Q1(c). Squaric acid; acylation of trans-2-butene

Introduction. (i) A is squaric acid, 3,4-dihydroxycyclobut-3-ene-1,2-dione. Its strength comes from the stability of its anions. (ii) is a Friedel–Crafts acylation of an alkene (Darzens–Nenitzescu reaction).

(i) Squaric acid

1. The O–H is a vinylogous carboxylic acid: O=C–C=C–OH. The charge of the anion is delocalised over the C=O through the ring C=C.
2. After both protons are lost, the dianion C₄O₄²⁻ has D₄ₕ symmetry: all four C–C and all four C–O bonds are equal, and each O carries half a negative charge.
3. The four-membered ring then holds 2π electrons (Hückel 4n+2, n=0): the ring is aromatic, like the cyclopropenyl cation.

Figure: Squarate dianion: four equal C–O bonds, each O about -12; 2π aromatic ring.

1. The sp² ring carbons and the electron-withdrawing C=O groups also stabilise the anion inductively.
2. Result: pK₁ is about 1.5 and pK₂ about 3.5 (March): even the second proton is lost more easily than the proton of acetic acid (pKₐ 4.76). In water the first ionisation is nearly complete, which is why A is called almost as strong as H₂SO₄.

(ii) trans-2-Butene and CH₃COCl/AlCl₃

1. AlCl₃ takes Cl⁻ from acetyl chloride: acylium ion CH₃C#O⁺ (AlCl₄⁻ counter-ion).
2. The C=C attacks the acylium carbon: a secondary carbocation β to the new C=O.
3. The cation is trapped by Cl⁻ (from AlCl₄⁻): 4-chloro-3-methylpentan-2-one (two new stereocentres, mixture of diastereomers, racemic).
4. On warming or on distillation (or with a base) HCl is eliminated: the C=C forms in conjugation with C=O, giving the α,β-unsaturated ketone 3-methylpent-3-en-2-one. Direct loss of H⁺ from the cation gives the same enone and some β,γ-isomer.

Scheme: Electrophilic addition of the acylium ion, then loss of HCl.

Conclusion. (i) Squaric acid is very strong because its mono- and di-anions are fully delocalised and the dianion is a 2π aromatic, symmetric ring (pK₁ ≈ 1.5). (ii) trans-2-Butene is acylated: the acylium ion adds to give 4-chloro-3-methylpentan-2-one, which loses HCl to the conjugated enone 3-methylpent-3-en-2-one.

Application. Squaric acid is the starting material for squaraine dyes (near-infrared absorbers used in imaging and solar cells); aliphatic Friedel–Crafts acylation of alkenes is a direct route to α,β-unsaturated ketones.