<P> 1) Many reactions studied are solvolysis reactions where a solvent molecule--often an alcohol, is the nucleophile . While still a second order reaction mechanistically, the reaction is kinetically first order as the concentration of the nucleophile--the solvent molecule, is effectively constant during the reaction . This type of reaction is often called a pseudo first order reaction . </P> <P> 2) In reactions where the leaving group is also a good nucleophile--bromide for instance, the leaving group can perform an S 2 reaction on a substrate molecule . If the substrate is chiral, this inverts the configuration of the substrate before solvolysis, leading to a racemized product--the product that would be expected from an S 1 mechanism . In the case of a bromide leaving group in alcoholic solvent Cowdrey et al. have shown that bromide can have an S 2 rate constant 100 - 250 times higher than the rate constant for ethanol . Thus, after only a few percent solvolysis of an enantiospecific substrate, it becomes racemic . </P> <P> The examples in textbooks of secondary substrates going by the S 1 mechanism invariably involve the use of bromide (or other good nucleophile) as the leaving group have confused the understanding of alkyl nucleophilic substitution reactions at secondary carbons for 80 years . Work with the 2 - adamantyl system (S 2 not possible) by Schleyer and co-workers, the use of azide (an excellent nucleophile but very poor leaving group) by Weiner and Sneen, the development of sulfonate leaving groups (non-nucleophilic good leaving groups), and the demonstration of significant experimental problems in the initial claim of an S 1 mechanism in the solvolysis of optically active 2 - bromooctane by Hughes et al. have demonstrated conclusively that secondary substrates go exclusively (except in unusual but predictable cases) by the S 2 mechanism . </P> <P> A common side reaction taking place with S 2 reactions is E2 elimination: the incoming anion can act as a base rather than as a nucleophile, abstracting a proton and leading to formation of the alkene . This is more common when the incoming ion is sterically hindered in which case abstracting a proton is much easier . Elimination reactions are usually favoured at elevated temperatures because of increased entropy . This effect can be demonstrated in the gas - phase reaction between a sulfonate and a simple alkyl bromide taking place inside a mass spectrometer: </P>

Other type of reaction that is common for alkyl halide