Q4(c)(ii). Reimer–Tiemann reaction of p-cresol: normal and abnormal products

Introduction. CHCl₃ and OH⁻ generate dichlorocarbene (:CCl₂), an electrophile that attacks the electron-rich ring of the phenoxide (Reimer–Tiemann reaction). With p-cresol the para position is blocked by CH₃, so attack at the ortho carbon gives the normal aldehyde, while attack at the substituted para carbon gives a dienone that cannot re-aromatise.

CHCl₃ + OH⁻ ⇌ CCl₃⁻ + H₂O CCl₃⁻:CCl₂ + Cl⁻ (α-elimination)

Scheme: Normal (ortho-formyl) product and abnormal (4-dichloromethyl dienone) product.

1. Ortho attack (normal product). The phenoxide attacks:CCl₂ through its ortho carbon, giving a -CCl₂⁻ carbanion on a cyclohexadienone. Proton transfer restores the aromatic ring: an o-(dichloromethyl)phenoxide.
2. The benzal chloride is hydrolysed by OH⁻ (two SN steps via a quinone methide) to the aldehyde; acidification gives 2-hydroxy-5-methylbenzaldehyde (major).
3. Para attack (abnormal product). Attack at C-4, which carries CH₃, gives a dienone with a quaternary carbon. There is no H on C-4 to lose, so aromaticity cannot be restored, and the CHCl₂ group, attached to an sp3 carbon, resists hydrolysis: 4-(dichloromethyl)-4-methylcyclohexa-2,5-dien-1-one (minor).

Conclusion. The normal Reimer–Tiemann product is 2-hydroxy-5-methylbenzaldehyde (ortho formylation via:CCl₂); the abnormal product is 4-(dichloromethyl)-4-methylcyclohexa-2,5-dienone, formed by attack at the methyl-bearing para carbon.

Application. The Reimer–Tiemann reaction makes salicylaldehyde (for coumarin) and vanillin-type aldehydes; the abnormal dienones are the classical evidence for the carbene mechanism.