Tertiary carbocations are more stable than primary ones, but powerful stabilization is also provided when there is genuine conjugation between the empty p orbital and adjacent π or lone pair electrons. The allyl cation has a filled (bonding) orbital containing two electrons delocalized over all three atoms and an important empty orbital with coefficients on the end atoms only. It’s this orbital that is attacked by nucleophiles. The curly arrow picture tells us the same thing.

Allylic electrophiles react well by the SN1 mechanism because the allyl cation is relatively stable. Here’s an example of a reaction working in the opposite direction from most of those you have seen so far—we start with the alcohol and form the bromide. Treatment of cyclohex-enol with HBr gives the corresponding allylic bromide.

In this case, only one compound is formed because attack at either end of the allylic cation gives the same product. But when the allylic cation is unsymmetrical this can be a nuisance as a mixture of products may be formed. It doesn’t matter which of these two butenols you treat with HBr, you get the same delocalized allylic cation.

When this cation reacts with Br−, about 80% goes to one end and 20% to the other, giving a mixture of butenyl bromides. This regioselectivity (where the nucleophile attacks) is determined by steric hindrance: attack is faster at the less hindered end of the allylic system. Sometimes this ambiguity is useful. The tertiary allylic alcohol 2-methylbut-3-en-2-ol is easy to prepare and reacts well by the SN₁ mechanism because it can form a stable carbocation that is both tertiary and allylic. The allylic carbocation intermediate is unsymmetrical and reacts only at the less substituted end to give ‘prenyl bromide’.

The benzyl cation is about as stable as the allyl cation but lacks its ambiguity of reaction. Although the positive charge is delocalized around the benzene ring, to three positions in particular, the benzyl cation always reacts on the side chain so that aromaticity is preserved.

An exceptionally stable cation is formed when three benzene rings can help to stabilize the same positive charge. The result is the triphenylmethyl cation or, for short, the trityl cation. Trityl chloride is used to form an ether with a primary alcohol group by an SN₁ reaction. You will notice that pyridine is used as solvent for the reaction. Pyridine (a weak base: the pKa of its conjugate acid is 5.5—see Chapter 8) is not strong enough to remove the proton from the primary alcohol (pKa about 15), and there would be no point in using a base strong enough to make RCH₂O− as the nucleophile makes no difference to an SN₁ reaction. Instead, the TrCl ion-izes first to trityl cation, which now captures the primary alcohol and finally pyridine is able to remove the proton from the oxonium ion. Pyridine does not catalyse the reaction; it just stops it becoming too acidic by removing the HCl formed. Pyridine is also a convenient polar organic solvent for ionic reactions.

The table below shows the rates of solvolysis (i.e. a reaction in which the solvent acts as the nucleophile) in 50% aqueous ethanol for substituted allylic chlorides compared with benzylic chlorides and simple alkyl chlorides. The values give you an idea of the relative reactivity towards substitution of the different classes of compound. These rates are mostly SN1, but there will be some SN2 reactivity with the primary compounds.

مواضيع ذات صلة

A closer look at steric effects

Adjacent C=C or C=O π systems increase the rate of SN2 reactions

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