Alkene Isomers, Friedel–Crafts Acylation Mechanism & Combustion: 04-BS-12, December 2014, Q3

Question 3: Alkene Isomers, Friedel–Crafts Acylation Mechanism & Combustion (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) Cis/trans-3-heptene, C₇H₁₄. The C3=C4 double bond carries an ethyl group on one side and a propyl group on the other; cis places both alkyl chains on the same side of the double bond, trans places them on opposite sides.

Fig. Q3a — C7H14, both geometric isomers of 3-heptene

Both are C₇H₁₄ (one degree of unsaturation, the C=C); they are configurational (E/Z) isomers of each other, not constitutional isomers — identical connectivity, different 3-D arrangement about the fixed double bond.

Isomer	Formula	Geometry
trans-3-heptene (E)	C7H14	ethyl & propyl on opposite sides
cis-3-heptene (Z)	C7H14	ethyl & propyl on the same side

b) Friedel–Crafts acylation mechanism. AlCl₃ is a strong Lewis acid; it activates the acyl chloride into a resonance-stabilised acylium ion, which is electrophilic enough to attack the electron-rich benzene ring. The mechanism runs in three elementary steps.

→

Fig. Q3b — substrates: benzene + propanoyl chloride, AlCl3 catalyst

Step 1 — Lewis-acid activation of the acyl chloride. AlCl₃'s empty p-orbital accepts a lone pair from the chloride's Cl, polarising and then fully breaking the C–Cl bond and generating a resonance-stabilised acylium ion, CH₃CH₂C≡O⁺ (positive charge shared between the carbon and the oxygen), plus the tetrahedral counter-ion AlCl₄⁻: $$\mathrm{CH_3CH_2C(=O)Cl + AlCl_3 \longrightarrow CH_3CH_2C{\equiv}O^+ + AlCl_4^-}$$
Step 2 — electrophilic attack on the aromatic ring. A pair of π-electrons from the benzene ring attacks the electrophilic acylium carbon, forming a new C–C bond. This breaks the ring's aromaticity and generates the resonance-stabilised arenium (Wheland) intermediate: a cyclohexadienyl cation whose ipso carbon is now sp³ (bonded to both the acyl group and its original H), with the positive charge delocalised over the three remaining ring positions ortho/para to that carbon:
Fig. Q3b — sp3 ipso carbon, positive charge delocalised over 3 ring positions (only 2 of 3 resonance forms shown as double bonds here)
Step 3 — loss of H⁺ restores aromaticity. The tetrahedral AlCl₄⁻ counter-ion (or a second equivalent of the leaving Cl⁻) removes the ipso carbon's remaining H⁺; the C–H bonding pair becomes the new ring π-bond, regenerating the aromatic sextet and releasing HCl, and AlCl3 is regenerated (catalytic, not consumed overall): $$\mathrm{[\text{arenium}]^+ + AlCl_4^- \longrightarrow C_6H_5C(=O)CH_2CH_3 + HCl + AlCl_3}$$

Fig. Q3b — product: propiophenone (1-phenylpropan-1-one)

Overall: propiophenone (1-phenylpropan-1-one) + HCl.

\mathrm{C_6H_6 + CH_3CH_2C(=O)Cl \xrightarrow{AlCl_3} C_6H_5C(=O)CH_2CH_3 + HCl}

c) Combustion of benzene. Every carbon ends up as CO₂ and every hydrogen as H₂O; balance oxygen last.

$$\mathrm{2\,C_6H_6 + 15\,O_2 \longrightarrow 12\,CO_2 + 6\,H_2O}$$

(Balanced with a leading coefficient of 2 on benzene, since benzene's six carbons and six hydrogens both give an odd oxygen-atom count on a per-molecule basis; doubling everything clears the fraction.)

Part	Result
(a)	cis- and trans-3-heptene, both C7H14
(b)	propiophenone + HCl, via acylium → arenium → deprotonation
(c)	2 C6H6 + 15 O2 → 12 CO2 + 6 H2O

Concept & Theory

Topic. E/Z alkene isomerism; the Friedel–Crafts acylation mechanism (electrophilic aromatic substitution); combustion stoichiometry.

Key relations. Acylium ion formation: RC(=O)Cl + AlCl₃ → RC≡O⁺ + AlCl₄⁻. EAS always runs attack→arenium→deprotonation, never a concerted addition. Combustion: balance C, then H, then O; use a doubling coefficient whenever the parent hydrocarbon formula gives an odd oxygen count.

Why this works. An acylium ion is far less prone to the carbocation rearrangements that plague Friedel–Crafts alkylation (the resonance-stabilised C≡O⁺ is already a good electrophile without needing to shift), which is exactly why acylation is the preferred route whenever a clean, single, unrearranged product is required. The arenium intermediate's positive charge is only ever on carbons ortho/para to the new substituent, never the ipso carbon itself once deprotonation restores aromaticity.

Common pitfall. Drawing the arenium intermediate with the aromatic sextet still intact (it is not — the ipso carbon is genuinely sp³ and non-aromatic mid-mechanism); or forgetting that Friedel–Crafts acylation (unlike alkylation) never over-reacts, since the product ketone deactivates the ring toward further substitution.

Source. McMurry, Organic Chemistry, 9th ed., Ch. 7 (E/Z isomerism), Ch. 16 (Friedel–Crafts acylation mechanism), Ch. 3 (combustion).

Question 3 of 5: Alkene Isomers, Friedel–Crafts Acylation Mechanism & Combustion

Question 3: Alkene Isomers, Friedel–Crafts Acylation Mechanism & Combustion (20 marks)