x — Gattermann-Koch reaction: Benzene + CO + HCl → C₆H₅CHO (benzaldehyde). This reaction introduces an aldehyde group directly onto the benzene ring using CO/HCl with CuCl/AlCl₃ as catalyst.
y — Friedel-Crafts acylation: Benzene + CH₃COCl → C₆H₅COCH₃ (acetophenone). The acetyl group (COCH₃) is introduced by electrophilic aromatic substitution.
Benzaldehyde (x) has no α-hydrogen. Acetophenone (y) has α-hydrogens on the CH₃ group. With NaOH (alkali), this is a Claisen-Schmidt condensation (cross-aldol between an aldehyde and a ketone with no enolisable protons on the aldehyde side).
Gattermann-Koch reaction: Benzene + CO + HCl → Benzaldehyde, using CuCl/AlCl₃ as Lewis acid catalyst. This is a specialized electrophilic aromatic substitution that introduces −CHO directly. The formyl cation (CHO⁺) is the electrophile.
Friedel-Crafts acylation: Benzene + RCOCl → ArCOR. The acylium ion (RCO⁺) is the electrophile. With CH₃COCl, the product is acetophenone (C₆H₅COCH₃).
Claisen-Schmidt condensation: A special cross-aldol between an aromatic aldehyde (no α-H) and a carbonyl compound with α-H, under basic conditions. The aldol product immediately dehydrates to give an α,β-unsaturated carbonyl compound (chalcone here).
Chalcone (benzalacetophenone): Structure is C₆H₅−CH=CH−CO−C₆H₅. It is an enone (α,β-unsaturated ketone) with extended conjugation. Used as a precursor in synthesis of flavonoids.
Pi electrons counting: Each C=C counts 2, each C=O counts 2, each benzene ring counts 6 (3 double bonds). Do NOT count lone pairs on oxygen — only π bonds from double bonds.
Benzene + CO + HCl → benzaldehyde, using CuCl + AlCl₃. The active electrophile is the formyl cation CHO⁺ (or its equivalent). This is one of the few ways to directly introduce −CHO onto a benzene ring without side chain.
Normal aldol: two molecules of the same carbonyl compound. Claisen-Schmidt: aromatic aldehyde (no α-H, cannot form enolate) + aliphatic/aryl ketone (has α-H). The aromatic aldehyde acts as electrophile; enolate of ketone attacks it. Product always dehydrates to give conjugated enone.
In chalcone C₆H₅−CH=CH−CO−C₆H₅, all π systems (both rings, C=C, C=O) are conjugated through the carbon chain. This extended conjugation is responsible for the yellow color of chalcones and their UV absorption.
For any organic molecule: count every π bond. Benzene ring = 3 C=C = 6π. Alkene C=C = 2π. Carbonyl C=O = 2π. Sum all. In chalcone: 6+2+2+6=16π. Note: lone pairs on O are NOT π electrons unless explicitly forming a π system.