Chapter 10: Conjugation in Alkadienes and Allylic Systems

Bonding in Dienes

• The bonding in isolated dienes is the same as that in alkenes.
• Conjugated and cumulated dienes have interesting structural features worthy of closer attention.

The C-C single bond between conjugated double bonds is shorter than a typical alkane C-C This is due to the hybridization of the C atoms involved and the conjugation of the two p bonds.
Two conformations of conjugated dienes are possible, as shown for 1,3-butadiene right. The "s" refers to the fact that we are looking at a rotation about a single bond. The s-trans conformation is about 12 kJ/mol (2.8 kcal/mol) more stable than the s-cis due to the unfavourable steric interaction of substituents at C1 and C4 in the s-cis conformation, see the CHIME images.
Show steric interaction in s-cis conformation Reset

Cumulated Dienes

• Like the triple bond unit of an alkyne, the C=C=C unit of allenes are linear.
• Note the central sp hybridised C atom.
• The C=C bonds in allenes are slightly shorter (131 pm) than those in a typical alkene C=C (134 pm)
• Allenes are non-planar. Note the perpendicular nature of the C-H bonds in the CHIME image.