Resonance

Report
Conjugated systems
Compounds that have a p orbital
on an atom adjacent to a double
bond
© E.V. Blackburn, 2011
Ionic addition
CH3CH=CH 2
Br2
CH3CHBrCH 2Br
However, we have seen that X2 reacts with
alkanes, by a free radical mechanism, to form
substitution products:
250-400o
C H + X2
or h
C X + HX
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:
C C C + Br2
H
h
or 
C C C + HBr
Br
© E.V. Blackburn, 2011
Free radical substitution v
ionic addition
low T
CCl 4
CH3-CH=CH 2
Cl2
CH3CHClCH 2Cl
ionic addition
500 - 600o
CH2ClCH=CH 2
(gas phase)
radical substitution
© E.V. Blackburn, 2011
N-bromosuccinimide
Br
O
+
N Br
O
h, CCl 4
O
+
N H
O
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
Br2 which is produced by reaction between HBr and
NBS:
CH2=CHCH3 + Br
O
HBr +
N Br
O
CH2=CHCH 2 + Br2
CH2=CHCH2 + HBr
O
Br2 +
N H
O
CH2=CHCH 2Br + Br
© E.V. Blackburn, 2011
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 ?
© E.V. Blackburn, 2011
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:
© E.V. Blackburn, 2011
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:
© E.V. Blackburn, 2011
Properties of allylic radicals
• The propenyl radical is symmetric:
H
H
H
H
H
© E.V. Blackburn, 2011
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:
CH2=CH-CH 2
and
CH2-CH=CH 2
• The molecule is a hybrid of all the contributing
structures and cannot be adequately represented by
any one of these structures.
© E.V. Blackburn, 2011
The theory of resonance
and
O
H3C
and
O-
O
H3C
O-
OH
and
H3C
H3C
O
O+
OH
???
© E.V. Blackburn, 2011
The theory of resonance
• Resonance is important when these structures are of
about the same stability. For example,
O
H3C
H3C
OO
OH
Oand
H3C
O
OH3C +
OH
• The hybrid is more stable than any of the
contributing structures. This increase in stability is
called the resonance energy.
© E.V. Blackburn, 2011
The allyl radical - an example
of resonance stabilization
There are two structures which contribute to the hybrid:
CH2=CH-CH 2
and
CH2-CH=CH 2
They are of the same energy and contribute equally to
the hybrid.
© E.V. Blackburn, 2011
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:CH2=CH-CH 2
CH2-CH=CH 2
H
CH2
CH
CH2
H
C
H
C
C
H
H
© E.V. Blackburn, 2011
Structure of the allyl
(propenyl) radical
H
H
C
H
C
C
H
H
• The electron is delocalised and the molecule is
symmetric.
• The resonance energy is ~42 kJ/mol.
• We can explain the allylic rearrangement.
© E.V. Blackburn, 2011
Allylic rearrangement
CH3CH2CH=CH 2
CH3CHCH=CH 2
Br2
CH3CHBrCH=CH 2
CH3CHCH=CH 2
CH3CH=CHCH 2
Br2
CH3CH=CHCH 2Br
© E.V. Blackburn, 2011
Orbital representation
H
H
C
H
C
C
H
H
© E.V. Blackburn, 2011
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
© E.V. Blackburn, 2011
Diene classification
• 1,2-dienes - cumulated double bonds
CH2=C=CH2 - propadiene, allene
• 1,3-dienes - conjugated double bonds
2-methyl-1,3-butadiene,
isoprene
• Isolated double bonds
CH2=CH-CH2-CH=CH2 - 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:
C2-C3 = 1.48Å
H3C-CH3= 1.54Å
© E.V. Blackburn, 2011
Electrophilic addition reactions
of dienes
CH2=CH-CH 2-CH=CH 2
Br2
CH2Br-CHBr- CH2-CH=C H2
+ CH2Br-CHBr-CH2-CHBr-CH2Br
This is typical behavior for dienes having isolated
double bonds.
© E.V. Blackburn, 2011
Addition reactions of
conjugated dienes
CH2=CH-CH=CH 2
Br2
CH2BrCHBrCHBrCH 2Br
+ CH2BrCHBrCH=CH 2 + CH2BrCH=CHCH 2Br
1,2 addition
1,4 addition
© E.V. Blackburn, 2011
Addition reactions of
conjugated dienes
Try to predict the products of the following reaction:
CH3CH=CHCH=CHCH
HCl
?
3
+
CH3CH=CHCH=CHCH3 + H
+
CH3CH2CHCH=CHCH3
© E.V. Blackburn, 2011
Addition reactions of
conjugated dienes
Try to predict the products of the following reaction:
CH3CH=CHCH=CHCH
HCl
?
3
+
CH3CH=CHCH=CHCH3 + H
X
+
CH3CH2CHCH=CHCH3
+
CH3CHCH2CH=CHCH3
allylic carbocation
secondary carbocation
© E.V. Blackburn, 2011
Allylic carbocation
H3C CH2
CH
-
Cl
H3C-CH2-CHCl-CH=CH-CH3
1,2 addition
CH
+
CH
CH3
-
Cl
H3C-CH2-CH=CH-CHCl-CH3
1,4 addition
© E.V. Blackburn, 2011
1,2 v 1,4 addition
-80o
CH2=CHCH=CH 2
+ HBr
CH3CHCH=CH 2 + CH3CH=CHCH 2Br
Br
20%
80%
40o
40o
CH3CHCH=CH 2 + CH3CH=CHCH 2Br
Br
20%
80%
© E.V. Blackburn, 2011
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.
© E.V. Blackburn, 2011
E

Br

CH2CH=CHCH 3

H2C=CHCHCH 3
Br

+
H2C=CHCH=CH 2
+ HBr
H2C=CHCHBrCH 3
1,2 addition
BrCH 2CH=CHCH 3
1,4 addition
© 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, 2011
1,2-addition
DCl
D
+
2
+
D
D
2o
o
Cl
D
+
Cl
D
75,5%
24%
© E.V. Blackburn, 2011
Diels - Alder reaction
200C
diene dienophile
cyclohexene
Nobel Prize awarded in 1950
© E.V. Blackburn, 2011
Diels - Alder reaction
200C
diene dienophile
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
G
G
diene dienophile
G = -CO2H, -COR, -C=N
electron attracting substituants
© E.V. Blackburn, 2011
Diels - Alder reaction
NC
CN
NC
CN
+
25C
CN
CN
NC CN
© E.V. Blackburn, 2011
Diels - Alder reaction - a
stereospecific reaction
The configuration of the dienophile is retained in the
product.
H
C O2CH3
H CO2CH3
+
H
CO2CH3
H
C O2CH3
© E.V. Blackburn, 2011
Diels - Alder reaction - a
stereospecific reaction
The configuration of the diene is also retained in the
product.
H
+
H
NC
CN
NC
CN
H
CN
CN
H
CN
CN
© 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:
© E.V. Blackburn, 2011

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