(i) The reaction between carbonyl compounds and HCN proceeds via the nucleophilic attack of CN‌– on carbonyl carbon. Such an attack takes place faster if the carbonyl carbon is not attached to large alkyl groups. Less is the steric hindrance, morereactive is the compound.
∴ The order is :(ii) The presence of any electron withdrawing group or electronegative atom in carboxylic acid molecule increases the acidic strength of the acid. Moreover, the acidity decreases if the atom/group is bonded further off the α-carbon. Therefore, the acidstrength is : Between I and II the higher acidity of II is due to lesser number of –CH3 groups on the α-carbon. –CH3 group increases the electron density by +I effect on a-carbon which in turn exerts the same effect on COOH group and hinders release of H+ .
(iii) In benzoic acid, the attachment of electron withdrawing groups (EWG) on the benzene ring makes the O—H bond of COOH group polar and H is lost as H+.
Therefore, the order is,