The Effect of Carbonyl Containing Terminal Chains on Mesomorphic Properties in 4,4′- Disubstituted Phenylbenzoates and Phenylthiobenzoates. 4. Phenylbenzoates Containing A (CH₂)_nCO₂R'Group (n = 0–2) on the Phenolic End

Abstract

The effect of a (CH₂),CO₂R' group on the mesomorphic properties of the esters where X= alkyl or alkoxy, Y=(CH₂)nCO₂R'(R'=C₇ and C₉) and n = 0–2 has been studied by synthesizing these esters and determining their mesomorphic properties by hot-stage polarizing microscopy. The starting phenols were prepared by esterification of hydroxy protected 4-hydroxybenzoic, phenylacetic or phenylpropionic acids. Both the benzyl and methoxycarbonyl protecting groups were tried with the latter giving higher yields when n = 0 because of better solubility of the protected acid. No mesophases were observed in the esters when n = 1, nematic and smectic A phases occurred when n = 2 and smectic A and C phases when n = 0. A few 1,4-cyclohexane diesters were also prepared using these phenols, The mesomorphic properties of these esters followed the same trend observed in the phenylbenzoates except no C phases were observed.

Comparisons of the transition temperatures for these esters with those for Y=R' showed that both small increases or decreases were observed for Y=CO₂R'. However, the addition of a spacer methylene group (n = 1 and 2) always gave lower temperatures with the amount of lowering being much greater for n = 2 than n = 1. A comparison of transition temperatures for Y=CO₂R', OCOR', COR' and OR' indicated that these temperatures were highest when Y=COR' as expected from dipole moment considerations but were lower when Y=CO₂R' than when Y=OCOR' the opposite expected from these considerations. This trend was also observed in the cyclohexane diesters. Transition temperatures were always higher for the esters when Y has an oxygen atom adjacent to the benzene ring. Therefore, esters with Y=O(CH₂)_nCO₂R'. n = 1 and 2 were also synthesized. The phenols were prepared by alkylation of 4-benzyloxyphenol with the bromo esters followed by hydrogenolysis. However, these esters showed no mesophases except the cyclohexane diester with n= 2.