Dienes are unsaturated hydrocarbons containing two carbon-carbon double bonds per molecule. Their general formula is C_nH_2n-2. Dienes and alkynes are functional isomers.

Classification of dienes

Depending upon the relative positions of the two double bonds dienes are classified as:

Isolated dienes

In these dienes the double bonds are separated by more than one single bond and are called isolated dienes. For example:

H₂C = CH – CH₂ – CH = CH₂

           1, 4-pentadiene

Cumulated dienes

When the double bonds are present between successive carbon atoms such dienes are called cumulated dienes or allenes. For example:

      H₂C = C = CH₂

1, 2-Propadiene or Allene

Conjugated dienes

When the double bonds are separated by a single bond, dienes are termed as conjugated dienes. For example:

CH₂ = CH — CH = CH₂

      1, 3-Butadine

Relative stabilities of dienes

A conjugated diene is more stable as compared with non-conjugated dienes. The relative order of stability of dienes is:Conjugated diene > Isolated diene > Cumulated diene

The evidence in favour of maximum stability of conjugated dienes can be obtained from the values of heats of hydrogenation

CH₂ = CH — CH = CH — CH₃ + 2H₂ → CH₃ — (CH₂)₃ — CH₃              Δ H = -226 KJ

                1, 3-Pentadiene

CH₂ = CH — CH₂ — CH = CH₂ + 2H₂ → CH₃ — (CH₂)₃ — CH,            Δ H = -254 KJ

                1, 4-Pentadiene

As more heat is given out during hydrogenation of 1,4-pentadiene (an isolated diene) as compared with 1,3-pentadiene (a conjugated diene), it indicates that 1,4-pentadiene has more energy than 1,3-pentadiene and hence is less stable than it.