Chlorine inductive effect

The reduction of the C— Br and C—1 group moments from 1.10 and 0.90 in bromo- and iodo-benzene to about 0.80 and 0.50 in 2-bromo- and 2-iodo-thiophene has been ascribed to the larger weight of resonance forms such as (8) and (9) in the thiophene series. The chlorine, nuclear, quadrupole, resonance frequencies of chloro-substituted thiophenes are much higher than those of the corresponding benzene derivatives. This has been ascribed to a relayed inductive effect originating in the polarity of the C—S o-bond in thiophenes. The refractive indices, densities, and surface tension of thiophene, alkyl- and halo-thiophenes, and of some other derivatives have been... 

In the structure on the left, the charge is somewhat stabilized by the inductive effects of the neighboring chlorine atoms. In contrast, the structure on the right is destabilized by the presence of methyl groups. Therefore, the structure on the left is more stable. 

Since the (n-Bu)3Sn radical (R ) is nucleophilic (17), a partial negative charge must be produced at the methine carbon whose chlorine is being abstracted. The rate of this abstraction should clearly be enhanced by electron-withdrawing groups on R due to their stabilization of this charge by inductive effects. As observed, the removal of Cl from EW (or W diad) is expected to be more facile than from EVE (or VE diad) as a result of a -halogen effect in the former structure. 

Thus, the enhancements in chlorine removal from W diads compared to EV diads and from m-W diads compared to r-W diads observed in the (n-Bu)3SnH reduction of DCP, TCH, and PVC are consistent with the free-radical chain reaction mechanism. Inductive effects produced by neighboring 7-Cl s tend to favor the reduction of W diads relative to EV diads and steric interactions resulting from different preferred conformations in each isomer favor the removal of Cl from m-W diads relative to r-W diads. 

The effect of the chlorine atom s partial appropriation of the electrons of the carbon-chlorine bond is to leave C, slightly electron-deficient this it seeks to rectify by, in turn, appropriating slightly more than its share of the electrons of the a bond joining it to C2, and so on down the chain. The effect of Ct on C2 is less than the effect of Cl on Cl5 however, and the transmission quickly dies away in a saturated chain, usually being too small to be noticeable beyond C2. These influences on the electron distribution in [Pg.22]

The extra electron-withdrawing inductive effect of the electronegative chlorine atom is responsible for the greater acidity of chloroacetic acid by making the hydroxyl proton of chloroacetic acid even more positive than that of acetic acid. 

Also other Type B and C series from Table II are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silica-alumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37) in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,1-diarylethanes increased with the increasing basicity of the reactants (series 33). 

Chlorine withdraws electrons through inductive effect and releases electrons through resonance. Through inductive effect, chlorine destabUises the intermediate carbocahon formed during the electrophtiic substitution. 

Further inductive effects from other substituents enhance or counter these effects with predictable results. Thus, a halogen such as chlorine, with a strong inductive effect, produces stronger acids, especially in the case of the ortho derivative. Here, the extra inductive effect is correspondingly... 

It should be noted that the electron-donating resonance effects just considered are the result of lone pair electrons feeding in to the jr electron system. Potentially, any substituent with a lone pair might do the same, yet we did not invoke such a mechanism with chlorine substituents above. As the size of the atom increases, lone pair elechons will be located in orbitals of higher level, e.g. 3p rather than 2p as in carbon. Consequently, the ability to overlap the lone pair orbital with the it electron system of the aromatic ring will diminish, a simple consequence of how far from the atom the electrons are mostly located. Chlorine thus produces a low resonance effect but a high inductive effect, and the latter predominates. 

If there is a suitable electron-withdrawing substituent, hydrate formation may be favoured. Such a situation exists with trichloroacetaldehyde (chloral). Three chlorine substituents set up a powerful negative inductive effect, thereby increasing the 8- - charge on the carbonyl carbon and favouring nucleophilic attack. Hydrate formation is favoured, to the extent that chloral hydrate is a stable solid, with a history of use as a sedative. 

---