1. Compounds that have a p orbital on an atom adjacent to a double bond
Conjugated systems
Compounds that have a p orbital on an atom adjacent to a double bond

2. Ionic addition
However, we have seen that X2 reacts with alkanes, by a free radical mechanism, to form substitution products:
Perhaps we can brominate at the methyl position of propene.....

3. Free radical substitution
We must use conditions which favor free radical substitution reactions and are not favorable to ionic addition:

4. Free radical substitution v ionic addition

5. N-bromosuccinimide
N-bromosuccinimide (NBS) is used for the specific purpose of brominating alkenes at the allylic position.

6. N-bromosuccinimide
How does it work? NBS provides a low concentration of Br2 which is produced by reaction between HBr and NBS: CH
=CHCH + Br CH
=CHCH
+ HBr 2 3 2 2

7. Orientation and reactivity
vinyl hydrogens undergo very little substitution.
allylic hydrogens are particularly reactive.
the order of ease of hydrogen abstraction is: allylic > 3o > 2o > 1o >CH4 > vinylic
How can we explain the stability of allylic radicals ?

8. Properties of allylic radicals
We will find the answer in the concept of resonance. Let us start by examining some of the properties of allylic radicals:
Allylic radicals can rearrange:

9. Properties of allylic radicals
We will find the answer in the concept of resonance. Let us start by examining some of the properties of allylic radicals:
Allylic radicals can rearrange:

10. Properties of allylic radicals
The propenyl radical is symmetric:

11. The theory of resonance
Whenever a molecule can be represented by 2 or more structures which differ only in the arrangement of their electrons, there is resonance:
The molecule is a hybrid of all the contributing structures and cannot be adequately represented by any one of these structures.

12. The theory of resonance

13. The theory of resonance
Resonance is important when these structures are of about the same stability. For example,
The hybrid is more stable than any of the contributing structures. This increase in stability is called the resonance energy.

14. The allyl radical - an example of resonance stabilization
There are two structures which contribute to the hybrid:
They are of the same energy and contribute equally to the hybrid.

15. Structure of the allyl (propenyl) radical
The radical has no double bond because the two C - C bonds must be identical if the two structures contribute equally.
The radical is therefore represented by:-

16. Structure of the allyl (propenyl) radical
The electron is delocalised and the molecule is symmetric.
The resonance energy is ~42 kJ/mol.
We can explain the allylic rearrangement.

17. Allylic rearrangement

18. Orbital representation

19. Dienes - structure and nomenclature
The position of each double bond is indicated using an appropriate number:
CH2=C=CH-CH ,2-butadiene
CH2=CH-CH2-CH=CH2 1,4-pentadiene

20. Diene classification 1,2-dienes - cumulated double bonds
CH2=C=CH2 - propadiene, allene
1,3-dienes - conjugated double bonds
Isolated double bonds
CH2=CH-CH2-CH=CH2 - 1,4-pentadiene

21. Stability of conjugated dienes
The heat of hydrogenation of conjugated dienes is lower than that of other dienes. Why?
Bond lengths: C2-C3 = 1.48Å
H3C-CH3= 1.54Å

22. Electrophilic addition reactions of dienes
+ CH2Br-CHBr-CH2-CHBr-CH2Br
This is typical behavior for dienes having isolated double bonds.

23. Addition reactions of conjugated dienes

24. Addition reactions of conjugated dienes
Try to predict the products of the following reaction:
allylic carbocation

25. Allylic carbocation H3C-CH2-CHCl-CH=CH-CH3 H3C-CH2-CH=CH-CHCl-CH3
1,2 addition
1,4 addition

26. 1,2 v 1,4 addition

27. Thermodynamic v kinetic control
The more stable isomer is the product of a reaction under thermodynamic control.
However the product of a kinetically controlled reaction is determined by the transition state having the lower energy.
Thus, at higher temperatures, the more stable product is obtained as there is sufficient energy to cross both potential energy barriers.

29. 1,2-addition
There is another possible explanation for the favoring of 1,2-addition. After the initial protonation, the Br- is far closer to carbon 2 than carbon 4. Addition at carbon 2 may be due to proximity.
Norlander tested this using 1,3-pentadiene and DCl which gives only secondary allylic cations. He found that 1,2-addition was preferred!
It is a proximity effect.

30. 1,2-addition

31. Diels - Alder reaction
cyclohexene
Nobel Prize awarded in 1950

32. Diels - Alder reaction cyclohexene
This is a concerted reaction that involves a cyclic flow of electrons. Such a process is called a pericyclic reaction.

33. Diels - Alder reaction

34. Diels - Alder reaction

35. Diels - Alder reaction - a stereospecific reaction
The configuration of the dienophile is retained in the product.

36. Diels - Alder reaction - a stereospecific reaction
The configuration of the diene is also retained in the product.

37. Problems
Try problems 13.16, 13.19, – 13.24, and – in Solomons and Fryhle.