Loudon Chapter 4 & 5 Review: Reactions of Alkenes
CHEM 3311, Jacquie Richardson, Spring 2010 - Page 1
There are many different ways to think about splitting up the reactions that we’ve covered so far,
but one way that will be very useful later is dividing them up by what types of groups are added.
1) Reactions that add 2 non-H groups: All of these go by the same mechanism, forming a
halonium intermediate (a three-membered ring with a halogen cation in one corner).
Dihalogenation:
CH3
H
H
H
CH3
Halohydrin formation:
H
Haloether formation:
CH3 Br
H
H
Br H
Br2, CH2Cl2
Br2, H2O
CH3 Br
H
H
OH H
Br2, ROH
CH3 Br
H
H
RO H
H
H
CH3
H
H
H
Mechanism:
CH3
H
H
H
Br Br
Br
CH3
H
CH3 Br
H
H
Nu H
H
H
Nu
Depending on conditions, Nu can be either Br , H2O or ROH. It goes for the more substituted
carbon because there's a bigger partial positive charge there. If Nu is either ROH or H2O,
there's one more step at the end of the mechanism where it gets deprotonated.
2) Reactions that add 1 H and 1 non-H: These can be subdivided into those that follow
Markovnikov’s Rule, and those that don’t.
a. Markovnikov-style reactions: These are the ones we covered in Ch. 4.
CH3
Hydrogen halide addition:
H
Acid-catalyzed hydration:
H
HBr
H
CH3
H
H
H
CH3
Acid-catalyzed etherification:
H
CH3 H
H
H
Br H
H2O, H2SO4
H
ROH, H2SO4
H
CH3 H
H
H
OH H
CH3 H
H
H
RO H
Mechanism:
CH3
H
H
H
H+
CH3 H
H
H
H
rearrange?
CH3 H
H
H
H
Nu
This particular molecule won't rearrange, but some molecules will once they form the
o
carbocation - remember that 3 carbocations are best. Again, depending on
CH3 H
H
H
Nu H
Loudon Chapter 4 & 5 Review: Reactions of Alkenes
CHEM 3311, Jacquie Richardson, Spring 2010 - Page 2
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conditions, Nu can be either Br , H2O or ROH. If Nu is either ROH or H2O, there's one
more step at the end of the mechanism where it gets deprotonated.
The downside to these reactions is that they all go by carbocation mechanisms,
which can lead to unexpected rearrangements. On top of that, H2SO4 is a very brutal
reagent that can mess up other functional groups. A good workaround to the
problems of acid-catalyzed hydration is…
CH3 H
CH3
H 1) Hg(OAc)2, H2O, THF
Oxymercuration-reduction:
H
H
2) NaBH4, NaOH
H
H
OH H
Mechanism:
CH3
H
H
H
Hg OAc
OAc
OAc
Hg
CH3
H
OAc
CH3 Hg
H
H
H O H
H
H
H
OAc
CH3 Hg
H
H
HO H
HOH
The oxymercuration mechanism is very similar to what we saw for dihalogenation.
OAc
CH3 Hg
H
H
HO H
CH3 H
H
H
HO H
NaBH4, NaOH
The reduction mechanism is based on transition-metal chemistry, so you don't need to know it.
b. Anti-Markovnikov-style reactions:
CH3
Radical hydrogen halide addition:
H
Mechanism:
H
HBr, ROOR
H
CH3 Br
H
H
H H
1) Initiation
RO OR
RO
ROH + Br
H Br
2) Propagation
CH3
H
H
Br
H
CH3 Br
H
CH3 Br
H
H
H
+ Br
H
H H
H Br
3) Termination: any step that involves 2 radicals getting together, for example:
CH3 Br
H
H
H
CH3 Br
H
Br
H
H
Br
Please note that this works only when bromine is the halogen being used. If you try to
do the reaction with chlorine or iodine, it will ignore the radical initiator and just add
Markovnikov-style. Also, there are other options for what to use as the initiator –
AIBN is another choice, instead of ROOR.
This reaction, along with anything else that happens via radicals, can be broken
down into three types of steps.
Loudon Chapter 4 & 5 Review: Reactions of Alkenes
CHEM 3311, Jacquie Richardson, Spring 2010 - Page 3
1)
Initiation creates the radicals that will actually be used in the reaction.
Most initiation steps involve creating two new radicals from a non-radical,
but there can also be steps where the radical that actually does the
chemistry is being formed.
Propagation lets these radicals do their work on the molecule, so the
number of radicals in conserved from one step to the next. The radical
that you have going into the propagation section should be the same as
the radical you create at the end of the propagation section.
Termination involves any two radicals getting together and killing each
other off. There are many different possibilities, but showing one or two
is normally sufficient.
2)
3)
CH3
Hydroboration-oxidation:
H
Mechanism:
H
H
1) BH3-THF
2) H2O, H2O2, OH-
CH3 OH
H
H
H H
H BH2
H
CH3 BH2
CH3 OH
H
H H2O, H2O2, OH
H
H
H H
H H
H
H
BH2 prefers to add to the less substituted carbon here, mostly because the sterics
aren't so bad but also partly because of electron density. Since this step is concerted,
the H and the BH2 add to the same face at the same time. (As a side note, each of
the three Hs coming off boron can do this reaction on a separate molecule, so by the
end of the hydroboration step you can have up to three R groups dangling off of the
B. But I only show one, for the purposes of clarity.) The mechanism for the oxidation
step is not something you need to know, but the result of it is that the BH2 gets
replaced with an OH group that's pointing the same direction as the BH2 was before.
CH3
3) Reactions that add 2 H: We only know one reaction in this category.
CH3 H
CH3
H
H2, Pd/C
Hydrogenation:
H
H
H
H
H H
4) Reactions that break the C=C bond entirely
Reduce with (CH3)2S (a.k.a. DMS) or with Zn, H2O
H3C
H
O
O
H
Ozonolysis:
CH3
H
H
H
O3
H3C
H
O O
O
H
H
H
For both workups, the C=C double bond gets replaced
with 2 C=O bonds. But for the oxidative workup, one
other thing happens: for each C=O that has an H
directly attached, you replace one H with an OH.
H3C
H
O
O
HO
Oxidize with H2O, H2O2
OH