1. Dienes
Systems Conjugated

2. - Dienes are simply alkenes that contain two carbon-carbon double bonds.
Types of Dienes
Dienes are divided into two important classes according to the arrangement of the double bonds
Conjugated:
Double bonds that alternate with single bonds are said to be conjugated
Most stable

3. contains cumulated double bonds
: -Isolated
double bonds that are separated by more than one single bond are said to be isolated
Less stable than conjugated
-Cumulated :
contains cumulated double bonds
Least stable .

4. Dienes are named by the IUPAC system in the same way as alkenes , except that the ending –diene is used, with two numbers to indicate the position of the two double bonds.
This system is easily extended to compounds containing any number of double bonds.
1,3-butadiene
1,2-propadiene
1,4-pentadiene

5. Preparation of Dienes
Dienes are usually prepared by adaptations of the methods used to make simple alkenes. For example, the most important diene, 1,3-butadiene (used to make synthetic rubber), has been made by cracking process,

6. dehydration of an alcohol containing two –OH groups:
Conjugated dienes differ from simple alkenes in three ways: (a) they are more stable,
(b) they undergo 1,4-addition,
(c) toward free radical addition, they are more reactive.

7. Vinylic versus Allylic
carbon
vinylic carbons 4

8. Vinylic hydrogen's are attached to vinylic carbons.
Vinylic versus Allylic H C C H C H
Vinylic hydrogen's are attached to vinylic carbons. 4

9. Vinylic versus AllylicH
Allylic hydrogen's are attached to allylic carbons. 4

10. Vinylic versus AllylicX C C X C X
Vinylic substituents are attached to vinylic carbons. 4

11. Vinylic versus AllylicX X C C X C
Allylic substituents are attached to allylic carbons. 4

12. Conjugation, Resonance and Dienes
Drawing resonance structures for the allyl carbocation is a way to see how to use Lewis structures to illustrate how conjugation delocalizes electrons.
The true allyl cation is a hybrid of the two resonance forms.
In the hybrid, the positive charge is delocalized over the two terminal carbons.
Delocalizing electron density lowers the energy of the hybrid, thus stabilizing the allyl carbocation and making it more stable than a normal 10 carbocation.

13. Conjugation, Resonance and Dienes
Experimental data show that the stability of the allyl cation is comparable to a more highly substituted 20 carbocation.

14. Electrophilic Addition to dienes, 1,4-Addition
When 1,4-Pentadiene is treated with bromine under conditions that favor formation of the dihalide ,there is obtained the expected product, 4,5-dibromo-1-pentene. Addition of more bromine yield the 1,2,4,5-tetrabromopentane.

15. electrophilic addition of one equivalent of HBr to an isolated diene yields one product and Markovnikov’s rule is followed.

16. Stability of Conjugated Dienes
Conjugation, Resonance and Dienes
Stability of Conjugated Dienes
When hydrogenation gives the same alkane from two dienes, the more stable diene has the smaller heat of hydrogenation.

17. When 1,3-butadiene is treated with bromine under similar conditions, there is obtained not only the expected 3,4-dibromo-1-butene, but also 1,4-dibromo-2-butene. Treatment with HCl yields not only 3-chloro-1-butene, but also 1-chloro-2-butene. Hydrogenation yields not only 1-butene but also 2-butene.

18. Mechanism

19. Carbocations from Conjugated Dienes
-Addition of H+ leads to delocalized secondary allylic carbocation

20. Products of Addition to Delocalized Carbocation
Nucleophile can add to either cationic site
The transition states for the two possible products are not equal in energy

22. Example: suggest a mechanism

24. Diels-Alder Reaction
Reaction Type: Conjugate addition/electrophilic addition.
Overall Reaction: Diene + Dienophile (alkene)  Cyclic Alkene.

25. h e a t diene dienophile diene alkene diene alkyne diene dienophile
Diels-Alder Reaction
Simple Diels Alder Examples:
1,3-butadiene + ethene  cyclohexene h e a t
diene dienophile
diene alkene
1,3-butadiene + ethyne  1,4-cyclohexadiene
diene alkyne h e a t
diene dienophile 6

26. Conjugated dienes are more stable (~3/4 Kcal/mole) than predicted
Conjugated dienes are more stable (~3/4 Kcal/mole) than predicted. (Isolated dienes are as expected.)
Conjugated dienes are the preferred products of eliminations:
CH3CH2CHCH2CH=CH KOH(alc)  Br
CH3CH2CH=CHCH=CH2
only CH3CH=CHCH2CH=CH2
none!

27. isolated dienes : (as expected) 1,5-hexadiene
CH2=CHCH2CH2CH=CH H2, Ni  CH3CH2CH2CH2CH=CH2
CH2=CHCH2CH2CH=CH H2, Ni  CH3CH2CH2CH2CH2CH3
CH2=CHCH2CH2CH=CH Br2  CH2CHCH2CH2CH=CH2
Br Br
CH2=CHCH2CH2CH=CH HBr  CH3CHCH2CH2CH=CH2 Br
CH2=CHCH2CH2CH=CH HBr  CH3CHCH2CH2CHCH3
Br Br

28. conjugated dienes yield 1,2- plus 1,4-addition:
CH2=CHCH=CH H2, Ni  CH3CH2CH=CH2 + CH3CH=CHCH3
CH2=CHCH=CH H2, Ni  CH3CH2CH2CH3
CH2=CHCH=CH Br2  CH2CHCH=CH2 + CH2CH=CHCH2
Br Br Br Br
CH2=CHCH=CH HBr  CH3CHCH=CH2 + CH3CH=CHCH2
Br Br
peroxides
CH2=CHCH=CH HBr  CH2CH=CHCH CH2CH2CH=CH2
Br Br

29. which of the following statements are true and which are false
1- Allylic carbocation is more stable than secondary carbocation.F
2- isolated diene is more stable than cumlated diene.T
3- The transition states for the 1,2-addition are less energy than 1,4-addition.T
4- Diels-Alder Reaction produce cyclic alkane.F
5- Dehydrobromination of 4-bromo-1-hexene by alcoholic potassium hydroxide produce 1,4-hexadiene.F