Benzene charge transfer

In these compounds, the intermediate arylmethyl radical is long-lived. However, further excitation of the radicals in benzene results in production of the bromine atom-benzene charge transfer complex ( max 550 nm), an indication of photo-induced cleavage of the remaining C-Br bond. This is further supported by observation of the corresponding vinyl products. 

For example, a simple bimolecular complex might entail a dative interaction between two atomic or molecular species, such as the iodine-benzene charge-transfer complex, that features only minimal spatial organization. These complexes are weak and topologically indeterminate because the interacting pair... 

Like bromine, iodine is soluble in organic solvents, for example chloroform, which can be used to extract it from an aqueous solution. The iodine imparts a characteristic purple colour to the organic layer this is used as a test for iodine (p. 349). NB Brown solutions are formed when iodine dissolves in ether, alcohol, and acetone. In chloroform and benzene a purple solution is formed, whilst a violet solution is produced in carbon disulphide and some hydrocarbons. These colours arise due to charge transfer (p. 60) to and from the iodine and the solvent organic molecules. 

The dipole moment varies according to the solvent it is ca 5.14 x 10 ° Cm (ca 1.55 D) when pure and ca 6.0 x 10 ° Cm (ca 1.8 D) in a nonpolar solvent, such as benzene or cyclohexane (14,15). In solvents to which it can hydrogen bond, the dipole moment may be much higher. The dipole is directed toward the ring from a positive nitrogen atom, whereas the saturated nonaromatic analogue pyrroHdine [123-75-1] has a dipole moment of 5.24 X 10 ° C-m (1.57 D) and is oppositely directed. Pyrrole and its alkyl derivatives are TT-electron rich and form colored charge-transfer complexes with acceptor molecules, eg, iodine and tetracyanoethylene (16). 

The aromatic ring has high electron density. As a result of this electron density, toluene behaves as a base, not only in aromatic ring substitution reactions but also in the formation of charge-transfer (tt) complexes and in the formation of complexes with super acids. In this regard, toluene is intermediate in reactivity between benzene and the xylenes, as illustrated in Table 2. 

The structures of the black crystalline benzene solvate C6o-4C6H6, the black charge-transfer complex with bis(ethylenedithio)tetrathiafulvene, [C6o(BEDT-TTF)2], and the black ferrocene adduct [C6o Fe(Cp)2)2] (Fig. 8.7b) ) have also been solved and all feature the packing of Cso clusters. 

This result together with the preferred formation of para-products suggests that the attacking species is electrophilic and that consecutively are involved formation of a charge transfer complex, addition of Br" ", and elimination of H" ". This picture is supported by Raman studies (ref. 24) of the system Br2-benzene-NaX. 

Fig. 1 Progressive growth of the charge-transfer band (act = 285 nm) attendant upon the incremental addition of benzene (from 2 1 to 40 1 ratio) to 5 mM solution of dibromine in carbon tetrachloride. From [41]... 

Fig. 2 Electronic spectra of the carbon tetrachloride solutions of diiodine attendant upon addition of excess amounts of benzene (Act = 285 nm), mesitylene (Act = 327 nm) and hexamethylbenzene (Act = 369 nm) with the charge-transfer band showing significant red-shifts with increasing strength of the aromatic donors...

The benzene ring per se does not impart any particular pharmacological response to a drug. It is widely held that its planarity, its ability to bind to tissue receptors by Van der Waals and charge transfer mechanisms, and, particularly, its ability to serve as a conductor of electrons within a substance serve as modulators, enhancing or diminishing the intensity of response to a molecule that is otherwise inherently bioactive. 

That charge-transfer effects are not involved follows from the fact that the rate of triplet decay in perfluorobenzene is larger than that in benzene. If the benzophenone triplet were to act as acceptor and the benzene derivative as donor in a charge-transfer complex, the substitution of perfluorobenzene for benzene should render this type of process much less probable due to the strongly electron-withdrawing character of the fluorine atoms. 

The first example of a donor-acceptor molecular complex was noted in 1949 by Bensei and Hildebrand [137] in their studies involving charge transfer complexes between benzene and molecular iodine. Subsequently such complexes were studied by Mulliken [138] and now more recently have been used by Stoddart et al. [16,139] in designing novel self-assembling systems. 

Fig. 1 Mesomeric structures of a para-substituted benzene intramolecular charge transfer (ICT) complex in the ground state and in the dipolar excited state...

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