<Tr> <Td> S 1 mechanism non-stereospecific </Td> <Td> S 2 mechanism stereospecific </Td> </Tr> <P> For example, tertiary centres react almost exclusively by the S 1 mechanism whereas primary centres (except neopentyl centres) react almost exclusively by the S 2 mechanism . When a nucleophilic substitution results in incomplete inversion, it is because of a competition between the two mechanisms, as often occurs at secondary centres, or because of double inversion (as when iodide is the nucleophile). </P> <P> The addition of singlet carbenes to alkenes is stereospecific in that the geometry of the alkene is preserved in the product . For example, dibromocarbene and cis - 2 - butene yield cis - 2, 3 - dimethyl - 1, 1 - dibromocyclopropane, whereas the trans isomer exclusively yields the trans cyclopropane . </P> <P> This addition remains stereospecific even if the starting alkene is not isomerically pure, as the products' stereochemistry will match the reactants' . </P>

What is the difference between a stereoselective reaction and a stereospecific reaction