Chapter 10: Conjugation in Alkadienes and Allylic Systems

Bonding in Dienes

• Conjugated and cumulated dienes have interesting structural features worthy of closer attention.

Isolated Dienes

• The bonding in isolated dienes is the same as that in alkenes.

Conjugated Dienes

• The C-C single bond between conjugated double bonds is shorter than a typical alkane C-C
• This is due to the hybridisation of the C atoms involved and the conjugation of the two π bonds.

C-C bond lengths hybridisations

• Two conformations of conjugated dienes are possible, as shown for 1,3-butadiene below.
• The "s" refers to the fact that we are looking at a rotation about a σ 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 JMOL images.

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 JMOL image.

© Dr. Ian Hunt, Department of Chemistry, University of Calgary