Hydrolysis of Organic Compounds

Hydrolysis means 'split with water'.

A number of organic compounds undergo hydrolysis, such as amides, esters and halogenoalkanes.

Hydrolysis
In the overall process of hydrolysis, a bond in an organic molecule is broken, and an O-H bond in a water molecule also breaks. Then, from the water molecule, an O-H group adds to one part from the organic molecule, and an H atom to the other.

Hydrolysis is therefore the reaction of an organic compound with water. However, hydrolysis is often catalysed (in acidic or alkaline solution), and this is discussed later.


The hydrolysis of an amide is illustrated below:

Comment:

The amide and water molecules are about to react...


The products of hydrolysis depend upon what is being hydrolysed...

  1. The hydrolysis of a primary amide forms a carboxylic acid and ammonia.
  2. RCONH2 + H2O   ®   RCOOH + NH3

  3. The hydrolysis of a secondary amide forms a carboxylic acid and primary amine.
  4. RCONHR' + H2O   ®   RCOOH + R'NH2

  5. The hydrolysis of an ester forms a carboxylic acid and an alcohol.
  6. RCOOR' + H2O   ®   RCOOH + R'OH

  7. The hydrolysis of a halogenoalkane forms an alcohol.
  8. RBr + H2O   ®   ROH + H+ + Br-

Note that the chemical equations above illustrate a reaction with water.

However, few organic compounds react readily with water...

Their molecules are mostly much less polar than the very polar water molecule. Although it is possible industrially to bring about reaction with water using superheated steam, a catalyst and high pressure, in the laboratory hydrolysis reactions require a catalyst - an acid (H+ ions) or alkali (OH- ions). Hydrolysis reactions are said to be acid-catalysed or base-catalysed (alkaline hydrolysis). Hydrolysis might involve refluxing in the presence of dilute hydrochloric acid or sodium hydroxide solution.

The hydrolysis of an ester in both acidic and alkaline conditions is represented below:

Notice that in alkaline conditions the carboxylate ion is formed. The addition of a strong acid, such as dilute hydrochloric acid, is required to free the carboxylic acid molecule. In the base-catalysed hydrolysis above, you could think of the reaction as with water, and the carboxylic acid molecule formed losing a proton to a hydroxide ion.

The equation below represents the acid-catalysed hydrolysis of an amide. Notice that since R'NH3+Cl- is shown as a product, HCl is needed on the left of the equation.

The hydrolysis of a halogenoalkane such as bromoethane is shown below. You may have discussed this reaction as a nucleophilic substitution reaction.


Some other organic compounds undergo hydrolysis as well as those above...

These include the following homlogous series:

Acyl chlorides react vigorously with water becoming rapidly hydrolysed. Steamy white fumes of hydrogen chloride can be seen.

RCOCl(l) + H2O(l)   ®   RCOOH(aq) + HCl(g)

The hydrolysis of amides are referred to above. In the acid-catalysed hydrolysis of a nitrile a primary amide is formed, which in turn is hydrolysed to the ammonium salt and then the carboxylic acid.

It is useful to remember that the 'hydrolysis of a nitrile produces a carboxylic acid'. Here is an equation to represent the overall reaction.

R-CºN(l) + 2H2O(l) + HCl(aq)   ®   R-COOH(aq) + NH4+Cl-(aq)

Had the nitrile been heated with dilute aqueous sodium hydroxide, OH- ions would have removed protons from the ammonium ions of the ammonium salt to form ammonia gas and the sodium carboxylate salt. Adding an acid such as hydrochloric acid or sulphuric acid to the sodium carboxylate would free the carboxylic acid.

The benzenediazonium ion is very unstable, especially if the temperature of its aqueous solution is allowed to rise above 5 °C. Its decomposition results in the loss of the very stable N2 molecule, and the carbocation then formed allows a water molecule nucleophile to add to it forming phenol. The hydrolysis of benzenediazonium chloride is given below:


Acid Hydrolysis of Proteins...

The primary structure of a protein is the sequence of amino acid units that make up the polypeptide chain. Peptide bonds (or links) form between amino acid molecules as a result of condensation reactions.

Proteins are natural polyamides. If a polypeptide or protein is refluxed for about 24 hours with moderately concentrated (say 6 mol dm-3) hydrochloric acid, all of the peptide bonds are hydrolysed. After neutralisation the free amino acids can be separated by paper chromatography or thin layer chromatography.