View source for Hydrolysis

'''Hydrolysis''' is a [[chemical reaction]] during which molecules of [[water (molecule)|water]] (H<sub>2</sub>O) are split into [[hydrogen]] [[cation]]s (H<sup>+</sup>) (conventionally referred to as [[proton]]s) and [[hydroxide]] [[anion]]s (OH<sup>−</sup>) in the process of a chemical mechanism.<ref>[[Compendium of Chemical Terminology]], [http://goldbook.iupac.org/H02902.html hydrolysis], accessed 2007-01-23.</ref><ref>[[Compendium of Chemical Terminology]], [http://goldbook.iupac.org/S05762.html solvolysis], accessed 2007-01-23.</ref> It is the type of reaction that is used to break down certain [[polymer]]s, especially those made by [[step-growth polymerization]]. Such [[polymer degradation]] is usually catalysed by either [[acid]], e.g., concentrated [[sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>), or [[alkali]], e.g., [[sodium hydroxide]] (NaOH). 

Hydrolysis is distinct from [[hydration reaction|hydration]]. In hydration, the hydrated molecule does not "lyse" (break into two new compounds). It should not be confused with [[hydrogenolysis]], a reaction of hydrogen.

==Types==

Hydrolysis is a chemical process in which a certain molecule is split into two parts by the addition of a molecule of water. One fragment of the parent molecule gains a [[hydrogen ion]] (H<sup>+</sup>) from the additional water molecule. The other group collects the remaining [[hydroxyl]] group (OH<sup>−</sup>). 

The most common hydrolysis occurs when a [[salt (chemistry)|salt]] of a [[weak acid]] or [[weak base]] (or both) is dissolved in water. Water autoionizes into negative hydroxyl ions and positive hydrogen ions. The salt breaks down into positive and negative ions. For example, [[sodium acetate]] dissociates in water into sodium and [[acetate]] ions. Sodium ions react very little with hydroxyl ions whereas acetate ions combine with hydrogen ions to produce neutral [[acetic acid]], and the net result is a relative excess of hydroxyl ions, causing a [[solution|basic solution]].

However, under normal conditions, only a few reactions between water and [[organic compound]]s occur. In general, [[strong acid]]s or bases must be added in order to achieve hydrolysis where water has no effect. The acid or base is considered a [[catalyst]]. They are meant to speed up the reaction, but are recovered at the end of it.

Acid–base-catalyzed hydrolyses are very common; one example is the hydrolysis of [[amide]]s or [[ester]]s. Their hydrolysis occurs when the [[nucleophile]] (a nucleus-seeking agent, e.g., water or hydroxyl ion) attacks the carbon of the [[Carbonyl|carbonyl group]] of the [[ester]] or [[amide]]. In an aqueous base, hydroxyl ions are better nucleophiles than dipoles such as water. In acid, the carbonyl group becomes protonated, and this leads to a much easier nucleophilic attack. The products for both hydrolyses are compounds with [[carboxylic acid]] groups.

Perhaps the oldest example of ester hydrolysis is the process called [[saponification]]. It is the hydrolysis of a [[triglyceride]] (fat) with an aqueous base such as [[sodium hydroxide]] (NaOH). During the process, [[glycerol]] is formed, and the [[fatty acid]]s react with the base, converting them to salts. These salts are called soaps, commonly used in households.

Moreover, hydrolysis is an important process in plants and animals, the most significant example being [[energy metabolism]] and storage. All living cells require a continual supply of energy for two main purposes: for the [[biosynthesis]] of small and macromolecules, and for 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 together) that release energy when needed. ATP can undergo hydrolysis in two ways: the removal of terminal phosphate to form [[adenosine diphosphate]] (ADP) and inorganic phosphate, or the removal of a terminal diphosphate to yield [[adenosine monophosphate]] (AMP) and [[pyrophosphate]]. The latter is usually cleaved further to yield two phosphates. This results in biosynthesis reactions, which do not occur alone, that can be driven in the direction of synthesis when the phosphate bonds have undergone hydrolysis.

In addition, in living systems, most biochemical reactions, including ATP hydrolysis, take place during the catalysis of [[enzyme]]s. The catalytic action of enzymes allows the hydrolysis of [[protein]]s, fats, oils, and [[carbohydrate]]s. As an example, one may consider [[protease]]s, enzymes that aid [[digestion]] by causing hydrolysis of [[peptide bond]]s in [[protein]]s. They catalyze the hydrolysis of interior peptide bonds in peptide chains, as opposed to [[exopeptidase]]s, another class of enzymes, that catalyze the hydrolysis of terminal peptide bonds, liberating one free amino acid at a time.

However, proteases do not catalyze the hydrolysis of all kinds of proteins. Their action is stereo-selective: Only proteins with a certain tertiary structure will be targeted. The reason is that 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 hydrolysis. This specificity preserves the integrity of other proteins such as [[hormone]]s, and therefore the biological system continues to function normally.

== Hydrolysis of amide links ==

In the hydrolysis of an [[amide]], it is converted into a [[carboxylic acid]] and an [[amine]] or [[ammonia]]. The carboxylic acid has a hydroxyl group derived from a water molecule and the amine (or ammonia) gains the hydrogen ion.
 
[[image:amide hydrolysis.png]]

A specific case of the hydrolysis of an amide link is the hydrolysis of [[peptide bond|peptides]] to smaller fragments or [[amino acid]]s. 

Many [[polyamide]] polymers such as [[nylon 6,6]] are attacked and hydrolysed in the presence of strong acids. Such attack leads to [[depolymerization]] and nylon products fail by fracturing when exposed to even small amounts of acid. Other polymers made by [[step-growth polymerization]] are susceptible to similar [[polymer degradation]] reactions. The problem is known as [[stress corrosion cracking]].

== Hydrolysis of polysaccharides {{anchor|saccharification}}== 
[[image:Sucrose-inkscape.svg|thumb|Sucrose. The glycoside bond is represented by the central oxygen atom, which holds the two monosaccharide units together.]]
[[Monosaccharide]]s can be linked together by [[glycosidic bond]]s, which can be cleaved by hydrolysis.  Two, three, several or many monosaccharides thus linked form [[disaccharide]]s, [[trisaccharide]]s, [[oligosaccharide]]s or [[polysaccharide]]s, respectively.  Enzymes that hydrolyse glycosidic bonds are called "[[glycoside hydrolase]]s" or "glycosidases".

The best-known disaccharide is [[sucrose]] (table sugar). Hydrolysis of sucrose yields [[glucose]] and [[fructose]]. [[Invertase]] is a [[sucrase]] used industrially for the hydrolysis of sucrose to so-called [[invert sugar]].  [[Lactase]] is essential for digestive hydrolysis of [[lactose]] in milk. Deficiency of lactase in humans causes [[lactose intolerance]]. 

The hydrolysis of polysaccharides to soluble sugars is called "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 [[brewing|produce beer]].  Other [[amylase]] enzymes may convert starch to glucose or to oligosaccharides.  [[Cellulose]] is converted to glucose or the disaccharide [[cellobiose]] by [[cellulase]]s.  Animals such as cows ([[ruminants]]) are able to digest cellulose because of symbiotic bacteria that produce cellulases.

== Irreversibility of hydrolysis under physiological conditions ==

Under physiological conditions (e.g., in dilute aqueous solution), a hydrolytic cleavage reaction, in which the concentration of a metabolic precursor is low (on the order of 10<sup>−3</sup> to 10<sup>−6</sup> molar), is essentially [[thermodynamics|thermodynamically]] irreversible. To give an example:

: A + H<sub>2</sub>O → X + Y

: <math>K_d = \frac{\left[X\right] \left[Y\right]} {\left[H_2O\right] \left[A\right]}</math>

Assuming that ''x'' is the final concentration of products, and that ''C'' is the initial concentration of A, and W = [H<sub>2</sub>O] = 55.5 molar, then ''x'' can be calculated with the equation:

: <math>\frac{x \times x}{W\left(C - x\right)} = K_d</math>

let K<sub>d</sub>×W = k:

then <math> x = \frac {-k + \sqrt {k^2 + 4kC} } {2}. </math>

For a value of C = 0.001 molar, and k = 1 molar, ''x''/C &gt; 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 [[anhydride|anhydrous]] [[pyrophosphate]] bonds.

== Hydrolysis of metal aqua ions ==

Metal ions are [[Lewis acid]]s, and in aqueous solution they form aqua ions, of the general formula M(H<sub>2</sub>O)<sub>n</sub><sup>m+</sup>. 
<ref>Burgess, J. ''Metal ions in solution'', (1978) Ellis Horwood, New York</ref> 
<ref>{{cite book | last = Richens | first =D. T. | title = The chemistry of aqua ions : synthesis, structure, and reactivity : a tour through the periodic table of the elements | publisher = Wiley | date = 1997 | isbn = 0471970581}}</ref>
The aqua ions undergo hydrolysis, to a greater or lesser extent. The first hydrolysis step is given generically as

: M(H<sub>2</sub>O)<sub>n</sub><sup>m+</sup> + H<sub>2</sub>O {{eqm}} M(H<sub>2</sub>O)<sub>n-1</sub>(OH)<sup>(m-1)+</sup> + H<sub>3</sub>O<sup>+</sup>

Thus the aqua ion is behaving as an acid in terms of [[Brønsted-Lowry acid-base theory]]. This 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 [[Acid dissociation constant|dissociation constant]], pK<sub>a</sub>, for this reaction is more or less linearly related to the charge-to-size ratio of the metal ion.<ref name="bm">Baes, C.F.; Mesmer, R.E. The Hydrolysis of Cations, (1976),Wiley, New York </ref> Ions with low charges, such as Na<sup>+</sup> are very weak acids with almost imperceptible hydrolysis. Large divalent ions such as Ca<sup>2+</sup>, Zn<sup>2+</sup>, Sn<sup>2+</sup> and Pb<sup>2+</sup> have a pK<sub>a</sub> of 6 or more and would not normally be classed as acids, but small divalent ions such as Be<sup>2+</sup> undergo extensive hydrolysis. Trivalent ions like Al<sup>3+</sup> and Fe<sup>3+</sup> are weak acids whose pK<sub>a</sub> is comparable to that of [[acetic acid]]. Solutions of salts such as BeCl<sub>2</sub> or Al(NO<sub>3</sub>)<sub>3</sub> in water are noticeably [[acidic]]; the hydrolysis can be [[le chatelier's principle|suppressed]] by adding an acid such as [[nitric acid]], making the solution more acidic. 

Hydrolysis may proceed beyond the first step, often with the formation of polynuclear species. 
<ref name="bm"/> Some "exotic" species such as Sn<sub>3</sub>(OH)<sub>4</sub><sup>2+</sup> <ref>{{greenwood&Earnshaw}} p384</ref> are well characterized. Hydrolysis tends to increase as [[pH]] rises leading, in many cases, to the precipitation of an hydroxide such as Al(OH)<sub>3</sub> or AlO(OH). These substances, the major constituents of [[bauxite]], are known as [[laterite]]s  and are formed by leaching from rocks of most of the ions other than aluminium and iron and subsequent hydrolysis of the remaining aluminium and iron.

Ions with a formal charge of four have undergone extensive hydrolysis and salts of Zr<sup>4+</sup>, for example, can only be obtained from strongly acidic solutions. With oxidation states five and higher the concentration of the aqua ion in solution is negligible. In effect, the aqua ion is a strong acid. For example, aqueous solutions of Cr(VI) contain CrO<sub>4</sub><sup>2-</sup>. 

:Cr(H<sub>2</sub>O)<sub>6</sub><sup>6+</sup> &rarr; CrO<sub>4</sub><sup>2-</sup> + 2 H<sub>2</sub>O + 8 H<sup>+</sup>

Note that reactions such as

: Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup> + H<sub>2</sub>O {{eqm}} 2 CrO<sub>4</sub><sup>2-</sup> + 2 H<sup>+</sup>

are formally hydrolysis reactions as water molecules are split up yielding hydrogen ions. Such reactions are common among [[polyoxometalate]]s.

== See also ==

* [[Adenosine triphosphate]]
* [[Dehydration synthesis]]
* [[Hydrolysis constant]]
* [[Polymer degradation]]
* [[Resomation]]
* [[Solvolysis]]
* [[Tissue Digestion]]

== References ==

<references/>

==External links==
{{commons|Collagen|Collagen}}

[[Category:Equilibrium chemistry]]
[[Category:Chemical reactions]]
[[category:Weathering]]

[[ar:حلمهة]]
[[bs:Hidroliza]]
[[bg:Хидролиза]]
[[ca:Hidròlisi]]
[[cs:Hydrolýza]]
[[da:Hydrolyse]]
[[de:Hydrolyse]]
[[et:Hüdrolüüs]]
[[el:Υδρόλυση]]
[[es:Hidrólisis]]
[[eo:Hidrolizo]]
[[eu:Hidrolisi]]
[[fa:آبکافت]]
[[fr:Hydrolyse]]
[[ko:가수분해]]
[[io:Hidrolizo]]
[[id:Hidrolisis]]
[[it:Idrolisi]]
[[he:הידרוליזה]]
[[ht:Idwoliz]]
[[lv:Hidrolīze]]
[[lt:Hidrolizė]]
[[mk:Хидролиза]]
[[nl:Hydrolyse]]
[[ja:加水分解]]
[[no:Hydrolyse]]
[[oc:Idrolisi]]
[[nds:Hydrolyys]]
[[pl:Hydroliza]]
[[pt:Hidrólise]]
[[ro:Reacția de hidroliză]]
[[ru:Гидролиз]]
[[sq:Hidrolizë]]
[[simple:Hydrolysis]]
[[sk:Hydrolýza]]
[[sr:Хидролиза]]
[[sh:Hidroliza]]
[[su:Hidrolisis]]
[[fi:Hydrolyysi]]
[[sv:Hydrolys]]
[[tr:Hidroliz]]
[[uk:Гідроліз]]
[[zh:水解]]
[[Category:2Fix]]