Pentadienes isomerisation

Mitsch and Neuvar have shown that perfluorovinyl cyclopropane isomerises at 131-183 °C to perfluorocyclopentene by a clean unimolecular reaction. The activation energy is about 15 kcal.mole less than for the reaction involving the parent hydrocarbon - . Perfluoroallylcyclopropane decomposes unimolecu-larly at 196-250 °C to perfluoro-l,4-pentadiene and perfluorocyclopropane plus perfluoroethylene (the latter two products presumably from difluorocarbene formed in the initial reaction). Arrhenius parameters are. <4 = 6.3 x 10 sec , E = 42.7 kcal.mole . Placzek et 499,500 reported on the rates of... 

Figure 8.2. Reaction scheme for the hydrogenation of 3,4-dimethyl-1,2-pentadiene configurations of the adsorbed reactant are shown both as tr- and di-a-structures (alkenes only isomerise after the reactant has disappeared).

Extension of the linear carbon chain to five or six atoms opens up additional possible reaction paths. Higher 1,3-alkadienes are expected to behave in a similar fashion to 1,3-butadiene, except the two double bonds may differ in reactivity under the influence of the alkyl substituent, and their reduction naturally affords distinguishable products. Thus for example 1,3-hexadiene can give 1- and 3-hexenes by respectively 3,4- and 1,2-addition as well as 2-hexenes by 1,4-addition or isomerisation. A new feature in the higher 1,3- alkadienes is the existence of geometrical isomerism Z- and -l,3-pentadienes have been examined separately. ... 

The well-known autoxidation of a Z,Z 1,4-pentadiene unit to a hydroperoxy E,Z conjugated diene is accompanied by isomerisation to... 

In unsaturated fatty acids, positional isomerisation is also possible, when the double bond shifts by one carbon atom either away from the carboxyl or towards the carboxyl. Positional isomerisation is usually associated with cis-trans isomerisation and takes place under similar conditions to the cis-trans isomerisation, on heating to temperatures higher than about 240 °C. Isolated double bonds are relatively stable, but the pentadiene systems easily isomerise to conjugated systems. Two conjugated isomers are formed from linoleic acid, whereas linolenic acid yields four conjugated isomers (Figure 3.14). 

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