Example 7.4

Example . . Esterification Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters. In substituted phenols, the presence of electron withdrawing groups such as nitro group, enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions. It is due to the effective delocalisation of negative charge in phenoxide ion when substituent is at ortho or para position. On the other hand, electron releasing groups, such as alkyl groups, in general, do not favour the formation of phenoxide ion resulting in decrease in acid strength. Cresols, for example, are less acidic than phenol. The greater the p K a value, the weaker the acid. Pyridine R/Ar +R ’ l COC R/Ar OCO R + HCl ’ The reaction with carboxylic acid and acid anhydride is carried out in the presence of a small amount of concentrated sulphuric acid. The reaction is reversible, and therefore, water is removed as soon as it is formed. The reaction with acid chloride is carried out in the presence of a base (pyridine) so as to neutralise HCl which is formed during the reaction. It shifts the equilibrium to the right hand side. The introduction of acetyl (CH CO) group in alcohols or phenols is known as acetylation. Acetylation of salicylic acid produces aspirin. (b) Reactions involving cleavage of carbon – oxygen (C–O) bond in alcohols The reactions involving cleavage of C–O bond take place only in alcohols. Phenols show this type of reaction only with zinc. . Reaction with hydrogen halides: Alcohols react with hydrogen halides to form alkyl halides (Refer Unit , Class XII). ROH + HX ® R–X + H O The difference in reactivity of three classes of alcohols with HCl distinguishes them from one another ( Lucas test ). Alcohols are soluble in Lucas reagent (conc. HCl and ZnCl ) while their halides are immiscible and produce turbidity in solution. In case of tertiary alcohols, turbidity is produced immediately as they form the halides easily. Primary alcohols do not