A -Chlorobenzyl methyl ether

We shall conclude this Section with an example of a reaction that undergoes an extreme rate acceleration with an increase in solvent polarity. Thermolysis of a-chlorobenzyl methyl ether in a series of non-nucleophilic, non-HBD solvents shows rate variations up to 10, encompassing a range of 30 kJ/mol (7 kcal/mol) [112], This dramatic solvent effect is best explained by a mechanism involving ionization of the C—Cl bond to form an ion pair, followed by a nucleophilic attack by Cl on CH3 to give an aldehyde and chloromethane cf. Eq. (5-41). 

The rate constants of the unimolecular thermolysis of a-chlorobenzyl methyl ether to yield benzaldehyde and chloromethane [cf. Eq. (5-41) in Section 5.3.2] have been determined in supercritical 1,1-difluoroethane (tc = °C pc = 4.5 MPa). Near the... 

A small change in the pressure of a SCF can produce a large change in the solvent strength (as measured by solubility parameter or solvatochromic polarity scales(2)), which can cause a large thermodynamic solvent effect on a rate or equilibrium constant. This phenomenon is unique to SCFs. An extremely pronounced pressure effect was discovered for the rate constant of the unimolecular decomposition of a-chlorobenzyl methyl ether in supercritical l,l>difluoroethane(12)... 

Johnston and Haynes (1987) demonstrate that the rate constant for the unimolecular, thermal decomposition of a chlorobenzyl methyl ether in 1,1-difluoroethane (P = 113.4°C, Pc = 45.0 bar) can be increased by an order of magnitude with a small change in pressure if the ether is very dilute and the... 

Variation and enhancement of reaction rates. The unimolecular decomposition of a-chlorobenzyl methyl ether in 1,1-difluoroethane (Tc = 113.4°C) was studied by Johnston and coworkers [54], monitored in situ spectroscopically from the formation of benzaldehyde ... 

Examples of the use of activation volume for the reactions in supercritical fluids is the unimolecular decomposition of a-chlorobenzyl methyl ether in 1,1-difluoroethane Jc = 113.4°C), studied by Johnston and coworkers [54] and the Diels-Alder reaction of isoprene and maleic anhydride in supercritical carbon dioxide [56], described in section 3.1.2.1. In the latter study the large variation in the rate coefficient at 35°C and near-critical pressures was quantified as AV = - 1.39 X lO cm moF in the highly compressible region, as compared with - 38.4 cm mol at 200 bar. Paulaitis and Alexander interpreted their results as a solvent effect stemming from an induced quadrupole moment in the carbon dioxide molecule. 

The importance of such direct-clustering effects on reaction has been further demonstrated for charge variation reactions in dipolar media. For such reactions, the dielectric properties of the solvent control the solvation and thus the height of the reaction activation barrier in that solvent. As a result, the reaction rate for such systems is directly correlated with the local dielectric properties of the solvent around the reacting solute. As an example, Johnston and Haynes showed that in SC 1,1-difluoroethane the rate of decomposition of a-chlorobenzyl methyl ether, which has an activated complex that is more polar than the reactant, can be explained by invoking a pressure-dependent effective dielectric constant which exceeds that of the bulk. ... 

Chemical forces, 102 Chemical reactions, 311-332 Chemotherapeutic agents, 304-307 Chen, S. J., 63 Chermin, H. A. G.,21 Chitin, 321, 322 Chlorinated hydrocarbons, 157 Chlorobenzyl methyl ether, 320 Chlorodifluoromethane, 194, 197, 205-210, 212... 

With substituted cycloproparenes, the regioselectivity of the silver(l)-catalyzed and the uncatalyzed reaction was markedly different. Thus, 2-methylbenzocyclopropene (2) reacted with hydrochloric acid to give predominantly w-xylene 3a (47%), but reacted with silver(I) nitrate in the presence of ethanol or aniline to give preferentially o-xylene derivatives 4b,c. 3-Methyl-benzocyclopropene (5a) gave predominantly the w-xylene 7a with hydrochloric acid, but p-xylenes 6b,c in the silver(I)-catalyzed reaction. 3-Chlorobenzocyclopropene (5b) underwent addition of hydrochloric acid to give a mixture of 3- and 4-chlorobenzyl chlorides 6d and 7d, ° but silver(I)-catalyzed methanolysis gave only the 4-chlorobenzyl ether 7e. ... 

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