Transition state for halogenation

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

Benzo[l,2-d 4,5-transition states for attack in the observed positions should be more stabilized than elsewhere. 

There is on the other hand a great deal of evidence showing that the electrochemical reduction of 1,2-dihalides to olefins can occur via a concerted pathway, i.e., via a transition state (39) in which both carbon-halogen bonds are partially broken and the carbon-carbon double bond is partially formed. An important, indeed critical, point of evidence supporting the conclusion that reduction is concerted lies in the remarkable ease with which vicinal dihalides are reduced. For example, the half-wave potentials of ethyl bromide and 1,2-dibromoethane are -2.08 V and -1.52 V (vs. s.c.e.), respectively 15 >46) those of ethyl iodide and /J-chloroethyl iodide are -1.6 V and -0.9 V, respectively 47). These very large differences must reflect the lower energy of delocalized transition state 39 relative to the transition state for reduction of an alkyl monohalide. 

Open-chain vicinal dihalides are apparently reduced conceitedly via a conformation in which the two halogen atoms are anti to each other. This conclusion is based upon the fact that meso isomers of structures 47 ait generally more easily reduced than the dl isomers, presumably because steric repulsions are less for meso than for dl in the transition state for electron transfer. The stereochemistry of the products of reduction of simple open chain vicinal di-... 

Finally it should be said that o-/p-ratios can be considerably influenced by the solvent in which the reaction is carried out. This can arise from changes in the relative stabilisation by solvent molecules of the transition states for o- and p-attack, but it may also involve the actual attacking electrophile being different in two different solvents the species actually added complexing with solvent molecules to form the electrophile proper—a different one in each case. This almost certainly occurs in halogenation without Lewis acid catalysts, e.g. in the chlorination of toluene at 25°, where f0./fp. ratios between 0-75 and 0-34 have been observed depending on the solvent. 

Facile SN2 substitution reactions of halogens are expected from the electron-attracting characteristics of the neighboring carbonyl function, which should make the transition state for attack by a nucleophilic reagent more favorable ... 

As discussed in Section 3.4.3.1, nucleophilic replacements of halogen atoms are facilitated by mesomeric stabilization in the transition state for some haloazoles, depending on the number and orientation of the ring heteroatoms and halogen. In addition to this, and just as in benzene chemistry, all types of halogen atoms are activated toward nucleophilic... 

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