Gray, Jorces, and Marson. 393 71. M~omorphism and Chemical Constitution. Part V I I P The Effect of 3'-Substituents on, the Maomorphim of the 4'-n-AEkoxydiphenyl-4-curboxylic Acids and their Alk: yl Esters.
Thirteen 4'-n-alkoxydiphenyl-4-carboxylic acids have been prepared and their mesomorphic behaviour is compared with that of the 4-n-alkoxybenzoic acids. They exhibit mesophases of much greater relative thermal stability than the simpler benzoic acids; this is attributed to the enhanced intermolecular cohesion arising from the second aromatic ring and to the greater molecular length of the diphenyl compounds. The high relative thermal stability of the mesophases is emphasised by the mesomorphic behaviour of the methyl, ethyl, and n-propyl alkoxydiphenylcarboxylates, and the absence of anisotropic melts in the analogous alkoxybenzoates.
The preparation and mesomorphic character of certain 4-rz-alkoxy-3-halogenobenzoic acids are described. Halogen-substitution results in an increase in the breadth of the molecule which is reflected in the relative thermal stabilities of the mesophases. With fluorine and chlorine the increase is not sufficient to eliminate the mesomofphisrn, but the thermal stabilities of the phases of the broader chloro-acids are lower than those for the corresponding fluoro-acids. The presence of one bromine atom, or two chlorine atoms, as in 4-n-alkoxy-3 : 5-dichlorobenzoic acids, is sufficient to destroy the mesomorphism of the unsubstituted acids.IT is generally recognised that, apart from such molecules as the esters of cholesterol, long rod-shaped molecules are the most likely to exhibit mesomorphism. This is true only if the intermolecular cohesion is sufficient to maintain the linear molecular arrangement in the mesophase at elevated temperatures. When the breadth of a mesomorphic compound is increased by substitution the intermolecular cohesion may be lowered, as a result of the increased separation of the molecules from one another. The melting points and the mesomorphic transition temperatures (smectic-nematic, and smectic-or nematicisotropic) should then be lower in the substituted than in the unsubstituted compounds.Such behaviour has been found for 4-~z-alkoxy-3-fluoro-, -3-chloro-, and -3-bromobenzoic acids, whose physical constants are tabulated below. Figures in parentheses *
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