Chlorobenzene triple ions

Figure 3. Dependence of the reciprocal conductance relaxation time on TBA— salt concentration in media and concentration range where conductance indicates an important fraction of triple ions. TBA-bromide in benzene-nitrobenzene (3, 22 vol % D = 2, 90) at 298 ( ). TBA-picrate in benzene-chlorobenzene (16 vol % D = 2, 78) at 298 (0).

The temperature- and pressure-dependence of the conductance relaxation in TBAP solutions in benzene-chlorobenzene (16 vol%) corroborates importantly the description of the phenomena by eq. 14b implying an ionic recombination process between the triple ion and a simple ion. Apparently this picture is still in conflict with conductance data which who the presence of two kinds of triple ions. This discrepancy remains as yet unresolved. 

In o-dichlorobenzene (oDCB), BuaNHPic has a very low A° probably because it dissociates according to the equilibria 5.6.1. The bromide and iodide in oDCB and the picrate in chlorobenzene form triple ions. ... 

When chlorobenzene is treated with the strong base sodamide, Scheme 9.6 NaNH2, aniline is formed. This reaction is more complex than it appears, since if the chlorobenzene is labelled at C-l with the isotope l4C (indicated by an asterisk), then the product consists of equal amounts of aniline with the label at C-l and C-2. The reaction proceeds by abstraction .He-of the weakly acidic hydrogen atom on the carbon atom next to the ,l C-halogen group by the strong base, NH, followed by loss of halide ion. Ammonia then adds to the apparent triple bond of the resulting . . 

Why is benzyne so strained Recall that alkynes normally adopt a linear structure, a consequence of the sp hybridization of the carbons making up the triple bond (Section 13-2). Becanse of benzyne s cyclic strnctnre, its triple bond is forced to be bent, rendering it nnnsnally reactive. Thns, benzyne exists only as a reactive intermediate under these conditions, being rapidly attacked by any nucleophile present. For example (step 2), amide ion or even ammonia solvent can add to furnish the product benzenamine (aniline). Becanse the two ends of the triple bond are equivalent, addition can take place at either carbon, explaining the label distribntion in the benzenamine obtained from " C-labeled chlorobenzene. 

An intermediate known as benzyne, which has a triple bond, accounts for the equivalence of two carbon atoms toward the nucleophile. This intermediate was proposed on the basis of isotopic labeling studies using chlorobenzene labeled with C at C-1. Reaction with amide ion gives a mixture of two differently labeled anihnes in equal amounts. The following equation shows location of the C with an asterisk. 

---