© E.V. Blackburn, 2011 Conjugated systems Compounds that have a p orbital on an atom adjacent to a double bond

© E.V. Blackburn, 2011 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.....

© E.V. Blackburn, 2011 Free radical substitution We must use conditions which favor free radical substitution reactions and are not favorable to ionic addition:

© E.V. Blackburn, 2011 Free radical substitution v ionic addition ionic addition radical substitution

© E.V. Blackburn, 2011 N-bromosuccinimide N-bromosuccinimide (NBS) is used for the specific purpose of brominating alkenes at the allylic position.

© E.V. Blackburn, 2011 N-bromosuccinimide How does it work? NBS provides a low concentration of Br 2 which is produced by reaction between HBr and NBS: CH 2 =CHCH 3 +BrCH 2 =CHCH 2 + HBr

© E.V. Blackburn, 2011 Orientation and reactivity allylic hydrogens are particularly reactive. the order of ease of hydrogen abstraction is: allylic > 3 o > 2 o > 1 o >CH 4 > vinylic How can we explain the stability of allylic radicals ? vinyl hydrogens undergo very little substitution.

© E.V. Blackburn, 2011 Properties of allylic radicals Allylic radicals can rearrange: We will find the answer in the concept of resonance. Let us start by examining some of the properties of allylic radicals:

© E.V. Blackburn, 2011 Properties of allylic radicals Allylic radicals can rearrange: We will find the answer in the concept of resonance. Let us start by examining some of the properties of allylic radicals:

© E.V. Blackburn, 2011 The propenyl radical is symmetric: Properties of allylic radicals

© E.V. Blackburn, 2011 The theory of resonance The molecule is a hybrid of all the contributing structures and cannot be adequately represented by any one of these structures. Whenever a molecule can be represented by 2 or more structures which differ only in the arrangement of their electrons, there is resonance:

© E.V. Blackburn, 2011 The theory of resonance

© E.V. Blackburn, 2011 The hybrid is more stable than any of the contributing structures. This increase in stability is called the resonance energy. The theory of resonance Resonance is important when these structures are of about the same stability. For example,

© E.V. Blackburn, 2011 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.

© E.V. Blackburn, 2011 The radical is therefore represented by:- The radical has no double bond because the two C - C bonds must be identical if the two structures contribute equally. Structure of the allyl (propenyl) radical

© E.V. Blackburn, 2011 The electron is delocalised and the molecule is symmetric. The resonance energy is ~42 kJ/mol. We can explain the allylic rearrangement. Structure of the allyl (propenyl) radical

© E.V. Blackburn, 2011 Allylic rearrangement

© E.V. Blackburn, 2011 Orbital representation

© E.V. Blackburn, 2011 Dienes - structure and nomenclature CH 2 =C=CH-CH 3 1,2-butadiene CH 2 =CH-CH 2 -CH=CH 2 1,4-pentadiene The position of each double bond is indicated using an appropriate number:

© E.V. Blackburn, 2011 Diene classification CH 2 =C=CH 2 - propadiene, allene 1,3-dienes - conjugated double bonds 1,2-dienes - cumulated double bonds Isolated double bonds CH 2 =CH-CH 2 -CH=CH 2 - 1,4-pentadiene

© E.V. Blackburn, 2011 Stability of conjugated dienes The heat of hydrogenation of conjugated dienes is lower than that of other dienes. Why? Bond lengths: C 2 -C 3 = 1.48Å H 3 C-CH 3 = 1.54Å

© E.V. Blackburn, 2011 Electrophilic addition reactions of dienes This is typical behavior for dienes having isolated double bonds. + CH 2 Br-CHBr-CH 2 -CHBr-CH 2 Br

© E.V. Blackburn, 2011 Addition reactions of conjugated dienes 1,2 addition 1,4 addition

© E.V. Blackburn, 2011 Addition reactions of conjugated dienes Try to predict the products of the following reaction:

© E.V. Blackburn, 2011 Addition reactions of conjugated dienes Try to predict the products of the following reaction: allylic carbocation

© E.V. Blackburn, 2011 Allylic carbocation 1,4 addition H 3 C-CH 2 -CHCl-CH=CH-CH 3 1,2 addition H 3 C-CH 2 -CH=CH-CHCl-CH 3

© E.V. Blackburn, ,2 v 1,4 addition

© E.V. Blackburn, 2011 Thermodynamic v kinetic 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. The more stable isomer is the product of a reaction under thermodynamic control.

© E.V. Blackburn, 2011

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.

© E.V. Blackburn, ,2-addition

© E.V. Blackburn, 2011 Diels - Alder reaction cyclohexene Nobel Prize awarded in 1950

© E.V. Blackburn, 2011 Diels - Alder reaction cyclohexene This is a concerted reaction that involves a cyclic flow of electrons. Such a process is called a pericyclic reaction.

© E.V. Blackburn, 2011 Diels - Alder reaction

© E.V. Blackburn, 2011 Diels - Alder reaction

© E.V. Blackburn, 2011 Diels - Alder reaction - a stereospecific reaction The configuration of the dienophile is retained in the product.

© E.V. Blackburn, 2011 Diels - Alder reaction - a stereospecific reaction The configuration of the diene is also retained in the product.

© E.V. Blackburn, 2011 Identify the diene and dienophile necessary to synthesize the following compound:

© E.V. Blackburn, 2011 Identify the diene and dienophile necessary to synthesize the following compounds: