EFFECTS OF STRUCTURE ON COMPETING SUBSTITUTION AND ELIMINATION

EFFECTS OF STRUCTURE ON COMPETING SUBSTITUTION AND ELIMINATION REACTIONS 

Substitution and elimination reactions compete with one another, giving rise to a wide variety of product mixtures. The reactions depend to a large extent on the structure of the substrate. We will consider the reactions of haloalkanes and consider them by type primary, secondary, and tertiary. 

Competing Substitution and Elimination Reactions of Tertiary Haloalkanes 

Tertiary haloalkanes undergo substitution reactions only by an S l mechanism because there is too much steric hindrance for an Sj 2 reaction to occur. However, a tertiary haloalkane can undergo an elimination reaction by either an E2 or an El process. The mechanism depends on the basicity of the nucleophile and the polarity of the solvent. If the nucleophile is a weak base, S l and El processes compete, and the amounts of the two types of products depend only on the stability of the carbocation that forms as an intermediate. For example, 2-bromo-2-methylbutane reacts in ethanol to give about 64% of an ether product from an S l process. The ratio of S l to El products is about 2 1. 

However, if sodium ethoxide, a strongly basic nucleophile, is added to the ethanol, an E2 reaction competes with the substitution reaction. The amount of elimination product is increased to a total of about 93% of the product mixture. Only 7% of the ether product is formed. Most of the elimination product is derived from the E2 reaction. 

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