Reactions at Saturated and Unsaturated Carbons

Keywords SN2 and SN2 reactions Baldwin mles Oxirane opening Activation of a cyclopropane 1,2-rearrangement Ring contraction Ring expansion Solvolysis Neighboring group participation Oxirane rearrangement Classical and nonclassical carbocations 5-exo- vis-a-vis 5-endo cyclization Addition and elimination reactions 

Steric and Stereoelectronic Effects in Organic Chemistry, 

The evidence as to the fact that the nucleophile is indeed aligned in a collinear arrangement to the leaving group is provided from the transformation 4 5, which 

We shall now turn to the internal SN2 reaction on a sp3 carbon. Purely on account of geometrical constraints imposed in achieving the collinear alignment, 

While all the reactions from 3-exo-tet to 7-exo-tet are favored, the 5-endo-tet and 6-endo-tet reactions are disfavored on stereoelectronic grounds, i.e., on account of the collinear requirement of SN2 reactions. The relative ease of these processes is 3-exo-tet 4-exo-tet 5-exo-tet 6-exo-tet 7-exo-tet. It is important to recognize that the intramolecular SN2 process proceeding through the transition structure resembling 6 constitutes a 6-endo-tet process and, hence, unfavorable. 

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According to equation (2), A log. K will have the same sign (i. e. the same nucleophilic order will prevail) except for large values of a of equation 3 (a) (very low values of x) i. e. for very hard acids. This is shown by the values of koH-/kF- for reactions at saturated and unsaturated carbon atoms (71), and by the high value of this ratio for the combination of OH-and F- ions with a proton. 

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