The first elimination of the chapter (t-BuBr plus hydroxide) illustrates something very important: the starting material is a tertiary alkyl halide and would therefore substitute only by SN1, but it can eliminate by either E2 (with strong bases) or E1 (with weak bases). The steric factors that disfavour SN2 at hindered centres don’t exist for eliminations. Nonetheless, E1 can occur only with substrates that can ionize to give relatively stable carbocations—tertiary, allylic, or benzylic alkyl halides, for example. Secondary alkyl halides may eliminate by E1, while primary alkyl halides only ever eliminate by E2 because the primary carbocation required for E1 would be too unstable. The chart below summarizes the types of substrate that can undergo E1—but remember that any of these substrates, under the appropriate conditions (in the presence of strong bases, for example), may also undergo E2. For completeness, we have also included in this chart three alkyl halides that cannot eliminate by either mechanism simply because they do not have any hydrogens to lose from carbon atoms adjacent to the leaving group.

Can a proton just ‘fall off’ a cation? In E1 mechanisms, once the leaving group has departed almost anything will serve as a base to remove a proton from the intermediate carbocation. Weakly basic solvent molecules (water or alcohols), for example, are quite sufficient, and you will often see the proton just ‘falling off’ in reaction mechanisms, with the assumption that there is a weak base somewhere to capture it. We showed the loss of a proton like this in the last example, and in the chart on this page.

The cation is stable because counterions such as BF4 − and SbF6 − are not only non-nucleophilic but also so non-basic that they won’t even accept a proton. This fact tells us that despite this common way of writing the E1 mechanism, some sort of weak base is necessary even for E1.

Polar solvents also favour E1 reactions because they stabilize the intermediate carbocation. E1 eliminations from alcohols in aqueous or alcohol solution are particularly common and very useful. An acid catalyst is used to promote loss of water, and in dilute H₂SO₄, H₃PO₄, or HCl the absence of good nucleophiles ensures that substitution does not compete. With phosphoric acid, for example, the secondary alcohol cyclohexanol gives cyclohexene.

But the best E1 eliminations of all are with tertiary alcohols. The alcohols can be made using the methods of Chapter 9: nucleophilic attack by an organometallic on a carbonyl compound. Nucleophilic addition, followed by E1 elimination, is an excellent way of making this substituted cyclohexene, for example. Note that the proton required in the fi rst step is recovered in the last—the reaction requires only catalytic amounts of acid.

Cedrol is important in the perfumery industry—it has a cedar wood fragrance. Corey’s syn thesis includes this step—the acid catalyses both the E1 elimination and the hydrolysis of the acetal.

At the end of the last chapter, you met some bicyclic structures. These sometimes pose problems for elimination reactions. For example, this compound will not undergo elimination by either an E1 or an E2 mechanism.

We shall see shortly what the problem with E2 is, but for E1 the hurdle to be overcome is the formation of a planar carbocation. The bicyclic structure prevents the bridgehead carbon becoming planar so, although the cation would be tertiary, it is very high in energy and does not form. You could say that the non-planar structure forces the cation to have an empty sp³ orbital instead of an empty p orbital, and we saw in Chapter 4 that it is always best to leave the orbitals with the highest possible energy empty.

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

E1 and E2 mechanisms

Size