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Chemistry LibreTexts

s-cis Conformation

[ "article:topic", "authorname:ggunawardena" ]
  • Page ID
    42804
  • Conjugated dienes are resonance stabilized.

    eg: 1,3-butadiene (1)

     

     

    stability: 1a >> 1b = 1c
    contribution to the hybrid: 1a >> 1b = 1c

     

     

    Since 1a contributes significantly more to the hybrid than 1b or 1c, the internal pi bond in 1 is considerably weaker than the terminal ones.

     

     

    It is weak enough to allow free rotation, due to which 1 exists in two limiting conformations.  The two double bonds are on the same side of the single bond in the Lewis structure of one of them, which is called the s-cis conformation (s-cis: cis with respect to the single bond); they are on the opposite sides of the single bond in the Lewis structure of the other, which is called the s-trans conformation (s-trans: trans with respect to the single bond).

     

     

    Some conjugated dienes are locked either in the s-cis conformation (2) or in the s-trans conformation (3).