Answer

Organic Chemistry Transformations

Note: If a transformation can proceed via an ester or a ketone, always choose the ester.

a. Methyl cyclohexanecarboxylate → Cyclohexylmethyl bromide

Reagents:
1. LiAlH₄ – reduces ester to primary alcohol
2. PBr₃ – converts alcohol to alkyl bromide

Explanation: LiAlH₄ reduces the ester to cyclohexylmethanol. PBr₃ then replaces the OH with Br via an SN2 mechanism.

b. 1-Methylcyclohexanol → 1-Phenyl-1-methylcyclohexanol

Reagents:
1. H₂SO₄ / Heat – dehydration to alkene
2. PhMgBr – Grignard addition
3. H₃O⁺ – protonation of alkoxide

Explanation: Dehydration yields 1-methylcyclohexene. Phenylmagnesium bromide adds across the double bond, followed by acidic workup to form the tertiary alcohol.

c. Isobutyl bromide → 3,3-Dimethyl-2-butanol

Reagents:
1. Mg/ether – forms Grignard reagent
2. Acetone – electrophilic carbonyl
3. H₃O⁺ – acidic workup

Explanation: The Grignard reagent from isobutyl bromide reacts with acetone. Acidic workup gives the tertiary alcohol.

d. 3-Phenyl-1-propanol → 1-Cyclohexyl-3-phenyl-1-propanone

Reagents:
1. PBr₃ – converts alcohol to alkyl bromide
2. Mg/ether – forms Grignard reagent
3. Cyclohexyl ester – for nucleophilic addition
4. H₃O⁺ – workup

Explanation: Bromination forms a suitable Grignard precursor. The Grignard adds to the ester, yielding a ketone after acidic workup.

e. 4-Bromotoluene → 4-Methylbenzoic acid

Reagents:
1. Mg/ether – Grignard formation
2. CO₂ (dry ice) – carboxylation
3. H₃O⁺ – acid workup

Explanation: Grignard reagent reacts with CO₂ to give the carboxylate intermediate. Protonation gives the carboxylic acid.

f. 1-Methylcyclohexanol → 1-Ethoxy-1-methylcyclohexane

Reagents:
1. H₂SO₄ / Heat – dehydration to alkene
2. EtOH / H⁺ – Markovnikov addition of ethanol

Explanation: Dehydration gives 1-methylcyclohexene. Electrophilic addition of ethanol across the double bond forms the ether.