Specificity of Intramolecular Hydrogen Abstraction

The most direct evidence that stereoelectronic effects are also important in these reactions follows from the specificity observed in hydrogen atom abstraction from confoiTnatioually constrained compounds. C-H bonds adjacent to oxygen or nitrogen and which subtend a small dihedral angle with a lone pair orbital ( 30°) are considerably activated in relation to those where the dihedral angle is or approaches 90 . fhus, the equatorial H in 20 is reported to be 12 times more reactive towards /-butoxy radicals than the axial 11 in 21. 

A further example of the importance of this type of stcrcoclcctronic effect is seen in the reactions of /-butoxy radicals with spiro[2,n]alkanes (22) where it is found that hydrogens from the position a- to the cyclopropyl ring arc specifically abstracted. 1 his can be attributed to the favorable overlap of the breaking C-H bond with the cyclopropyl a bonds.No such specificity is seen with bicyclo[n, 1,0]alkanes (23) where geometric constraints prevent overlap. 

Even though dissociation energies for X-H bonds appear insensitive to solvent changes,the nature of the reaction medium and the reaction 

It has been proposed that aromatic solvents, carbon disulfide, and sulfur dioxide forni a complex with atomic chlorine and that this substantially modifies both its overall reactivity and the specificity of its reactions.For example, in reactions of Cl with aliphatic hydrocarbons, there is a dramatic increase in the specificity for abstraction of tertiary or. secondary over primary hydrogens in benzene as opposed to aliphatic solvents. At the same time, the overall rate constant for abstraction is reduced by up to two orders of magnitude in the aromatic solvent. The exact nature of the complex responsible for this effect, whether a n -complex (24) or a chlorocyclohexadienyl radical (25), is not yet resolved. -  

Significant, though smaller, solvent effects have also been reported for alkoxy radical reactions (Section 3.4.2.J).  

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