Aromatic positions, deuterium-hydrogen

Aromatic positions, deuterium-hydrogen exchange reaction of tetralin-di2 and diphenylmethane, I62t,l63t Aromatic stretching, FTIR of vitrinite, 103-12 Aromatic-to-aliphatic ratios, carbon-13 NMR, 80-95 Aromaticity, 73... 

It is clear from the results that there is no kinetic isotope effect when deuterium is substituted for hydrogen in various positions in hydrazobenzene and 1,1 -hydrazonaphthalene. This means that the final removal of hydrogen ions from the aromatic rings (which is assisted either by the solvent or anionic base) in a positively charged intermediate or in a concerted process, is not rate-determining (cf. most electrophilic aromatic substitution reactions47). The product distribution... 

The a-position in naphthalene (and other condensed polycyclic aromatics) is sterically hindered. Hodges and Garnett have shown, for example, that at 100°C and in the presence of Pt(II)-salts the /1-hydrogen exchanges with deuterium 28 times faster than the a-hydrogen (55). This could suggest that Pt-catalyzed direct C6-cyclization will also favor the /3-position. 

An indication of the importance of the steric inhibition of resonance in aromatic substitution reactions has been gained by a study of the exchange reaction of dialkylanilines with deuterium oxide.21 With di-methylaniline, for example, the ortho and para hydrogen atoms equilibrate readily with deuterium oxide. This reaction, like most aromatic substitution reactions (see Chapter 13), is believed to proceed by the attack of a positively charged fragment (in this case a hydrogen or deuterium ion) at a point of high electron density. Consequently, the ease with which equilibration occurs can be taken as an approximate measure of the contribution of forms such as XLV or XLVI to the structure of the... 

The aromatized head-to-tai1 dimers 7 and 8 formed by the decomposition of l-ds on the silicas all have precisely the six deuterium atoms required by an exclusively geminate reaction. The product from the decomposition of non-deuterated azocumene on partially deuterated Po i or PiHq silica gives the aromatized dimers in which only the methine hydrogen position contain.s deuterium (Fig. 4). The lower yield is consistent with a late-determining transfer of a deuteron from the silica to the methine pos i t i on. 

---