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Conjugated systems

Conjugated systems. Compounds that have a p orbital on an atom adjacent to a double bond. Ionic addition. However, we have seen that X 2 reacts with alkanes, by a free radical mechanism, to form substitution products:. Perhaps we can brominate at the methyl position of propene.

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Conjugated systems

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  1. 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 ionic addition radical substitution

  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 • 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 ? • vinyl hydrogens undergo very little substitution.

  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-CH3 1,2-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 1,4 addition 1,2 addition

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

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

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

  27. 1,2 v 1,4 addition

  28. 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. Identify the diene and dienophile necessary to synthesize the following compound:

  38. Identify the diene and dienophile necessary to synthesize the following compounds:

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