Difference between hyperconjugation and inductive effect?

[SephirothXR]

My understanding is that for a tertiary carbocation, hyperconjugation refers to the methyl groups donating electrons while the inductive effect is like the oxygen on a carbonyl stealing electrons from an alpha hydrogen. Can inductive effect ever refer to donating electron density (like hyperconjugation)?

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[ponyo]

My understanding is that for a tertiary carbocation, hyperconjugation refers to the methyl groups donating electrons while the inductive effect is like the oxygen on a carbonyl stealing electrons from an alpha hydrogen. Can inductive effect ever refer to donating electron density (like hyperconjugation)?

Hyperconjugation is based on nearby molecular orbitals (p or pi) whereas inductive effect has to do with differences in electronegativity.

Inductive effect can be electron withdrawing or electron donating. Someone please correct me if I'm wrong but a tertiary carbocation is not only stabilized by hyperconjugation but also by the inductive electron donating character of the nearby alkyl groups, although the latter is less influential.

 

[DDW]

I came here looking for the answer to the same question, but I think I have found the answer, so I might as well share it.

The main difference between the inductive effect, and hyperconjugation is the orbitals through which the effect acts. The ethyl carbocation, shown in the image below, illustrates this well.

The inductive effect acts through the carbon-carbon sigma bond (green). In this effect, the cation draws stabilizing electron density toward itself, due to unequal sharing of electron density within the sigma bond.

In contrast, in hyperconjugation, the donation of electron density occurs through overlapping p-orbitals, forming something like a partial pi-bond. The CH3 carbon partially rehybridizes from sp3 to sp2 and some of the electron density in the vertical pz orbital is donated to the pz orbital on the carbocation. It might be helpful to think of hyperconjugation in this case like a partial elimination of the CH3 proton.

Lastly, yes, a tertiary carbocation would be expected to be stabilized by both the inductive effect, and hyperconjugation. The only real limitation in this case would be if the molecular structure constrained the orientation such that the two adjacent p-orbitals could not achieve sufficient overlap. An example of this can be seen in the ring structure of pentose sugars, where glycosidic bonds are stabilized by hyperconjugation in the alpha, but not beta orientations, leading to orientations opposite to those predicted from steric hindrance.

Of course, this is all a little bit beyond the MCAT, but hopefully someone else is curious and finds it useful.