Carbocation Structure and Stability

To understand wliy Markovnikov s rule works, we need to learn more about the structure and stability of carbocations and about the general nature of reactions and transition states. The first point to explore involves structure. 

The second point to explore involves carbocation stability. 2-Methyl-propene might react with H to form a carbocation having three alkyl substituents (a tertiary ion, 3°), or it might react to form a carbocation having one alkyl substituent (a primary ion, 1°). Since the tertiary alkyl chloride, 2-chloro-2-methylpropane, is the only product observed, formation of the tertiary cation is evidently favored over formation of the primary cation, rhermodynamic measurements show that, indeed, the stability of carbocations increases with increasing substitution so that the stability order is tertiary secondary primary methyl. 

ThomsonNiO Click Organic Interactive to rank the stability of carbocation intermediates. 

The electronic structure of a carbocation. The trivalent carbon is sp -hybridized and has a vacant p orbital extending perpendicular to the plane of the carbon and three attached groups. 

2-chloro-2-methylpropane, is the only product observed, formation of the tertiary cation is evidently favored over formation of the primary cation. Thermodynamic measurements show that, indeed, the stability of carbocation s increases with increasing substitution More highly substituted car-bocations are more stable than less highly substituted ones. 

A plot of dissociation enthalpy versus substitution pattern for the gas-phase dissociation of aikyi chlorides to yield carbocations. More highly substituted alkyl halides dissociate more readily than less highly substituted ones. These enthalpies are calculated in the follovring way  

3° Three alkyl groups 2 Two alkyl groups 1° One alkyl group Methyl No alkyl groups 

What alkenes would you start with to prepare the following prodncts  

A great deal of experimental evidence has shown that carbocations are planar. The trivalent carbon is sp -hybridized, and the three substituents are oriented toward the corners of an equilateral triangle, as indicated in Fignre 7.7. Because there are only six valence electrons on carbon and all six are nsed in the three a bonds, the p orbital extending above and below the plane is unoccupied. 

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