Activation energy entropy

Table 5 Arrhenius activation energies, entropies of activation and rate constants at 308 K for reaction of A-acetoxy-A-alkoxybenzamides 25, iV-acetoxy-iV-butoxybenzamides 26, iV-benzoyloxy-iV-benzyloxybenzamides 2842,43 and other iV-acyloxy-iV-alkoxyamides with A-methylaniline in d4-methanol...

It would be of interest to measure the values of k5, such as activation energy, entropy, etc. Measurements of C from the steady-state studies are slow and not highly accurate. 

The Arrhenius relation given above for Are temperature dependence of air elementary reaction rate is used to find Are activation energy, E, aird Are pre-exponential factor. A, from the slope aird intercept, respectively, of a (linear) plot of n(l((T)) against 7 The stairdard enAralpv aird entropy chairges of Are trairsition state (at constairt... 

An overview of some basic mathematical techniques for data correlation is to be found herein together with background on several types of physical property correlating techniques and a road map for the use of selected methods. Methods are presented for the correlation of observed experimental data to physical properties such as critical properties, normal boiling point, molar volume, vapor pressure, heats of vaporization and fusion, heat capacity, surface tension, viscosity, thermal conductivity, acentric factor, flammability limits, enthalpy of formation, Gibbs energy, entropy, activity coefficients, Henry s constant, octanol—water partition coefficients, diffusion coefficients, virial coefficients, chemical reactivity, and toxicological parameters. 

Equation (5-43) has the practical advantage over Eq. (5-40) that the partition functions in (5-40) are difficult or impossible to evaluate, whereas the presence of the equilibrium constant in (5-43) permits us to introduce the well-developed ideas of thermodynamics into the kinetic problem. We define the quantities AG, A//, and A5 as, respectively, the standard free energy of activation, enthalpy of activation, and entropy of activation from thermodynamics we now can write... 

Reaction rate constants, k,x , are also related to free energies. As before, if entropy contributions can be neglected, the rate constant can be obtained directly from the activation energy, AE, by ... 

Kinetic studies on 2-, 3-, and 4-chloro-l-methylpyridinium salts showed a 30 10 ratio of the reaction rates at 50° with 4-nitro-phenoxide ion in methanol. The activation energy for reaction at the 4-position is one kilocalorie lower ( 8-fold higher rate) than for reaction at the 2-position. The reversal in rates relative to the corresponding halopyridines is the result of a much higher entropy of activation for the 2-chloro compound. The 3-chloro compound has a favorable entropy of activation also, but the energy of activation is about 13 kcal higher than that of the isomers (cf. Table II and Section III, A, 2). 

In a series of reactions for which an acceUrative decrease in the activation energy is accompanied by a decelerative decrease in the entropy of activation (Compensation Law ), or the two increase together, there wiU be an isokinetic temperature (between 0-200° C for three-fourths of the 79 reactions tabulated by Leffler ). The rate vs. temperature curves for all the reactions in the series pass through this single point. Comparisons are affected since the isokinetic temperature is a point of inversion of relative reactivity in the series. 

Line No. Pyridine substituents Nucleophile (solvent) Rate constant" (temp. °C) 10 fc liter mole- seo-i Activation energy kcal mole-1 Entropy of activation cal mole i deg-i Frequency factor logioA Ref. 

The Arrhenius activation energy,E. c Entropy of activation, 8 jjgt. 

Line No. Pyrimidine substituents Nucleophile in 99.8% EtOH Rate constant (temp. °C) 10 k liter mole i sec Activation energy kcal mole i Entropy of activation cal mole deg-i Frequency factor logic A Ref. 

The general principle that activation of para substitution is greater than of ortho substitution holds true also for an azinium moiety in the one instance studied. Thus, the activation energy for the 4-chloropyridine quaternary salt 280 (Table II, line 9) is 1 kcal lower than that for the 2-isomer (line 5). The rate relation (2- > 4-isomer) is controlled by the entropies of activation in this reaction due to electrostatic attraction in the transition state (281). The reverse rate relation (4- > 2-position) is predicted for aminations of such quaternary compounds due to electrostatic repulsion (282) plus the difference in E. A kinetic study of the 2- and 4-pyridine quaternary salts... 

In the comparison of 285, 286, and 287 the lower energy of activation for the para azine-nitrogen compounds 285 and 287 was responsible for their more rapid reaction. Both ratios of rates are about the same, the reactivity being greater for the azine-nitrogen compound in each case. Another relationship of moieties in the ortho-position is derived from comparison of 290 and 288, the rate difference being entirely due to the relationship of the activation energies. This ratio is essentially the same as that for 283 and 284, which involved both the entropy and... 

The rate differences result primarily from the lowering of the activation energies, but in a few cases small entropy increases also contribute. The relatively high rate of reaction of 8-bromoquinoline (346) is postulated to be due to hydrogen bonding of the solvent, piperidine, to the nearby azine-nitrogen in the ground state and... 

Table XIV, line 3). The rates are equal (only at 20°) due to a large, compensating difference between the entropies of activation. In piperidino-dechlorination, 4-chloroquinoline (Table XI, line 3) has a higher and a lower rate (by about 200-fold at 20°) than 1-chloroisoquinoline (Table XIV, line 1). This reversal of reactivity and of the relationship of the activation energies is attributed to the factors in amination reactions mentioned above. The relative reactivity of the chloro groups in 2,4-dichloroquinoline with methanolic methoxide is given as a 2 1 rate ratio of 4- to 2-displacement. 

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