.
The famous Macy's fireworks display usually held over the East River in New York City has been televised nationwide on NBC since 1976. In 2009, the fireworks display was returned to the Hudson River for the first time since 2000 to commemorate the 400th anniversary of Henry Hudson's exploration of that river.[31]
The Boston Pops Orchestra has hosted a music and fireworks show over the Charles River Esplanade called the "Boston Pops Fireworks Spectacular" annually since 1973.[32] The event was broadcast nationally from 1991 until 2002 on A&E, and since 2002 by CBS and its Boston station WBZ-TV. WBZ/1030 and WBZ-TV broadcast the entire event locally, and from 2002 through 2012, CBS broadcast the final hour of the concert nationally in primetime. The national broadcast was put on hiatus beginning in 2013, which Pops executive producer David G. Mugar believed was the result of decreasing viewership caused by NBC's encore presentation of the Macy's fireworks.[33][34] The national broadcast was revived for 2016, and expanded to two hours.[35] In 2017, Bloomberg Television took over coverage duty, with WHDH carrying local coverage beginning in 2018.[36]
On the Capitol lawn in Washington, D.C., A Capitol Fourth, a free concert broadcast live by PBS, NPR and the American Forces Network, precedes the fireworks and attracts over half a million people annually.[37]

text: Other countries

text:
The Philippines celebrates July 4 as its Republic Day to commemorate that day in 1946 when it ceased to be a U.S. territory and the United States officially recognized Philippine Independence.[38]
July 4 was intentionally chosen by the United States because it corresponds to its Independence Day, and this day was observed in the Philippines as Independence Day until 1962. In 1964, the name of the July 4 holiday was changed to Republic Day. Rebild National Park in Denmark is said to hold the largest July 4 celebrations outside of the United States.[39]


document_title: History of Texas (1845–1860)

question_text: river that mexico claimed was the texas border

text: History of Texas (1845–1860)

text: Annexation[edit]

text: The Republic of Texas had formed in 1836, after breaking away from Mexico in the Texas Revolution. The following year, an ambassador from Texas approached the United States about the possibility of becoming an American state. Fearing a war with Mexico, which did not recognize Texas independence, the United States declined the offer.[1] In 1844, James K. Polk was elected the United States president after promising to annex Texas. Before he assumed office, the outgoing president, John Tyler, entered negotiations with Texas. On February 26, 1845, six days before Polk took office, the U.S. Congress approved the annexation. The Texas legislature approved annexation in July 1845 and constructed a state constitution. In October, Texas residents approved the annexation and the new constitution, and Texas was officially inducted into the United States on December 29, 1845.[2]

text: Mexican–American War[edit]

text: When Texas was annexed, Mexico broke diplomatic relations with the United States. The annexation bill did not specifically define the boundaries of Texas. The former republic claimed the Rio Grande as its southern border, while Mexican authorities had always considered the Nueces River, situated further north, to be the boundary of Mexican Texas. The United States sent John Slidell to negotiate with the Mexican government, offering $25 million ($707,115,385 today) to set the Texas border at the Rio Grande and to purchase Mexico's provinces of Alta California and Santa Fe de Nuevo México. Popular sentiment in Mexico was against any sale, and the army deposed President José Joaquín de Herrera when he appeared inclined to negotiate with Slidell.

text: The United States positioned troops along the Rio Grande. On April 25, 1846, in an event known as the Thornton Affair, a large contingent of Mexican cavalry attacked an American patrol in the area between the Rio Grande and the Nueces, killing 16 Americans. On May 3, Mexican troops initiated the siege of Fort Texas, bombarding a makeshift American fort along the Rio Grande. On May 8, Zachary Taylor led 2,500 U.S. troops to relieve the fort. He was intercepted by Mexican troops, leading to the Battle of Palo Alto. Mexican troops retreated a short distance to regroup, and the following day the two sides fought fiercely in the Battle of Resaca de la Palma. The U.S. cavalry captured the Mexican artillery, and the Mexican soldiers retreated.

text: The United States officially declared war against Mexico on May 13. Mexico declared war against the U.S. on July 7. Throughout the official hostilities, the United States maintained two fronts—one in the Mexican interior south of the Rio Grande, and one in California. There was no further fighting in Texas.

text: The war ended on February 2, 1848, with the signing of the Treaty of Guadalupe Hidalgo. Mexico ceded claims to Texas, and the border was set at the Rio Grande.

text: Compromise of 1850[edit]

text: The expansion of the United States after the Mexican–American War led to tensions between the slave and free states as to how to maintain the balance between the opposing viewpoints. Texas had been admitted to the United States as a slave state, yet Texas claimed territory north of the 36°30' demarcation line for slavery set by the 1820 Missouri Compromise. According to the annexation agreement, if Texas were to be subdivided into multiple states, those north of the compromise line would become free states. Following the conclusion of the Mexican–American War, Texas also tried to exert control over much of New Mexico.

text: In an effort to avoid some states seceding from the United States, Congress passed the Compromise of 1850. Texas gave up much of the western territories it had claimed in exchange for $10 million to pay off previous debts.

text: Settlement[edit]

text: Post-war Texas grew rapidly as migrants poured into the cotton lands of the state.[3] German immigrants started to arrive in the early 1840s because of economic, social and political conditions in their states. In 1842, German nobles organized the Adelsverein, banding together to buy land in central Texas to enable German settlement. The Revolutions of 1848 acted as another catalyst for so many immigrants that they became known as the "Forty-Eighters." Many were educated artisans and businessmen. Germans continued to arrive in considerable numbers until 1890.[4]

text: The first Czech immigrants started their journey to Texas on August 19, 1851 headed by Jozef Šilar. The rich farmland of Central Texas attracted the Czech immigrants. The counties of Austin, Fayette, Lavaca, and Washington had early Czech settlements. The Czech-American communities are characterized by a strong sense of community and social clubs were a dominant theme of Czech-American life in Texas. By 1865, the Czech population numbered 700 and climbed to over 60,000 Czech-Americans by 1940.[5]

text: With their investments in cotton cultivation, Texas planters imported enslaved blacks from the earliest years of settlement. They established cotton plantations mostly in the eastern part of the state, where labor was done by enslaved African Americans. The central area of the state had more subsistence farmers.

text: Indian wars[edit]

text: In the late 1850s, settlers continued to push west and north, and by 1856 had begun settling, parts of the Comancheria in large numbers. Angry at the loss of their traditional hunting grounds, several bands of Comanche conducted raids on Texas settlers. In an effort to stop the violence and subdue the Comanche, in 1858 the Texas Rangers paired with members of the Tonkawa tribe—traditionally, enemies of the Comanche—for the Antelope Hills Expedition. Federal law promised Indian tribes safety in Indian Territory, located just north of Texas. Nevertheless, the Rangers crossed into Indian Territory and attacked a Comanche village at the Battle of Little Robe Creek. This was the first time any American forces had penetrated to the heart of the Comancheria, attacked Comanche villages with impunity, and successfully made it home. The expedition exhausted the annual Texas defense budget, and the governor disbanded the Rangers.


document_title: Ugarte

question_text: where does the last name ugarte come from

text: Ugarte

text:
Ugarte or uharte is a Basque word meaning "island".
It can also be spelt Huarte or Hugarte in Spanish. Notable people with the surname include:

text:
John Ugarte (born 1977), Military Leader
Adriana Ugarte (born 1985), Spanish actress
Agustín Jerónimo de Iturbide y Huarte (1807–1866), Prince Imperial of Mexico
Alfonso Ugarte (1847–1880), Peruvian military commander
Alirio Ugarte Pelayo (1923–1966), Venezuelan politician
Ana Carolina Ugarte (born 1992), Venezuelan model and beauty pageant titleholder
Augusto José Ramón Pinochet Ugarte (1915–2006), Chilean military officer and dictator
Jose de la Torre Ugarte (1798–1878), Peruvian lyricist
José Humberto Ugarte (born 1980), Costa Rican footballer
John Huarte (born 1944), American football quarterback
Juan Huarte de San Juan (c. 1530–1592), Spanish physician and psychologist
Juan Ugarte (born 1982), Spanish footballer
Julio Ugarte y Ugarte (1890–1949), Peruvian writer
Manuel Ugarte (1875–1951), Argentinian author
Nicolás Armentia Ugarte (1845–1909), Spanish missionary and bishop
Renzo Pi Hugarte (1934–2012), Uruguayan anthropologist
Salvador de Iturbide y Huarte (1820–1856), prince of Mexico
Víctor Ugarte (1926–1995), Bolivian footballer
Víctor Ugarte (voice actor), Mexican voice actor
Xóchitl Ugarte (born 1979), Mexican voice actress

text: Fictional characters:

text: Guillermo Ugarte, a character in the film Casablanca