Olah found that he could measure the spectrum of the tert-butyl cation, but he was never able to observe the methyl cation in solution. Why do those extra substituents stabilize the cationic centre? Any charged organic intermediate is inherently unstable because of the charge. A carbocation can be formed only if it has some extra stabilization, and extra stabilization can come to the planar carbocation structure from weak donation of σ bond electrons into the empty p orbital of the cation. In the t-butyl cation, three of these donations occur at any one time: it doesn’t matter if the C–H bonds point up or down; one C–H bond of each methyl group must be parallel to one lobe of the empty p orbital at any one time. The first diagram shows one overlap in orbital terms and the second and third diagrams, three as dotted lines.

There is nothing special about the C–H bond donating electrons into an empty orbital: a C–C bond is just as good and some bonds are better (C–Si, for example). But there must be a bond of some sort—a hydrogen atom by itself has no lone pairs and no σ bonds so it cannot stabilize a cation. Planarity is so important to the structure of a carbocation that if a tertiary cation cannot become planar, it is not formed. A classic case is the structure in the margin, which does not react with nucleophiles either by SN₁ or by SN₂. It does not react by SN₁ because the cation cannot become planar, nor by SN2 because the nucleophile cannot approach the carbon atom from the right direction. In general, though, simple tertiary structures undergo efficient SN₁ substitution reactions. With good leaving groups such as halides, substitutions can be done under neutral conditions; with less good leaving groups such as alcohols or ethers, acid catalysis is required. The following group of reactions give an idea of the types of SN₁ reactions that work well.

مواضيع ذات صلة

A closer look at steric effects

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

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

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