Mr. Sayad Imran Asst. Professor YBCCPA Conjugated Dienes Mr. Sayad Imran Asst. Professor YBCCPA http://sayadimranpoc.blogspot.in/

Dienes The compound that contain two C=C bond is called Dienes. Depending upon the relative position of the double bonds the dines are classified into three type. Conjugated dienes is one in which the double bonds are separated by one single bond. Unconjugated Dienes is one in which the double bonds are separated by two or more single bonds.   c. Cumulated Dienes are two double bond connected to a similar atom.

Which of these molecules contain conjugated double bonds? Solution Compounds (b) and (c) contain conjugated double bonds. The double bonds in compounds (a) and (d) are unconjugated.

Stability of conjugated dienes Conjugated dienes are more stable than unconjugated and Cumulated dienes due to factors such as delocalization of electron through resonance. The electrons are delocalized across the four carbons in this conjugated diene. This delocalization of electrons stabilizes the conjugated diene:

Along with resonance, hybridization energy effect the stability of the compound. For example in 1,3-butadiene the C2 and C3 with the single bond are sp2 hybridized unlike in nonconjugated dienes where the carbons with single bonds are sp3 hybridized. This difference in hybridization shows that the conjugated dienes have more % of 's' character and draw in more of the pi electrons, thus making the single bond stronger and shorter than an ordinary alkane C-C single bond Bond length α 1 Bond Energy Bond Energy α Stability Thus C-C single bond between conjugated double bonds is shorter than a typical alkane C-C

Heats of Hydrogenation The higher the heat of hydrogenation the less stable the compound. Heat of hydrogenation 𝛼 1 𝑆𝑡𝑎𝑏𝑖𝑙𝑖𝑡𝑦 Here is an energy diagram comparing different types of bonds with their heats of hydrogenation to show relative stability of each molecule: Stability of alkene is directly proportional to number of α hydrogen atom i.e. Hyperconjugation.

Different conformations of Conjugated Dienes There are two different conformations of conjugated dienes which are s-cis and s-trans conformations. s-cis is when the double bonds are cis in reference to the single bond and s-trans is when the two double bonds are trans in reference to the single bond. The cis conformation is less stable due to the steric interation of hydrogens on carbon.

Preparation of Conjugated Dienes Dienes are prepared from the same reactions that form ordinary alkenes. Dehydration of diols The generation of either an isolated or conjugated system depends on the structure of the original reactants. Vicinal diols, which have two hydroxyl groups on adjacent carbon atoms, and vicinal dihalides, which have halogen substituents on adjacent carbons, always become conjugated systems in elimination reactions.

2. Dehydrohalogenation of dihalides

Dehydrogenation of butane Thermal cracking of butane using a chromium oxide/ aluminum oxide catalyst

4. Unsaturated alcohols and alkyl halides