Arene hydrates, dehydration

The substituent effects for the dehydration of arene hydrates have been studied." The kinetics of dehydration were studied with a series of substituted arene-hydrates (84) in dilute HCIO4 solutions. The results are consistent with a rate-limiting step involving formation of the arenium ion (86) from the oxonium ion intermediate (85). Rapid deprotonation then provides the dehydration product (87). The Hammett plot of second-order... 

Rate constants for the acid-catalysed dehydration of different types of monocyclic arene hydrates have been determined by UV spectrophotometry. General acid catalysis was not observed, and linear plots of pseudo-first-order rate constants for dehydration against hydronium concentration were obtained. A Hammett plot of the second-order rate constants for acid-catalysed dehydration shows an excellent correlation with <7+ values and yields a large negative p-value of -6.5. The results are consistent with ratedetermining formation of a benzenium ion in which direct mesomeric interaction with the substituent occurs, presumably permitted by the coplanar arrangement of the diene and carbocation centre in the intermediate. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

Charge-transfer complexes of neutral molecules in zeolites have also been examined. Transient experiments with 1,2,4,5-tetracyanobenzene (TCNB) as acceptor and arene donors have been reported. For naphthalene, transient absorption bands centered at 470 and 680 nm due to TCNB and naphthalene radical were observed [138]. The decay was found to be biphasic and was 10 times slower in dehydrated zeolite Y than in the hydrated sample, indicating a strong interaction with the framework. 

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