Question 11 of 13

Question 11 (11/13)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Approach. For every multi-substituted target, first classify each substituent as an activating ortho,para-director (alkyl, NH₂/NHAc, after reduction) or a deactivating meta-director (NO₂, SO₃H, acyl/C=O), then choose an installation order in which each new group's directing effect is either reinforced by, or at minimum compatible with, the group(s) already on the ring — and always install any group that would poison a later Friedel–Crafts step (a strong deactivator) dead last.

a) Simple electrophilic bromination: benzene + Br₂/FeBr₃ → bromobenzene directly.

b) Brominate toluene first (Br₂/FeBr₃, take the ortho product), then nitrate: CH₃ is a stronger activator than Br and dominates the regiochemistry, directing NO₂ to its own para position — which is simultaneously meta to the already-installed Br, giving exactly the target substitution pattern.

c) Friedel–Crafts acylation must precede nitration here, since a nitro group would deactivate the ring against any further AlCl₃-catalysed step. Alkylate benzene to cumene, then acylate (directed ortho/para by isopropyl; the required target needs the ortho acylation product specifically, which is a real, isolable minor product alongside the sterically preferred para isomer). The final nitration is doubly convergent: isopropyl's para preference and the newly installed acetyl group's meta-directing preference point to the identical ring carbon, so the required regiochemistry is obtained essentially without ambiguity.

d) PABA (4-aminobenzoic acid) is best built by installing the carbon that will become the carboxyl group first (via Friedel–Crafts acylation, since direct carboxylation of benzene is not practical), nitrating para to that acyl group (a meta-director, but here the required NO₂ position for a 1,4-relationship to the eventual amine is reached via the acyl group's own meta preference measured from the correct ring carbon), reducing the nitro group to the amine (protecting it as the acetanilide before any further reaction to avoid over-oxidation), and finally oxidising the acyl side chain through to the carboxylic acid.

e) Alkylate benzene to n-propylbenzene, sulfonate (SO₃/H₂SO₄, directed para to the propyl group — the sterically preferred site for a bulky electrophile reacting with an alkylbenzene), then neutralise the sulfonic acid with NaOH to the sodium salt.

f) A single alkyl group can never deliver its own meta product directly (alkyl groups are o,p-directors only) — but here Br is required para to the amine, which the amine's own (very strong) directing power supplies once installed. Alkylate benzene to propylbenzene, nitrate ortho to the propyl group, reduce to the amine, protect as the acetanilide (to moderate reactivity and avoid polybromination), brominate (now directed para by the dominant amido group), and hydrolyse the amide to reveal the free amine — 4-bromo-2-propylaniline.

g) Build the vinyl group by Friedel–Crafts acylation (CH₃COCl/ AlCl₃) followed by carbonyl reduction (NaBH₄) and acid-catalysed dehydration (H₂SO₄, Δ) to give styrene; a second Friedel–Crafts alkylation installs the allyl/propenyl side chain, and electrophilic chlorination (Cl₂/FeCl₃), directed ortho by the ring-activating vinyl group, installs the chlorine adjacent to it.