Ring-closing reactions entropy

Entropy of activation is also responsible for the difficulty in closing rings larger then six membered. Consider a ring-closing reaction in which the two groups that must interact are situated on the ends of a 10-carbon chain. In order for reaction to... 

Ring-closure and hydrogen transfer in the hex-5-enyl, hept-6-enyl, and oct-7-enyl radicals have been studied. Ring-closing reactions decrease in importance as the methylene chain gets longer the preferred formation of methylcycloalkenes rather than simple cycloalkanes by ring-closure was shown to be mainly due to relative activation enthalpies rather than relative activation entropies. ... 

Ring-closing metathesis (RCM) is an extremely powerful method for transforming acyclic dienes into unsaturated cyclic systems [1], The process is believed to proceed via the catalytic cycle outlined in Scheme 1, and the principal driving force for the reaction is the gain in entropy resulting from the loss of ethene. 

The study of the initiation of Hoveyda-type complexes has been hampered by the issues involved in treating the entropy of intermolecular reactions. Opinions differ on the best way to treat this issue, with some researchers preferring to consider a pre-activated complex in which the two reactants are present in the same initial DFT calculation, while others prefer to treat the two reagents at infinite separation.Calculations of the thermodynamics of ring-closing have met similar issues, with a correction being required to calibrate DFT data (using experimental EM data) to obtain sensible EM values. " When no such correction is applied, DFT... 

Aldol reactions are often used to close five- and six-membered rings. Because of the favorable entropy (p. 211), such ring closures generally take place with ease, even where a ketone condenses with a ketone. An important example is the Robinson annulation reaction which has often been used in the synthesis of steroids and terpenes. In this reaction a cyclic ketone is converted to another cyclic ketone, with one additional six-membered ring containing a double bond. The substrate is treated with methyl vinyl ketone (or a simple derivative of methyl vinyl ketone) and a base.551 The enolate ion of the substrate adds to the methyl vinyl ketone in a Michael reaction (5-17) to give a diketone that undergoes or... 

Two excellent reviews <71AHC(13)235, 72IJS(C)(7)6l) have dealt with quantitative aspects of electrophilic substitution on thiophenes. Electrophilic substitution in the thiophene ring appears to proceed in most cases by a mechanism similar to that for the homocyclic benzene substrates. The first step involves the formation of a cr-complex, which is rate determining in most reactions in a few cases the decomposition of this intermediate may be rate determining. Evidence for the similarity of mechanism in the thiophene and benzene series stems from detailed kinetic studies. Thus in protodetritiation of thiophene derivatives in aqueous sulfuric and perchloric acids, a linear correlation between log k and —Ho has been established the slopes are very close to those reported for hydrogen exchanges in benzene derivatives. Likewise, the kinetic profile of the reaction of thiophene derivatives with bromine in acetic acid in the dark is the same as for bromination of benzene derivatives. The activation enthalpies and entropies for bromination of thiophene and mesitylene are very similar. 

AS X 2, and so an increase in entropy will be expected. Since A 5 A Se, the activated complex closely resembles the reactant, and the C—C bond breaking is not at all well developed. There is little strain to be released in the four-membered ring. 

The intramolecular aldol condensation is well known, and aldol reactions are often used to close five- and six-membered rings. Because of the favorable entropy (p. 303), such ring closures generally take place with ease when using hydroxide or alkoxide bases in protic solvents. In aprotic solvents with amide bases,... 

The cleavage of both of the weak C-N bonds and the formation of the nitrogen/ nitrogen triple bond, coupled with the great increase in entropy, help drive this reaction. The initial biradical, which is formed by the loss of the nitrogen molecule, then closes the ring to give the cyclic product. 

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