Chemistry · General Principles Of Organic Chemistry · NEET
It is the sideways sharing of the electrons in a C-H single bond (sigma bond) with an empty p orbital or a pi bond right next to it. The C-H electrons do not stay only between C and H; they help the neighbour. This spreading of electrons makes the whole species more stable. It only needs a hydrogen on the carbon directly next to the positive centre or double bond.
An alpha carbon is the carbon directly attached to the positive carbon (or to a double-bond carbon). The hydrogens on that alpha carbon are the alpha hydrogens. Only alpha C-H bonds can take part in hyperconjugation. So you count alpha hydrogens: more alpha hydrogens means more hyperconjugation and more stability. Tert-butyl cation has 9 alpha hydrogens.
Resonance moves pi electrons or lone pairs between real bonds and atoms. Hyperconjugation moves sigma (C-H single bond) electrons, which is why it is weaker and is nicknamed 'no-bond resonance' — in the extra structure the C-H bond appears broken. Resonance needs pi bonds or lone pairs; hyperconjugation needs an alpha C-H bond. Both delocalise charge and add stability.
When you draw the extra (canonical) structure for hyperconjugation, the alpha C-H bond is shown as broken: the H becomes a positive ion and its two electrons form a new pi bond. Because one bond disappears in that picture, it is called 'no-bond resonance'. It is only a way to draw the delocalisation; the H does not really leave.
No. Hyperconjugation needs at least one C-H sigma bond on the alpha carbon (the carbon next to the positive centre or double bond). If the alpha carbon has no hydrogen, there is no C-H bond to donate electrons, so no hyperconjugation. This is why a neopentyl-type cation with no alpha H cannot use hyperconjugation.
The inductive effect (+I) is electron push through the sigma bonds of the whole chain and gets weaker with distance. Hyperconjugation is a specific sideways donation of the alpha C-H bond electrons into an empty p orbital or pi system. Both stabilise carbocations, but hyperconjugation is usually the stronger, main reason for the order primary < secondary < tertiary.
Yes. In an alkene, the alpha C-H bonds of an attached alkyl group donate into the pi bond. So more alkyl groups (more alpha hydrogens) around a double bond make the alkene more stable. This is why a more substituted alkene is more stable and is often the major product in elimination (Saytzeff rule).
A tertiary butyl carbocation is more stable than the secondary butyl carbocation because of which one of the following?
The most stable carbocation among the following is (options are cyclohexyl-type cations of different substitution).
The carbocation C6H5-CH(+)-CH3 (1-phenylethyl cation) is stabilized by the interaction of the empty p orbital with:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Count the number of alpha C-H bonds. Each alpha C-H bond gives one hyperconjugation structure. For the tert-butyl cation there are 9 alpha C-H bonds, so 9 hyperconjugative structures.
For explaining carbocation and alkene stability, hyperconjugation is usually considered the stronger, main factor, with the +I effect adding extra support.
Yes, it needs a C-H bond on the alpha carbon. Without an alpha hydrogen there is no sigma bond to donate, so no hyperconjugation.
More alkyl groups mean more alpha C-H bonds donating into the pi bond by hyperconjugation, which lowers the energy and makes the alkene more stable.
