Suprafacial sigma bonds

The problem may be restated in terms of the symmetry of all of the orbitals involved. In the transition state the pair of electrons forming the sigma bond to hydrogen may now be considered part of the n system the hydrogen will move suprafacially or antarafacially depending on whether the HOMO is a or b. For (4n + 2)77- electrons a thermal change is allowed, and for (471)77- electrons a first-excited-state process is allowed. 

SHMO, 153 Substituent types, 99 see C substituents see X substituents see Z substituents Sudden polarization, 272 a-Sulfonyl carbanion, 277-278 Sulfuric acid, 100 Suprafacial, 163 examples, 164 sigma bonds, 167... 

For conrotatory ring opening of cyclobutene there is retention at C3 and inversion at C4. Sigma bond thus participates in antarafacial manner. Addition across rt-bond is then suprafacial and this pathway is classified as o s+Ji s t e... 

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