Dienes or Diolefins or Alkadienes

Hydrocarbons having two double bonds ae known as alkadienes. Alkadienes are classified into three categories on the basis of location of two double bonds.

(i) Cumulative dienes: Two double bonds are on adjacent carbon atoms e.g.

                                    CH₂ = C = CH₂ allene or 1, 2-propadiene

(ii)Conjugated dienes: Molecules having alternate single & double bonds e.g.

                                    CH₂ = CH-CH = CH₂ buta-1, 3-diene

                                    CH₂ = C-CH = CH₂ 2-methylbuta-1, 3-diene or isoprene

                                                 |

                                                CH₃

(iii)Isolated diene : Molecules having two double bonds separated by more than one single bond.

                        CH₂=CH-CH₂-CH=CH₂                     1, 4-pentadiene

                        CH=CH-CH₂-CH₂-CH=CH₂                1, 5-hexadiene

Among the three types of dienes, conjugated alkadienes have some characteristic nature and undergo addition reactions in a peculiar manner. The simplest conjugated alkadiene is buta-1, 3-diene. It has following note worthy features.

(i)All carbon atoms in CH₂=CH-CH=CH₂ (buta-1, 3-diene) are sp² hybridized

(ii)The delocalization of Π electrons results in resonance in molecule to show extra stable nature than corresponding non conjugated alkadienes (Resonance energy of buta-1, 3-diene is 3 kcal mol^-1)

            CH₂=CH-CH=CH₂ ↔ CH₂⁺ -CH=CH-CH₂ ↔ CH₂-CH=CH-CH₂

(iii)The addition of H₂, Br₂ or HBr ........ etc on conjugated alkadienes takes place in two ways either 1, 2 or 1, 4-addition.

              

 

(iv)Non ionizing solvent favours 1, 2-addition whereas ionizing solvent favours 1, 4-addition. However in each case mixture of both type of addition products are formed, the one predominating on the other as the case may be.

(v)It undergoes polymerization in presence of peroxides to give polybutadiene (Buna Rubber).

                        nCH₂=CH-CH=CH₂     (CH₂-CH=CH-CH₂)_n

                                                                         ^{Buna rubber}