Lecture 2
Wednesday, January 7, 2015
2:16 PM
Allylic rearrangement
The substitution product in which the double bond changes the position is said to be formed by allylic rearrangement.
SN2 reactions in allylic systems
Presence of a double bond also effects the rate of Sn2 reactions when the leaving group is in allylic position. SN2
reactions of allylic electrophiles are faster than reactions of non-allylic ones. You can see that in the foloowing figure
that shows the relative rates of Sn2 reactions of alkyl chlorides.
So, the following reactions is faster than it would be in the absence of the double bond.
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Lecture 2
Wednesday, January 7, 2015
2:22 PM
Why are Sn2 reactions faster? Looking at the starting material and the product it looks like the double bond does not
participate in the reaction. It turns out that the electrons from a double bond can stabilize the transition state for the SN2
reaction. Another way to think about it is that the presence of the double bond changes the LUMO of the alkyl halide and that
changes the orbital interaction necessary for the reaction and stabilizes the transition state. The picture below shows the
LUMO of the allylic chloride. Notice that the LUMO stretches across not only the carbon with the leaving group but also the
carbons with the double bond.
Unlike in Sn1 reactions, in SN2 reactions allylic rearrangement does not happen. So, double bond does
not change the position.
Allylic radicals as intermediates in substitution reactions.
Allylic radicals are more stable than alkyl radicals. As a result they are easier to generate:
Radical allylic substitution can be useful transformations:
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Lecture 2
Wednesday, January 7, 2015
3:35 PM
Mechanism
Initiation:
Propagation:
Analogous bromination is also possible using NBS (N-bromosuccinimide).
N-bromosuccinimide
Again, the reaction proceeds through the allylic radical intermediate. As you can see hydrogen atoms from both position 3
and position 6 can be abstracted and we get both allylic radicals. In this case their reaction with the Br leads to the formation
of the same product. In cases where the structure of the starting material is less symmetrical we can get a complex mixture
of products.
For example: In the following reaction we can get two different allylic radical intermediates which are of roughly similar
stabilities. They are both secondary allylic radicals. Furthermore, both radicals have an additional resonance structure. As a
result of all of that we can get 4 different bromide products. Write the structure of the 4 possible products. In this case the
for products are formed roughly in the same amounts.
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Lecture 2
Wednesday, January 7, 2015
3:44 PM
Allylic anions:
Allylic anions are more stable than the analogous alkyl anions. As a result allylic positions are more acidic than simple alkanes.
It is easier to deprotonate the allylic position than it is to deprotonate the alkane. The satability of the allylic anions comes
from the fact that the two electrons of the anion can be delocalized across the 3 atom fragments by conjugation with the
double bond. That is reflected in the shown resonance structure. An additional stabilization of the allylic anion, relative to the
simple alkyl anion, comes from the fact that the anion is next to the carbon that is sp2 hybridized and not sp3 hybridized (see
in red). Remember sp2 carbon atoms are more electronegative than the sp3 carbons.
Dienes
Dienes are molecules that have two double bonds. When two double bonds are found in the same molecule they can have
a profound effect on each other's stability and activity. What effect they will have on each other depends on their relative
position in the molecule. Based on the relative position double bonds have in the diene dienes can be divided into:
Isolated dienes: at least one sp3 atom between the double bonds
Conjugated dienes: double bonds next to each other.
Cumulated dienes or allenes: share the same carbon.
Stability of the dienes can follows the following trend. Isolated dienes behave and have the stability that
is directly related to the stability and reactivity of simple alkenes. Dienes are more stable and allenes are
less stable than simple alkenes.
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Lecture 2
Wednesday, January 7, 2015
3:57 PM
Structure of conjugated dienes
Shown are the two most stable  molecular orbitals (MOs) in a conjugated diene. These are obtained by a linear combination
of the 4 2p atomic orbitals on the 4 consecutive carbon atoms of the diene. As you can see, the most stable MO 1 is the one
with no nodes that covers the all four carbon atoms. The next most stable one, 2 has one node. So electrons in this orbital
can be found between atoms C1&C2 and C3&C4 but they cannot be found between C2&C3. Diene has 4 electrons in  orbitals.
2 are in 1 and 2 are in 2. The shape of 1 suggests that there is a partial double bond character between C2&C3 and that is
consistent with the fact that C2-C3 bond is shorter than the normal C-C bond. Notice that when we draw the structure of a
diene
this partial double bond character is not obvious. The structure we draw simply says that there are two
 electrons between C1&C2 and two  electrons between C3&C4. It says that there are no  electrons between C2&C3. In a
sense, the structure we use corresponds to the shape of the HOMO  orbital. The fact that the 1 stratches over 4 atoms with
no nodes contributes to the increased stability of the conjugated dienes, relative to isolated dienes.
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