Question 4 of 5: Friedel–Crafts Acylation Mechanism & Alkene Stability

Question 4: Friedel–Crafts Acylation Mechanism & Alkene Stability (20 marks)

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.

a) Friedel–Crafts acylation mechanism. AlCl₃ activates acetyl chloride toward ionization, generating a resonance-stabilized acylium ion that then attacks the aromatic ring:

1. Acylium ion formation. AlCl₃ (a strong Lewis acid) coordinates the chlorine of acetyl chloride, and the C–Cl bond heterolyzes: $$\mathrm{CH_3COCl + AlCl_3 \longrightarrow CH_3CO^+ + AlCl_4^-}$$
2. Electrophilic attack on the ring — the Wheland (arenium) intermediate. The acylium ion's electrophilic carbon is attacked by a pair of π electrons from the aromatic ring, breaking aromaticity and generating a resonance-stabilized cyclohexadienyl cation with the positive charge delocalized over three ring positions.
3. Deprotonation restores aromaticity. AlCl₄^- removes the proton from the sp³ ring carbon that bears the new acetyl group, regenerating the aromatic ring and AlCl₃: $$\mathrm{C_6H_6 + CH_3CO^+ \longrightarrow [\text{arenium}]^+ \xrightarrow{-H^+} C_6H_5COCH_3}$$

Overall: $$\mathrm{C_6H_6 + CH_3COCl \xrightarrow{AlCl_3} C_6H_5COCH_3 + HCl}$$ (verified atom-balanced, C8H9OCl on both sides). Product: acetophenone. Unlike Friedel–Crafts alkylation, the acylium ion cannot rearrange (no hydride/alkyl shifts) and the ring is deactivated after one acylation, so the reaction cleanly stops at mono-substitution.

b) Alkene stability by substitution count. All three are C₈H₁₆ constitutional isomers of dimethylhexene; the ranking depends on how many carbon substituents sit directly on the two sp² (alkene) carbons themselves — a branch one bond away (allylic) does not count.

1. 4,5-Dimethyl-2-hexene: disubstituted. The double bond is at C2=C3; the methyl branches sit at C4 and C5, which are not on the alkene carbons (C4 is allylic to C3, one bond removed). The alkene itself carries only its ordinary chain substituents (C1 on one side, C4 chain on the other) — a plain disubstituted alkene, same substitution level as a simple internal alkene. Least stable.
2. 3,4-Dimethyl-2-hexene: trisubstituted. The double bond is at C2=C3; C3 (an alkene carbon) carries a directly-attached methyl branch in addition to the C4 chain — three total alkyl substituents across the two alkene carbons (C1 on C2; methyl branch and C4-chain on C3). Intermediate stability.
3. 2,3-Dimethyl-2-hexene: tetrasubstituted. Both C2 and C3 carry a directly-attached methyl branch as well as their chain continuations — every position on both alkene carbons is a carbon substituent, none is H. Most stable (maximum hyperconjugative stabilization from four alkyl groups).