Question

Which of the following is the least reactive substrate in an E2 reaction?

a) Tertiary alkyl halide
b) Secondary alkyl halide
c) Primary alkyl halide
d) Allylic halide

Answer 1

The least reactive substrate in an E2 reaction is c) Primary alkyl halide. This is because the E2 reaction rate depends on the degree of substitution of the alkyl halide, with more substituted substrates being more reactive12. This is due to the increased stability of the alkene product and the ease of breaking the carbon-halogen bond in more substituted substrates12. Allylic halides are also very reactive in E2 reactions because they can form resonance-stabilized allylic carbocations3. Therefore, primary alkyl halides are the least reactive substrates in E2 reactions.

Answer 2

A primary alkyl halide is the least reactive substrate in an E2 reaction because it has minimal steric hindrance and is more likely to undergo an SN2 reaction instead.

Step-by-step explanation:

The reactivity of different substrates in an E2 reaction is influenced by the degree of steric crowding around the α-carbon (the carbon to which the leaving group is attached). In E2 reactions, tertiary alkyl halides are generally the most reactive due to the ease with which the base can abstract a proton, leading to elimination and formation of an alkene.

Conversely, primary alkyl halides are the least reactive in the context of E2 reactions because of minimal steric hindrance, which makes them more prone to undergoing substitutions via the SN2 mechanism instead. Therefore, in the given list, a primary alkyl halide would be the least reactive substrate in an E2 reaction, since it favors SN2 over E2 when compared to secondary and tertiary alkyl halides.