Physical and Bond Properties

Isolated dienes do not show any properties different from those of plain alkenes. However, cumulated and conjugated dienes do.

Types of dienes: isolated (CH₂=CHCH₂CH=CH₂), conjugated (CH₂=CHCH=CH₂) and cumulated, or allene (CH₂=C=CH₂)

Cumulated dienes (allenes)

In order to explain the structure of an allene (cumulated diene), the central carbon must be considered as sp hybridized, NOT sp².

Structure of an allene: sp2 terminal carbons and sp central carbon, with the two π bonds perpendicular; 1.31 Å C=C bonds

The higher s character of the hybrid sp orbitals of the central carbon makes the double C=C bonds shorter than those of plain alkenes.

Axial chirality of 2,3-pentadiene: object and mirror image are not superimposable — 2,3-Pentadiene has two enantiomeric structures. Each of them displays optical rotation.

Allenes can be chiral.

Although allenes do not strictly possess any stereocenter, those with different substituents at both ends of the cumulated C=C double bonds happen to bear a chiral axis that makes object and mirror image be non-superimposable.

In order to assign the R/S configuration to a chiral allene, the four substituents should be appropriately numbered. Start with the carbon in the front and then with that in the back of the allene following the usual priority rules. Then draw a tetrahedron as it is shown in the Figure, place the least important substituent (4) to the back and look at the clockwise (R) or anti-clockwise (S) turn necessary to go from 1 to 2 and 3.

Assignment of the S configuration of an allene: the substituents of the front carbon are numbered first and a tetrahedron is drawn

Conjugated dienes

The central bond of a conjugated diene has a partial double-bond character.

Conjugated diene: the weak overlap between the central carbons shortens the central single bond to 1.47 Å

The length of the central bond is intermediate between true single C–C and double C=C bonds.

π molecular orbitals of 1,3-butadiene: π1 and π2 are occupied; π2 is the HOMO and π3 the LUMO

The lateral overlap of the four atomic p orbitals produces four molecular π orbitals. The π₁ orbital delocalizes some electronic density along the four atoms involved in the conjugation. However, the π₂ orbital doesn't. This explains the partial double bond character of the central bond.

The small contribution of the charge-separated resonance forms also explains the partial double bond nature of the central bond.

Charge-separated resonance forms of 1,3-butadiene

The twisting of the central bond of a conjugated diene displays different features to those of a true single C-C bond of alkanes:

In order to make the p orbitals overlap in the diene framework, the whole system has to be planar. Yet, the central bond could be twisted giving rise to two conformers named as s-trans (more stable) and s-cis.

Energy diagram of the rotation of the central bond of butadiene: the s-cis conformer lies 2.8 kcal/mol above the s-trans and the barrier is 6.7 kcal/mol

This bond twisting is more costly (6.7 kcal/mol) than in an alkane, due to the fact that in the TS the π₁ orbital overlap is destroyed because the planes of the double C=C bonds are perpendicular.

Molecular model of 1,3-butadiene — Blue: transition state (TS)
Red: steric hindrance in the *s-cis* conformation.