Reactions, Types Of | Research & Encyclopedia Articles

Reactions, Types Of

The number of possible chemical reactions is so vast that chemists have divided them into categories. Although there is no single, universally recognized classification schemes, there are relatively few classification schemes and some very important names of types of reaction are used generally. Most chemists divide reactions into groups based on structural features of molecules and ions that allow them to predict the products that would result if a particular molecule or ion undergoes a class of reaction. Often a chemist can use the classification of reactions and information about a molecule or ion's structure to predict how readily it would react with another molecule or ion. Five general types of reactions cover most situations: acid-base, oxidation-reduction (redox), addition, substitution, elimination, and rearrangement (also called metathesis) reactions. The first two are commonly used categories, but chemists divide other reactions in several different ways. Some of the other ways of classifying of reactions that area common include complexation, hydrolysis and polymerization reactions.

Any chemist's list of important types of reactions would include acid-base reactions. There have been several different definitions of acids and bases, but the most useful definitions of acids and bases comes from Brønsted-Lowry theory, which defines acid-base reactions as proton transfer reactions and views acids as proton donors and bases as proton acceptors. The Brønsted-Lowry theory was proposed in 1923 by two chemists working independently, Johannes Nicolaus Brønsted (1879-1947) in Denmark and T. M. Lowry (1874-1936) in England.

Materials have been classified as acids and bases from the time of the great expansion of the Moslem Empire 1,300 years ago. The familiar term alkaline, which refers to basic substances, is derived from Arabic, as is the term alchemist, which originally referred to someone skilled in the art of producing useful substances such as sulfuric acid. Despite the recognition and use of acids and bases for many centuries, a theory to explain and predict acid-base reactions did not exist until the Periodic Table provided the framework to explain chemical behavior. Acids were first described as substances that tasted sour, tart, or biting. Alkaline substances did not have a sour taste but were caustic and felt slippery. Since it is not a good idea to use taste or touch to identify an unknown substance, it was a great improvement for the health and well being of experimentalists when the affect of acids and bases on the color of vegetable extracts became the operational definition. Litmus, which turns red in the presence of acid and blue in base, is the most commonly used of these dyes. Another characteristic of acids and bases has been recognized since their early identification: mixing one with the other can neutralize or destroy the characteristics of both.

Many important acid-base reactions take place in aqueous media. Because the free proton, H, does not exist in water, the proton in aqueous solution is depicted as H₃O⁺ (aq), the hydrated proton or hydronium ion. The species donated by a Brønsted acid and accepted by a Brønsted base is the proton, H⁺. The term proton in acid/base reactions refers to H⁺, the species transferred. The symbol H₃O⁺ (aq) refers to the proton as it exists in water, as a hydrated ion.

Any substance that dissociates in solution to produce hydroxide ions or that reacts in solution to produce hydroxide ions is a Brønsted base. This definition encompasses salts that contain the OH- ion, other anions that react with water to form the OH- ion, and some neutral molecules that we will learn about during our study of acids and bases. The hydroxide ion accepts protons to form water and is a very important base, although not the only base recognized in the Brønsted-Lowry theory.

In the nineteenth century, scientists observed that even the purest water conducted electricity. This means that ions must be present to carry the charge. If the water is pure, the water molecules themselves must be the source of the ions. Autoionization, self-ionization or autoprotolysis are three terms that describe the process by which one water molecule interacts with another to produce hydroxide ion and the hydronium ion in solution. In pure water, the number of hydronium ions produced is exactly equal to the number of hydroxide ions produced. The concentration of ions in pure water is very small. At 77°F (25°C), [H₃O⁺(aq)] = [OH^-(aq)] = 1.0 x 10^-7 M. When the concentration of hydronium ions in a solution equals the concentration of hydroxide ions, [H₃O⁺(aq)] = [OH^-(aq)], as in the case of pure water, the result is a neutral solution, neither acidic or basic.

The concentration of hydronium ions and hydroxide ions changes when solutions become either acidic or basic. To focus on the relationship between the concentrations of those ions, chemists use the relationship between the concentrations of ions as defined by the K_w of water. The K_w defines a quantitative characteristic of all aqueous solutions at 77°F (25°C): no matter what the concentrations of hydronium and hydroxide ions are, their product must always equal 1.0 x 10^-14. This fact is the basis of the pH scale, the negative of the exponent of ten corresponding to the hydronium ion concentration, a series of numbers derived from the concentration of hydronium ion and describing the acidity or basicity of a solution. The pH of a solutions is the negative of the exponent of 10, corresponding to the hydronium ion concentration. If [H₃O⁺(aq)] = [OH^-(aq)], the solution is neutral and the pH is 7.0. If an acid is added to water, the hydronium concentration will be greater than it is in pure water and the pH is below 7.0. If a base containing the hydroxide ion, such as sodium hydroxide, is added to water, the hydroxide ion concentration will be greater than it is in pure water and the pH will be above 7.0.

To understand the relationship how acid-base reactions affect the pH of a solution, it is useful to distinguish strong acids and strong bases from weak acids and bases.Svante Arrhenius (1859-1927) and others used the observation that solutions of some substances conduct electric current to establish the existence of ions. Very early quantitative work on measuring the amount of electric current passing through solutions led to an improved understanding of the behavior of acids and bases.Carboxylic acids like acetic acid and many other species that do not ionize completely in aqueous medium are called weak acids. Weak acids vary in strength from those which exist primarily as hydrated molecules and only ionize to a very small extent to those which ionize substantially and are in equilibrium with only a small amount of undissociated molecules. The extent of their ionization is defined by the value of the equilibrium constant for their reaction. The K_eq values for the ionization of weak acids are designated as K_a, or acid dissociation constants. Strong acids, such as HCl, HBr, HI, and acids formed from certain polyatomic ions that have several oxygen atoms, such as perchloric acid, HClO₄, chloric acid, HClO₃, sulfuric acid, ₄ and nitric acid, HNO₃ essentially ionize completely in water to form the hydronium ion. Although the other hydrogen halides are strong acids in water, HF is weak. Weak acids have dissociable protons like strong acids, but they simply do not dissociate completely. Most of the common acids formed from oxoanions other than those listed above are weak. All common carboxylic acids are weak acids. Another weakly acidic group commonly found in biologically important molecules is the phosphoric acid group, -PO₃H. Some of the molecules with this acidic group are the polynucleic acids, DNA and RNA, and molecules called phospholipids that compose membranes.

Metallic oxides are bases because the oxide ions accept protons from water molecules, thereby generating hydroxide ions in solution. Because of this behavior upon hydrolysis, they are known as basic anhydrides. Although most metallic oxides are insoluble or only very slightly soluble in water, they readily react with acidic solutions to form hydroxide ions that are then neutralized. Common bases in water are the hydroxides of Group I metals and of calcium, strontium, and barium in Group II.

Metal oxides are not the only substances that hydrolyze to produce hydroxide ions and basic solutions. The compound lithium hydride, LiH, is a polar covalent solid that reacts with water to liberate hydrogen gas and form basic solutions of the metal hydroxide. Other metal hydrides react the same way with water. In fact, all ionic metal hydrides are strong bases. Another important non-ionic hydride is also basic in water. If ammonia, NH₃(g), is dissolved in pure water, the solution is basic because the concentration of hydroxide ion in the medium increases because ammonia competes the hydroxide ion formed by the autoionization of water, forming the ammonium ion (NH₄) and leaving more hydroxide than hydronium ion in the solution. Ammonia and the organic amines, molecules in which one or more of the hydrogen atoms of ammonia are replaced by organic groups, are weak bases.

Although acid-base reactions are often called neutralization reactions, they do not all yield neutral solutions. If a strong acid is added to a solution of a strong base, each hydronium ion will react with a hydroxide ion to form a water and ions of a neutral salt. When the total amount of hydronium ion added equals the amount of hydroxide originally present (the equivalence point), the solution will be neutral, with a pH of 7.0. If more acid is added, the pH will be lower by the same amount as if the additional acid were being added to pure water. Addition of a strong base to a solution of a strong acid is just the reverse process. However, if a weak acid is added to a strong base to the equivalence, the salt that results will not be neutral. The anion formed by removal of the dissociable hydrogen atom of a weak acid will compete with the hydronium ion formed from the autoionization of water, leaving an excess of hydroxide ion. The pH of the solution will be greater than 7.0. It will be the same pH as one would observe if the salt itself were dissolved at the same concentration. If a strong acid is added to a weak base to the equivalence point, the resulting solution would be acidic. When a weak acid and weak base react, the resulting salt can be either basic or acidic depending on the relative weakness of the acid and base.

Oxidation-reduction (redox) reactions are processes in which the number of electrons assigned to atoms involved in the reaction change during the course of the reactions. The original definition of oxidation centered on the class of reaction in which one or more oxygen atoms from oxygen itself to another species was added to an element or compound. It became apparent, however, that other substances besides oxygen could add oxygen atoms to molecules. Species that cause oxidation are called oxidants or oxidizing agents. They remove electrons from the species being oxidized and are themselves reduced. Species that cause reduction are reductants or reducing agents. They contribute electrons to the species being reduced and are themselves oxidized. Oxidation and reduction always occur simultaneously and to the same extent (the same number of electrons is taken from the reductant as are added to the oxidant).

Oxidizing and reducing agents are identified by changes in the oxidation number that occur as a result of a reaction. Atoms and ions are assigned oxidation numbers based on the total number of electrons available in the entire molecule or ion and the atom's electronegativity. For example, electrons in a covalent bond are assigned to the more electronegative atom. Oxidation numbers are not ionic charges. They can be computed for atoms in any compound, ionic or covalent.

Oxidation-reduction reactions cover a wide range of chemical changes, including reactions of atoms of the same element in different oxidation states. Three categories that encompass all these different oxidation-reduction reactions are: atom transfer reactions, electron transfer reactions, and disproportionation reactions. In an atom transfer oxidation-reduction reaction, the atom being oxidized or reduced is bonded with a different species in the product than in the reactant; it has been chemically "transferred" to form a new species. Reactions of non-metallic elements to form covalent compounds are atom transfer reactions. Because we exist in an oxygen rich atmosphere, the most prevalent class of atom transfer oxidation-reduction reactions involves oxygen. Such reactions include combustion, oxidative corrosion of metals, and metabolic oxidation of foods.

Reactions in which oxidation states change without the transfer of either the reductant or oxidant atoms are defined as electron transfer reactions. Electron transfer reactions occur in the process of metabolism and photosynthesis and are also used in large scale industrial processes when appropriate oxidants and reductants are mixed together. Electrochemical processes are also electron transfer reactions. Unlike most other types of chemical reactions, the reactants in an electrochemical process can be spatially separated as long as there is a means for electrons to flow from the reductant to the oxidant.

Reactions in which atoms of the same element are both oxidized and reduced are disproportionation reactions. An example is the reaction of two molecules of hydrogen peroxide to form two molecules of water and one molecule of oxygen. The oxidation number for both oxygen atoms in hydrogen peroxide is -1. In the products the oxidation number of the oxygen atom in water is -2, while that in molecular oxygen is 0.

Addition (combination) reactions are processes in which two molecules or ions combine to form a single new species with no atoms left over. Organic molecules with double or triple bonds (alkenes and alkynes) undergo addition reactions. Examples include the addition of hydrogen to an unsaturated fat to make a saturated fat and addition of molecules such as ₂, HCl or HBr to a double or triple carbon-carbon bond. Addition reactions of inorganic molecules occur when an atom has more than one valence. For example, phosphorus atoms can bind to three or five halogen atoms so PCl₃ can react with Cl₂ to form PCl₅.Sulfur can bond to two, three, or four oxygen atoms so sulfur dioxide can undergo an addition reaction to form sulfur trioxide and, in turn, sulfur trioxide can react with water to form a molecule of sulfuric acid (more properly designated dihydrogen sulfate if undissociated). From these examples, you may see that many inorganic addition reactions can also be classified as redox reactions.

In substitution reactions, one or more atoms or groups of atoms attached to a particular atom of one reactant is transferred to an atom of another reactant. One of the most important types of substitution reactions of metallic compounds is precipitation. These reactions have tremendous utility in the preparation of commercial products and in the testing and analysis of ions of clinical and environmental significance. They are also behind many day to day irritations: the formation of stains on porcelain, the encrusting of heating elements in coffee makers, and the formation of bathtub rings. In natural water systems, many common minerals are formed by anion substitution-precipitation reactions, among them carbonates, phosphates and the sulfate containing rocks. Both qualitative solubility rules and quantitative expressions of equilibrium constants for solubility, K_sp, are useful in discussing the chemistry behind these occurrences and in controlling them. They also enable us to make reliable predictions about what will happen when systems containing ions are perturbed by the addition of other ions, by the presence of dissolved gases, or by changing pH.

The dissolution of varying amounts of a substance in water results in what are qualitatively described as unsaturated, saturated, and supersaturated solutions. For systems not at equilibrium, determining the solubility reaction quotient, Q_sp, is useful for predicting what happens as the system returns to equilibrium. The value of Q_sp is compared to the K_sp for salts in solution. If Q_sp K, no precipitate will form in the solution; if Q_sp K_sp, one predicts a precipitate will form. When the calculated Q_sp is less than the K_sp, the solution is unsaturated. An unsaturated solution can dissolve more solute. When the concentrations of the ions in solution are such that Q_sp = K_sp, the solution is saturated. No more solute can dissolve in the solvent; it is "holding" the maximum amount. When the concentrations are such that Q_sp is greater than K_sp, either of two results are observed. In the simplest cases, a precipitate forms until Q_sp = K_sp: until the concentration of ions in solution as defined by the equilibrium constant is reached. At this point, the solution is saturated. A solution is saturated with respect to a solid if some of the solid is visible in the bottom of the container. The maximum concentration of ions in the solution is in equilibrium with the precipitated solid.

Sometimes, however, no precipitate forms until Q_sp greatly exceeds K_sp or until the solution is disturbed. When Q_sp K_sp for a solution in such cases, the solution is supersaturated. The ions in the solution are at a higher concentration than would be predicted by the K_sp for the dissolved material.

Ion substitution reactions are put to great practical use in a process known as ion exchange. Ions causing hard water can be removed from solution and replaced by other ions through the intermediacy of inorganic zeolites or organic ion-exchange resins.

Substitution reactions of non-metallic elements are of two major types: nucleophilic and electrophilic substitution. In a large class of reactions called nucleophilic substitutions, reactions occur when a species with available electrons (a nucleophile), either in the form of an anionic charges or an unshared pair of electrons, is attracted to an electropositive site in a molecule. During the reaction, a new bond is formed between the nucleophile and another bond in the molecule is broken, thereby releasing a new species into the reaction medium. Nucleophilic substitution reactions can be envisioned by using two different mechanisms--one unimolecular and one bimolecular--and the type of substitution reaction that a given molecule undergoes depends on its structure and the conditions under which the reaction is carried out.

Another type of substitution reaction involves free radicals. These reactions are industrially important for the production of some useful small molecules and also play a major role in atmospheric chemistry.

Aromatic compounds like benzene that contain delocalized electrons undergo electrophilic substitution reactions. Electrophiles are electron-poor species that are attracted to the electron cloud of aromatic compounds and other species with loosely held electrons.

Inorganic compounds held together by covalent bonds also undergo substitution reactions. Compounds and ions containing nitrogen and sulfur are frequently nucleophilic. Phosphorus behaves similarly to nitrogen but has a more extensive reactivity because of its ability to expand its octet to form compounds with five or even six bonds. Substitution reactions of boron-containing compounds are directed by the ability of boron to form electron deficient molecules that can readily bond with electron rich species, especially those containing a pair of non-bonding electrons. The chemistry of silicon compounds is similar to that of carbon, but silicon forms extremely stable silicon-oxygen bonds that results in compounds like sand and quartz that are resistant to further chemical modification.

In elimination reactions, larger molecules expel smaller molecules like water, hydrogen halides, carbon dioxide and other diatomic gases to form new substances. Both organic and inorganic compounds experience elimination reactions.

Rearrangement reactions are isomerization. They include both intramolecular changes in covalent bond connectivity or orientation that requires bond breaking and reformation (including "rotation" about a carbon-carbon double bond) and structural changes caused by an intermolecular reaction, often catalytic. Many organic molecules undergo structural changes as a result of catalytic reactions with acid or base. For example, 1-butene (CH₃CH₂CH=CH₂) is converted to the more stable isomer, 2-butene (CH₃CH=CHCH₃), in the presence of acid.

Complexation reactions are processes in which molecules of ion (called legends) bond to a metal atom (usually a transition metal) by forming coordinate (dative) covalent bonds. In a coordinate covalent bond, both electrons are donated by the Logan, a Lewis base, to the metal atom, a Lewis acid. Complexation is often used to make a metal atom or ion more soluble. Metals in dietary supplements, for example, are often chelated (a chelate is a ligand that has more than one atom that bonds to the metal atom) to increase solubility. Iron(III) citrate, formed by adding a solution of citric acid or a citrate salt to a solid iron(III) salt, is very soluble, whereas uncomplexed Fe(III) is highly insoluble in water. Insoluble silver halides on photographic film can be dissolved by adding ammonia, which forms a silver diammine complex ion, Ag(NH₃).

Hydrolysis is the reaction of a substance in solution with the solvent water. It is a specific case of a more general reaction called solvolysis, which simply refers to reaction of a dissolved species with solvent. Many other fluids can serve as solvents for chemical reactions, and if the solvents interact chemically with materials dissolved in them, the reactions are named in a corresponding fashion. If ammonia is the solvent, for example, the reaction with the solvent is called ammonolysis.

It is important to distinguish hydration, a physical process whereby water molecules surround a species in solution, and hydrolysis, a chemical reaction occurring between solvent and dissolved species. The species to whose symbols we have appended "(aq)" are hydrated; they are physically surrounded by water. When hydrolysis occurs, we will write a chemical equation to describe it, and new species will be formed in solution.

Polymerization reactions are addition reactions, prinicpally of two types: condensation reactions and free-radical reactions. In a condensation reaction, atoms dissociate from the ends of two molecules, forming a new small molecule and allowing the remaining fragment of the two molecules to form a covalent linkage and, hence, a new larger molecule. The process can continue until very large polymers are formed. The monomer units can be the same or different. The largest class of condensation reactions is dehydration, in which water is the small molecule that is dissociated. Geopolymers such as silicate minerals and metal oxides and biopolymers such as proteins, nucleic acids, and polysaccharides are formed by dehydration condensation reactions.Ceramics, synthetic fibers and many other products are also produced by condensations reactions.

In free radical polymerization, monomers can add to each other without formation of a small molecule. A free radical has an atom with unpaired electron. Free radicals can react with molecules to form a new, larger free radical or can react with other free radicals to form a covalent bond. The process of free radical polymerization is initiated by a species that can remove an atom from a monomer and produce a free radical. The free radical adds to a monomer, producing another free radical that, in turn, adds to another monomer, increasing the length of the polymer chain. The growth is terminated by a reaction of free radicals that produces a molecule that is not a free radical. Synthetic polymers such as polyethylene and polypropylene are produced by free radical addition reactions.

The essence of chemistry is understanding and applying chemical reactions. Chemists have divided chemical reactions into several categories based upon the type of changes that occur to atoms and molecules during each type of process. Acid-base reactions involve the transfer of protons, typically resulting in some extent of salt formation and neutralization. Oxidation-reduction reactions result in a change in the number of electrons assigned to some of the atoms involved in the process. Addition reactions result in the formation of one molecule or polyatomic ion from more than one. In substitution reactions, atoms or groups of atoms originally bound to one particular atom are replaced by another atom or group of atoms. In elimination reactions, one group of atoms is removed from a molecule or polyatomic ion. Rearrangement processes result in isomerization. Complexation is the formation of a Lewis acid-Lewis base combination where a metal atom is the Lewis acid and ligands that form coordinate covalent bonds to the metal act as Lewis bases. Hydrolysis is the addition of a hydrogen atom and a hydroxyl group to separate atoms of a molecule or ion. Polymerization is an addition reaction, typically either a condensation reaction involving dehydration or a free radical process, in which many units of a molecule or molecular fragment are covalently combined.

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