Alcohol Arrhenius equation

Schwab (5) points out a relationship between the parameters E and A of the Arrhenius equation which varies from one catalyst to the other for a definite reaction, for instance, for the dehydrogenation of alcohol, as expressed by... 

Note that r and the diffusion coefficient D have cancelled from Equation 2.29, because D is inversely proportional to the molecular radii r /2. Hence the rate constant kd depends only on temperature and solvent viscosity in this approximation. A selection of viscosities of common solvents and rate constants of diffusion as calculated by Equation 2.29 is given in Table 8.3. The effect of diffusion on bimolecular reaction rates is often studied by changing either the temperature or the solvent composition at a given temperature. For many solvents,54-56 although not for alcohols,57 the dependence of viscosity on temperature obeys an Arrhenius equation, that is, plots of log rj versus 1 IT are linear over a considerable range of temperatures and so are plots of log(kdr]/T) versus 1/T.56... 

In order to compare the experimental values of with the calculated ones accordingly to Eqs. (9) and (13), we described the temperature dependence of viscosity for a series of the -alkanes and -alcohols, and also for water on the basis of referenced data via the Arrhenius equation in form ... 

The chemistry for a stoichiometrically balanced reaction suggests that m = 1 and n = 2 in Eq. (2.86). For real systems, values are often close to these values but not identical. In the epoxy-amine reaction the alcohol, which may be present initially in small concentrations but is also a product of the reaction, catalyzes further reaction, resulting in autocatalysis. Since there are four unknowns ki, k2, m, and n) nonlinear regression analysis must be employed, although ki can be evaluated independently as the extrapolated reaction rate at a = 0. Autocatalytic kinetics are usually evaluated by the derivative form of the autocatalytic rate equation [Eq. (2.86)] with data coUected by isothermal method 1 measurements. Activation energy E and preexponential factor A are measured from the Arrhenius equation... 

In general, polar solvents speed up the quaternization reaction322-4 . To some extent the effect of solvents follows their dipole moments322 but the connection is not general, and with solvents of high polarity (tetramethylene sulphone, D = 42 propylene carbonate, D = 65 1) examples are known of quaternizations which are faster in the less polar medium3246. The reaction of pyridine with alkyl halides has been used to elucidate the ways in which the parameters of the Arrhenius equation vary with changes in solvent 323, 324, 325 YoT preparative purposes alcohol is often used as the solvent, but for slow reactions nitrobenzene is useful. 

Using the same experimental approach, a family of enantiomerically pure oxonium ions, i.e., O-protonated 1-aryl-l-methoxyethanes (aryl = 4-methylphenyl ((5 )-49) 4-chlorophenyl ((5)-50) 3-(a,a,a-trifluoromethyl)phenyl ((5)-51) 4-(a,a,a-trifluoromethyl)phenyl ((S)-52) 1,2,3,4,5- pentafluorophenyl ((/f)-53)) and 1-phenyl-l-methoxy-2,2,2-trifluoroethane ((l )-54), has been generated in the gas phase by (CH3)2Cl -methylation of the corresponding l-arylethanols. ° Some information on their reaction dynamics was obtained from a detailed kinetic study of their inversion of configuration and dissociation. Figs. 23 and 24 report respectively the Arrhenius plots of and fc iss for all the selected alcohols, together with (/f)-40) of Scheme 23. The relevant linear curves obey the equations reported in Tables 23 and 24, respectively. The corresponding activation parameters were calculated from the transition-state theory. 

The departure from Arrhenius behavior of the relaxation time arises, in the Adam-Gibbs theory, from the temperature dependence of the Sc term in equation (1), which itself is a consequence of the excess heat capacity ACp of equation (2). For constant Ap in equation (2), the degree of non-Arrhenius character, now called the fragility, is determined by the magnitude of ACp. A general but incomplete accord seems to exist between the fragility and ACp and exceptions, like the alcohols, can be rationalized by the presence of unusual Ap terms. 

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