Time to stop mentioning alkyl group inductive effects

A computational study by researchers in the UK has shown that alkyl groups are inductively electron-withdrawing relative to hydrogen, and not electron-donating as described in some textbooks.

Chemistry students are usually taught that alkyl groups are electron-donating and hence that tertiary carbocations are more stable than primary carbocations. This understanding is often used to rationalise why, for example, a tertiary alkyl halide undergoes a S_N1 rection more readily than a primary alkyl halide. Some older organic chemistry textbooks, however, attribute this carbocation stability to inductive effects and not to hyperconjugation.

‘It’s very common for people to say that methyl groups donate electron density, but in fact it’s a combination of two different effects,’ says Jonathan Goodman, a computational chemist and lecturer at the University of Cambridge, UK, who was not involved in the study. ‘The hyperconjugation effect is electron-releasing in a big way but the inductive effect … is a much smaller effect. What exactly [one] means by an inductive effect is a very precise question.’

Now, a computational investigation into the inductive effect in aliphatic and aromatic hydrocarbons by Mark Elliott and colleagues at Cardiff University has generated fresh evidence that alkyl groups are inductively electron-withdrawing relative to hydrogen and that this effect is small compared to other factors such as hyperconjugation. ‘[We wanted] to unpick older understandings and apply modern methods to get more depth into the broader context of organic chemistry,’ comments Cardiff team member Benjamin Ward.

The team defined the inductive effect as the ‘ground state polarisation of σ-bonds in neutral molecules as a result of the ability of groups at either end of the bond to draw electron-density towards themselves’. They found that the charge on a given carbon atom became less negative as the number of alkyl groups attached to it increased and that this occurred for alkanes, alkenes, alkynes and arenes. This finding indicated that alkyl groups are inductively electron-withdrawing relative to hydrogen, rather than electron-donating. Additionally, and contrary to some early textbooks, the team concluded that alkyl groups inductively destabilise tertiary carbocations, relative to primary carbocations, and hence that hyperconjugation should always be invoked as the reason for the carbocation stability.

‘You could say “Who cares? It’s small!” But it’s interesting in a calming sort of way that, having read it, you now know a bit more about how molecules work,’ adds Goodman.

The Cardiff team now intends to investigate charged species such as carbocations and carbanions.