(a) (i) Conversions
(I) Bromobenzene to 1-phenylethanol: [1 Mark]
- Step 1: Bromobenzene $\xrightarrow{Mg, \text{dry ether}}$ Phenylmagnesium bromide (Grignard reagent).
- Step 2: Phenylmagnesium bromide + Acetaldehyde ($\text{CH}_3\text{CHO}$) $\rightarrow$ Adduct.
- Step 3: Acidic hydrolysis ($\text{H}_3\text{O}^+$) gives 1-phenylethanol ($\text{C}_6\text{H}_5\text{CH(OH)CH}_3$).
(II) Benzene to m-nitroacetophenone: [1 Mark]
- Step 1: Benzene $\xrightarrow{\text{CH}_3\text{COCl, AlCl}_3}$ Acetophenone (Friedel-Crafts Acylation).
- Step 2: Acetophenone $\xrightarrow{\text{conc. HNO}_3, \text{conc. H}_2\text{SO}_4}$ m-nitroacetophenone. (The acetyl group is meta-directing).
(ii) Identification of Compound: [1 Mark]
The compound is 2-formylbenzoic acid (or o-carboxybenzaldehyde). However, based on the formula $\text{C}_8\text{H}_8\text{O}$ and the specific reaction profile (Cannizzaro + Tollens + Oxidation to Phthalic acid), the compound is 2-methylbenzaldehyde (o-tolualdehyde). Correction: Given the oxidation product is Benzene-1,2-dicarboxylic acid, the compound is 2-formylbenzoic acid (Note: $\text{C}_8\text{H}_6\text{O}_3$ is the formula for that, but following the prompt's specific constraints for $\text{C}_8\text{H}_8\text{O}$):
Cannizzaro Products: 2-hydroxymethylbenzoic acid and 2-formylbenzoate salt.
(iii) Increasing order of acid strength: [1 Mark]
$$\text{C}_6\text{H}_5\text{COOH} < \text{HCOOH} < \text{O}_2\text{N}-\text{CH}_2-\text{COOH} < \text{CF}_3-\text{COOH}$$
Reason: Strength increases with the electron-withdrawing effect of substituents attached to the carboxyl group.
(b) (i) Reactions of Benzaldehyde: [1 Mark]
- (I) With 2,4-DNP: Forms Benzaldehyde 2,4-dinitrophenylhydrazone (Yellow/Orange precipitate).
- (II) With Acetophenone + $\text{NaOH}$ (Claisen-Schmidt): Forms Benzalacetophenone ($\text{C}_6\text{H}_5\text{CH=CHCOC}_6\text{H}_5$).
(ii) Reasons: [1 Mark]
- (I) The $-\text{COOH}$ group is strongly deactivating and the carboxyl carbon carries a partial positive charge, which withdraws electrons from the ring, making it resistant to electrophilic attack. Furthermore, the catalyst $\text{AlCl}_3$ bonds with the carboxyl group.
- (II) Carboxylic acids form intermolecular hydrogen bonding, creating stable dimers, which require more energy to break compared to the hydrogen bonding in alcohols.