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

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.....

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

Free radical substitution v ionic addition

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

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

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 ?

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:

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:

Properties of allylic radicals The propenyl radical is symmetric:

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.

The theory of resonance

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.

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.

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:-

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.

Allylic rearrangement

Orbital representation

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

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

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

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

Addition reactions of conjugated dienes

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

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

1,2 v 1,4 addition

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.

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.

1,2-addition

Diels - Alder reaction cyclohexene Nobel Prize awarded in 1950

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

Diels - Alder reaction

Diels - Alder reaction

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

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

Problems Try problems 13.16, 13.19, 13.22 – 13.24, and 13.33 – 13.35 in Solomons and Fryhle.