Question

An ester has a molecular mass of 102. On aqueous hydrolysis, it produces a monobasic acid and alcohol. If 0.185 g of the acid produced completely neutralises 25 mL of 0.1 N NaOH, find out the structural forulae of the alcohol produced and the ester with proper reasoning.

Answer 1

(i) From the available data,
25 mL of 0.1 N NaOH neutralise acid =0.185 g
1000 mL of 1.N NaOH will neutralise acid `=overset(0.185xx1000)underset(25xx0.1)=74.0g`
1000 mL of 1 N NaOH contain gram equivalent of it and it and it must react with gram equivalent of acid.
`therefore`Equivalent mass of monobasic acid =74.0
Molecular mass of monobasic acid =74.0
(ii) The monobasic acid is represented as RCOOH and the molecular mass from the molecular formula is
`=RCOOH=R+12+32+1+R+45`
`therefore Now R+45=74 or R=74-45=29`.
This indicates that R is ethyl group `(CH_(3)CH_(2))` and the acid is `CH_(3)CH_(2)COOH.` (Propanoic acid).
(iii) The molecular mass of ester `(CH_(3)CH_(2)COOR)` is 102.
Mass of alkyl group (R-)=(102-73)=29
This indicates that the alkyl group R is also `C_(2)H_(5)` group and the ester is `CH_(3)CH_(2)COOC_(2)H_(5).`
`underset("Ethyl propionate")underset()(CH_(3)CH_(2)COOC_(2)H_(5))+H_(2)Ooverset(H^(+))rarrunderset("Propionic acid")underset()(CH_(3)CH_(2)COOH)+underset("Ethyl alcohol")underset()(C_(2)H_(5)OH)`