Residual affinity, theory

Durrans in 1919 attempted to develop a theory based on the examination of the odours of substances considered class by class, and expressed the opinion that, from a chemical point of view, odour is caused primarily by the presence of unsatisfied or residual afiinity, but that the possession or otherwise of an odour by a body depends on physiological and physical as well as chemical properties. This theory, which is named the Residual Affinity Theory of Odour, demands that if a substance has an odour, it must answer to the following requirements — ... 

The second of the premises of the residual affinity theory has already been dealt with here, the third is obvious, it remains therefore onlj to consider the first. 

The effect of a particular element on the odour of its compound seems also to lend support to the residual affinity theory, for it is only the elements which possess residual affinity in certain of their compounds, which function as osmophores. Oxygen, nitrogen, sulphur, phosphorous, halogens, arsenic, antimony, bismuth, etc., whose valencies vary under certain conditions are powerfully osmophoric whereas carbon, hydrogen, and many others which have a constant valency are practically non-osmophoric, and it is very instructive to note that the element is osmophoric when it is not employing its full number of valencies and therefore has free affinity. 

The Residual Affinity Odour Theory can have both a qualitative and a quantitative conception since the nature, distribution, and amount of affinity may vary from substance to substance. It is well known that bodies of similar type and construction frequently have similar odours. This fact was drawn attention to by Parry who instanced the various types of odoriferous alcohols —... 

This theory takes account of the view that the sensation of odour is the result of a chemical reaction in the nose, since in order for a body to be able to enter into chemical reaction the possession of residual affinity is probably a sine qua non, but it should be noted that the converse does not hold. 

The anomalous behaviour of butadiene on halogenation has been recognized for some time. It would be expected that the addition of bromine across one of the double bonds would produce only the molecule CH2BrCHBr CH=CH2. However, in addition to the 1,2-dibromo, compound there is a considerable yield of the 1,4-dibromo compound CH2Br—CH=CH—CH2Br, which contains a double bond in the 2,3 position. In order to explain this behaviour, Thiele proposed a theory of partial valencies. In this theory the second link in a double bond does not fully neutralize the affinity of the two carbon atoms so that they are left with a residual affinity In a system of alternate double and single bonds, the residual affinities of the inner atoms combine across the single bonds whilst those of the outer atoms remain free to attract other atoms. Thiele s formula for butadiene was therefore... 

Just before the nineteenth century ended Thiele applied his theory of partial valencies to explain the properties of benzene [21], in particular its reluctance to take part in the usual addition reactions of alkenes. He considered that simple multiple bonds had residual affinities or partial valencies which were responsible for the addition reactions. This could be depicted thus ... 

Mellor traced this idea of association of molecules by residual affinities to Mercer and Playfair it was adopted by H. E. Armstrong, and J. W. Walker. The idea of an intermediate complex appears in the work of W. Higgins (1789 see Vol. HI, p. 745) and in Dalton s note-books. Kekule s scheme of reaction (3) implies the modern theory of the intermediate complex , since he said ... 

A. Rep.y x88 (1886), 945 iVoc. Roy. Soc.y x886, xl, 268-91 (Electrolytic Co uction in relation to Molecular Composition, Valency and the nature of Chemical Change being an Attepipt to Apply a Theory of Residual Affinity) Proc. Chem. Soc.y X887, iii, X27 J. Chem. iSoc.i 1888, liii, X25. 

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