temperature effect on elimination

A practical rule of thumb in the lab is that turning up the heat tilts a reaction away from substitution and toward elimination. Run an alkyl halide with a base cold and you may favour substitution; run the same combination hot and you tend to favour the alkene. Temperature is one of the simplest dials a chemist turns to steer the SN versus E competition.

The reason lies in entropy. An elimination takes one molecule and produces several pieces — the alkene plus the protonated base plus the leaving group — so it increases disorder; its transition state has a more positive entropy of activation than substitution, where the nucleophile and substrate combine into roughly the same number of particles. Free energy is G = H - TS, so the favourable entropy term (the TS part) grows with temperature. As T rises, the entropy advantage of elimination is amplified, lowering its effective free-energy barrier relative to substitution and making the alkene win more often.

This is why elimination reactions are so often written 'with heat' and why dehydrations and dehydrohalogenations are run at reflux or hotter, while substitutions are sometimes kept cool to suppress the alkene. It is a tendency, not a guarantee — substrate and reagent still dominate the outcome — but when products are competing closely, raising the temperature is a reliable nudge toward elimination.