Carbocation intermediates
Addition reactions involving both unsymmetrical alkenes and reagents give two addition products. The preference for formation of one of these over the other can be rationalised on the basis of the relative energies of the two possible carbocation intermediates.
In general: Charged species are of lower energy (more stable) if their charge is delocalised over more atoms.
Carbocations are classified according to the number of saturated carbon atoms bonded to the carbon bearing the positive charge. The charge density maps indicate through the blue colour that the positive charge is delocalised to a greater extent on tertiary and secondary carbocations. Thus alkyl groups are better able to delocalise charge than the hydrogens.
In addition reactions of asymmetric reagents to asymmetric alkenes, two products are possible.
In general: Charged species are of lower energy (more stable) if their charge is delocalised over more atoms.
Carbocations are classified according to the number of saturated carbon atoms bonded to the carbon bearing the positive charge. The charge density maps indicate through the blue colour that the positive charge is delocalised to a greater extent on tertiary and secondary carbocations. Thus alkyl groups are better able to delocalise charge than the hydrogens.
 | 2CHPlus.jpg) | 3CPlus.jpg) |
| primary (one carbon attached) | secondary (two carbons attached) | tertiary (three carbons attached) |
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In addition reactions of asymmetric reagents to asymmetric alkenes, two products are possible.
The major product of an addition reaction arises from the lower energy intermediate.
Thus tertiary halides are formed in preference to primary or secondary ones, and secondary halides are formed in preference to primary ones.
Thus tertiary halides are formed in preference to primary or secondary ones, and secondary halides are formed in preference to primary ones.