Torsional Strain and Related Stereoelectronic Effects

Torsional strain refers to the component of total molecular energy which results from nonoptimal arrangement of vicinal bonds. Ethane in the eclipsed conformation is strained by 2.8kcal/mol relative to the staggered conformation and this is an example of torsional strain. This energy barrier is not primarily the result of van der Waals interactions but arises instead from stereoelectronic effects. The basis for the preference for the staggered conformation was considered in Sections 1.7 and 3.2. 

For example, cyclohexanone is reduced by sodium borohydride 23 times faster than cyclopentanone. The explanation for this difference is believed to lie in the 

Torsional effects can also influence the stereochemistry of reactions. One system which has been examined particularly closely with regard to torsional effects are additions to cyclohexanones. There are two possible directions for nucleophilic attack on cyclohexanones, from the axial and equatorial directions. From many 

SECTION 3.10. TORSIONAL STRAIN AND RELATED STEREOELECTRONIC EFFECTS 

There is another stereoelectronic explanation for the preferred axial approach by small unencumbered nucleophiles. The tt orbitals of the carbonyl group are aligned to interact with the bond between C-2 and C-3 and C-6 and C-5. The most stabilizing interactions are between the cr orbitals and the tt orbital and between the TT orbital and cr, since these interactions each involve one filled and one unfilled level. An important feature of these interactions is that they will distort the tt and TT orbitals. In particular the tt orbital will be distorted such that it will have its largest density on the side of the carbon which maximizes overlap with the filled a orbitals. This orbital is the LUMO for the carbonyl group and it would therefore be expected that nucleophiles would attack preferentially from the axial direction. In contrast, the occupied tt orbital, the HOMO, will have increased concentration on the equatorial side to maximize interaction with the cr orbitals. 

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