Halogens, stabilizing effect

The effects of fluonnation on carbanion stability are largely deduced from C-H acidity data (p. 988) [64], a-Halogens stabilize carbanions in the order Br > Cl > F, which IS opposite the inductive electron-withdrawing order and reflects the... 

This is observed in the case of the secondary alcohol illustrated, where a secondary carbocation would be generated. A methyl migration would merely lead to another secondary carbocation, and this serves no stabilizing effect. However, a hydride migration produces a tertiary carbocation, so this process will stabilize the system. This is what actually happens, and the major product is a bromide where the halogen appears to have attacked the wrong position. 

A series of chloromethyl cations were observed, including phenylchloromethyl cations382-384,525 526 and perchlorotriphenylmethyl ion 256.527 West and Kwito-wski528 have characterized the perchloroallyl cation 257. A series of chloro- as well as bromo- and iodomethyl cations have been observed (258-260) and the general stabilizing effect of halogen attached to carbocation center has been demonstrated.529... 

On the other hand, for RCXJ and R2CF+ the cation stability increases along with the increase of resonance (crR) effect of a halogen F > Cl > Br > I [59]. The significant stabilizing effect of fluorine substituent was explained as a result of back-donation of an unshared electron pair of F on the vacant orbital of carbon. Stability of substituted fluoromethyl cations in gas phase increases going from CF3 to 12 [15] ... 

The effect of fluorine, chlorine, or bromine as a substituent is unique in that the ring is deactivated, but the entering electrophile is directed to the ortho and para positions. This can be explained by an unusual competition between resonance and inductive effects. In the starting material, halogen-substituted benzenes are deactivated more strongly by the inductive effect than they are activated by the resonance effect. However, in the intermediate carbocation, halogens stabilize the positive charge by resonance more than they destabilize it by the inductive effect. 

Tables 3-11 are organized according to the a-heteroatom. Where there are two different hetero-substituted groups present in the carbanions, the listing in the tables is in accord with the priority sequence N, P, O, S, Se and halogen (as heteroatoms), COjR, CN, and metalloids. This takes no regard of the stabilizing effectiveness of the hetero-substituted groups present. Table 12 lists lithioformamides and -thioformamides.

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