Studies in organic reaction mechanisms

Abstract

The thesis entitled: "STUDIES IN ORGANIC REACTION MECHANISMS" "STUDIES ON SOME PSEUDO AND NORMAL ACID CHLORIDES" is divided into five chapters. Chapter I gives an introduction to the investigations described in the later chapters. Various interactions such as neighbouring group participation, neighbouring group catalysis, steric hindrance and steric assistance, field effects, and ring-chain tautomerism, which may be brought into play when two groups are in close proximity, are briefly discussed. Literature on the systematic studies on ring-chain tautomerism of ? and ?-aldehydo and keto-acids, amides, acid chlorides and esters is presented. Chapter II: Tautomerism of pseudo and normal acid chlorides. This chapter discusses the ambiguities resulting from the tautomerism of normal and pseudo acid chlorides. A number of obscurities existing in literature, like the question of existence or otherwise of equilibrium, have been cleared up. The factors that favour the pseudo form and the normal form in ? and ?-keto acid chlorides have been delineated. In a table containing about 75 acid chlorides described in literature and prepared by us, structural assignments have been examined. The mechanism of isomerization encountered in the reactions of monoester acid chloride of dicarboxylic acids has been studied. It has been shown that oxonium salt is the intermediate in these reactions, e.g., O=C–COOCH? O–COCl

Chapter III: Reaction of pseudo acid chlorides with nucleophiles. This chapter gives a review on the reactions of pseudo acid chlorides with different nucleophiles. Formation of more than one product is quite common in the reactions of pseudo acid chlorides with nucleophiles. This material, it is felt, will be useful to get a total view of the reactivity pattern of pseudo acid chlorides. Chapter IV: Solvolysis of pseudo acid chlorides. This chapter describes the first report, to the best of our knowledge, on the solvolysis of pseudo acid chlorides, viz., levulinyl chloride (I) and p-benzoylpropionic acid chloride (II), o-benzoylbenzoic acid chloride (III) and its A and B ring-substituted derivatives (IV a–g) and (V a–c), tetrachloro acid chloride (VI) and 3,3-dichlorophthalide (XI) [Chart I, II], have been examined in three solvent systems: dioxane–water, acetone–water and dimethylformamide–water. Influence of structure, change in solvent polarity (m values), solvent effects, common-ion effects, solvent isotope effects on the rates has been examined. The kinetics of these solvolysis reactions were followed by conductometric method. It has been shown that by and large the pseudo acid chlorides solvolyze in aqueous solvents by a unimolecular mechanism. There are however some exceptions. Quantitative measure of solvent polarity of aqueous DMF had not been reported. A Y-value scale for aqueous DMF solutions has been determined. Chapter V is divided into two parts: Part I: A review on the solvolysis of normal acid chlorides. It is hoped that this would be useful as background information for comparison with solvolysis of pseudo acid chlorides. Part II: Solvolysis of a few normal acid chlorides, namely p-benzoylbenzoyl chloride (VII), o-acetyl-salicyloyl chloride (VIII), fluorene-9-one-1-carboxylic acid chloride (IX) and phthaloyl chloride (X), have been examined to obtain information under conditions comparable to the solvolysis of pseudo acid chlorides. It has been shown that in contrast to the pseudo acid chlorides, normal acid chlorides solvolyze by a bimolecular mechanism. The question of non-existence of equilibrium between pseudo and normal forms of phthaloyl chlorides (X) and (XI) (tacitly assumed in many textbooks) has been established.