We have already established that methyl and primary alkyl compounds react well by the SN2 mechanism, while secondary alkyl compounds undergo SN2 reactions only reluctantly. But there are other important structural features that also encourage the SN2 mechanism. Two of these, allyl and benzyl groups, also encourage the SN1 mechanism. Allyl bromide reacts well with alkoxides to make ethers, and shown below is the typical SN2 mechanism for this reaction. Also shown is the transition state for this reaction. Allyl com pounds react rapidly by the SN2 mechanism because the π system of the adjacent double bond can stabilize the transition state by conjugation. The p orbital at the reaction centre (shown in brown, and corresponding to the brown orbital in the diagram on p. 340) has to make two partial bonds with only two electrons—it is electron deficient, and so any additional electron density it can gather from an adjacent π system will stabilize the transition state and increase the rate of the reaction.

The benzyl group acts in much the same way using the π system of the benzene ring for conjugation with the p orbital in the transition state. Benzyl bromide reacts very well with alkoxides to make benzyl ethers. Among the fastest of all SN2 reactions are those where the leaving group is adjacent to a car bonyl group. With α-bromo carbonyl compounds, two neighbouring carbon atoms are both powerfully electrophilic sites. Each has a low-energy empty orbital—π* from C=O and σ* from C–Br (this is what makes them electrophilic)—and these can combine to form a new LUMO (π* + σ*) lower in energy than either. Nucleophilic attack will occur easily where this new orbital has its largest coefficient, shown in orange on the diagram.

The effect of this interaction between antibonding orbitals is that each group becomes more electrophilic because of the presence of the other—the C=O group makes the C–Br bond more reactive and the Br makes the C=O group more reactive. In fact, it may well be that the nucleophile will attack the carbonyl group, but this will be reversible whereas displacement of bromide is irreversible. There are many examples of this type of reaction. Reactions with amines go well and the aminoketone products are widely used in the synthesis of drugs.

Quantifying structural effects on SN2 reactions , Some actual data may help at this point. The rates of reaction of the following alkyl chlorides with KI in acetone at 50 °C broadly illustrate the patterns of SN2 reactivity we have just analysed. These are relative rates with respect to n-BuCl as a ‘typical primary halide’. You should not take too much notice of precise figures but rather observe the trends and notice that the variations are quite large—the full range from 0.02 to 100,000 is eight powers of ten.

مواضيع ذات صلة

A closer look at steric effects

An adjacent C=C π system stabilizes a carbocation: allylic and benzylic carbocations

Alkyl substituents stabilize a carbocation

Shape and stability of carbocations

A closer look at the SN1 reaction

? How can we decide which mechanism (SN1 or SN2) will apply to a given organic compound

Significance of the SN1 rate equation

Significance of the SN2 rate equation

Mechanisms for nucleophilic substitution

Resolutions using diastereoisomeric salts

Separating enantiomers is called resolution

Chiral compounds with no stereogenic centres