Some physico-chemical themes / by Alfred W. Stewart with five plates and thirty-seven diagrams in the text.

  • Alfred Walter Stewart
Date:
1922
Catalogue details

Licence: In copyright

Credit: Some physico-chemical themes / by Alfred W. Stewart with five plates and thirty-seven diagrams in the text. Source: Wellcome Collection.

    26/450 (page 10)
    Thiele’s conception of the mechanism of addition reactions is as follows : In the first place, the two reacting molecules approach each other as in (I.) ; thereafter the partial valencies come into operation, (II.); and finally, a rearrangement of affinity takes place which changes the partial valency into a normal single bond (III.). H2C Br H2C Br H2C—Br II I II I H2C Br H2C Br H2C—Br “ (I.) (ii.) (in.) On this basis, it is clear that the point of attack of an external atom upon an unsaturated molecule is represented by the partial valency; and the symbolical representation by means of the dotted line is really only a short way of expressing the ex¬ perimentally established facts and can be employed apart from any theory at all. Let us now turn to a case in which two unsaturated centres occur in the same molecule and again let us represent the doubly-linked atoms as having partial valencies :— R—CH=CH—CH2—CH=-CH—R Owing to the two ethylenic linkages in this molecule, two molecules of bromine can be added on to the hydrocarbon; and it is found that if the bromine molecules be allowed to act one at a time the first product is a compound—- R . CHBr . CHBr . CH2. CH : CH . R which is then attacked by the second bromine molecule to yield the tetrabromo-derivative— R . CHBr . CHBr . CH2 . CHBr . CHBr . R This behaviour is perfectly normal. When a molecule of the following type is examined— R—CH=CH—CH=-CH—R its action with a single molecule of bromine might be expected to be the formation of the dibromo-derivative—• R . CHBr. CHBr. CH : CH . R one of the double bonds being saturated and the other left