Aromatic halides, electron attachment

Aromatic halides don t react unless an electron-withdrawing group is attached to the ring. For example, bromobenzene doesn t react. An example showing the reaction when an electron-withdrawing group is present is illustrated in Figure 13-10, the nucleophilic substitution attack on p-bromonitrobenzene. 

A simple diagram depicting the differences between these two complementary theories is shown in Fig. 1, which represents reactions at zero driving force. Thus, the activation energy corresponds to the intrinsic barrier. Marcus theory assumes a harmonic potential for reactants and products and, in its simplest form, assumes that the reactant and product surfaces have the same curvature (Fig. la). In his derivation of the dissociative ET theory, Saveant assumed that the reactants should be described by a Morse potential and that the products should simply be the dissociative part of this potential (Fig. Ib). Some concerns about the latter condition have been raised. " On the other hand, comparison of experimental data pertaining to alkyl halides and peroxides (Section 3) with equations (7) and (8) seems to indicate that the simple model proposed by Saveant for the nuclear factor of the ET rate constant expression satisfactorily describes concerted dissociative reductions in the condensed phase. A similar treatment was used by Wentworth and coworkers to describe dissociative electron attachment to aromatic and alkyl halides in the gas phase. ... 

In GC-MS, negative chemical ionization is an often used but improper term that refers to electron attachment. This ionization mode is generally used for the analysis of particularly electrophilic compounds such as certain halides and aromatics— compounds that in classical gas chromatography are often detected with a device known as an electron capture detector (ECD). 

In support of this mechanism it has been found that the rate of decomposition of substituted benzamides (and presumably the ease of rearrangement) is more rapid when electron-releasing groups arc introduced into the aromatic ring.74 Thus the separation of the halide ion must be the controlling step for the reaction. When there is an asymmetric carbon atom attached to the carbonyl group, configuration is retained and virtually no racemization occurs.75 (+)a-Methyl plienyl-... 

Bromine Water. Aromatic amines, since they possess an electron-rich aromatic ring, can undergo electrophilic aromatic substitution with bromine, to yield the corresponding arylamino halide(s). Therefore, if elemental tests indicate that an aromatic group is present in an amine, treatment with the bromine water reagent may indicate that the amine is attached to an aromatic ring. 

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