Protic media, alkenes

The type of solvent or diluent should be specified in reporting a Ziegler-Natta catalyst system. Alkene polymerisations are usually carried out in inert solvents, such as aliphatic or aromatic hydrocarbons (e.g. some gasoline fractions or toluene). The use of protic or aprotic polar solvents or diluents instead of the hydrocarbon polymerisation medium can drastically alter the reaction mechanism. This usually results in catalyst deactivation for alkene coordination polymerisation. Modern alkene polymerisation processes are carried out in a gas phase, using fluidised-bed catalysts, and in a liquid monomer as in the case of propylene polymerisation [28,37]. 

Bromine Addition to Alkenes. Alumina can advantageously replace protic solvents thus avoiding secondary reactions due to their nucleophdicity. This situation is evidenced in the bromation of alkenes [14]. When performed in methanol, bromine addition leads to a mixture of a frans-dibromo adduct and a trans-bromo ether compound. The latter results from competitive attack by pro-tic solvent on the bromonium ion intermediate. This byproduct can be suppressed using Br2/alumina, as the support behaves as a non-nucleophilic polar medium (Scheme 3). 

A number of reviews have been devoted to the photoisomerism of alkenes, with the photochemistry of cycloalkenes treated as a subcategory.The geometrical photoisomerization of alkenes was reviewed also in the previous edition." However, the photobehavior of cycloalkenes is not a simple extension of that of acyclic alkenes, particularly in the cases of small- and medium-sized cycloalkenes. Medium-sized (E)-cycloalkenes produced photochemicaUy are highly constrained and rapidly react with alcohols in the presence, or even in the absence, of dilute acid. An account of the photoprotonation-driven cationic reactions, such as addition, isomerization, rearrangement, and fragmentation of cyclohexenes and cyclo-heptenes, in protic solvents has been published. The protonation of strained (E)-cycloalkenes was also reviewed in the previous edition. ... 

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