Now that you have seen a few examples of elimination reactions, it is time to return to our discussion of the two mechanisms for elimination. To summarize what we have said so far:

• E1 describes an elimination reaction (E) in which the rate-determining step is unimolecular (1) and does not involve the base. The leaving group leaves in this step, and the proton is removed in a separate second step.

• E2 describes an elimination (E) that has a bimolecular (2) rate-determining step that must involve the base. Loss of the leaving group is simultaneous with removal of the proton by the base.

There are a number of factors that affect whether an elimination goes by an E1 or E2 mechanism. One is immediately obvious from the rate equations: only the E2 is affected by the concentration of base, so at high base concentration E2 is favoured. The rate of an E1 reaction is not even affected by what base is present—so E1 is just as likely with weak as with strong bases, while E2 goes faster with strong bases than weak ones: strong bases at whatever concentration will favour E2 over E1. If you see that a strong base is required for an elimination, it is certainly an E2 reaction. Take the fi rst elimination in this chapter as an example.

With less hindered alkyl halides hydroxide would not be a good choice as a base for an elimination because it is rather small and still very good at SN2 substitutions (and even with tertiary alkyl halides, substitution outpaces elimination at low concentrations of hydroxide). So what are good alternatives?

We have already mentioned the bulky t-butoxide—ideal for promoting E2 as it’s both bulky and a strong base (pKa of t-BuOH = 18). Here it is at work converting a dibromide to a diene with two successive E2 eliminations. Since dibromides can be made from alkenes (you will see how in the next chapter), this is a useful two-step conversion of an alkene to a diene.

The product of the next reaction is a ‘ketene acetal’. Unlike most acetals, this one can’t be formed directly from ketene (ketene, CH2=C=O, is too unstable), so instead the acetal is made by the usual method from bromoacetaldehyde and then HBr is eliminated using t-BuOK.

Among the most commonly used bases for converting alkyl halides to alkenes is one that you met in Chapter 8: DBU. This base is an amidine—delocalization of one nitrogen’s lone pair onto the other, and the resulting stabilization of the protonated amidiniumion, makes it particularly basic, with a pKa (of the protonated amidine) of about 12.5. There is not much chance of getting those voluminous fused rings into tight corners—so they pick off the easy to-reach protons rather than attacking carbon atoms in substitution reactions.

DBU will generally eliminate HX from alkyl halides to give alkenes. In these two examples, the products were used as intermediates in the synthesis of natural products.

مواضيع ذات صلة

Distinguishing acid derivatives by carbon NMR is difficult

E1cB eliminations in context

Anion-stabilizing groups allow another mechanism—E1cB

The regioselectivity of E2 eliminations

E2 elimination from vinyl halides: how to make alkynes

E2 eliminations from cyclohexanes

E2 eliminations can be stereospecific

E2 eliminations have anti-periplanar transition states

E1 reactions can be stereoselective

The role of the leaving group

Substrate structure may allow E1

Size