Q.(a) Show how each of the following compounds can be converted into benzoic acid ?

Ethylbenzene, acetophenone, and bromobenzene each convert to benzoic acid by a different route (side-chain oxidation, haloform cleavage, and Grignard carboxylation respectively); –COOH is meta-directing and deactivating, and electron-withdrawing substituents like Cl raise acidity. Aldol condensation needs an α-H; Cannizzaro's reaction needs its absence.

1. Conversion of each compound to benzoic acid:
   1. Ethylbenzene → benzoic acid: Any alkyl side chain on a benzene ring, however long, is oxidised all the way down to a single –COOH group directly attached to the ring when treated with a strong oxidising agent such as alkaline KMnO4 (or acidic K2Cr2O7/K2Cr2O7-H2SO4), under reflux, followed by acidification: C6H5–CH2CH3 —[alk. KMnO4, Δ; then H3O+]→ C6H5–COOH (The rest of the side chain is lost as CO2/other oxidation products; only the ring-attached carbon survives, as the carboxyl carbon.)
   2. Acetophenone → benzoic acid: Acetophenone, C6H5–CO–CH3, is a methyl ketone. Treating it with I2 and NaOH (or NaOI, generated in situ) — the haloform reaction — iodinates and then cleaves the C–C bond next to the carbonyl, splitting off the methyl group as iodoform (CHI3, a pale yellow precipitate, positive iodoform test) and leaving the rest as sodium benzoate: C6H5COCH3 + 3I2 + 4NaOH → C6H5COONa + CHI3↓ + 3NaI + 3H2O Acidifying the sodium benzoate with dilute HCl then gives benzoic acid: C6H5COONa + HCl → C6H5COOH + NaCl
   3. Bromobenzene → benzoic acid: Bromobenzene is first converted to its Grignard reagent by reacting with magnesium in dry ether: C6H5Br + Mg —(dry ether)→ C6H5MgBr (phenylmagnesium bromide) This Grignard reagent is then treated with solid carbon dioxide (dry ice), which inserts into the C–Mg bond to form the magnesium salt of the carboxylic acid, which on acidic hydrolysis (dilute HCl) gives benzoic acid: C6H5MgBr + CO2 → C6H5COOMgBr —[H3O+]→ C6H5COOH
2. Explanations:
   1. Electrophilic substitution in benzoic acid occurs at the meta position: The –COOH group is strongly electron-withdrawing, both inductively (−I, through the sigma bonds) and by resonance (−M/−R, the carbonyl oxygen pulls electron density from the ring through conjugation). This withdrawal of electron density is greatest at the ortho and para positions relative to –COOH (the positions directly conjugated with the group), leaving these positions comparatively more electron-poor and destabilised towards attack by an electrophile (which needs electron density to attack). The meta position, not being directly conjugated with the withdrawing group in the relevant resonance structures, retains comparatively more electron density and gives a less destabilised (i.e. relatively more favourable, though still deactivated overall) intermediate arenium ion on electrophilic attack. Hence –COOH (like –NO2, –CN, –SO3H) is classified as a meta-director as well as a ring-deactivator, so incoming electrophiles preferentially substitute at the meta position of benzoic acid.
   2. Chloroacetic acid is a stronger acid than acetic acid: Acid strength depends on how stable/favoured the conjugate base (the carboxylate anion) is — the more stable the anion, the more readily the acid gives up its proton, and the stronger the acid. In chloroacetic acid, Cl–CH2–COOH, the electronegative chlorine atom exerts an electron-withdrawing inductive (−I) effect, which pulls electron density away from the carboxylate oxygen atoms in the conjugate base (ClCH2COO−), delocalising/dispersing its negative charge and thus stabilising it. In plain acetic acid, CH3–COOH, the methyl group is a (weak) electron-donating (+I) group, which intensifies (concentrates) the negative charge on the conjugate base (CH3COO−) rather than dispersing it, making it comparatively less stable. Since a more stable conjugate base means a stronger acid, chloroacetic acid (Ka ≈ 1.4 × 10⁻³) is significantly more acidic than acetic acid (Ka ≈ 1.8 × 10⁻⁵). OR — (a) Aldol condensation and (b) Cannizzaro's reaction: …