The alcoholate-promoted dehydrohalogenation of 5-nonyl fluoride and cyclododecyl fluoride, typical straight-chain and, resp., medium-size cyclic substrates, leads to cis- and trans-alkenes in an approx. ratio of 1:3. With bulky bases such as lithium diisopropylamide the trans isomer is obtained almost exclusively. In general, the elimination of hydrogen fluoride proceeds very slowly. Increase of the base strength has only a moderate effect on the rates. Electrophilic assistance as provided by lithium cations in media of low polarity can, however, considerably accelerate the reaction. Cyclododecyl fluoride cannot be prepd. from cyclododecanol. It is, however, readily accessible by bromofluorination of cyclodecene and subsequent redn. of the adduct with stannane. [on SciFinder (R)]