Wheland intermediate, formation

Strong differences in the reactivity of the aromatic C=C double bond compared to the reactivity of the C=C double bond of olefins are observed olefinic electrophilic additions are faster than aromatic electrophilic substitutions. For instance, the addition of molecular bromine to cyclohexene (in acetic acid) is about 1014 times faster than the formation of bromobenzene from benzene and bromine in acetic acid113,114. Nevertheless, the addition of halogens to olefins parallels the Wheland intermediate formation in the halogenation of aromatic substrates. 

The present evidence on these points comes from published work (8)of Moodie, Schofield and their group at the University of Exeter. They have studied the nitration of pseudocumene and have shown that there is about a 9 1 selectivity of attack on positions 5 and 6 provided that one makes the reasonable assumption that any ipso-attack is not followed by rearrangement under the chosen conditions. Both positions are reactive enough to achieve the limiting rate and have similar steric requirements. This deduction rules out the explanation in terms of early transition states of the positionally oriented type and also precludes the possibility of detectable positionally oriented intermediates prior to the Wheland intermediates. It means that there must be a common intermediate prior to Wheland intermediate formation, but the extent of intramolecular selectivity is certainly not sufficient to make it necessary to postulate any attractive interaction in this intermediate. 

Significantly, the pre-exponential factors decrease with increasing reactivity, and this suggests that the Wheland intermediate is more nearly formed in the transition state, the more reactive the compound. Or, considered another way, the position along the reaction co-ordinate at which a given amount of carbon-halogen bond formation occurs is nearer to the ground state the more reactive the compound. 

What we shall be doing in the discussion that follows is comparing the effect that a particular Y would be expected to have on the rate of attack on positions o-/p- and m-, respectively, to the substituent Y. This assumes that the proportions of isomers formed are determined entirely by their relative rates of formation, i.e. that the control is wholly kinetic (cf. p. 163). Strictly we should seek to compare the effect of Y on the different transition states for o-, m- and p-attack, but this is not usually possible. Instead we shall use Wheland intermediates as models for the transition states that immediately precede them in the rate-limiting step, just as we have done already in discussing the individual electrophilic substitution reactions (cf. p. 136). It will be convenient to discuss several different types of Y in turn. 

The formation of the Wheland intermediate from the ion-radical pair as the critical reactive intermediate is common in both nitration and nitrosation processes. However, the contrasting reactivity trend in various nitrosation reactions with NO + (as well as the observation of substantial kinetic deuterium isotope effects) is ascribed to a rate-limiting deprotonation of the reversibly formed Wheland intermediate. In the case of aromatic nitration with NO, deprotonation is fast and occurs with no kinetic (deuterium) isotope effect. However, the nitrosoarenes (unlike their nitro counterparts) are excellent electron donors as judged by their low oxidation potentials as compared to parent arene.246 As a result, nitrosoarenes are also much better Bronsted bases249 than the corresponding nitro derivatives, and this marked distinction readily accounts for the large differentiation in the deprotonation rates of their respective conjugate acids (i.e., Wheland intermediates). 

Accordingly, the role of water might be explained in the following way in the absence of water, protonation of toluene can induce arylation, whereas, in the presence of water, the acidity of the clay is just sufficient to protonate nitric acid and to favour the formation of an ipsosubstituted Wheland intermediate. The most reasonable reaction sequence compatible with our observations is depicted in scheme 1 and eq. 4. 

The generally accepted mechanism of the electrophilic aromatic substitutions120121 involves the direct formation of the cr-complex (the Wheland intermediate) in the RDS (Scheme 29). The reversibility of the formation of the Wheland intermediate is under investigation. Many cases of reversibility of the steps shown in Scheme 29 have long been noted. 

Formation of the Wheland intermediate from the ion-radical pair as the critical reactive intermediate is common to both nitration and nitrosation processes. The nitrosoarenes (unlike their nitro counterparts) are excellent electron donors, as judged by their low Eox° as... 

It corresponds to addition followed by elimination, and is symbolised by AE + De. The departing X+ is often a proton, while Z is a general substituent. The key step in this scheme is the formation of an intermediate arenium ion (Wheland intermediate, a complex), and the relative stability of this species is crucial to the outcome of the reaction. Isolable arenium ions are known, and the benzenonium ion itself C6Hy has been inferred from NMR of strongly acidic solutions [255],... 

Carbenium ions react readily with alcohols, ethers, ketones, anhydrides, and various aromatic compounds. Transfer constants to most alcohols are approximately Cx 1 in styrene polymerizations [308]. Anhydrides are more reactive, with transfer constants Cx 10 ketones and ethers are less reactive (Cx 0.1) [120]. In general, the transfer constants to aromatic compounds listed in Table 17 increases as their nucleophilicity increases. This indicates that the rate-determining step in Friedel-Crafts alkylation is formation of arenium ions ( Wheland intermediates) [Eq. (119)] rather than reinitiation. 

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