Usually<unk>  is a chemical process in which a molecule of water is added to a substance. Sometimes this addition causes both the substance and water molecule to split into two parts. In such reactions, one fragment of the target molecule (or parent molecule) gains a hydrogen ion. It breaks a chemical bond in the compound. Salts A common kind of<unk>  occurs when a salt of a weak acid or weak base (or both) is dissolved in water. Water spontaneously ionizes into hydroxide anions and hydronium cations. The salt also dissociates into its constituent anions and cations. For example, sodium acetate dissociates in water into sodium and acetate ions. Sodium ions react very little with the hydroxide ions whereas the acetate ions combine with hydronium ions to produce acetic acid. In this case the net result is a relative excess of hydroxide ions, yielding a basic solution. Strong acids also undergo<unk> . For example, dissolving sulfuric acid (H2SO4) in water is accompanied by<unk>  to give hydronium and bisulfate, the sulfuric acid's conjugate base. For a more technical discussion of what occurs during such a<unk> , see Brønsted–Lowry acid–base theory. Esters and amides Acid–base-catalysed<unk>  are very common; one example is the<unk>  of amides or esters. Their<unk>  occurs when the nucleophile (a nucleus-seeking agent, e.g., water or hydroxyl ion) attacks the carbon of the carbonyl group of the ester or amide. In an aqueous base, hydroxyl ions are better nucleophiles than polar molecules such as water. In acids, the carbonyl group becomes protonated, and this leads to a much easier nucleophilic attack. The products for both<unk>  are compounds with carboxylic acid groups. Perhaps the oldest commercially practiced example of ester<unk>  is saponification (formation of soap). It is the<unk>  of a triglyceride (fat) with an aqueous base such as sodium hydroxide (NaOH). During the process, glycerol is formed, and the fatty acids react with the base, converting them to salts. These salts are called soaps, commonly used in households. In addition, in living systems, most biochemical reactions (including ATP<unk>  take place during the catalysis of enzymes. The catalytic action of enzymes allows the<unk>  of proteins, fats, oils, and carbohydrates. As an example, one may consider proteases (enzymes that aid digestion by causing<unk>  of peptide bonds in proteins). They catalyze the<unk>  of interior peptide bonds in peptide chains, as opposed to exopeptidases (another class of enzymes, that catalyze the<unk>  of terminal peptide bonds, liberating one free amino acid at a time). However, proteases do not catalyze the<unk>  of all kinds of proteins. Their action is stereo-selective: Only proteins with a certain tertiary structure are targeted as some kind of orienting force is needed to place the amide group in the proper position for catalysis. The necessary contacts between an enzyme and its substrates (proteins) are created because the enzyme folds in such a way as to form a crevice into which the substrate fits; the crevice also contains the catalytic groups. Therefore, proteins that do not fit into the crevice will not undergo<unk> . This specificity preserves the integrity of other proteins such as hormones, and therefore the biological system continues to function normally. Mechanism for acid-catalyzed<unk>  of an amide. Upon<unk> , an amide converts into a carboxylic acid and an amine or ammonia (which in the presence of acid are immediately converted to ammonium salts). One of the two oxygen groups on the carboxylic acid are derived from a water molecule and the amine (or ammonia) gains the hydrogen ion. The<unk>  of peptides gives amino acids. Many polyamide polymers such as nylon 6,6 hydrolyze in the presence of strong acids. The process leads to depolymerization. For this reason nylon products fail by fracturing when exposed to small amounts of acidic water. Polyesters are also susceptible to similar polymer degradation reactions. The problem is known as environmental stress cracking. ATP<unk>  is related to energy metabolism and storage. All living cells require a continual supply of energy for two main purposes: the biosynthesis of micro and macromolecules, and the active transport of ions and molecules across cell membranes. The energy derived from the oxidation of nutrients is not used directly but, by means of a complex and long sequence of reactions, it is channeled into a special energy-storage molecule, adenosine triphosphate (ATP). The ATP molecule contains pyrophosphate linkages (bonds formed when two phosphate units are combined) that release energy when needed. ATP can undergo<unk>  in two ways: Firstly, the removal of terminal phosphate to form adenosine diphosphate (ADP) and inorganic phosphate, with the reaction: : ATP + → ADP + Pi Secondly, the removal of a terminal diphosphate to yield adenosine monophosphate (AMP) and pyrophosphate. The latter usually undergoes further cleavage into its two constituent phosphates. This results in biosynthesis reactions, which usually occur in chains, that can be driven in the direction of synthesis when the phosphate bonds have undergone<unk> . Polysaccharides Sucrose. The glycoside bond is represented by the central oxygen atom, which holds the two monosaccharide units together. Monosaccharides can be linked together by glycosidic bonds, which can be cleaved by<unk> . Two, three, several or many monosaccharides thus linked form disaccharides, trisaccharides, oligosaccharides, or polysaccharides, respectively. Enzymes that hydrolyze glycosidic bonds are called "glycoside hydrolases" or "glycosidases". The best-known disaccharide is sucrose (table sugar).<unk>  of sucrose yields glucose and fructose. Invertase is a sucrase used industrially for the<unk>  of sucrose to so-called invert sugar. Lactase is essential for digestive<unk>  of lactose in milk; many adult humans do not produce lactase and cannot digest the lactose in milk. The<unk>  of polysaccharides to soluble sugars can be recognized as saccharification. Malt made from barley is used as a source of β-amylase to break down starch into the disaccharide maltose, which can be used by yeast to produce beer. Other amylase enzymes may convert starch to glucose or to oligosaccharides. Cellulose is first hydrolyzed to cellobiose by cellulase and then cellobiose is further hydrolyzed to glucose by beta-glucosidase. Ruminants such as cows are able to hydrolyze cellulose into cellobiose and then glucose because of symbiotic bacteria that produce cellulases. Metal aqua ions Metal ions are Lewis acids, and in aqueous solution they form metal aquo complexes of the general formula M(H2O)nm+. The aqua ions undergo<unk> , to a greater or lesser extent. The first<unk>  step is given generically as :M(H2O)nm+ + H2O M(H2O)n−1(OH)(m−1)+ + H3O+ Thus the aqua cations behave as acids in terms of Brønsted–Lowry acid–base theory. This effect is easily explained by considering the inductive effect of the positively charged metal ion, which weakens the O-H bond of an attached water molecule, making the liberation of a proton relatively easy. The dissociation constant, pKa, for this reaction is more or less linearly related to the charge-to-size ratio of the metal ion. Ions with low charges, such as Na+ are very weak acids with almost imperceptible<unk> . Large divalent ions such as Ca2+, Zn2+, Sn2+ and Pb2+ have a pKa of 6 or more and would not normally be classed as acids, but small divalent ions such as Be2+ undergo extensive<unk> . Trivalent ions like Al3+ and Fe3+ are weak acids whose pKa is comparable to that of acetic acid. Solutions of salts such as BeCl2 or Al(NO3)3 in water are noticeably acidic; the<unk>  can be suppressed by adding an acid such as nitric acid, making the solution more acidic.<unk>  may proceed beyond the first step, often with the formation of polynuclear species via the process of olation. Some "exotic" species such as Sn3(OH)42+ are well characterized.<unk>  tends to proceed as pH rises leading, in many cases, to the precipitation of a hydroxide such as Al(OH)3 or AlO(OH). These substances, major constituents of bauxite, are known as laterites and are formed by leaching from rocks of most of the ions other than aluminium and iron and subsequent<unk>  of the remaining aluminium and iron. Mechanism strategies Acetals, imines, and enamines can be converted back into ketones by treatment with excess water under acid-catalyzed conditions: RO·OR-H3O-O; NR·H3O-O; RNR-H3O-O.
Hydrolysis
Demethylation
Redox
Phosphorylation
Hydroxylation
Glucuronidation
Saponification
Fragmentation 