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Origin of Carbocation Stability Profile: Hyperconjugation

Recall, the way to stabilize positive charge is to add negative charge to it. Carbocations can "steal charge" from adjacent bonds, if they exist. This phenomenon is referred to as hyperconjugation. It's "conjugation" in the same sense as resonance, but it is not full-blown resonance.

H

C--H bonding orbital

have orbital ovrlap here

H H

empty 2p orbital

a 2-electron, 2-orbital interaction

Notice that this is a 2-electron, 2-orbital interaction, a net bonding interaction (recall bond order). Now the carbocation stability ranking is shown below. As the number of methyl (or alkyl) groups increases the carbocation becomes more stable. The reason for this rests with the ability of each methyl group to donate electron density to the empty 2p-orbital via hyperconjugation interaction.

CH3 H3C C CH3 3o most stable H H H

>

H3C

C CH3 2o

>

H3C

C H 1o

>

H

C H methyl

decreasing stability

least stable

The figure below shows a 3 carbocation with three F(C-H) º 2p (empty) hyperconjugative interactions. The more electron density one can donate to the 2p-orbital, the more the positive charge is attenuated, the more it is spread out onto the hydrogen atoms, and the more stable the

°

H H

H

the other C-H bonds are left off for clarity.

H H

3 C-H bonding oprbitals donating e--density a 3o carbocation-the most stable

cationic species. Note that there is a stereoelectronic requirement for the hyperconjugation phenomena, namely, that the F(C-H) bond engaged in the interaction be parallel to the empty 2porbital. So, only a single F(C-H) bond of the three available on any one methyl group is actually involved in the hyperconjugation process. Now, if a pair of electrons that is part of a F-bond can donate electron density to an empty 2porbital, what about a lone pair of electrons? This is shown below for a carbocation flanked by a chlorine atom. This is an example of simple resonance, but it occurs by a mechanism similar to hyperconjugation: overlap of two orbitals; a 2-orbital/2-electron interaction.

3sp3 2p

Cl

R R

Cl

R R

resonance contributor #1

C--Cl pi-bond resonance contributor #2

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