Negative entropy, of activation

Unimolecular reactions that take place by way of cyclic transition states typically have negative entropies of activation because of the loss of rotational degrees of freedom associated with the highly ordered transition state. For example, thermal isomerization of allyl vinyl ether to 4-pentenal has AS = —8eu. ... 

Reductions by NaBKt are characterized by low enthalpies of activation (8-13kcal/mol) and large negative entropies of activation (—28 to —40eu). Aldehydes are substantially more reactive than ketones, as can be seen by comparison of the rate data for benzaldehyde and acetophenone. This relative reactivity is characteristic of nearly all carbonyl addition reactions. The reduced reactivity of ketones is attributed primarily to steric effects. Not only does the additional substituent increase the steric restrictions to approach of the nucleophile, but it also causes larger steric interaction in the tetrahedral product as the hybridization changes from trigonal to tetrahedral. 

The mechanism of the cycloaddition of phenyl azide to norbornene has been shown to involve a concerted mechanism with a charge imbalance in the transition state (199). In a similar manner the cycloaddition of phenyl azide to enamines apparently proceeds by a concerted mechanism (194, 194a). This is shown by a rather large negative entropy of activation (—36 entropy units for l-(N-morpholino)cyclopentene in benzene solvent at 25°C), indicative of a highly ordered transition state. Varying solvents from those of small dielectric constants to those of large dielectric constants has... 

A comparison of the second-order rate coefficients for nitration of 2,4,6-tri-methylpyridine and 1,2,4,6-tetramethylpyridinium ion (both at the 3-position) shows similarity of profile in the common acidity region and a rapidly increasing rate with acidity for the trimethyl compound at acidities below 90 wt. % (where the usual maximum is obtained). These two pieces of evidence show reaction to occur on the conjugate acid as also indicated by the large negative entropy of activation. Surprisingly, the tetramethyl compound is less reactive than the trimethyl compound so maybe this is an example of steric hindrance to solvation. Calculation of the encounter rate also showed that reaction on the free base was unlikely. 

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. 

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... 

A kinetic study of the deuteration of pyridones and quinolones by deuterated sulphuric acid yielded the data in Table 148sl0. For the 4-pyridones, the rapid rise in rate with increasing acidity in strongly basic solutions, and the levelling off in rate at about H0 = 0 is consistent with reaction on the free base as is the small negative entropy of activation. The similarity in rate between 4-pyridone and its 1-methyl derivative shows reaction to take place on the form (XII) and not (XIII), viz. 

This is, of course, an even more highly organized transition state and entails a more negative entropy of activation, but this need not necessarily be prohibitive. 

On the basis of the evidence presented above as well as some other pertinent data (e.g. negative entropies of activation), Darwish and Braverman have suggested that the rearrangement of allylic 2,6-dimethylbenzenesulfinates (6a-f) to corresponding sulfones (7a-f) proceeds by a cyclic intramolecular mechanism involving a five-membered transition state which may be represented by a resonance hybrid (8) of the following resonance structures. 

Some of these reactions have been shown to exhibit negative entropies of activation, indicating that the molecules are more restricted in geometry in the transition state than they are in the starting compound. 

C and 2.4 — 5.3 for Ae hydration of arylpropiolic acids at 25°C in 50% sulfuric acid. The rates of both reactions give a linear correlation when plotted against Ho, with a slope of near unity and both reactions have comparable high negative entropies of activation. 

The evidence presented includes the observed solvent isotope effect of kHjSO./ DjSO =2.2-3.9, the high negative rho of -4.21, and the high negative entropies of activation (-27 e.u. to -35 e.u.). The negative entropy of activation is in line with values observed for other rate-limiting transfers to unsaturated carbons (29). 

As a model for the insertion process in the polymerization of ethylene, the reaction of Cp ScMe with 2-butyne was investigated. The reaction was revealed to have a relatively small enthalpy of activation and a very large negative entropy of activation a highly ordered four-centered transition state (117) was proposed [111, 112]. 

An unusually negative entropy of activation (—16.1 e.u.) was also observed for the 1,2-H shift.60... 

In aqueous dioxan aryl sulfinyl sulfones undergo rapid (tin = 43 s for PhS(0)S02Ph at 21° in 60% dioxan) uncatalyzed hydrolysis to sulfinic acid (135). This spontaneous hydrolysis has a low activation energy ( a = 9.4 kcal mol-1 for Ar = /j tolyl), but a large negative entropy of activation (/IS4 =... 

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