chemical reaction

Any chemical process in which substances are changed into different ones, with different properties, as distinct from changing position or form (phase).

Chemical reactions involve the rupture or rearrangement of the bonds holding atoms together (see bonding), never atomic nuclei. The total mass and number of atoms of all reactants equals those of all products, and energy is almost always consumed or liberated (see heat of reaction). The speed of reactions varies (see reaction rate). Understanding their mechanisms lets chemists alter reaction conditions to optimize the rate or the amount of a given product; the reversibility of the reaction and the presence of competing reactions and intermediate products complicate these studies. Reactions can be syntheses, decompositions, or rearrangements, or they can be additions, eliminations, or substitutions. Examples include oxidation-reduction, polymerization, ionization (see ion), combustion (burning), hydrolysis, and acid-base reactions.

* * *

Introduction

 a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements (chemical element) or compounds (chemical compound). A chemical reaction rearranges the constituent atoms (atom) of the reactants to create different substances as products.

 Chemical reactions are an integral part of technology, of culture, and indeed of life itself. Burning fuels, smelting iron, making glass and pottery, brewing beer, and making wine and cheese are among many examples of activities incorporating chemical reactions that have been known and used for thousands of years. Chemical reactions abound in the geology of Earth, in the atmosphere and oceans (ocean), and in a vast array of complicated processes that occur in all living systems.

      Chemical reactions must be distinguished from physical changes. Physical changes include changes of state, such as ice melting to water and water evaporating to vapour. If a physical change occurs, the physical properties of a substance will change, but its chemical identity will remain the same. No matter what its physical state, water (H₂O) is the same compound, with each molecule composed of two atoms of hydrogen and one atom of oxygen. However, if water, as ice, liquid, or vapour, encounters sodium metal (Na), the atoms will be redistributed to give the new substances molecular hydrogen (H₂) and sodium hydroxide (NaOH). By this, we know that a chemical change or reaction has occurred.

Historical overview

      The concept of a chemical reaction dates back about 250 years. It had its origins in early experiments that classified substances as elements (chemical element) and compounds (chemical compound) and in theories that explained these processes. Development of the concept of a chemical reaction had a primary role in defining the science of chemistry as it is known today.

  The first substantive studies in this area were on gases (gas). The identification of oxygen in the 18th century by Swedish chemist Carl Wilhelm Scheele (Scheele, Carl Wilhelm) and English clergyman Joseph Priestley (Priestley, Joseph) had particular significance. The influence of French chemist Antoine-Laurent Lavoisier (Lavoisier, Antoine-Laurent) was especially notable, in that his insights confirmed the importance of quantitative measurements of chemical processes. In his book Traité élémentaire de chimie (1789; Elementary Treatise on Chemistry), Lavoisier identified 33 “elements”—substances not broken down into simpler entities. Among his many discoveries, Lavoisier accurately measured the weight gained when elements were oxidized, and he ascribed the result to the combining of the element with oxygen. The concept of chemical reactions involving the combination of elements clearly emerged from his writing, and his approach led others to pursue experimental chemistry as a quantitative science.

      The other occurrence of historical significance concerning chemical reactions was the development of atomic theory. For this, much credit goes to English chemist John Dalton (Dalton, John), who postulated his atomic theory early in the 19th century. Dalton maintained that matter is composed of small, indivisible particles, that the particles, or atoms (atom), of each element were unique, and that chemical reactions were involved in rearranging atoms to form new substances. This view of chemical reactions accurately defines the current subject. Dalton's theory provided a basis for understanding the results of earlier experimentalists, including the law of conservation of matter (matter is neither created nor destroyed) and the law of constant composition (all samples of a substance have identical elemental compositions).

      Thus, experiment and theory, the two cornerstones of chemical science in the modern world, together defined the concept of chemical reactions. Today experimental chemistry provides innumerable examples, and theoretical chemistry allows an understanding of their meaning.

Basic concepts of chemical reactions

Synthesis (chemical synthesis)

      When making a new substance from other substances, chemists say either that they carry out a synthesis (chemical synthesis) or that they synthesize the new material. Reactants are converted to products, and the process is symbolized by a chemical equation. For example, iron (Fe) and sulfur (S) combine to form iron sulfide (FeS).

Fe(s) + S(s) {long right arrow} FeS(s)

      The plus sign indicates that iron reacts with sulfur. The arrow signifies that the reaction “forms” or “yields” iron sulfide, the product. The state of matter of reactants and products is designated with the symbols (s) for solids (solid), ( l) for liquids (liquid), and (g) for gases (gas).

The conservation of matter (mass, conservation of)

      In reactions under normal laboratory conditions, matter is neither created nor destroyed, and elements are not transformed into other elements. Therefore, equations depicting reactions must be balanced; that is, the same number of atoms (atom) of each kind must appear on opposite sides of the equation. The balanced equation for the iron-sulfur reaction shows that one iron atom can react with one sulfur atom to give one formula unit of iron sulfide.

      Chemists ordinarily work with weighable quantities of elements (chemical element) and compounds (chemical compound). For example, in the iron-sulfur equation the symbol Fe represents 55.845 grams of iron, S represents 32.066 grams of sulfur, and FeS represents 87.911 grams of iron sulfide. Because matter is not created or destroyed in a chemical reaction, the total mass of reactants is the same as the total mass of products. If some other amount of iron is used, say, one-tenth as much (5.585 grams), only one-tenth as much sulfur can be consumed (3.207 grams), and only one-tenth as much iron sulfide is produced (8.791 grams). If 32.066 grams of sulfur were initially present with 5.585 grams of iron, then 28.859 grams of sulfur would be left over when the reaction was complete.

      The reaction of methane ( CH₄, a major component of natural gas) with molecular oxygen (O₂) to produce carbon dioxide (CO₂) and water can be depicted by the chemical equation

CH₄(g) + 2O₂(g) {long right arrow} CO₂(g) + 2H₂O( l)

      Here another feature of chemical equations appears. The number 2 preceding O₂ and H₂O is a stoichiometric factor. (The number 1 preceding CH₄ and CO₂ is implied.) This indicates that one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water. The equation is balanced because the same number of atoms of each element appears on both sides of the equation (here one carbon, four hydrogen, and four oxygen atoms). Analogously with the iron-sulfur example, we can say that 16 grams of methane and 64 grams of oxygen will produce 44 grams of carbon dioxide and 36 grams of water. That is, 80 grams of reactants will lead to 80 grams of products.

      The ratio of reactants and products in a chemical reaction is called chemical stoichiometry. Stoichiometry depends on the fact that matter is conserved in chemical processes, and calculations giving mass relationships are based on the concept of the mole. One mole of any element or compound contains the same number of atoms or molecules, respectively, as one mole of any other element or compound. By international agreement, one mole of the most common isotope of carbon (carbon-12) has a mass of exactly 12 grams (this is called the molar mass) and represents 6.02214179 × 10²³ atoms ( Avogadro's number). One mole of iron contains 55.847 grams; one mole of methane contains 16.043 grams; one mole of molecular oxygen is equivalent to 31.999 grams; and one mole of water is 18.015 grams. Each of these masses represents 6.0221 × 10²³ molecules.

Energy considerations

       energy plays a key role in chemical processes. According to the modern view of chemical reactions, bonds (chemical bonding) between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved on making new bonds, and the net result is the absorption of energy. Such a reaction is said to be endothermic if the energy is in the form of heat. The opposite of endothermic is exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic (energy evolved) and endoergic (energy required) are used when forms of energy other than heat are involved.

      A great many common reactions are exothermic. The formation of compounds from the constituent elements is almost always exothermic. Formation of water from molecular hydrogen and oxygen and the formation of a metal oxide such as calcium oxide (CaO) from calcium metal and oxygen gas are examples. Among widely recognizable exothermic reactions is the combustion of fuels (such as the reaction of methane with oxygen mentioned previously).

      The formation of slaked lime (calcium hydroxide, Ca(OH)₂) when water is added to lime (CaO) is exothermic.

CaO(s) + H2O (l) {long right arrow} Ca(OH)₂(s)

      This reaction occurs when water is added to dry portland cement to make concrete, and heat evolution of energy as heat is evident because the mixture becomes warm.

      Not all reactions are exothermic (or exoergic). A few compounds, such as nitric oxide (NO) and hydrazine (N₂H₄), require energy input when they are formed from the elements. The decomposition of limestone (CaCO₃) to make lime (CaO) is also an endothermic process; it is necessary to heat limestone to a high temperature for this reaction to occur.

CaCO₃(s) {long right arrow} CaO(s) + CO₂(g)

      The decomposition of water into its elements by the process of electrolysis is another endoergic process. Electrical energy is used rather than heat energy to carry out this reaction.

2 H₂O(g) {long right arrow} 2 H₂(g) + O₂(g)

      Generally, evolution of heat in a reaction favours the conversion of reactants to products. However, entropy is important in determining the favourability of a reaction. Entropy is a measure of the number of ways in which energy can be distributed in any system. Entropy accounts for the fact that not all energy available in a process can be manipulated to do work.

      A chemical reaction will favour the formation of products if the sum of the changes in entropy for the reaction system and its surroundings is positive. An example is burning wood. Wood has a low entropy. When wood burns, it produces ash as well as the high-entropy substances carbon dioxide gas and water vapour. The entropy of the reacting system increases during combustion. Just as important, the heat energy transferred by the combustion to its surroundings increases the entropy in the surroundings. The total of entropy changes for the substances in the reaction and the surroundings is positive, and the reaction is product-favoured.

      When hydrogen and oxygen react to form water, the entropy of the products is less than that of the reactants. Offsetting this decrease in entropy, however, is the increase in entropy of the surroundings owing to the heat transferred to it by the exothermic reaction. Again because of the overall increase in entropy, the combustion of hydrogen is product-favoured.

Kinetic considerations

      Chemical reactions commonly need an initial input of energy to begin the process. Although the combustion of wood, paper, or methane is an exothermic process, a burning match or a spark is needed to initiate this reaction. The energy supplied by a match arises from an exothermic chemical reaction that is itself initiated by the frictional heat generated by rubbing the match on a suitable surface.

 In some reactions, the energy to initiate a reaction can be provided by light. Numerous reactions in Earth's atmosphere are photochemical, or light-driven, reactions initiated by solar radiation. One example is the transformation of ozone (O₃) into oxygen (O₂) in the troposphere. The absorption of ultraviolet light (hν) from the Sun to initiate this reaction prevents potentially harmful high-energy radiation from reaching Earth's surface.

      For a reaction to occur, it is not sufficient that it be energetically product-favoured. The reaction must also occur at an observable rate. Several factors influence reaction rates (reaction rate), including the concentrations of reactants, the temperature, and the presence of catalysts (catalyst). The concentration affects the rate at which reacting molecules collide, a prerequisite for any reaction. temperature is influential because reactions occur only if collisions between reactant molecules are sufficiently energetic. The proportion of molecules with sufficient energy to react is related to the temperature. Catalysts affect rates by providing a lower energy pathway by which a reaction can occur. Among common catalysts are precious metal compounds used in automotive exhaust systems that accelerate the breakdown of pollutants such as nitrogen dioxide into harmless nitrogen and oxygen. A wide array of biochemical catalysts are also known, including chlorophyll in plants (plant) (which facilitates the reaction by which atmospheric carbon dioxide is converted to complex organic molecules such as glucose) and many biochemical catalysts called enzymes (enzyme). The enzyme pepsin, for example, assists in the breakup of large protein molecules during digestion.

Classifying chemical reactions

      Chemists classify reactions in a number of ways: (a) by the type of product, (b) by the types of reactants, (c) by reaction outcome, and (d) by reaction mechanism. Often, a given reaction can be placed in two or even three categories.

Classification by type of product

Gas-forming reactions

      Many reactions produce a gas such as carbon dioxide, hydrogen sulfide (H₂S), ammonia (NH₃), or sulfur dioxide (SO₂). An example of a gas-forming reaction is that which occurs when a metal carbonate such as calcium carbonate (CaCO₃, the chief component of limestone, seashells, and marble) is mixed with hydrochloric acid (HCl) to produce carbon dioxide.

CaCO₃(s) + 2 HCl(aq) {long right arrow} CaCl₂(aq) + CO₂(g) + H₂O (l)

      In this equation, the symbol (aq) signifies that a compound is in an aqueous, or water, solution.

 Cake-batter rising is caused by a gas-forming reaction between an acid and baking soda, sodium hydrogen carbonate ( sodium bicarbonate, NaHCO₃). tartaric acid (C₄H₆O₆), an acid found in many foods, is often the acidic reactant.

C₄H₆O₆(aq) + NaHCO₃(aq) {long right arrow} NaC₄H₅O₆(aq) + H₂O (l) + CO₂(g)

      In this equation, NaC₄H₅O₆ is sodium tartrate.

      Most baking powders contain both tartaric acid and sodium hydrogen carbonate, which are kept apart by using starch as a filler. When baking powder is mixed into the moist batter, the acid and sodium hydrogen carbonate dissolve slightly, which allows them to come into contact and react. Carbon dioxide is produced, and the batter rises.

Precipitation reactions

      Formation of an insoluble compound will sometimes occur when a solution containing a particular cation (a positively charged ion) is mixed with another solution containing a particular anion (a negatively charged ion). The solid that separates is called a precipitate.

      Compounds (chemical compound) having anions such as sulfide (S²⁻), hydroxide (OH⁻), carbonate (CO₃²⁻), and phosphate (PO₄³⁻) are often insoluble in water. A precipitate will form if a solution containing one of these anions is added to a solution containing a metal cation such as Fe²⁺, Cu²⁺, or Al³⁺.

Fe²⁺(aq) + 2 OH⁻(aq) {long right arrow} Fe(OH)2(s)

Al³⁺(aq) + PO₄³⁻(aq) {long right arrow} AlPO⁴(s)

      Minerals are water-insoluble compounds. Precipitation reactions in nature can account for mineral formation in many cases, such as with undersea vents called “black smokers” that form metal sulfides.

Classification by types of reactants

      Two types of reactions involve transfer of a charged species. Oxidation-reduction reactions (oxidation–reduction reaction) occur with electron transfer between reagents. In contrast, reactions of acids with bases (base) in water involve proton (H⁺) transfer from an acid to a base.

Oxidation-reduction reactions (oxidation–reduction reaction)

      Oxidation-reduction (redox) reactions (oxidation–reduction reaction) involve the transfer of one or more electrons from a reducing agent to an oxidizing agent. This has the effect of reducing the real or apparent electric charge on an atom in the substance being reduced and of increasing the real or apparent electric charge on an atom in the substance being oxidized. Simple redox reactions include the reactions of an element with oxygen. For example, magnesium burns in oxygen to form magnesium oxide (MgO). The product is an ionic compound, made up of Mg²⁺ and O²⁻ ions. The reaction occurs with each magnesium atom giving up two electrons and being oxidized and each oxygen atom accepting two electrons and being reduced.

      Another common redox reaction is one step in the rusting of iron in damp air.

2Fe(s) + 2H₂O(l) + O₂(g) {long right arrow} 2Fe(OH)₂(s)

      Here iron metal is oxidized to iron dihydroxide (Fe(OH)₂); elemental oxygen (O₂) is the oxidizing agent.

      Redox reactions are the source of the energy of batteries (battery). The electric current generated by a battery arises because electrons are transferred from a reducing agent to an oxidizing agent through the external circuitry. In a common dry cell and in alkaline batteries, two electrons per zinc atom are transferred to the oxidizing agent, thereby converting zinc metal to the Zn²⁺ ion. In dry-cell batteries, which are often used in flashlights, the electrons given up by zinc are taken up by ammonium ions (NH₄⁺) present in the battery as ammonium chloride (NH₄Cl). In alkaline batteries, which are used in calculators (calculator) and watches (watch), the electrons are transferred to a metal oxide such as silver oxide (AgO), which is reduced to silver metal in the process.

Acid-base reactions (acid–base reaction)

      Acids (acid) and bases (base) are important compounds in the natural world, so their chemistry is central to any discussion of chemical reactions. There are several theories of acid-base behaviour.

The Arrhenius theory

      The Arrhenius theory, named after Swedish physicist Svante August Arrhenius (Arrhenius, Svante August), views an acid as a substance that increases the concentration of the hydronium ion (H₃O⁺) in an aqueous solution and a base as a substance that increases the hydroxide ion (OH⁻) concentration. Well-known acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), and acetic acid ( CH₃COOH). Bases includes such common substances as caustic soda (sodium hydroxide, NaOH) and slaked lime (calcium hydroxide, Ca(OH)₂). Another common base is ammonia (NH₃), which reacts with water to give a basic solution according to the following balanced equation.

NH₃(aq) + H₂O(l) {long right arrow} NH₄⁺(aq) + OH⁻(aq)

      (This reaction occurs to a very small extent; the hydroxide ion concentration is small but measurable.)

      A large number of natural bases are known, including morphine, cocaine, nicotine, and caffeine; many synthetic drugs are also bases. All of these contain a nitrogen atom bonded to three other groups, and all behave similarly to ammonia in that they can react with water to give a solution containing the hydroxide ion.

      Amino acids (amino acid), a very important class of compounds, are able to function both as acids and as bases. Amino acid molecules (molecule) contain both acidic (−COOH) and basic (−NH₂) sites. In an aqueous solution, amino acids exist in both the molecular form and the so-called "zwitterionic" form, H₃N + CH₂CO₂⁻. In this structure the nitrogen atom bears a positive charge, and the oxygen atom of the acid group bears a negative charge.

      According to the Arrhenius theory, acid-base reactions (acid–base reaction) involve the combination of the hydrogen ion (H⁺) and the hydroxide ion to form water. An example is the reaction of aqueous solutions of sodium hydroxide and hydrochloric acid.

HCl(aq) + NaOH(aq) {long right arrow} NaCl(aq) + H₂O (l)

The Brønsted-Lowry theory

      A somewhat more general acid-base theory, the Brø nsted-Lowry theory (Brønsted–Lowry theory), named after Danish chemist Johannes Nicolaus Brø nsted (Brønsted, Johannes Nicolaus) and English chemist Thomas Martin Lowry, defines an acid as a proton donor and a base as a proton acceptor. In this theory, the reaction of an acid and base is represented as an equilibrium reaction.

acid (1) + base (2) ⇌ base (1) + acid (2)

      (The double arrows, ⇌, indicate that the products can re-form the reactants in a dynamic process.)

      Acid (1) and base (1) are called a conjugate acid-base pair, as are acid (2) and base (2). The advantage of this theory is its predictive capacity. Whether the equilibrium lies toward the reactants (reactant-favoured) or the products (product-favoured) is determined by the relative strengths of the acids and bases.

      The Brønsted-Lowry theory is often closely associated with the solvent water. Dissolving an acid in water to form the hydronium ion and the anion of the acid is an acid-base reaction. Acids are classified as strong or weak, depending on whether the equilibrium favours the reactants or products. Hydrochloric acid, a strong acid, ionizes completely in water to form the hydronium and chlorine (Cl⁻) ions in a product-favoured reaction.

HCl(aq) + H₂O (l) {long right arrow} H₃O⁺(aq) +Cl⁻(aq)

      Using the Brønsted-Lowry theory, the reaction of ammonia and hydrochloric acid in water is represented by the following equation:

NH₃(aq) + HCl(aq) {long right arrow} NH₄⁺(aq) + Cl⁻(aq)

      Hydrochloric acid and the chlorine ion are one conjugate acid-base pair, and the ammonium ion and ammonia are the other. The acid-base reaction is the transfer of the hydrogen ion from the acid (HCl) to the base (NH₃). The equilibrium favours the weaker acid and base, in this case the products. Note that the hydroxide ion does not appear in this equation, a point differentiating the Arrhenius and Brønsted-Lowry theories.

The Lewis theory

      A still broader acid and base theory was proposed by American physical chemist Gilbert Newton Lewis (Lewis, Gilbert N.). In the Lewis theory, bases are defined as electron-pair donors and acids as electron-pair acceptors. Acid-base reactions involve the combination of the Lewis acid and base through sharing of the base's electron pair.

      Ammonia is an example of a Lewis base. A pair of electrons located on the nitrogen atom may be used to form a chemical bond to a Lewis acid such as boron trifluoride (BF₃). (In the following equation, the colon represents an electron pair.)

H₃N: + BF₃ {long right arrow} H₃N− BF₃

      Ammonia, water, and many other Lewis bases react with metal ions to form a group of species known as coordination compounds (coordination compound). The reaction to form these species is another example of a Lewis acid-base reaction. For example, the light blue colour of a solution of Cu²⁺ ions in water is due to the [Cu(H₂O)₆]²⁺ ion. If ammonia is added to this solution, the water molecules attached to copper are replaced by ammonia molecules, and the beautiful deep blue ion [Cu(NH₃)₄]²⁺ is formed.

Classification by reaction outcome

      Chemists often classify reactions on the basis of the overall result. Here several commonly encountered reactions are classified. As previously noted, many reactions defy simple classification and may fit in several categories.

Decomposition reactions

      Decomposition reactions are processes in which chemical species break up into simpler parts. Usually, decomposition reactions require energy input. For example, a common method of producing oxygen gas in the laboratory is the decomposition of potassium chlorate (KClO₃) by heat.

2KClO₃(s) {long right arrow} 2KCl(s) + 3O₂(g)

      Another decomposition reaction is the production of sodium (Na) and chlorine (Cl₂) by electrolysis of molten sodium chloride (NaCl) at high temperature.

2NaCl (l) {long right arrow} 2Na (l) + Cl₂(g)

      A decomposition reaction that was very important in the history of chemistry is the decomposition of mercury oxide (HgO) with heat to give mercury metal (Hg) and oxygen gas. This is the reaction used by 18th-century chemists Carl Wilhelm Scheele (Scheele, Carl Wilhelm), Joseph Priestley (Priestley, Joseph), and Antoine-Laurent Lavoisier (Lavoisier, Antoine-Laurent) in their experiments on oxygen.

2HgO(s) {long right arrow} 2Hg (l) + O₂(g)

Substitution, elimination, and addition reactions

      These terms are particularly useful in describing organic reactions. In a substitution reaction, an atom or group of atoms in a molecule is replaced by another atom or group of atoms. For example, methane (CH₄) reacts with chlorine (Cl₂) to produce chloromethane (CH₃Cl), a compound used as a topical anesthetic. In this reaction, a chlorine atom is substituted for a hydrogen atom.

      Substitution reactions are widely used in industrial chemistry. For example, substituting two of the chlorine atoms on chloroform (CHCl₃) with fluorine atoms produces chlorodifluoromethane (CHClF₂). This product undergoes a further reaction when heated strongly.

2CHClF₂(g) {long right arrow} F₂C=CF₂(g) + 2HCl(g)

      This latter reaction is an example of an elimination reaction, a hydrogen atom and a chlorine atom being eliminated from the starting material as hydrochloric acid (HCl). The other product is tetrafluoroethylene, a precursor to the polymer known commercially as Teflon.

      Addition reactions (addition reaction) are the opposite of elimination reactions. As the name implies, one molecule is added to another. An example is the common industrial preparation of ethanol (CH₃CH₂OH). Historically, this compound was made by fermentation. However, since the early 1970s, it has also been made commercially by the addition of water to ethylene.

C₂H₄+ H₂O {long right arrow} CH₃CH₂OH

polymerization reactions

      Polymers (polymer) are high-molecular-weight compounds, fashioned by the aggregation of many smaller molecules called monomers (monomer). The plastics (plastic) that have so changed society and the natural and synthetic fibres (fibre, man-made) used in clothing are polymers. There are two basic ways to form polymers: (a) linking small molecules together, a type of addition reaction, and (b) combining two molecules (of the same or different type) with the elimination of a stable small molecule such as water. This latter type of polymerization combines addition and elimination reactions and is called a condensation reaction .

      An example of the first type of reaction is the union of thousands of ethylene molecules that gives polyethylene.

nH₂C=CH₂ {long right arrow} [−CH₂CH₂−]_n

      Other addition polymers include polypropylene (made by polymerizing H₂C=CHCH₃), polystyrene (from H₂C=CH C₆H₅), and polyvinyl chloride (from H₂C=CHCl).

      Starch and cellulose are examples of the second type of polymer. These are members of a class of compounds called carbohydrates (carbohydrate), substances with formulas that are multiples of the simple formula CH₂O. Both starch and cellulose are polymers of glucose, a sugar with the formula C₆H₁₂O₆. In both starch and cellulose, molecules of glucose are joined together with concomitant elimination of a molecule of water for every linkage formed.

nC₆H₁₂O₆ {long right arrow} −[−C₆H₁₀O₅−]−n + nH₂O

      The synthetic material nylon is another example of this type of polymer. Water and a polymer (nylon-6,6) are formed when an organic acid and an amine (a compound derived from ammonia) combine.

      The natural fibres of proteins such as hair, wool, and silk are also polymers that contain the repeating unit (-CHRCONH-), where R is a group of atoms attached to the main polymer. These form by joining amino acids with the elimination of a water molecule for each CONH or peptide linkage formed.

solvolysis and hydrolysis

      A solvolysis reaction is one in which the solvent is also a reactant. Solvolysis reactions are generally named after the specific solvent—for example, the term hydrolysis when water is involved. If a compound is represented by the formula AB (in which A and B are atoms or groups of atoms) and water is represented by the formula HOH, the hydrolysis reaction may be represented by the reversible chemical reaction

AB + HOH ⇌ AH + BOH.

      Hydrolysis of an organic compound is illustrated by the reaction of water with esters (ester). Esters have the general formula RCOOR′, R and R′ being combining groups (such as CH₃). The hydrolysis of an ester produces an acid and an alcohol. The equation for the reaction of methyl acetate and water is

CH₃COOCH3(aq) + H₂O(l) {long right arrow} CH₃COOH(aq) + CH₃OH(aq).

      Hydrolysis reactions play an important role in chemical processes that occur in living organisms. Proteins are hydrolyzed to amino acids, fats (fat) to fatty acids (fatty acid) and glycerol, and starches and complex sugars to simple sugars. In most instances, the rates of these processes are enhanced by the presence of enzymes, biological catalysts.

      Hydrolysis reactions are also important to acid-base behaviour. Anions of weak acids dissolve in water to give basic solutions, as in the hydrolysis of the acetate ion, CH₃C OO⁻.

CH₃COO⁻(aq) + H₂O (l) {long right arrow} CH₃COOH(aq) + OH⁻(aq)

      Although this is a reactant-favoured reaction, it occurs to an extent sufficient to cause a solution containing the acetate ion to exhibit basic properties (e.g., turning red litmus paper blue).

      Hydrolysis reactions account for the basic character of many common substances. Salts (salt) of the borate, phosphate, and carbonate ions, for example, give basic solutions that have long been used for cleaning purposes. Many food products also contain basic anions such as tartrate and citrate ions.

Classification by reaction mechanism

      Reaction mechanisms (reaction mechanism) provide details on how atoms are shuffled and reassembled in the formation of products from reactants. Chain and photolysis reactions are named on the basis of the mechanism of the process.

Chain reactions (chain reaction)

      Chain reactions (chain reaction) occur in a sequence of steps, in which the product of each step is a reagent for the next. Chain reactions generally involve three distinct processes: an initiation step that begins the reaction, a series of chain-propagation steps, and, eventually, a termination step.

      Polymerization reactions are chain reactions, and the formation of Teflon from tetrafluoroethylene is one example. In this reaction, a peroxide (a compound in which two oxygen atoms are joined together by a single covalent bond) may be used as the initiator. Peroxides readily form highly reactive free-radical species (a species with an unpaired electron) that initiate the reaction. There are a number of different ways to terminate the chain, only one of which is shown. (In the following equations, the dots represent unpaired electrons, and R is a generic organic group.)

●

      Decomposition of a peroxide to radicals:

ROOR {long right arrow} 2 RO∙

●

      Chain initiation:

RO∙ + F₂C=CF₂ {long right arrow} ROCF₂CF₂∙

●

      Chain-propagation steps:

ROCF₂CF₂∙ + F₂C=CF₂ {long right arrow} ROCF₂CF₂CF₂CF₂∙

ROCF₂CF₂CF₂CF₂∙ + (n−2)F₂C=CF₂ {long right arrow} RO−(CF₂CF₂∙)_n−

●

      A possible chain-termination step:

RO−(CF₂CF₂∙)_n− + ∙OR {long right arrow} RO(CF₂CF₂)_nOR

photolysis reactions

      Photolysis reactions are initiated or sustained by the absorption of electromagnetic radiation. One example, the decomposition of ozone to oxygen in the atmosphere, is mentioned above in the section Kinetic considerations (chemical reaction). Another example is the synthesis of chloromethane from methane and chlorine, which is initiated by light. The overall reaction is

CH₄(g) + Cl₂(g) + hυ {long right arrow} CH₃Cl(g) + HCl(g),

      where hυ represents light. This reaction, coincidentally, is also a chain reaction. It begins with the endothermic reaction of a chlorine molecule (Cl₂) to give chlorine atoms, a process that occurs under ultraviolet irradiation. When formed, some of the chlorine atoms recombine to form chlorine molecules, but not all do so. If a chlorine atom instead collides with a methane molecule, a two-step chain propagation occurs. The first propagation step produces the methyl radical (CH₃). This free-radical species reacts with a chlorine molecule to give the product and a chlorine atom, which continues the chain reaction for many additional steps. Possible termination steps include combination of two methyl radicals to form ethane (CH₃CH₃) and a combination of methyl and chlorine radicals to give chloromethane.

●

      Chain-initiation step:

Cl₂ ⇌ 2 ∙Cl

●

      Chain-propagation steps:

CH₄ + ∙Cl {long right arrow} ∙CH₃+ HCl

∙CH₃+ Cl₂ {long right arrow} CH₃Cl + ∙Cl

●

      Possible chain-termination steps:

∙CH₃+ ∙CH₃ {long right arrow} CH₃CH₃

∙CH₃+ ∙Cl {long right arrow} CH₃Cl

John C. Kotz Paul M. Treichel

Additional Reading

John C. Kotz, Paul M. Treichel, and John R. Townsend, Chemistry & Chemical Reactivity, 7th ed. (2009), intended for university students, contains examples of chemical reactions woven into descriptions of chemical principles. William H. Brown, Christopher S. Foote, Brent L. Iverson, and Eric Anslyn, Organic Chemistry, 5th ed. (2009), describes the principles of organic chemistry, with chemical reactions organized by the type of molecule undergoing reaction and by the type of reaction. N.N. Greenwood and Alan Earnshaw, Chemistry of the Elements, 2nd ed. (1997), is an advanced textbook and reference book for inorganic chemistry that presents a broad overview of chemical reactions, organized by element.John C. Kotz Paul M. Treichel

* * *