You wish to alkylate a ketone as shown. Mixing methyl bromide and cyclohexane in

Question

You wish to alkylate a ketone as shown. Mixing methyl bromide and cyclohexane in ethanol with a catalytic amount of sodium tetroxide produces very poor yields of the target compound. However, treatment of cyclohexane with LDA in THF followed by reaction with methyl bromide generates far better yields. Explain these observations by pointing out the problems of the first method and how the second set of reaction conditions helps alleviate those problems. [the mass spectrum of the products produced in the first method shows one fragment corresponding to m/z = 178]

Explanation / Answer

when lithium diisopropylamide (LDA) is used to remove the alpha hydrogen

(the of LDA’s conjugate acid is about 35), essentially all the carbonyl compound

is converted to enolate because LDA is a much stronger base than the base being

formed. Therefore, LDA is the base of choice for those reactions that

require the carbonyl compound to be completely converted to enolate before it reacts

with an electrophile.

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