Halogenation stability

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... 

CHBr3 or iodoform, CHI3). Note that the second step of the reaction is a nucleophilic acyl substitution of CX3 by OH. That is, a halogen-stabilized carbanion acts as a leaving group. 

We may ask How does Y know which side will give the more stable carbocation As in the similar case of electrophilic aromatic substitution (p. 681), we invoke the Hanunond postulate and say that the lower energy carbocation is preceded by the lower energy transition state. Markovnikov s rule also applies for halogen substituents because the halogen stabilizes the carbocation by resonance ... 

Structurally Characterized Compounds X2(L)E-E(L)X2 (E = Al, Ga, In X = Halogen) Stabilized by Organic Donor Molecules (E-E Bond Lengths in pm Stretching Vibrations ve e in cnT1)... 

Thus, halogens stabilize an adjacent positive charge by resonance. 

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. 

The addition of a carbon atom to the 0=N double bond has been reported using sulfur (equation 35)133 and arsenic ylides, halogen-stabilized carbenes, and diazo compounds.i i-i The best substrates are imines in which the nitrogen is substituted with an aromatic ring, substituted oximes, and hydrazones. 

Reactions of halogen-stabilized carbenoids with imines have been carried out using preformed lithium species e.g. equation or via a carbenoid generated from diiodomethane utilizing zinc-copper... 

Reactions of halogen-stabilized carbenoids with imines have been carried out using prefoimed lithium species (e.g. equation 36), or via a carbenoid generated from diiodomethane utilizing zinc-copper couple (Simmons-Smith conditions). A stereospecific ring closure is observed after the addition of lithiodichloromethane to a benzaldimine (equation 37).The addition of lithiochloro(phenylsulfon-yl)methane to aromatic imines affords 2-phenylsulfonyl-substituted aziridines, which can be deproton-ated and alkylated in excellent yield (Scheme 20). 

Carbanions bear many substituents that can affect their structure and reactivity and can influence the acidity of a parent C-H precursor. Halogens stabilize carbanions in the order Br > Cl > F. A prominent I—)i repulsion between the F and carbanionic center causes some destabilization in a-fluorinated carbanions. The magnitude of the destabilization depends on the carbanion structure. The destabilization is maximized as the carbanion structure approaches a planar configuration. Thus, fluorinated carbanions possess pyramidal structures with high barriers to inversion. 

Induction—electron-withdrawing groups, such as halogens, stabilize a nearby negative charge via induction. 

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