Q.(a)

(a) A Grignard reagent adds to dry-ice $CO_2$ and hydrolyses to a carboxylic acid; biodegradable polymers like PHBV are broken down by microbes. (b) Tracing the clue chain (Clemmensen reduction, then nitration, then nitroalkane hydrolysis to acid + hydroxylamine) identifies A-E as acetaldehyde, ethane, nitroethane, nitromethane, and acetic acid.

(a)(i) Acetic acid from a Grignard reagent: Methylmagnesium iodide, $CH_3MgI$, is added to solid carbon dioxide (dry ice); the nucleophilic methyl carbanion of the Grignard reagent adds to the electrophilic carbon of $CO_2$, forming a magnesium carboxylate salt, which on acidic hydrolysis (dilute acid, $H_3O^+$) liberates the free carboxylic acid: $CH_3MgI + CO_2 \rightarrow CH_3COOMgI \xrightarrow{H_3O^+} CH_3COOH\ (\text{acetic acid}) + Mg(OH)I$

(a)(ii) Biodegradable polymers: These are polymers that can be broken down (degraded) by the action of microorganisms (bacteria, fungi) into simple, non-toxic products such as $CO_2$, water, and biomass, unlike conventional plastics which persist in the environment for very long periods. Example: PHBV (poly-$\beta$-hydroxybutyrate-co-$\beta$-hydroxyvalerate), a copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic (valeric) acid, used in specialty packaging and controlled drug-release/orthopaedic applications; another example is nylon-2-nylon-6.

OR (b) Identifying A-E:

Compound A ($C_2H_4O$) reacted with $Zn$-$Hg$/conc.HCl (Clemmensen reduction, which reduces a carbonyl group fully to $-CH_2-$) — this points to A being an aldehyde/ketone that is reduced to an alkane, so $A=$ acetaldehyde, $CH_3CHO$. Clemmensen reduction converts it to $B=$ ethane, $CH_3CH_3$: $CH_3CHO \xrightarrow[\text{conc. HCl}]{Zn(Hg)} CH_3CH_3$

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