Entropy of activation determination

Second-order rate coefficients have been obtained for chlorination of alkyl-benzenes in acetic acid solutions (containing up to 27.6 M of water) at temperatures between 0 and 35 °C, and enthalpies and entropies of activation (determined over 25 °C range) are given in Table 63 for the substitution at the position indicated266. 

An investigation has been made of the reactions of p-nitrophenoxide ion in methanol with equimolar quantities of chloropyrazine and (other monochloro-diazabenzenes) in the presence of a tenfold excess of p-nitrophenol (908). The second-order rate coefficient for 2-chloropyrazine at 80° was determined as 9.57 X 10 1/mol sec and as 23.4kcal/mol and logiofias 10.5 and its reactivity (at 50°) compared to that of chlorobenzene was 4.1 x 10, whereas the estimated value for 2-chloropyridine (relative to chlorobenzene) was 2.7 x 10 (908). Kinetics have also been measured for the reaction of 2-chloropyrazine with piperidine in toluene and the energy of activation and entropy of activation determined as 13.2 and 46.2 (909) the corresponding values for 2-chloropyridine are 17.1 and 42.2, respectively. 

The energies and entropies of activation determined by Maron and Berens (22) (30.5 kcal/mole and 9.7 e.u.) include the energy and entropy of formation of sulfamic acid (i.e., both reactions 24 and 25). The latter values can be estimated from the data of King and King (24) as +0.9 kcal/mole and +-9 e.u., and this will mean values for the energy and entropy of activation of hydrolysis of sulfamic acid (Eq. 25) of 29.6 kcal/mole and 0.7 e.u. 

A number of groups have criticized the ideas of Dauben and Noyce, especially the concept of PDC. Kamernitzsky and Akhrem, " in a thorough survey of the stereochemistry of addition reactions to carbonyl groups, accepted the existence of SAC but not of PDC. They point out that the reactions involve low energies of activation (10-13 kcal/mole) and suggest that differences in stereochemistry involve differences in entropies of activation. The effect favoring the equatorial alcohols is attributed to an electrostatic or polar factor (see also ref. 189) which may be determined by a difference in the electrostatic fields on the upper and lower sides of the carbonyl double bond, connected, for example, with the uncompensated dipole moments of the C—H bonds. The way this polar effect is supposed to influence the attack of the hydride is not made clear. 

NO 2, MesN+j and Me2S+ leaving groups is determined by the entropy of activation the energies of activation of the first two are approximately the same while that of Me2S+ is substantially TiigTier and thus opposes the rate increase. 

Subsequently rates of benzoylation of a range of aromatics were determined under the same conditions (Table 105)407. The high negative entropy of activation is consistent with the high degree of ordering required for the polarised acyl chloride-aluminium chloride complex to be the electrophile. 

An exceptionally badly reported kinetic study in which a linear correlation of rate coefficient with acidity function was claimed was that of Mackor et al. 11, who studied the dedeuteration of benzene and some alkylbenzenes in sulphuric acid-trifluoroacetic acid at 25 °C. Rates were given only in the form of a log rate coefficient versus —H0 plot and rate coefficients and entropies of activation (measured relative to p-xylene) together with heats of activation (determined over a temperature range which was not quoted) were also given (Table 129). However,... 

The detritiation of [3H]-2,4,6-trimethoxybenzene by aqueous perchloric acid was also studied, the second-order rate coefficients (107/c2) being determined as 5.44, 62.0, and 190 at 0, 24.6, and 36.8 °C, respectively, whilst with phosphate buffers, values were 3.75, 13.8, and 42.1 at 24.6, 39.9, and 55.4 °C, respectively. The summarised kinetic parameters for these studies are given in Table 134, and notable among the values are the more negative entropies of activation obtained in catalysis by the more negative acids. This has been rationalised in terms of proton transfer... 

Activation energies and log A values have been determined for some compounds over the temperature range 40.06-50.18 °C but the range of the former is barely outside the possible experimental error of 1.5 kcal.mole-1 for rates reproducible to 1.5 % (as quoted) for a 10 °C measurement range, and similar conclusions apply to the log A values, so that discussion of the variations is inappropriate, especially since the values depend upon the medium composition679 68°. The activation energies averaged 21.0 and the log A values ca. 11.5 (after correction of rates to sec-1) so that a concerted reaction (proposed earlier) would seem to be quite possible since the entropy of activation will be of the order of 7 e.u. 

Rate coefficients and kinetic parameters for iododeboronation were determined for the benzene- and thiophene-boronic acids, and the results are given in Table 256. The relative reactivities derived from this work correlated well with those obtained in a number of other electrophilic substitutions572, which is perhaps surprising in view of the large variation in the entropies of activation. These differences were explained by Brown et al.132 in terms of the transition state for the phenyl compound occurring earlier along the reaction coordinate than for the... 

The simplest overall interpretation of these data is in terms of a rate-determining dissociation. Entropies of activation are positive and the solvent-dependence for a better leaving group (Cl) is less marked than for a worse one (Br) in the case of reaction (38) . For the dimeric carbonyls, [M(CO)4X]2, bridge-breaking, essentially the same dissociation, could result in a rapid pre-equilibrium. If this were followed by a second dissociative step, then the kinetics could be first-order (as for Mn), while a rate-determining entry of L could produce second-order kinetics (as for Re). 

If k is expressed in liters per mole per second, the standard state for the free energy and entropy of activation is 1 mole/liter. If the units of k are cubic centimeters per molecule per second, the corresponding standard state concentration is 1 molecule/cm3. The magnitudes of AG and AS reflect changes in the standard state, so it is not useful to say that a particular reaction is characterized by specific numerical values of these parameters unless the standard states associated with them are clearly identified. These standard states are automatically determined by the units chosen to describe the reactant concentrations in the phenomenological rate expressions. 

Reaction mechanisms, in solution, entropies of activation and, 1, 1 Reaction mechanisms, use of volumes of activation for determining, 2,93 Reaction velocities and equilibrium constants, NMR measurements of, as a function of temperature, 3, 187... 

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