The Heck reaction is a coupling reaction that combines an unsaturated halide with an alkene in the presence of base and palladium catalyst to provide a substituted alkene. It an important reaction as it forms a carbon-carbon bond and allows substitution on sp² hybridized carbons.
In this example, we have bromobenzene reacting with propene and are told that two constitutional isomers are formed. The expected product is the first one shown which has the benzene substitute onto the terminal end of the alkene. The isomer by-product that is formed is substitution onto the central carbon of the propene which leads to a product with a terminal alkene.
