(i) Give reasons :
(a) HCHO is more reactive than `CH_3-CHO` towards addition of HCN .
(b) pKa of `O_2N-CH_2-COOH` is lower than that of `CH_3-COOH`.
(c) Alpha hydrogen of aldehydes and ketones is acidic in nature.
(ii) Give simple chemical tests to distinguish between the following pairs of compounds :
(a) Ethanal and Propanal
(b) Pentan-2 one and Pentan-3-one

Explanation

(i)(a) In ethanl , the `(+)` ve charge on the carbonyl carbon is lowerd by high `+I` effect of two alkyl groups in comparison to methanol in which 2 hydrogens are present . Further increase in the no. of alkyl groups also increases the steric hindrance making it less reactive. Thus, in general, HCHO is more reactive than `CH_3CHO` towards addition of nucleophile HCN.
(b) `NO_2` is withdrawing group, which disperse the negative charge on carbonlate ion (decreases electron density) stabilize the carboxylate anion and thus, increases the acidity of carboxylic acid. Greater the acidic character, lower will be the pKa value. Hence, `O_2N-CH_2COOH` is having lower value of pKa than `CH_3COOH`.
(c) `alpha`-hydrogen of aldehyde and ketone is attached to carbonyl carbon. After removal of `alpha`-Hydrogen, conjugated base so obtained is resonance Hence `alpha-H` in aldehyde and ketone is acidic in nature.
(ii) `overset(|)underset(Theta)underset (..)(C)-overset(..)overset(O: )ChArr-overset(|)C=overset(-)overset( :O: )overset(|)(C)-`
(ii) (a) Ethanal `(CH_2CHO)` and propanal `(CH_3CH_2CHO)`
`CH_3CHO+l_(2)+NaOHoverset("Iodoform")underset("Reaction")to` no reaction (NO yellow ppt)
(b) Penton -2-one pentan-3-one
`underset("Penton-2-one")(C-C-C-overset(O)overset(||)C-C+l_2)+NaOHoverset("Iodoform")underset("Reaction")tounderset("Yellow ppt")(CHI_3)+CH_3CH_2CH_2-COONa+Nal+H_2O`
`C-C-C-overset(O)overset(||)underset("Penton-2-one")(C-C+l_2+NaOH)overset("Iodoform")underset("Reaction")tounderset("No Yellow ppt")("No reaction")`