Conjugated dienes are a class of organic compounds which contain carbon-carbon double bond (C=C ) units joined by a single bond. These dienes are more stable than any other dienes. This is because of the presence of resonance.
The resonating structures of 1,3 butadiene are as follows:
Dienes are extensively used in organic synthesis. The conjugated dienes undergo many chemical reactions to produce useful products such as polymerization reactions.
For example, in 1,3-butadiene is used to produce EPDM (ethylene-propylene-diene rubber) rubber.
Nomenclature of conjugated Diene
The naming of the conjugated dienes is almost similar to the naming of alkenes.
The following steps can be used for this purpose:
- First of all, select the longest chain containing both carbons of the double bond.
- Give the lowest possible number to the double-bonded carbon atom.
- The suffix ‘diene’ is used at the end.
- At the start of the name, use two numbers to locate the position of double bonds.
Due to the wide application of conjugated dienes in industry, they are prepared at a large scale by using the methods given below:
By the elimination reaction
Conjugated dienes can be prepared by the elimination reaction of the unsaturated alcohol and alkyl halide. However, the conjugated diene produced more yield than the isolated diene. This is because dehydration of alcohols and dehydrohalogenation of alkyl halide is regioselective. This led to the formation of a more stable double bond. Hence, the conjugated diene is more stable than the isolated diene.
By the dehydrogenation of n-butane
Conjugated diene like 1,3-butadiene can also be prepared by dehydrogenation of the n-butane.
Properties of Conjugated dienes
The double bond in conjugation makes a system of delocalized pi-electron system. These arrangements make them more stable and assigned other specific properties.
Stability of conjugated diene
The stability of conjugated dienes can be determined by considering the number of pi electrons in the conjugated system. The additional pi electrons in conjugation increase the delocalization of electrons through resonance, which results in even increased stability.
It is of great importance that the stability of conjugated dienes can also be affected by the presence of substituents on the double bonds, such that an electron-withdrawing group decreases stability, and, electron-donating groups increase their stability. Dienes have relatively low thermal stability and tend to decompose at relatively low temperatures.
If we compare the stability of 1,4-pentadiene and 1,3-pentadiene. The heat of hydrogenation for the two double bonds in the 1,4-pentadiene is 252kJ/mol. Whereas, the heat of hydrogenation of 1,3-pentadiene is 111kJ/mol.
The carbon-carbon single bond length in conjugated dienes is relatively short than in alkanes. This is because in alkanes all carbons are sp3 hybridized whereas in conjugated dienes both double-bonded carbon atoms are sp2 hybridized. In fact, the single bond is not purely single as well as a double bond is not a complete double bond. The decrease in bond length is due to the resonance.
Conjugated dienes have unique optical properties due to the delocalization of the pi electrons. They tend to absorb light in the visible region of the electromagnetic spectrum, which may result in various complementary colors. In fact, this is the basis of azo dyes and pigments.
Conjugated dienes are reactive towards electrophiles to give addition reaction. Their reactivity is due to the formation of stable allylic carbonation intermediate during the reaction. Therefore, they give diels-alder’s reaction and polymerization.
Reactions of conjugated dienes
Addition of hydrogen halide
Conjugated dienes undergo an addition reaction like alkenes. For example, when hydrogen chloride is added to the diene unit of the 1,3-cyclopentadiene. It will give 3-chlorocyclopentene. The mechanism of this is analogous to the electrophilic addition of hydrogen chloride HCl to the alkene.
Addition of halogens
When conjugated dienes are treated with bromine or chlorine it gives the mixture of 1,2 and 1,4 additional products. This reaction takes place in the presence of chloroform (CHCl3).
The Diels-Alder reaction is the conjugated addition of alkene to a diene. The alkene which adds is known as a dienophile. However, the reaction is known as a cycloaddition reaction. The product formed contains a cyclohexene ring.
1,3-butadiene polymerize to form rubber which is used in tires.
How can we prepare cycloalkane from conjugated dienes?
Diels-alder reaction can be used for the preparation of cycloalkenes from resonating dienes.
Why are conjugated dienes more reactive than alkenes and alkynes towards electrophilic addition reaction?
when conjugated dienes give an electrophilic addition reaction they form allylic carbonation as an intermediate. Since allylic carbonation is more stable due to resonance.