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