Chapter 10: Conjugation in Alkadienes and Allylic Systems

Bonding in Dienes

• Of the three subtypes of dienes, the 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 difference is the hybridisation of the C atoms involved (check the % s character) and the conjugation of the two π bonds.

C-C bond lengths hybridisations

• Two conformations of conjugated dienes are important, s-cis and s-trans, 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 of 1,3-butadiene 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 (i.e. those at the ends of the diene unit) in the s-cis conformation, see the JSMOL images. Remember that steric interactions are best visualised in the spacefilling models.

	s-trans	s-cis
	Toggle to/from spacefill	Toggle to/from spacefill Highlight steric interaction

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