JEE Main & Advanced Chemistry Hydrocarbons / हाइड्रोकार्बन Dienes

These are hydrocarbon with two carbon-carbon double bonds. Dienes are of three types

(1) Conjugated dienes : Double bonds are seperated by one single bond.

Ex : \[C{{H}_{2}}=CH-CH=C{{H}_{2}}\] (1, 3-butadiene)

(2) Cumulative dienes : Double bonds are adjacent to each other.

Ex : \[C{{H}_{2}}=C=C{{H}_{2}}\] Propadiene [allene]

(3) Isolated or Non-conjugated : Double bonds are separated by more than one single bond.

Ex : \[C{{H}_{2}}=CH-C{{H}_{2}}-CH=C{{H}_{2}}\] (1, 4 pentadiene)

The general formula is \[{{C}_{n}}{{H}_{2n-2}}\].  The predominant member of this class is 1, 3-butadiene.

(1) Method of preparation

(i) From acetylene :

\[2HC\equiv CH\underset{N{{H}_{4}}Cl}{\mathop{\xrightarrow{C{{u}_{2}}C{{l}_{2}}}}}\,\underset{\text{Vinyl acetylene}}{\mathop{HC\equiv C-CH=C{{H}_{2}}}}\,\underset{Pd/BaS{{O}_{4}}}{\mathop{\xrightarrow{{{H}_{2}}}}}\,\]   \[\underset{\text{1, 3-Butadiene}}{\mathop{C{{H}_{2}}=CH-CH=C{{H}_{2}}}}\,\]

(ii) From 1, 4-dichlorobutane :

\[\underset{\text{1,4-Dichlorobutane}}{\mathop{\overset{Cl\,\,\,}{\mathop{\overset{|\,\,\,\,\,}{\mathop{C{{H}_{2}}}}\,}}\,C{{H}_{2}}C{{H}_{2}}\overset{Cl\,\,\,}{\mathop{\overset{|\,\,\,\,\,}{\mathop{C{{H}_{2}}}}\,}}\,}}\,\xrightarrow{Alc.\,KOH}\underset{\text{1, 3-Butadiene}}{\mathop{C{{H}_{2}}=CH-CH=C{{H}_{2}}}}\,\]

(iii) From 1,4-butanediol :

\[\underset{\text{1, 4-Butanediol}}{\mathop{\overset{OH}{\mathop{\overset{|\,\,\,\,\,}{\mathop{C{{H}_{2}}}}\,}}\,C{{H}_{2}}C{{H}_{2}}\overset{OH}{\mathop{\overset{|\,\,\,\,\,}{\mathop{C{{H}_{2}}}}\,}}\,}}\,\underset{\text{heat}}{\mathop{\xrightarrow{{{H}_{2}}S{{O}_{4}}}}}\,\underset{\text{1, 3-Butadiene}}{\mathop{C{{H}_{2}}=CH-CH=C{{H}_{2}}}}\,\]

(iv) From butane :

\[\underset{\text{n-Butane}}{\mathop{C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}C{{H}_{3}}}}\,\underset{{{600}^{o}}C}{\mathop{\xrightarrow{\text{Catalyst}}}}\,\underset{\text{1, 3-Butadiene}}{\mathop{C{{H}_{2}}=CH-CH=C{{H}_{2}}}}\,\]

(\[C{{r}_{2}}{{O}_{3}}\] used as catalyst.)

(v) From cyclohexene :

(2) Physical property : 1,3-butadiene is a gas.

(3) Chemical properties

(i) Addition of halogens :

(ii) Addition of halogen acids :

(iii) Addition of water :

(iv) Polymerisation :

\[\underset{\text{1, 3-Butadiene}}{\mathop{nC{{H}_{2}}=CHCH=C{{H}_{2}}}}\,\xrightarrow{\text{Peroxide}}{{[-\underset{\text{Buna rubber}}{\mathop{C{{H}_{2}}CH=CHC{{H}_{2}}}}\,-]}_{n}}\]

Diels-alder reaction :

Stability of conjugated dienes : It is explained on the basis of delocalisation of electron cloud between carbon atoms.

The four \[\pi \] electrons of 1, 3-butadiene are delocalised over all the four atoms. This delocalisation of the \[\pi \] electrons makes the molecule more stable.

(v) Ozonolysis :

\[C{{H}_{2}}=CHCH=C{{H}_{2}}+2{{O}_{3}}\xrightarrow{Zn/{{H}_{2}}O}2HCHO+OHCCHO\]