Reactions at an Allylic Carbon Atom

Displacement Reactions at an Allylic Carbon Atom. It has long been known that reactions carried out with compounds of type I or II 

The most logical interpretation of such results is to consider that the reaction proceeds by the simultaneous operation of two processes.3 One is a normal Sn2 type displacement which gives only primary products from primary starting materials and secondary products from secondary starting materials. The other is an Sn 1-like process in which reaction of the intermediate carbonium ion can occur through the resonance structures VI or VII to give crotyl or methylvinylcarbinyl products  

It is a consequence of this concept of displacement at an allylic position that conditions which favor an Sy2 attack should tend to decrease the amount of the other isomer formed. Correspondingly, conditions which would be expected to produce a preliminary dissociation should favor rearrangement. In the extreme case, when the reaction proceeds exclusively by ionization into a carbonium ion, the same proportion of products should be obtained regardless of whether compounds of type I or II were employed as starting materials. These consequences have been very nicely verified for the crotyl-methylvinylcarbinyl system which has already been mentioned. In acetic anhydride or acetone (solvents 

With silver acetate in acetic acid, however, we should expect the route involving a carbonium ion to predominate (see p. 82), and we find, in fact, that the ratio of products is almost the same from either starting material  

An exception to these general principles which has been reported is the reaction of a- or 7-ethylallyl alcohol with thionyl chloride. The primary isomer gives principally the secondary chloride, and from the secondary isomer the primary chloride predominates.4 It has been pointed out, however,5 that these results may be explained by an Sni type reaction of the intermediate chlorosulfinic ester in which a six-membered ring is involved. Thus for the transformation of a-ethylallyl alcohol (VIII) to 7-ethallyl chloride (IX), we have  

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