HYDROLYSIS

HYDROLYSIS

The term hydrolysis is applied to the reaction in which water effects a double decomposition with another compound, hydrogen goes to one of the product, hydroxyl to the other. Breaking down of proteins, the saponification of fats and other esters are the examples of hydrolysis. It is very difficult to carry out effective hydrolysis with water alone even at high temperature and pressure. For completion and speedy hydrolysis an accelerating agent of some kind is always added. The most important accelerating agents are alkali, acid and hydrolyzing enzyme, although ion-exchange resins have also been used. The enzymes (organic catalyst) are of surpassing importance in vital processes, both animal and plant.

                                                       XY +H2O HY + XOH

If XY were an inorganic compound, this would be the reverse of neutralization. But in organic chemistry, hydrolysis has a wider scope, which includes:

1. Inversion of sugar
2. Breaking down of proteins
3. Saponification of fats and oils

Different examples of hydrolysis reaction are as follows.

In the hydrolysis of either organic or inorganic compounds, water and another compound undergo double decomposition to form two products. The hydrogen from the water goes to one product while the hydroxyl goes to the other product as illustrated in the following equation:

DIFFERENT HYDROLYZING AGENTS USED FOR HYDROLYSIS

The important hydrolysis agents are

1. Water
2. Water - acid (acid hydrolysis)
3. Water-alkali (alkali hydrolysis)
4. Enzymes (enzymatic hydrolysis)

1. Water

Although the word hydrolysis means decomposition by water, cases in which water unaided brings about effective hydrolysis are rare, but are there few specific reaction where water alone acting as effective hydrolysis agent. Hydrolysis of Grignard reagent takes place completely and quickly by water alone. The acid anhydrides, lactones, lactides and other internal anhydrides such as ethylene oxide are readily hydrolyzed by water.

Alkyl halide of complex composition reacts with water alone.

                        e.g. CIH4C2 - S - C2H4CI + H2O                        OH-H4C2 - S - C2H4OH + 2HCI

Diazonium salt when heated with water hydrolyzed completely.

                          C6H5-N+ NCI¯ + H2O                                  C6H5OH + HCl + N2

Ethyl hydrogen sulfate, sodium salt of acetoacetic ester can be completely hydrolyzed by water.  A slight degree of hydrolysis seems to occur with numerous materials such as esters carbohydrates etc, but it should be noted that water by itself is able to bring about omplete and rapid hydrolysis of only very few substances.

The use of steam rather that water has brought good result. Benzene sulfonic acid with steams yields benzene and sulfuric acid similarly α-naphthalene sulfonic acid hydrolyzes to naphthalene.

C6H5SO3H + H2O (Steam)                             C6H6 + H2SO4

C10H7SO3H + H2O (Steam)                               C10H8 + H2SO4

        2. Acid hydrolysis

The use of acid in hydrolysis is rapidly extended to organic materials such as esters, sugar, amide etc. it was found that acid accelerates the reaction. It also initiates the reaction where water alone fails to complete the hydrolysis. This effect is due to hydrogen ion content of acid.

Hydrochloric and sulfuric acids are the most commonly used. Formic acid and trichloro acetic acid appears to be lower in activity whereas oxalic and benzenesulfonic acid are more active.

Sulfuric acid is particularly useful because it forms, with many types of organic substances, intermediate compound that themselves readily undergo hydrolysis; sulfuric acid exhibits a specific action, distinct from its hydrogen ion concentration, and cannot be replaced by other acids. Organic esters of all kinds, including the esters of carbohydrates are quire subject to hydrolysis with acid.

       3.Alkali hydrolysis

Sodium hydroxide is most commonly used, but alkali carbonates and bicarbonates as well as the alkaline earth hydroxides all find occasional application. Potassium hydroxide shows no advantage over other hydroxides except in some case permits the use of a lower temperature. Ammonium hydroxide and ammonium salts are rarely used.

Hydroxyl ion catalyzes the hydrolysis. Acid is one of the products of hydrolysis that mediately reacts with hydroxyl ion.

Alkali hydrolysis is distinguished into three cases.

1.The use of low concentration of alkali in the hydrolysis of esters and similar material. 

NaOH R-COOC2H5     gives       RCOONa + C2H5OH

2.The use of sufficient caustic under pressure and in high concentrations to unite with all the acid produced.

C6H5CI + NaOH      gives         C6H5OH + NaCI + H2O
C6H5OH + NaOH     gives         C6H5ONa + H2O

3.The fusion of organic material with caustic soda or potash. 

MECHANISM OF HYDROLYSIS