Question 4 of 5: Alkene Geometric Isomers, a Friedel–Crafts Mechanism, and Alkene Stability
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Notes on this paper
National Exam 04-BS-12, Organic Chemistry — May 2013. 3 hours, closed-book
examination; no calculator allowed. Answer ALL FIVE problems; each problem is of equal
value (20 points), and the lettered/numbered sub-parts of a given problem may be treated
independently.
(a) Cis and trans 2-hexene. Both isomers share the molecular formula
$\text{C}_6\text{H}_{12}$ (one C=C, four degrees of freedom otherwise saturated); they
differ only in the spatial arrangement of the two non-hydrogen groups across the C2=C3 double
bond.
Fig. Q4a — cis-2-hexene (methyl and
propyl chain on the same side of the C=C) vs. trans-2-hexene (opposite sides). Both are
C6H12.
(b) Friedel–Crafts acylation mechanism. AlCl3 is a Lewis
acid catalyst that first activates the acyl chloride into an electrophilic acylium ion; the
aromatic ring then attacks that electrophile, and the resulting non-aromatic arenium
(Wheland) intermediate loses a proton to restore the aromatic sextet, giving acetophenone.
Fig. Q4b — Friedel–Crafts acylation of
benzene with acetyl chloride/AlCl3: (1) acylium generation, (2) electrophilic attack
by the ring, (3) loss of H+ restores aromaticity, regenerating AlCl3.
Step 1 — generate the electrophile. The carbonyl chlorine's lone pair
attacks the empty orbital on Al; the C–Cl bond heterolyses, giving the resonance-stabilised
acylium cation $[\text{CH}_3\text{-C}{\equiv}\text{O}]^+$ and $\text{AlCl}_4^-$.
Step 2 — electrophilic attack by the ring. A pair of the ring's
delocalised π electrons attacks the acylium carbon, forming a new C–C bond and a
cyclohexadienyl (arenium) cation whose positive charge is delocalised over three ring
carbons; this carbon is now sp3, temporarily breaking full ring aromaticity.
Step 3 — re-aromatisation. $\text{AlCl}_4^-$ removes the H
on the sp3 ring carbon as H+, re-forming the aromatic sextet and giving
$\boxed{\text{acetophenone, C}_6\text{H}_5\text{C(=O)CH}_3}$, plus HCl; AlCl3
is regenerated (it is a true catalyst, not a stoichiometric reagent).
(c) Ranking the three 1,2-diethylcyclohexene isomers by stability. Alkene
stability increases with the number of carbon substituents directly on the two double-bond
carbons themselves (more substitution → more hyperconjugative/inductive stabilisation of
the π system) — a ring-carbon or alkyl-chain substituent on the alkene counts, but a
substituent one bond further away (allylic, not on the alkene itself) does not add to this
count.
Fig. Q4c — the same 1,2-diethylcyclohexane
skeleton with the ring's one C=C in three different positions relative to the two ethyl
groups.
(a) Trisubstituted double bond. The C=C sits between one ethyl-bearing
carbon and its unsubstituted ring neighbour. Counting carbon substituents directly on the two
alkene carbons: the ethyl-bearing alkene carbon contributes its ring-neighbour and its
ethyl group (2), the other alkene carbon contributes only its own ring-neighbour (1) plus an H
— $2+1=\boxed{3}$ carbon substituents on the double bond (trisubstituted).
(b) Tetrasubstituted double bond. The C=C sits directly between the two
ethyl-bearing carbons. Each alkene carbon now carries both its ring-neighbour and its
own ethyl group, with no H left on either alkene carbon: $2+2=\boxed{4}$ carbon substituents
(tetrasubstituted) — the most stable of the three.
(c) Disubstituted double bond. The C=C sits on the far side of the ring,
away from both ethyl-bearing carbons; each alkene carbon carries only its own ring-neighbour
plus an H: $1+1=\boxed{2}$ carbon substituents (disubstituted) — the fewest of the
three, hence the least stable. The two ethyl groups here are merely spectator
(allylic-or-further) substituents that do not touch the double bond at all.
