Progress in physical organic chemistry

Progress in Physical Organic Chemistry, edited by A. SXXE~TWIESER AND R. W. TAFT, Interscience, New York, Vol. 8, 1971, pp. vii+359, price &lO.SS. This volume of Progress in Physicd Organic Chemistry contains a series of four articles of varying length and merit. The first article is an extensive survey concerned with a critical evaluation of the concept of fluorine hyperconjugation. The purpose of this review appears to be to investigate one special aspect of the relative importance of the inductive and conjugative abilities of fluorine when substituted in an organic molecule, i.e. the relevance of fluorine no-bond resonance to the observed reactivity of organo-fluorine compounds. Several miscellaneous topics and physical methods that comment on fluorine hyperconjugation are dealt with first_These include molecular orbital calcuiations, ESR and NMR data, information derived from bond lengths, the~o~hemic~, and dipole moment measurements. Reactivity relationships in aromatic and aliphatic systems including much recent unpublished work are finally discussed at some length. It is hoped that this work previously unreported elsewhere will be published and not remain hidden in this extensive review. The conclusions reached based on present avaiIabIe information indicate that the use of no-bond resonance is unnecessary in rationalizing the chemistry of organic fluorine compounds. Such a statement or claim made by the author seems to remove the main arguments in favour of such a review as this and refute somewhat the claims made by the editors in the introduction as to the value of speculation,. The second review is of the structure-reactivity relationships in homogeneous gas-phase reactions. Examples of all the gas-phase reactions involving quantitative data for a selected number of compounds have been reported here. Representative examples of product analysis dataqare given to demonstrate the scope of the reaction and, where appropriate, reference is made to liquid-phase reactions for the compounds in question. The material selected involves mainly thermolysis and thermal rearrangements and in particular the thermolysis of esters is discussed in the greatest detail. Reasons given for this are that this reaction has been studied extensively by the authors and many other researchers. Also ester thermolytic reactivity is midway between the thermolysis of primary halides, chloroformates and rearrangements of allylic esters and the thetmolysis and rearrangements of hydrocarbons and #I-hydroxyolefins. Most of the compounds selected thermally decompose and rearrange by unimolecular dissociation. Comparative rates and Arrhenius parameters are tabulated for homogeneous unimolecular gas-phase reactions. General observations are made on the data presented as to how me~ingful comparisons in reactivity are in such cases. The purpose of the third article is to provide a quantitative treatment of the ortho substituent effect by means of linear free energy relationships the most 464

J. Mol. sfrzJcfurt?, 13 (1972)

useful ofwhich is the Hammett equation. This is described and discussed here in terms of its application to ortho substituent derivatives and in its validity in such systems. In this context ortho substituent constants, steric effect substituent constants and the extended Hammett equation all receive consideration. Examples of the most frequently studied reactions, i.e. involving proton transfer, are given and discussed due to the vast source of quantitative data supplied from this standpoint on the behaviour of ortho substituted components. Variation in the ortho effect is discussed in terms of the solvent, side chain and proximity electrical effects. Reaction rate data for ortho substituted benzenes is included and a quantitative description of this data by means of linear free energy relationships is an objective of this work. The various reaction types are dealt with separately and these include ester hydrolysis and esterification, carbonyl reactions other than these, elimination reactions, nucleophilic substitution, side chain reactions and electrophilic aromatic substitution. Correlations of physical data with various cr constants are finally reported and discussed, i.e. polarographic half-wave potentials’ with the a, constant of Mammalis and Rydon and Taft’s Go* constant and NMR chemical shift data with ap and oO* constants. The summary of this article should be read by all since here the author puts forward strong arguments which should deter researchers from trying to evaluate a true ortho substituent constant. The final article in this volume is concerned with the electron spin resonance of nitrenes. Comparison is made of the nitrenes with the family of methylenes as regards the nature of the ground state. The differences which exist are discussed in terms of proximity of orbitals on the nitrogen and carbon nuclei in these two cases. The nitrenes which receive attention are aliphatic, sulphonyl, aromatic and carbonyl in nature and reference is also made to dinitrenes and higher multiplets. The triplet state ESR field is a highly specialised and sophisticated one but this short concise article makes interesting reading. This volume continues in the excellent style of the past volumes and the editors have every right to commend the efforts of the contributors in the way they have done in the introduction. P. J. B.

Photochemistry-Third viii + 187, price fl5.00.

IUPAC

Symposium,

Butterworths,

London,

1971,

pp.

The Third IUPAC Symposium on Photochemistry was held in St. Moritz, Switzerland, in July 1970. The texts of the eleven plenary lectures are published in this volume. These are: diabatic and adiabatic processes in photochemistry (Th. FBrster); recent advances in the photochemistry of coordination compounds J. Mol.

Srructure,

I3 (1972)

465