Conjugated dienes

Conjugated dienes

• 1.
  B-Pharm Students Presented By-Sandhya Punetha
• 2.
   Conjugated dienesare carbon structures which maintain 2 double bond separated by a single bond.  The C-C single bond between conjugated double bonds is shorter than a typical alkane C-C.  This is due to the difference in the hydridisation of the C atom involved and the conjugation of the 2 pi bonds.
• 3.
   Two conformationsof conjugated dines are important – s-cis and s-trans. Eg- 1,3- butadiene  The s refers rotation about a sigma bond.
• 4.
   Conjugated dienes-Double bonds that alternate with single bonds. Eg- CH3-CH=CH-CH3  Isolated dienes- Double bond that are separated by more than one single bond Eg- CH2=CH-CH2-CH2-CH3  Allenes- Contains cumulated double bond. Eg- -C=C=C-
• 5.
   Conjugated dienesare more stable than non conjugated dienes due to factors such as delocalisation of charge through resonance and hybridisation energy.  This can also explain why allylic radicals are much more stable than secondary or even tertiary carbocations.  This is all due to the positioning the pi orbitals and ability for overlap to occur to strengthen the single bond between the 2 double bond.
• 6.
   The resonancestructure shown below gives a good understanding of how the charge is delocalized across the 4 carbons in this conjugated diene.  This delocalization of charges stabilizes the conjugated diene-
• 7.
   Along withresonance, hybridisation energy effect the stability of the compound.  Eg- 1,3- Butadiene the carbon with the single bond are sp2 hybridised unlike in non-conjugated dienes where the carbons with single bonds are sp3 hybridised.  This difference in hybridisation shows that the conjugated dienes have more s character and draw in more of the pi electrons, thus making the single bond stronger and shorter than an ordinary alkane C-C bond.
• 9.
   Another usefulresource to consider are the heat of hydrogenation of different arrangements of double bonds. Since, the higher the heat of hydrogenation the less stable the compound, it is shown below that conjugated dienes ( 54kcal) have a lower heat of halogenation than their isolated ( 60kcal) and cumulated diene (70kcal) counterparts.
• 10.
   Here isan energy diagram comparing different types of bonds with their heats of hydrogenation to show relative stability of each molecule.