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COMMUnICATION TO THE IDITOR
COMMUNICATION TO THE EDITOR A NOTE ON THE STRENGTH OF ORGANIC BASES Though the strength of organio acids is fairly well explained and understood from the electronic mechanism, the behavior of organic bases is very anomalous. This has led recently to a rejection by Brown of the thermodynamic dissociation constants of bases in aqueous solution to serve as a measure of base strength, attributing them to “complex solvation effects the importance of which can not yet be estimated”; he takes the strength of binding between a base and a boron trialkyl as a correct measure of base strength. The purpose of the present note is to suggest a new approach, based on classical concepts, which removes some of the anomalies and reconciles the theory and results of Brown with the question of base strength in aqueous solution. Alkylamines against hydrogen ion: Our main purpose in this note is to draw attention to two hitherto neglected or unnoticed features of the problem. The fiFst point is that the weakness of ammonia is contributed partly by the repulsion of the approaching hydrogen ion by the three positively charged hydrogen atoms around the central nitrogen atom. If one of the hydrogen atoms is replaced by an alkyl group, the approaching hydrogen ion then undergoes repulsion only by the remaining two hydrogen atoms, which are not symmetrically situated about the line of approach. This diminution of the repulsion to the approaching hydrogen ion is the main factor which causes all primary alkylamines to be a t least twenty times stronger than ammonia. Of course, the weak electrondonating (+ I ) effect of the alkyl groups also contributes partly to this increase of strength. The second point we want to make is offered to account for the fact that on passing from a secondary to a tertiary alkylamine the base strength invariably falls off very steeply-a fact which is just opposite to what is expected from the classical polar factor considerations. When a basic nitrogen atom captures a proton, it shares some of this positive charge. Now, carbon being electropositive to hydrogen, and an N-C bond being less polarizable than an N-H bond, the tertiary nitrogen atom will have less tendency to capture a proton than a secondary amine has, a fact which will make the former weaker. This idea, which we propose to call ‘polarstriction effect’, since it arises out of a restrictive effect on an atom to assume a charge owing to the unsuitable electronegativity of the adjoining atoms, is taken from Pauling, who proposed it to explain the unexpected weakness of dialkyl-substituted guanidines. This opposition by the carbon atom is strongest with the methyl group, since the natural tendency of the methyl group is to keep the nitrogen most negative by supplying it eIectrons by the “hyperconjugation” mechanism; hence, methylamines are the weakest alkylamines. Of course, steric factors depending on the size and shape of the alkyl groups are also operative, but we consider the polarstriction factor to be most important in the matter. Bases agaznst generalized acids: If the approaching acid is not the positively charged hydrogen ion but is a neutral molecule, e.g., a boron trialkyl, the power-
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ful electrical forces arising out of the charged bydrogen ion become practically absent, though not completely absent as form41 charges are now involved, and other factors begin to assume more importance. Brown has successfully explained the base strength under such circulhstances as due to an interplay between the classical polar factor and a steric factor originating out of the size of the alkyl groups present in the acid and/or the base. Though substantially agreeing with Brown’s theory we are inclined to think that his steric factor should also include the polarstriction effect we have discussed, which becomes more probable from the fact observed by Brown that the tertiary amine is invariably weaker than the secondary amine, though the relative strengths of the other amines vary with the nature of the alkyl group present in the acid and/or the base. SANTIR.PALIT. Polytechnic Institute of Brooklyn Brooklyn, New York April, 1947
SEW BOOKS Ozidation. A general discussion held by the Faraday Society, September, 1945. 398 pp. London: Gurney and Jackson, 1946. Price: 20 shillings. The appearance of this volume, containing thirty-two papers presented a t a symposium on oxidation, marks a welcome return by the Faraday Society to the policy i t had long followed before the war of holding symposia from time t o time on subjects of current interest in theoretical chemistry. The subject of oxidation affords a wide spread of topics in many fields of theory and practical application. The general discussions interspersed through the sessions form a most valuable part of the symposium. While the theoretical aspects of oxidation receive full attention, the subject of combustion naturally led t o the consideration of fuels and explosive gas reactions. S. C. LIND. Abridged Scienfific Publications from the Kodak Research Laboratories. Vol. XXVII. 314 pp. Rochester, Xew York: Eastman Kodak Company, 1945. This is a continuation of the publication in abridged form of papers which have been previously published in full in English in scientific periodicals. The present volume contains fifty papers published in American and British journals. S. C. LIND.
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Reactions at CarbonCarbon Double Bonds. By CHARLESC. PRICE. vi 120 pp. Rew York: Interscience Publishers, Inc., 1946. Price: $2.50. This interesting little book is the result of a group of lectures given by Professor Price a t Polytechnic Institute of Brooklyn under the direction of Dr. H. Mark. As stated in the Introduction, this book is the first volume of a series of “Lectures on Progress in Chemistry.” Written in the “eventful summer of 1945,” this work constitutes one of the finest reviews of the subject t o be found anywhere. While many researchers in physicoorganic Chemistry may not share the position taken by Dr. Price in his reluctance to accept the application of
