Arrhenius parameters with alcohols

Saunders and co-workers (Amin et al., 1990) determined the secondary tritium KIEs for the E2 reactions of [19] with sodium ethoxide in ethanol, [20] with potassium t-butoxide in t-butyl alcohol and [21] with potassium t-butoxide in t-butyl alcohol over a temperature range of 40°C. The Arrhenius parameters were found for each isotopic reactant and the AhIAt ratios were calculated (Table 41). The AH/AT ratios for the reactions of [19] and [21] are both less than unity, confirming that, in agreement with the model calculations, tunnelling is important in these reactions. The AHIAT ratio for the reaction of... 

In order to go further into the experimental check we constructed Arrhenius plots of the fluorescence quantum yield of BMPC in a few solvents (methanol, ethanol, propanol, hexanol and methylene chloride), all of which showed good linearity. The activation energies and A/kp ratios, calculated from the slopes and intercepts of those plots, are collected in Table 1. The smooth increase of both parameters in the alcohol series is mainly associated with the increase of solvent viscosity. On the other hand, decrease of the solvent dielectric constant from 32.7 (methanol) to 8.9 (dichloromethane) causes a small but significant increase of the activation energy also, this increase is probably somewhat compensated by the decrease of the viscous-flow... 

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. 

When the commodity chemical propylene oxide is heated to high temperature in the gas phase in a shock tube, unimolecular rearrangement reactions occur that generate the CsHgO isomers allyl alcohol, methyl vinyl edier, propanal, and acetone (Figure 15.9). Dubnikova and Lifshitz carried out a series of calculations to determine the mechanistic pathway(s) for each isomerization, with comparison of activation parameters to those determined from Arrhenius fits to experimental rate data to validate the theoretical protocol. 

Observed Arrhenius activation parameters for allylic alcohol isomerizations are also consistent with a unimolecular rate-limiting step. In aqueous ethanol and aqueous dioxane, differences in reactivity due to structural changes in the allylic system are reflected primarily in the energy of activation. Values of entropy of activation are generally near Harris... 

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. 

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