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

© 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 X2 reacts with alkanes, by a free radical mechanism, to form substitution products:

Perhaps we can brominate at the methyl position of propene.....

C H + X2

250-400o

or hC X + HX

CH3CH=CH2Br2

CH3CHBrCH2Br

© E.V. Blackburn, 2011

Free radical substitution

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

C CH

C C CBr

C+ Br2 + HBrh

or

© E.V. Blackburn, 2011

Free radical substitution v ionic addition

ionic addition

500 - 600o

CH2ClCH=CH2(gas phase)

radical substitution

CH3-CH=CH2

Cl2

low T

CCl4CH3CHClCH2Cl

© E.V. Blackburn, 2011

N-bromosuccinimide

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

N Br

O

O

N H

O

O

++h, CCl4

Br

© E.V. Blackburn, 2011

N-bromosuccinimideHow does it work? NBS provides a low concentration of Br2 which is produced by reaction between HBr and NBS:

N Br

O

O

N H

O

O

HBr + Br2 +

CH2=CHCH3 + Br CH2=CHCH2+ HBr

+ BrCH2=CHCH2 + Br2 CH2=CHCH2Br

© E.V. Blackburn, 2011

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.

© 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:

CH3CH2CH=CH2NBS

CH3CHBrCH=CH2

© 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:

CH3CH2CH=CH2NBS

CH3CHBrCH=CH2

+ CH3CH=CHCH2Br

© E.V. Blackburn, 2011

• The propenyl radical is symmetric:

H H

HHH

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:

CH2=CH-CH2 CH2-CH=CH2and

© E.V. Blackburn, 2011

The theory of resonance

and

H3CO-

OH3C

O

O-

and

H3COH

OH3C

OH

O-

and + ???

© 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,

H3CO-

OH3C

O

O-

and

H3COH

OH3C

OH

O-

+

© 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.

CH2=CH-CH2 CH2-CH=CH2and

© E.V. Blackburn, 2011

The radical is therefore represented by:-

CH2=CH-CH2 CH2-CH=CH2

CH2 CH CH2C

CH

H

H

CH

H

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

CC

H

H

H

CH

H

© E.V. Blackburn, 2011

Allylic rearrangement

CH3CH2CH=CH2 CH3CHCH=CH2

CH3CHCH=CH2 CH3CH=CHCH2

CH3CHBrCH=CH2

Br2 Br2

CH3CH=CHCH2Br

© E.V. Blackburn, 2011

Orbital representation

CC

H

H

H

CH

H

© E.V. Blackburn, 2011

Dienes - structure and nomenclature

CH2=C=CH-CH3 1,2-butadiene

CH2=CH-CH2-CH=CH2 1,4-pentadiene

The position of each double bond is indicated using an appropriate number:

© E.V. Blackburn, 2011

Diene classification

CH2=C=CH2 - propadiene, allene

• 1,3-dienes - conjugated double bonds

• 1,2-dienes - cumulated double bonds

• Isolated double bonds

CH2=CH-CH2-CH=CH2 - 1,4-pentadiene

2-methyl-1,3-butadiene, isoprene

© 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: C2-C3 = 1.48Å H3C-CH3= 1.54Å

© E.V. Blackburn, 2011

Electrophilic addition reactions of dienes

This is typical behavior for dienes having isolated double bonds.

CH2=CH-CH2-CH=CH2

Br2 CH2Br-CHBr- CH2-CH=C H2

+ CH2Br-CHBr-CH2-CHBr-CH2Br

© E.V. Blackburn, 2011

Addition reactions of conjugated dienes

1,2 addition

CH2=CH-CH=CH2

Br2 CH2BrCHBrCHBrCH2Br

+ CH2BrCHBrCH=CH2 + CH2BrCH=CHCH2Br

1,4 addition

© E.V. Blackburn, 2011

Addition reactions of conjugated dienes

Try to predict the products of the following reaction:

CH3CH=CHCH=CHCH 3 HCl

?

CH3CH2CHCH=CHCH3

+

CH3CH=CHCH=CHCH3 + H+H+

© E.V. Blackburn, 2011

Addition reactions of conjugated dienes

Try to predict the products of the following reaction:

CH3CH=CHCH=CHCH 3 HCl

?

X

allylic carbocation secondary carbocation

CH3CH2CHCH=CHCH3

+CH3CHCH2CH=CHCH3

+

CH3CH=CHCH=CHCH3 + H+H+

© E.V. Blackburn, 2011

Allylic carbocation

1,4 addition

H3C CH2 CH CH CH CH3

+Cl- Cl-

H3C-CH2-CHCl-CH=CH-CH3

1,2 addition

H3C-CH2-CH=CH-CHCl-CH3

© E.V. Blackburn, 2011

1,2 v 1,4 addition

-80o

CH2=CHCH=CH2

+ HBr

CH3CHCH=CH2

Br

CH3CH=CHCH2Br+

80% 20%

40o CH3CHCH=CH2

Br

CH3CH=CHCH2Br+

20% 80%

40o

© 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

H2C=CHCH=CH2

+ HBr

+

BrCH2CH=CHCH3 1,4 addition

H2C=CHCHBrCH3 1,2 addition

E

Br CH2CH=CHCH3

H2C=CHCHCH3

Br

© E.V. Blackburn, 2011

1,2-additionThere 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, 2011

1,2-addition

DClD

+

D+ D

+

2o 2o

D DCl Cl

75,5% 24%

© E.V. Blackburn, 2011

Diels - Alder reaction

cyclohexene

200C

diene dienophile

Nobel Prize awarded in 1950

© E.V. Blackburn, 2011

Diels - Alder reaction

cyclohexene

200C

diene dienophile

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

diene dienophile

G G

G = -CO2H, -COR, -C=N

electron attracting substituants

© E.V. Blackburn, 2011

NC CN

NC CN+

25C CN

CNCNNC

Diels - Alder reaction

© E.V. Blackburn, 2011

Diels - Alder reaction - a stereospecific reaction

The configuration of the dienophile is retained in the product.

+H CO2CH3

H CO2CH3 C

CH

HO2CH3

O2CH3

© E.V. Blackburn, 2011

Diels - Alder reaction - a stereospecific reaction

The configuration of the diene is also retained in the product.

+NC CN

NC CN C

CCN

CNH

H

HH N

N

© E.V. Blackburn, 2011

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

CO2CH3

CO2CH3

CO2CH3

H3CO2C

CO2CH3

CO2CH3

CO2CH3

H3CO2C

© E.V. Blackburn, 2011

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

O

H

H

O

OCN

H