Heterolysis of Bonds to Carbon Carbocations and Carbanions

Heterolysis of a bond to a carbon atom can lead to either of two ions either to an ion with a positive charge on the carbon atom, called a carbocation, or to an ion with a negatively charged carbon atom, called a carbanion  

The calculated lowest unoccupied molecular orbital (LUMO) for BF3 is shown by solid red and blue lobes. Most of the volume represented by the LUMO corresponds to the empty p orbital in the sp -hybridized state of BF3 (located perpendicular to the plane of the atoms). This orbital is where electron density fills (bonding occurs) when BF3 is attacked by NH3. The van der Waals surface electron density of BF3 is indicated by the mesh. As the structure shows, the LUMO extends beyond the electron density surface, and hence it is easily accessible for reaction. 

The highest occupied molecular orbital (HOMO) of ammonia, where the nonbonding pair resides, is shown by red and blue lobes in its structure. When the reaction occurs, the electron density from the HOMO of ammonia is transferred to the LUMO of boron trifluoride. This interaction 

In this way they are like BF3 and AICI3. Most carbocations are also short-lived and highly reactive. They occur as intermediates in some organic reactions. Carbocations react rapidly with Lewis bases—with molecules or ions that can donate the electron pair that they need to achieve a stable octet of electrons (i.e., the electronic configuration of a noble gas)  

Carbocation Anion (a Lewis acid) (a Lewis base) 

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Other nice examples of well-studied solvent-dependent ionization equilibria of ionogens are azidocycloheptatriene tropylium azide [282, 283] and (triphenylcyclo-propen-l-yl) (4-nitrophenyl)malononitrile ("2a) triphenylcyclopropenium dicyano(4-nitrophenyl)methide (3a), the latter being one of the first examples of direct heterolysis of a weak carbon-carbon bond to a carbocation and carbanion in solution [284],... 

Another linear correlation between AH values and between AG values has been proposed to correlate the heats of heterolysis for the carbon-carbon <7 bond with pX R+ values of the cations and pK values of the conjugate acids of the anions by Arnett et al. (1987a, 1990a). From the results of calorimetry for the coordination of resonance-stabilized carbocations and carbanions in sulfolane or acetonitrile, these workers demonstrated that (28) and (29), for secondary and tertiary cations, respectively, can be used for predicting heats of heterolysis of the carbon-carbon a bond. 

Removal of a proton from carbon atom 4 of aldol was considered not easy (point) In the carbocation, however, this proton should be much more acidic as again the carbanion can be stabilised by the adjacent positive charge. Thus, this bond becomes reactive and the point representing this heterolysis is again shifted more to the right of the reactivity space. However, the product is a p,y-unsaturated carbonyl compound, which is not as stabilised as the a,p-isomer. This is reflected in the fact that point 12 is not as far to the right as point 11. 

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