hydrolysis

 

(hīdrŏl'ĭsĭs) , chemical reaction of a compound with water, usually resulting in the formation of one or more new compounds. The most common hydrolysis occurs when a salt of a weak acid or weak base (or both) is dissolved in water. Water ionizes into negative hydroxyl ions (OH⁻) and positive hydrogen ions (H⁺), which become hydrated to form positive hydronium ions (H₃O⁺). The salt also breaks up into positive and negative ions. For example, when sodium acetate is dissolved in water it readily dissociates into sodium and acetate ions. Because sodium hydroxide is a strong base, the sodium ions react only slightly with the hydroxyl ions already present in the water to form sodium hydroxide molecules. Acetic acid is a weak acid, so the acetate ions react readily with the hydrogen ions present in the water to form neutral acetic acid molecules. The net result of these reactions is a relative excess of hydroxyl ions, causing an alkaline solution. A chemical reaction has actually taken place between the water and the dissolved salt. There are relatively few instances in which water reacts directly with organic compounds under ordinary conditions. It does react with acid halides, acid anhydrides, and organometallic compounds, e.g., Grignard reagents. The addition of strong acids or bases or the use of steam will often bring about hydrolysis where ordinary water has no effect. Some industrially important hydrolysis reactions are the synthesis of alcohols from olefins (e.g., ethanol, CH₃COOH, from ethene, CH₂CH₂) in the presence of a strong acid catalyst, the conversion of starches to sugars in the presence of a strong acid catalyst, and the conversion of animal fats or vegetable oils to glycerol and fatty acids by reaction with steam. Hydrolysis is an important reaction in plants and animals (see metabolism). The catalytic action of certain enzymes allows the hydrolysis of proteins, fats, oils, and carbohydrates.

Not to be confused with electrolysis

Hydrolysis is a chemical reaction or process in which a chemical compound is broken down by reaction with water.^[1][2] This is the type of reaction that is used to break down polymers. Water is added in this reaction.

In organic chemistry, hydrolysis can be considered as the reverse or opposite of condensation, a reaction in which two molecular fragments are joined for each water molecule produced. As hydrolysis may be a reversible reaction, condensation and hydrolysis can take place at the same time, with the position of equilibrium determining the amount of each product.

In inorganic chemistry, the word is often applied to solutions of salts and the reactions by which they are converted to new ionic species or to precipitates (oxides, hydroxides, or salts). The addition of a molecule of water to a chemical compound, without forming any other products is usually known as hydration, rather than hydrolysis.

In biochemistry, hydrolysis is considered the reverse or opposite of dehydration synthesis. In hydrolysis, a water molecule (H2O), is added. Where as in dehydration synthesis, a molecule of water is removed.

Examples

Hydrolysis of metal salts

Many metal ions are strong Lewis acids, and in water they may undergo hydrolysis to form basic salts. Such salts contain a hydroxyl group that is directly bound to the metal ion in place of a water ligand. For example, aluminium chloride undergoes extensive hydrolysis in water, such that the pH of the solution become quite acidic.

This means that if solutions of AlCl₃ are evaporated, hydrogen chloride is lost and the residue is a basic salt (in this case an oxychloride) in place of AlCl₃. Such behaviour is also seen with other metal chlorides such as ZnCl₂, SnCl₂, FeCl₃ and lanthanide halides such as DyCl₃. With some compounds such as TiCl₄, the hydrolysis may go to completion and form the pure hydroxide or oxide, in this case TiO₂.

Hydrolysis of an ester link

In a hydrolysis reaction that involves breaking an ester link, one hydrolysis product contains a hydroxyl functional group, while the other contains a carboxylic acid functional group.

The carbonyl is attacked by a hydroxide anion (or a water molecule, which is rapidly deprotonated). The resulting tetrahedral intermediate breaks down. The alkoxide fragment breaks off from the tetrahedran carbon and becomes an alcohol by protonation, leaving the acyl fragment with the attacking hydroxide, to produce a carboxylic acid. This is the reverse of the esterification reaction, yielding the original alcohol and carboxylic acid again. In a basic solution, the carboxylic acid is deprotonated, such that the basic hydrolysis is irreversible, while acidic hydrolysis is not.

There are two main methods for hydrolysing esters, basic hydrolysis and acid-catalysed. With acid-catalysed hydrolysis a dilute acid is used to protonate the carbonyl group in order to activate it towards nucleophilic attack by a water molecule. However the more usual method for ester hydrolysis involves refluxing the ester with an aqueous base such as NaOH or KOH. Once the reaction is complete, the carboxylate salt is acidified to release the free carboxylic acid.

An important example of this reaction is the release of fatty acids from glycerol in triglyceride hydrolysis, as occurs during saponification.

Hydrolysis of amide links

In other hydrolysis reactions, such as hydrolysis of an amide link into a carboxylic acid and an amine product or ammonia, only the carboxylic acid product has a hydroxyl group derived from the water. The amine product (or ammonia) gains the remaining hydrogen ion. A more specific case of the hydrolysis of an amide link is hydrolyzing the peptide links of amino acids.

Hydrolysis of cellulose (Cellulolysis)

Main article: Cellulase

Cellulolytic is relating to or causing the hydrolysis of cellulose (i.e. cellulolytic bacteria, fungi or enzymes).

The hydrolysis into glucose (i.e. of cellulose or starch) is called saccharification.

Irreversibility of hydrolysis under physiological conditions

Under physiological conditions (i.e. in dilute aqueous solution), a hydrolytic cleavage reaction, where the concentration of a metabolic precursor is low (on the order of 10^-3 to 10^-6 molar), is essentially thermodynamically irreversible. To give an example:

A + H₂O → X + Y

Assuming that x is the final concentration of products, and that C is the initial concentration of A, and W = [H₂O] = 55.5 molar, then x can be calculated with the equation:

let K_d×W = k:

then

For a value of C = 0.001 molar, and k = 1 molar, x/C > 0.999. Less than 0.1% of the original reactant would be present once the reaction is complete.

This theme of physiological irreversibility of hydrolysis is used consistently in metabolic pathways, since many biological processes are driven by the cleavage of anhydrous pyrophosphate bonds.

See also

• Adenosine triphosphate
• Dehydration synthesis
• Solvolysis

References

1. ^ Compendium of Chemical Terminology, hydrolysis, accessed 2007-01-23.
2. ^ Compendium of Chemical Terminology, solvolysis, accessed 2007-01-23.

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