Reactions of Alkyl Halides:
Substitution vs. Elimination

The reactions of alkyl halides typically involve either substitution or elimination pathways. Depending on a variety of factors, the reaction mechanism can be either SN1, SN2, E1, E2, or some combination of these.  By understanding the influences different factors have on each of these mechanisms, it is usually possible to predict which mechanism will predominate.

SN2

The key feature of these reactions is a "five-coordinate" transition state. This requires that the central carbon atom is NOT crowded. In practice, SN2 reactions are limited to primary and secondary alkyl halides. The nature of the attacking nucleophile is an important factor determining the rate of these reactions.   Polar, aprotic solvents are best.

SN1

In this mechanism, the leaving group is "lost" (or at least partially dissociated) in the rate-determining step, so the nature of the attacking nucleophile is not an important factor. Since tertiary carbon centers give the most stable carbocations, these are by far the most reactive alkyl halides via this mechanism. Protic solvents (H2O, ROH, RNH2, etc.) are best due to the fact that they help remove/stabilize the leaving group.

E2

Elimination reactions following E2 mechanism are preferred for all secondary and tertiary alkyl halides when a strong base (OH-, RO-, R-CºC:-, NH2-) is used. For primary alkyl halides, the E2 pathway is preferred if a bulky base (such as (CH3)3CO-) is used.

E1

The initial, rate-determining step for this mechanism is the same as for the SN1 mechanism. The best conditions for E1 reactions involve using a weakly basic nucleophile (strong bases react via E2), protic solvents, and high temperatures. This is not a generally useful synthetic route.

Summary

  1° alkyl halide 2° alkyl halide 3° alkyl halide
Strong, hindered base E2 E2 E2
Strong base SN2 E2 E2
Good nucleophile
(only weakly basic)		 SN2 SN2 SN1
Poor nucleophile,
protic solvent		 NR SN1 or NR SN1 & E1
Poor nucleophile,
aprotic solvent		 NR SN2 or NR NR