Advices For Studying Organic Chemistry

1) A charged system is stabilized when the charge is dispersed or delocalized.

2) Alkyl groups, when compared to hydrogen atoms, are electron releasing.

Figure 6.9 How a methyl group helps stabilize the positive charge of a carbocation.
Electron density from one of the carbon-hydrogen sigma bonds of the
methyl group flows into the vacant p orbital of the carbocation because
the orbitals can partly overlap. Shifting electron density in this way
makes the sp^2 -hybridized carbon of the carbocation somewhat less
positive, and the hydrogens of the methyl group assume some of the
positive charge. Delocalization (dispersal) of the charge in this way
leads to greater stability. This interaction of a bond orbital with a p
orbital is called hyperconjugation.

2. The delocalization of charge and the order of stability of carbocations
   parallel the number of attached methyl groups.

H 3 C C

CH 3

CH 3

is

more

stable

than

H 3 C C

CH 3

H

H 3 C C

H

H

H C

H

H

δ+ δ+ δ+ δ+ δ+

δ+ δ+

δ+ δ+

δ+

is

more

stable

than

is

more

stable

than

tert-Butyl cation Isopropyl cation Ethyl cation Methyl cation
(3°) (most stable) (2°) (1°) (least stable)

3. The relative stabilities of carbocations is 3° > 2° > 1° > methyl.


4. The electrostatic potential maps for carbocations: