ELIMINATION VERSUS SUBSTITUTION
The ratio of substitution and elimination products formed from alkyl halides depends on the reaction conditions as well as the nature of the nucleophile and the alkyl halide.
Primary alkyl halides will usually undergo SN2 substitution reactions in preference to E2 elimination reactions. However, the E2 elimination reaction is favored if a strong bulky base is used with heating.
Substitution by the SN2 mechanism is favored over the E2 elimination if the nucleophile is a weak base and the solvent is polar and aprotic. E2 elimina-tion is favored over the SN2 reaction if a strong base is used in a protic solvent. Elimination is further favored by heating. SN1 and E1 reactions may be possible when dissolving secondary alkyl halides in protic solvents.
E2 elimination occurs virtually exclusively if a tertiary alkyl halide is treated with a strong base in a protic solvent. Heating a tertiary alkyl halide in a protic solvent is likely to produce a mixture of SN1 substitution and E1 elimination products, with the former being favored.
Alkyl halides can undergo both elimination and substitution reactions and it is not unusual to find both substitution and elimination products present. The ratio of the products will depend on the reaction conditions, the nature of thenucleophile and the nature of the alkyl halide.
Primary alkyl halides undergo the SN2 reaction with a large range of nucleophiles (e.g. RS- , I- , CN- , NH3, or Br- ) in polar aprotic solvents such as hexamethyl- phosphoramide (HMPA; [(CH3)2N]3PO). However, there is always the possibility of some E2 elimination occurring as well. Nevertheless, substitution is usually favored over elimination, even when using strong bases such as HO- or EtO-. If E2 elimination of a primary halide is desired, it is best to use a strong bulky base such as tert-butoxide [(CH3)3C–O- ]. With a bulky base, the elimination product isfavored over the substitution product since the bulky base experiences more steric hindrance in its approach to the electrophilic carbon than it does to the acidic β-proton.
Thus, treatment of a primary halide (Fig. 1) with an ethoxide ion is likely to give a mixture of an ether arising from SN2 substitution along with an alkene arising from E2 elimination, with the ether being favored. By using sodium tert-butoxide instead, the preferences would be reversed.
Increasing the temperature of the reaction shifts the balance from the SN2 reaction to the elimination reaction. This is because the elimination reaction has a higher activation energy due to more bonds being broken. The SN1 and E1 reactions do not occur for primary alkyl halides.
Secondary alkyl halides can undergo both SN2 and E2 reactions to give a mixture of products. However, the substitution product predominates if a polar aprotic solvent is used and the nucleophile is a weak base. Elimination will predominate if a strong base is used as the nucleophile in a polar, protic solvent. In this case, bulky bases are not so crucial and the use of ethoxide in ethanol will give more elimination product than substitution product. Increasing the temperature of the reaction favors E2 elimination over SN2 substitution as explained above.
If weakly basic or nonbasic nucleophiles are used in protic solvents, elimination and substitution may occur by the SN1 and E1 mechanisms to give mixtures.
Tertiary alkyl halides are essentially unreactive to strong nucleophiles in polar, aprotic solvents – the conditions for the SN2 reaction. Tertiary alkyl halides can undergo E2 reactions when treated with a strong base in a protic solvent and will do so in good yield since the SN2 reaction is so highly disfavored. Under nonbasic conditions in a protic solvent, E1 elimination and SN1 substitution both take place.
A tertiary alkyl halide treated with sodium methoxide could give an ether or an alkene (Fig. 2). A protic solvent is used here and this favors both the SN1 and E1 mechanisms. However, a strong base is also being used and this favors the E2 mechanism. Therefore, the alkene would be expected to be the major product with only a very small amount of substitution product arising from the SN2 reaction. Heating the same alkyl halide in methanol alone means that the reaction is being carried out in a protic solvent with a nonbasic nucleophile (MeOH). These condi-tions would result in a mixture of substitution and elimination products arising from the SN1 and E1 mechanisms. The substitution product would be favored over the elimination product.