substitution versus elimination competition

Hand the same alkyl halide to the same reagent and you may get a substitution product, an elimination product, or a mixture — because four reactions are all eyeing the same molecule: SN1, SN2, E1, and E2. A reagent that can act as a nucleophile attacks the carbon and substitutes; the very same reagent acting as a base grabs a beta proton and eliminates. Which pathway dominates is one of the central predictive questions of introductory organic chemistry.

Three inputs decide the contest. The substrate: methyl and primary halides cannot make carbocations, so they avoid E1 and SN1 and lean to SN2 (or E2 only with a very bulky base); tertiary halides cannot do SN2 (too crowded), so they go SN1, E1, or E2; secondary halides can do almost anything, which makes them the trickiest. The reagent: a strong, bulky base favours E2; a strong, small nucleophile/base can do SN2 or E2; a good nucleophile that is a weak base favours SN2; a weak nucleophile/base in an ionizing solvent gives SN1/E1. The conditions: heat favours elimination (more molecules, higher entropy in the transition state), and polar protic solvents favour the carbocation routes (SN1/E1) while polar aprotic solvents favour SN2.

Rather than memorize a giant table, reason it out. First classify the substrate (primary/secondary/tertiary). Then ask if the reagent is a strong base (push to elimination), a good nucleophile but weak base (push to substitution), or weak at both (push to the unimolecular, carbocation routes). Then let temperature and solvent fine-tune. Getting fluent at this triage is precisely what 'predicting an elimination' means in practice — and why these four reactions are always taught together.