(E)-o-stilbenecarboxylic acid and its p-methyl, chloro and methoxy derivatives

The crystal structures of the title compounds, (E)-stilbene-1-carboxylic acid, C₁₅H₁₂O₂, (Ia), (E)-4′-methylstilbene-1-carboxylic acid, C₁₆H₁₄O₂, (Ib), (E)-4′-chlorostilbene-1-carboxylic acid, C₁₅H₁₁ClO₂, (Ic) and (Ic′), and (E)-4′-methoxystilbene-1-carboxylic acid, C₁₆H₁₄O₃, (Id), have been determined. The chloro derivative shows polymorphism, with prisms, (Ic), being obtained from ether/hexane solution and needle crystals, (Ic′), being obtained from ethanol solution. The latter is isostructural with (Id). The selectivities of the products to the addition reaction of bromine are different, however, suggesting that the bromonium ion intermediate is stabilized by a resonance contribution of the p-MeO group.