To cite this article: S. M. Kelly (1993) 2-[4-Alkenyloxy)phenyl]-5-alkylpyrimidines The relationship between position and nature (E/Z) of the double bond and transition temperatures, Liquid Crystals, 14:3, 675-698,The 5-n-alkyl-2-[4-(n-alkoxy)phenyl]pyrimidines are essential components of most commercial chiral smectic C mixtures for eiectrooptic display devices based on ferroelectric effects. This is due to their generally relatively low melting points, enantiotropic, relatively wide range smectic C mesophases, low viscosity and ease of preparation. An unsaturated carbon-carbon double bond has now been introduced into the terminal alkoxy chain of the 5-n-alkyl-2-[4-(alkoxy)phenyl]pyrimidines to produce the corresponding alkenyloxy substituted derivatives. The position and nature ( E / Z ) of the double bond has been varied systematically and the effect on the liquid crystal transition temperatures studied. A number of homologous series of the most interesting alkenyloxy substituted materials has been prepared and evaluated. The position and nature ( E / Z ) of the double bond changes the conformation of the alkenyloxy chain substantially. This can result in significantly higher smectic C transition temperatures for compounds with a trans double bond (E) at an even number of carbon atoms from the molecular core. Significantly lower transition temperatures (including the melting point) are observed for materials with a cis double bond ( Z ) at an odd number of carbon atoms from the molecular core. Comparisons with the corresponding alkoxy substituted materials (i.e. without a double bond) are made.