Compounds of the formula ##STR1## wherein ring B stands for 1,4-phenylene or a trans-1,4-disubstituted cyclohexane ring and A stands for a group with 1 to 3 six-membered rings, which together with ring B represents a central group of formulae II-XIII; R.sup.1 signifies a straight-chain alkyl group containing 1 to 9 carbon atoms or when R.sup.1 is attached to a benzene, monofluorinated benzene or pyrimidine, ring R.sup.1 also signifies a straight-chain alkoxy group with 1 to 9 carbon atoms; and R.sup.2 signifies hydrogen, cyano or a straight-chain alkyl group containing 1 to 7 carbon atoms, their manufacture, liquid crystalline mixtures which contain said compounds and their use for electro-optical purposes are described. The compounds of formula I are especially valuable as components in liquid crystal mixtures and for the most part themselves have liquid crystalline properties.
A process for effectively producing a 4-(4-alkylcyclohexyl)benzaldehyde, 4-(cyclohexyl)benzaldehyde, a 4-(trans-4-alkylcyclohexyl)benzaldehyde and a (trans-4-alkylcyclohexyl)benzene useful for electronic material applications such as liquid crystals and for pharmaceutical and agrochemical applications, etc., are disclosed. The present invention provides (1) a process for producing a 4-(4-alkylcyclohexyl)benzaldehyde or 4-(cyclohexyl)benzaldehyde by formylating a (4-alkylcyclohexyl)benzene or cyclohexylbenzene with carbon monoxide, (2) a process for producing a 4-(trans-4-alkylcyclohexyl)benzaldehyde by formylating a (4-alkylcyclohexyl)benzene having a cis/trans molar ratio of 0.3 or less with carbon monoxide, and (3) a process for producing a (trans-4-alkylcyclohexyl)benzene by isomerizing a mixture of the cis and trans isomers of a (4-alkylcyclohexyl)benzene, all of the processes being performed in the presence of HF and BF
3
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