Conjugated dienes

BP-202T, PHARMACEUTICAL ORGANIC CHEMISTRY, CONJUGATED DIENES, DIEL'S ALDER REACTION, FREE RADICAL REACTION, CLASSIFICATION OF CONJUGATED DIENES, ALLYLIC REARRANGEMENT REACTION, STABILITIES OF DIENES: DELOCALISATION OF CHARGE, RESONANCE EXPLANATION, MOLECULAR ORBITAL EXPLANATION, PREPARATION OF DIENES, CHEMICAL REACTION OF DIENES, BIRCH REDUCTION

1. Dienes sushma somkuwar associate professor school of pharmacy, lnct university, bhopal

2. Dienes :- In organic chemistry, dienes is a hydrocarbon that contains two carbon pi bonds (double bonds). Conjugated dienes are functional groups, with a general formula of C n H 2n-2 . Dienes and alkynes are functional isomers. Dienes occur occasionally in nature but are widely used in the polymer industry.

3. Classification of dienes :- Dienes can be divided into three classes, depending on the relative location of the double bonds: 1. Cumulated dienes / Allenes or cumulenes :   have the double bonds sharing a common atom as in a group of compounds called  allenes . Eg : penta-1,2-diene 2. Conjugated dienes : have conjugated double bonds separated by one single bond. Eg : penta-1,3-diene

4. continued …. 3. Unconjugated /Non-conjugated/isolated dienes : have the double bonds separated by two or more than two single bonds. They are usually less stable than isomeric conjugated dienes . Eg : penta-1,4-diene - Compounds that contain more than two double bonds are called  polyenes . Polyenes and dienes , share many of their properties.

5. Stabilities of dienes : Delocalisation of charge: Conjugated dienes are more stable than non-conjugated dienes (both isolated and cumulated) due to factors like delocalization of charge through resonance and hybridization energy. These stability can be seen in the differences in the hydrogenation between isolated and conjugated dienes of alkenes. Since the higher the heat of hydrogenation the less stable the compound it is shown below the conjugated dienes (~54 kcal) lower heat of hydrogenation than their isolated (~60 kcal) and cumulated dienes (~ 70 kcal) counterparts.

6. Continued…. Here is an energy diagram comparing different types of bonds with their heats of hydrogenation to show relative stability of each molecule.

7. Stabilities of conjugated dienes : The resonance explanation Conjugated double bonds are separated by single bond 1,3-dienes are an excellent example of conjugated system. Each carbon in 1,3-dienes are sp2 hybridized and therefore has one p-orbital. The four p- orbital in 1,3-dienes overlap to form a conjugated system. The resonance structure shown below gives a good understanding of how the pi electrons are delocalized across the four carbon in this conjugated diene . This delocalization of electrons stabilizes the conjugated diene .

8. Stabilities of conjugated dienes : The molecular orbital explanation: The molecular orbital for 1,3-butadiene is shown below. Note that the lobe of four p orbital component in each pi-orbital are coloured and carry a plus and minus sign. This distinction refers to different phases defined by a mathematical wave equations for such orbital. Regions in which adjacent orbital lobes undergoes a phase change are called nodes. Orbital electron density is zero in such region. Thus a single p-orbital has a node at the nucleus and all pi orbital shown has a nodal plane that is defined by the atoms of the dienes . These is the only nodal surface in the lowest energy pi orbital, n 1 . Higher energy pi orbital have an increasing number of nodes. Since 1,3-butadiene has four pi electrons. The two bonding molecular orbital are filled to explain the measurable stability of conjugated double bonds.

9. Continued….

10. Preparation of dienes :- 1. Retro- diels Alder reaction ( pyrolysis of cycloalkene ) : when cyclohexene vapours is passed over heated nichrome alloy then 1,3-Butadiene is produced.

11. 2. Catalytic hydrogenation : 1,3-Butadiene is prepared by catalytic hydrogenation of n-butane, 1-butene or 2-butene. Preparation of dienes : continued….

12. Preparation of dienes : continued…. 3. Dehydrogenation of butane-1,3-diol : acid catalysed dehydration of butane-1,3-diol gives 1,3-butadiene. 1,3-butadiene

13. Preparation of dienes : continued…. 4. Dehydrohalogenation of 1,3-dichlorobutanes : when 1,3-dichloro-3-methylbutane is reacted with alcoholic KOH then isoprene (2-methylbuta-1,3-diene) is formed.

14. Chemical reactions of dienes :- Some important chemical reactions of dienes are as follows: 1. Electrophilic addition to dienes : a) Addition of Halogens: When 1,3-Butadiene is treated with 1 mole of bromine it gives a mixture of 1,2 and 1,4- addition products. 1,2-addition product 1,4-addition product Br 2

15. Mechanism :- Step-I: Br + ( electrophile ) adds to the terminal carbon atom and forms stable 2˚ carbocation . Addition to second carbon does not occur because it gives less stable 1˚ carbocation . I II

16. Continued….. Step-II : In resonance hybrid positive charge is equally distributed between carbon atoms(I and II), the attack of bromine ion can occur on either of these.

17. Continued….. b) Addition of Halogen acids: Halogen acids (HI, HCl , HBr ) react with 1,3-Butadiene gives a mixture of 1,2 and 1,4-addition products. 1,4-addition product 1,2-addition product

18. Mechanism : Step-I Step-II I II

19. 2. Free radical addition reaction: Conjugated dienes undergo free radical addition reactions to give a mixture of 1,2 and 1,4- addition products. Peroxide

20. Mechanism : Step-I : Peroxide dissociates to give free radicals. C 6 H 5 CO-O-O-COC 6 H 5 C 6 H 5 CO-O Benzoyl peroxide C 6 H 5 · + CO 2 Phenyl radical C 6 H 5 · + Br-CCl 3 C 6 H 5 - Br + CCl 3 · Bromobenzene trichloromethyl radical

21. Continued…. Step-II : The CCl 3 radical add to one of the terminal carbon atoms since it yields the resonance stabilized allyl radical. CH 2 =CH-CH- CH 2 + CCl 3 · CH 2 =CH- ·CH-CH 2 - CCl 3 ·CH 2 -CH=CH-CH 2 - CCl 3 I II

22. Continued…. Step-III : The allyl radical formed is step-II then abstracts a bromine atom from BrCCl 3 to complete the addition and generation a new · CCl 3 radical which propagates the chain. If addition occurs at C 2 then 1,2-addition product and if addition occurs at C4, then 1,4-addition product is formed. Br CH 2 =CH- ·CH-CH 2 - CCl 3 + Br- CCl 3 CH 2 =CH-CH-CH 2 - CCl 3 + ·CCl 3 CH 2 =CH- ·CH-CH 2 - CCl 3 + Br- CCl 3 BrCH 2 -CH-CH=CH 2 - CCl 3 + ·CCl 3 1,2-addition product 1,4-addition product

23. 3. Birch reduction: 1,3-Butadiene are readily reduced to 1, 4-dihydro derivatives in absence of ethyl alcohol which donate proton.

24. 4. Diels alder reaction: It is also known as [4+2] cycloaddition reaction. It involves addition of a conjugated diene (4 π electron system) to a substituted alkene (2 π electron system) also known as dienophile to form a six memebered cyclic alkene . This reaction is particularly useful in synthetic organic chemistry as it is reliable method for preparation of six membered system. Dienophiles may be any acetylenic compound containing electron withdrawing groups or alpha, ß-unsaturated, ester, anhydrides, aldehydes , ketones and nitro compounds. Diels Alder reaction normally do not require any catalyst and occurs on heating.

25. Mechanism : Diels- Alder reaction is an example of concerted pericyclic reaction as it proceeds via a single, cyclic transition state with no intermediates generated during reaction. The reaction proceeds through the suprafacial interaction of a 4 π electron system with a 2 π electron system and this interaction is thermally allowed. Cyclic transition state

26. Allylic rearrangement : Free radical substitution in alkenes or at allylic positions lead to allylic rearrangement. For example, when 2-butene is reacted with NBS (n- bromo succinimide ) then, two products are expected, 3-bromo-2-butene and 3-bromo-1-butene. Major product, more stable with substituted double bond. But get rearranged product as well

27. Continued….

28. Continued…. An interesting competition is occurring consider the allylic radical