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Antarafacial and suprafacial are two topological concepts in organic chemistry describing the relationship between two simultaneous chemical bond making and/or bond breaking processes in or around a reaction center.〔IUPAC Gold Book definition (Link )〕 The reaction center can be a p-orbital, a conjugated system or even a sigma bond. * This relationship is ''antarafacial'' when opposite faces are involved (think ''anti''). * It is ''suprafacial'' when both changes occur at the same face. Many sigmatropic reactions and cycloadditions can be either suprafacial or antarafacial, and this determines the stereochemistry. An example is the ()-hydride shift, in which the interacting frontier orbitals are the allyl free radical and the hydrogen 1s orbitals. The suprafacial shift is symmetry-forbidden because orbitals with opposite algebraic signs overlap. The symmetry allowed antarafacial shift would require a strained transition state and is also unlikely. In contrast a symmetry allowed and suprafacial ()-hydride shift is a common event.〔F.A. Carey, R.J. Sundberg, ''Advanced Organic Chemistry'' Part A ISBN 0-306-41087-7〕 : ==References== 〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Antarafacial and suprafacial」の詳細全文を読む スポンサード リンク
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