Part 7: MECHANISMS

Note that no other reagents are needed in order to complete any of these sequences, you should only be using what is there.

I Alcohols can be both the electrophilic side and the nucleophilic side of substitution reactions. In this example, they are both ... this can be used for the synthesis of symmetrical ethers.

II The reaction of an alcohol with a hydrogen halide such as HBr is typically SN1 in character provided a reasonable carbocation is being formed. Here, thats means a resonance stabilised secondary carbocation. The two products arise due to the possibility of the bromide nucleophile attacking either of the electrophilic carbocations revealed by the resonance contributors.

III If an alkyl bromide is heated with strong base (here it's hydroxide), then a 1,2-elimination to give an alkene occurs. Alkyl halide eliminations are typically E2. In this case the base is small so the Zaistev elimination is going to dominate. E2 reactions prefer an anti arrangement of the H and the leaving group - this dictates the stereochemistry of the alkene product.

IV Both reactions are eliminations, but the alcohol dehydration is E1 and the alkyl halide dehydrohalogenation is E2. E1 reactions tend to give the more stable (more highly substituted alkene as the major product = Zaitsev's rule) while E2 are controlled other factors including the preference for an anti arrangement of the H and the leaving group.


The base removes the H at 180o to the Br leaving group, this defines the alkene location.

[Home]Return to Homepage