The s-trans(single-trans)conformation is about 2.3kcal lower in energy than the s-cis conformation,which arises from the steric repulsions of the hydrogens. The barrier to rotation for these conformers is 4.9kcal This low energy difference means at room temperature these conformations are easily and rapidly interconverting. Allylic Cations Allylic cations are stabilized by resonance with the adjacent double bond,which delocalizes the positive charge over two carbon atoms. 入Br The delocalized cation can be represented by the two resonance structures or the combined structure. 1.2-and 1.4-Additions Allylic cations are often intermediates when there is electrophilic addition to conjugated dienes. Consider the case of electrophilic H-Br addition to 1.3-butadiene: Br Br H-Br 1,2-addition 1,4-addition 1,2-Addition and 1,4-addition refer to the relationship of the carbon atoms to which the H and Br are added. Ch15 Conjugated Systems (landscape) Page 9 Ch15 Conjugated Systems (landscape) Page 9 The s-trans (single-trans) conformation is about 2.3kcal lower in energy than the s-cis conformation, which arises from the steric repulsions of the hydrogens. The barrier to rotation for these conformers is 4.9kcal. This low energy difference means at room temperature these conformations are easily and rapidly interconverting. Allylic Cations Allylic cations are stabilized by resonance with the adjacent double bond, which delocalizes the positive charge over two carbon atoms. The delocalized cation can be represented by the two resonance structures or the combined structure. 1,2- and 1,4-Additions Allylic cations are often intermediates when there is electrophilic addition to conjugated dienes. Consider the case of electrophilic H-Br addition to 1,3-butadiene: 1,2-Addition and 1,4-addition refer to the relationship of the carbon atoms to which the H and Br are added. Br + -Br - + = 1 /2+ 1 /2+ H-Br Br Br 1,2-addition 1,4-addition