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MORE REACTIONS OF ALKENES
THE OVERALL REACTION
The acid catalyzed addition of water to a general alkene andalso to isobutene is illustrated below:
o Note that this is not a reaction mechanism, but an
equation for the overall reaction.
o Hydronium ion is a required catalyst. Since it is not
consumed in the reaction, it is not a part of the equation
per se, but is placed in parentheses over the reaction
arrow.
o Note also that the product is an alcohol.o Note that the Markovnikov Rule is followed for this
reaction, i.e., the proton adds to the less substituted
carbon of the alkene double bond (the methylene
carbon, making it a methyl group), and the hydroxyl
group adds to the more highly substituted carbon atom
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of the alkene double bond. So the regiochemistry of
the hydration reaction is closely parallel to that
previously discussed for the hydrochlorination reaction.
MECHANISM OF ACID CATALYZED HYDRATION
o
In the addition of HCl, the acid which transfers theproton to the alkene to form a carbocation is, of course,
HCl. In acid catalyzed addition of water (hydration), the
acid which actually transfers the proton to the alkene is
normally the solvated hydronium ion. The acid used is
often sulfuric acid, but any strong acid will usually
work. Water, itself, is too weak an acid to transfer
the proton to the alkene, so acid catalysis is essential.
o The reaction is also similar to the addition of HCl in
that a carbocation is formed in the rds. Again, two
bonds are broken and just one is formed, so the reaction
is endothermic. It is therefore the rds, and the TS
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strongly resembles the product carbocation
(Hammond Principle). However, it differs in that the
carbocations, which are very reactive and so can react
even with a weak nucleophile like water, which is
present in great abundance, react with water to give theconjugate acid of the product alcohol.
o In the final step of the mechanism , this conjugate acid
transfers a proton to water, regenerating the original
hydronium ion catalyst. It is important to note that
this step is an equilibrium step, since proton
transfers from oxygen to oxygen are typically very
fast. IT IS VERY IMPORTANT, IN WRITING A
CORRECT REACTION MECHANISM, TO
SPECIFY BY THE USE OF AN EQUILIBRIUM
ARROW ANY STEP WHICH IS REVERSIBLE. IT
IS ALSO IMPORTANT TO SPECIFY THE RDS.
FURTHER, YOU MUST USE ELECTRON FLOW
ARROWS TO SHOW THE FORMATION OF
NEW BONDS. The product alcohol would be isolatedby making the solution basic or by extracting it into an
organic solvent or distilling it off.
TRANSITION STATE MODEL FOR HYDRATION
The development of a TS model for the rds of hydration
using resonance theory is illustrated below. You should be
able to show this same kind of development for any alkene.
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As before, the TS is represented as a resonance hybrid of
reactant-like (R) and product-like structures, presented in the
correct geometry for reaction. The transformation of the R
structure into the P structure is illustrated by electron flow
arrows. No nuclei are allowed to change position (includinga proton). The R structure consists of the alkene and the
hydronium ion , properly oriented, and the P structure
consists of the carbocation and the alcohol in the same
geometry as in the R structure.
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The dotted lines in the DL/PC (partial bonds) are the partial
pi bond, which is being broken (broken in the product but
formed in the reactant), the O-H bond of the hydronium ion,which is also broken in the product structure, and the C-H
bond, which is broken in the reactant (i.e., not formed) and
formed in the product. There is a partial positive charge on
the passive carbon (which has a unit positive charge in the
product and zero formal charge in the reactant) . These things
are all parallel to those seen in the addition to HCl. One
specific difference is that in the hydration TS there is a
partial positive charge on the oxygen atom , because thisoxygen had a unit positive charege in R and zero charge in P.
This constrasts with the partial negative charge on chloride in
the HCl addition, because the negatively charged chloride ion
is being formed.
The DL/PC is then characterized: It has extensive
(Hammond Postulate) positive charge on the passivecarbon, which in this case, is extensive tertiary
carbocation character.
RATES OF HYDRATION
The same regiochemistry is observed in hydration as in the
addition of HCl, because the TS develops the same character.
You should be able to use the Method of Competing TS's to
show that the rates of hydration of isobutene, propene, and
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ethene are in the same order as we saw before (isobutene
fastest).
The Markovnikov Rule is followed, i.e., the regiochemistry is
analogous to that for the HCl addition and for the same
reasons. Again, you should be able to use the Method of
Competing TS's to show that the hydration of isobutene, for
exsample, preferentially gives tert-butyl alcohol rather than
isobutyl alcohol.
You should be able to predict the regiochemistry of
hydration of various alkenes, such as 1-
methylcyclohexene and methylenecyclohexane.
ELECTROPHILES AND ELECTROPHILIC ADDITION
ELECTROPHILES: Species which are able to form a
covalent bond by contributing a vacant orbital (nucleus) to
the bond. They thus contribute no electrons to the bond.
They are essentially equivalent to Lewis Acids. Examples of
electrophiles we have encountered already are the proton, the
hydronium ion, hydrogen chloride, and boron trifluoride.
Carbocations are also very strong electrophiles, since they
have a vacant 2p orbital.
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NUCLEOPHILES-Species which are able to form a covalent
bond by contributing an electron pair to the bond. They are
thus essentially equivalent to Lewis Bases.
IMPORTANT: ELECTROPHILES REACT WITHNUCLEOPHILES, one species contributing 2 electrons and
the other no electrons, to give a strong, 2 electron bond.
Examples of nucleophiles which we have encountered are the
chloride ion, water, and ammonia. Pi bonds are
nucleophiles, since the electron pair of the pi bond is in a
bond which is not especially strong.
In the reaction of HCl with an alkene, the alkene is the
nucleophile, since it supplies the electron pair which forms
the C-H bond. HCl is the electrophile. In particular the H is
transferred to the alkene without its electrons, i.e., as a
proton. Similarly , in the addition of water to an alkene, the
alkene is the nucleophile and the electrophile is the
hydronium ion. Both of these reactions are considered, by the
organic chemist, to be ELECTROPHILIC ADDITIONS,because the organic species (the alkene) reacts with an
electrophile. The convention is that if the organic species
reacts with an electrophile, the reaction is considered to be
electrophilic. If the organic species reacts with a nucleophile,
it is a nucleophilic reaction. Thus, the point of view is that of
the organic species. What happens when both reacting
species are organic?? We will see about this later.
The full reaction symbol for both of these reactions is
AdE.
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We should also note that in the second step of both reactions,
a carbocation (an electrophile) reacts with a nucleophile
(chloride ion or water). Thus this reaction is nucleophilic.
CONSEQUENCES OF CARBOCATION INTERMEDIATES
IN ADDITION REACTIONS
CARBOCATION REARRANGEMENTS
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We have already seen that the order of carbocation stabilityof alkyl carbocationis is
tertiary>secondary>primary>methyl. We have also seen
that the differences in stability are relatively large (ca. 20
kcal/mol). Therefore it might not be surprising to find that a
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less stable carbocation species can rearrange readily to a
more stable species as shown in the illustration above.
Thus, the addition of HCl or water to the two alkenes shown
in the illustration (and many others) does not lead exclusively
or even primarily to the simple product expected from
Markovnikov addition, although this product is often
observed as a minor product. The main product is one of
rearrangement of an alkyl group or hydrogen, whichever
leads to the more stable carbocation. Thus, in the second
example, the hydrogen migrates rather than a methyl group,
because hydrogen migration yields a tertiary carbocation,
while methyl migration yields a secondary carbocation.
Because the hydrogen or methyl group is migrating to a
carbocation site which provides zero electrons (vacant 2p
orbital), it must migrate with an electron pair, rather than as
a proton or methyl cation, these are called hydridemigrations or methide migrations.
You should be able to predict the product of HCl or water
addition to alkenes, being aware of the possibility of
carbocation migrations.
IT IS AN IMPORTANT CHARACTERISTIC OF
CARBOCATION MECHANISMS THAT THEY
PROVIDE THE POSSIBILITY OFREARRANGEMENTS, WHICH MAY OR MAY NOT
BE DESIRABLE FROM A STANDPOINT OF
ORGANIC SYNTHESIS
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STEREOCHEMICAL CONSEQUENCES OF
CARBOCATION INTERMEDIATES
The scheme shown below illustrates another consequence of the
involvement of carbocation intermediates in organic reactions, inparticular addition reactions:
Carbocations are planar, sp2 hybridized, and have a vacant
2p orbital, the latter being the specific site ofelectrophilicreactivity--i.e., reactivity toward a nucleophile such as
chloride ion or water. Recall that a 2p orbital has two
equivalent lobes, one above and one below the trigonal
plane. Consequently the nucleophile can react with either
lobe of the carbocation, yielding, in appropriate cases, a
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mixture of products. A standard way of expressing this is to
say that the carbocation can react equally from eitherface
(e.g., the top face or the bottom face) of the carbocation.
Therefore the addition of HCl or water to an alkene such
as 1,2-dimethylcyclohexene yields a mixture of the cis and
trans diastereoisomers.
ELECTROPHILIC ADDITION OF HALOGENS TOALKENES
GENERAL CONSIDERATIONS:
Not all electrophilic additions necessarily involve
carbocations, although they typically would involve the
development of positive charge on the alkene, because it is
serving as a nucleophile.
As we can see in the illustration below, the addition of
bromine, chlorine , or iodine to an alkene pi bond proceeds
via an intermediate which has the positive charge mainly on
the halogen. Presumably this is because the positive charge is
more stable on the halogen than on carbon. This may seem
puzzling because the electronegativity of carbon is much
lower than that of the halogens, but the primary reason is thatan extra bond (a carbon-halogen bond) is formed in this
intermediate, which is called an EPIHALONIUM ION.
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Although not a carbocation, the epihalonium ion is
nevertheless electrophilic, but it cannot react at the
halogen atom, where most of the positive charge formally
exists, because the halogen atom cannot expand its valence to
four (it has no vacant orbital to react with a nucleophile).Since the halonium ion also has some charge on both carbons
of the former double bond, it ends up reacting at carbon, i.e.,
like a carbocation.
In contrast to a carbocation, the epihalonium ion reacts
stereospecifically from the side opposite the halogen
bridge, resulting in net trans addition of the two halogen
atoms. None of the cis adduct is formed. This is nicely
illustrated by the addition of bromine to cyclohexene. This is
termed"anti stereospecific addition". Incidentally, this
kind of reaction mechanism is also an AdE mechanism, like
the hydration and hydrochlorination of alkenes.
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THE EPIBROMONIUM ION IS A RESONANCE HYBRID OF
BROMONIUM
AND CARBOCATION STRUCTURES
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ANTI STEREOSPECIFIC ADDITION TO ACYCLIC
ALKENES
THE STRONG TENDENCY OF BROMINE AND OTHER
HALOGENS TO ADD TO OPPOSITE FACES OF AN
ALKENE DOUBLE BOND IS ESPECIALLY WELL
ILLUSTRATED BY THE ADDITION OF BROMINE
TO CIS- AND TRANS-2-BUTENE
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The addition of bromine, for example, to trans-2-butene
yields only meso-2,3-dibromobutane and no traces of the
enantiomeric pair. In contrast, cis-2-butene yields only
the enantiomeric pair as a racemate, and none of the
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meso isomer. By comparing the structure of the alkene
(which we have drawn as a Newman projection) with that
of the product corresponding to it, we can see that the
bromines had to add to opposite faces of the double bond.
This is termed anti stereospecific addition.
It should be noted that if the bromine atoms had both
entered from the same side (face) of the pi bond, the
opposite result would have been observed. That is, trans-
2-butene would have given the racemate and cis-2-butene
would have given the meso compound. A carbocation
mechanism would have allowed both modes of addition
(addition to the same face is called syn addition). In sucha mechanism, both 2-butene isomers would have given
both sets of products, i.e., trans-2-butene would have
given both meso and dl and cis-2-butene would have
given both.
The fact that the reaction is anti stereospecific is an
outstanding characteristic of the reaction which
immediately allows us to rule out a carbocation
mechanism.
Another characteristic of this type of mechanism is that
carbocation character is developed upon both carbons of
the original alkene. Although more of the positive charge
is on the halogen, the high electronegativity of positively
charge halogen assures that the C-Br bonds are very
highly polar, placing substantial positive charge on both
carbons. It is on both carbons because the two C-Br
bonds in the epibromonium ion are equivalent. As a
result, the reactivity of 2-butene, with one alkyl group on
each carbon of the epibromonium ion , is much higher
than that of propene, which has an alkyl group on only
one carbon of the epibromonium ion. This is in contrast
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to hydration or HCl addition, where 2-butene is not much
more reactive than propene.
HYDROBORATION/OXIDATION
THE OVERALL REACTION OF HYDROBORATION AND
OXIDATION IS SHOWN BELOW:
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The reagent is usually borane, BH3, but an organoborane
can also be used, as long as at least one B-H bond is
present. In the laboratory, the borane-THF complex,
dissoved in tetrahydrofuran (THF) is often used. All three
hydrogens of borane are usable. In our illustrations, for
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simplicity, we will usually designate the borane as R2BH,
where R can be alkyl or hydrogen.
The initial product of addition of borane or an
organoborane across a carbon-carbon pi bond is anorganoborane, where a new B-C bond has been made,
along with a new C-H bond. These two bonds are formed
and the B-H and C-C pi bonds are broken, all in concert,
i.e., in a single reaction step with no intermediates being
involved.
These organoboranes are not stable in air, reacting rather
rapidly with oxygen. Instead of isolating them, they are
normally treated in situ (i.e., in place) with alkaline
hydrogen peroxide, a treatment which converts the B-C
bond to a C-O bond (and a B-O bond). We are not going
to take up the mechanism of this latter reaction, but we
will note that the overall result of these two steps
(hydroboration plus oxidation) is to convert an alkene to
an alcohol. This reaction was discovered by Professor
H.C. Brown of Purdue University and is an important
enough synthetic conversion that he was awarded theNobel Prize for Chemistry primarily based upon this
work.
We should note that the net addition of water which
occurs during hydroboration/oxidation is in the anti-
Markovnikov regiochemical sense, with propene giving 1-
propanol, rather than the 2-propanol which is generated
by the acid catalyzed, electrophilic hydration mechanism.
TRANSITION STATE FOR HYDROBORATION
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In this example, we again use the resonance method to set
up a transition state model for hydroboration, but we
extend the resonance method a little from previous
examples. You recall that previously we have described
the TS as a resonance hybrid of R and P-like structures.However, we know that according to resonance theory, a
chemical species is best described as a resonance hybrid
of all of the reasonable, relatively low energy structures
which can be written using the rules (canons) of valence.
In the case of hydroboration, the boron atom is
electrophilic (vacant 2p AO), so that proceeding from the
reactant-like structure,and using electron flow arrows, wecan derive a structure in which boron is tetravalent and
negatively charged (a reasonable valence state for B) and
in which carbon is positively charged (specifically, the
carbon not bonding to B). Not being either an R or P-like
structure, this is designated as an "X" structure. To get a
better model of the TS than available from just the
treatment using R and P structures, we should include it.
So, there are three relatively reasonable structures to
write.
As a result, the DL/PC structure shows carbocation
character at the carbon not bonding to B, and negative
partial charge on B. The carbocation character, however,
is not so extensive because the X structure is not as
favorable as the R and P structures, and so does not
contribute as much. Recall that the true structure more
closely resembles the lower energy structure. Usually,charge separated structures are higher in energy than
neutral ones.
We can then rationalize the regiochemistry of the
hydroboration reaction, e.g., using propene as an
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example. We , of course, use the Method of Competing
Transition States. We see that the favored TS has
secondary carbocation character, while the disfavored
one has primary carbocation character. The favored one
leads to 1-propanol when the B is replaced by O in theoxidation step.
MORE COMPLEX EXAMPLES OF HYDROBORATION
/OXIDATION
The following examples illustrate the use of
hydroboration/oxidation for the net anti-Markovnikov,syn stereospecific hydration of alkenes. You should be able
to predict the structure of the alcohol obtained in the
hydration of such alkenes, specifying the regiochemistry
and the stereochemistry.
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Note that the H and B add syn to the double bond, and
the B adds to the less highly substituted carbon. This
results in the oxygen being at the less highly substituted
carbon of the original double bond.
Note also that syn addition to 1-methylcyclohexene yields
exclusively the trans isomer of 2-methylcyclohexanol.
CATALYTIC HYDROGENATION
ONE OF THE MOST GENERAL AND USEFUL
REACTIONS OF ALKENES IS THE ADDITION OF
MOLECULAR HYDROGEN (DIHYDROGEN) TO GIVEALKANES. THIS PROCESS IS CALLED CATALYTIC
HYDROGENATION.
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We will not stress the mechanism of this reaction in all of
its details, but keep in mind that it is a heterogeneous
reaction, i.e., it occurs on the surface of the catalyst (an
insoluble solid).
Both hydrogen atoms are delivered from the surface of
the catalyst. Since the alkene can only present one face to
the catalyst, both hydrogens are added in a syn
stereospecific manner.See the example of 1,2-
dimethylcyclohexene, which gives cis-1,2-
dimethylcyclohexane.
The hydrogenation of a C=C of an unsaturated fat (or oil)gives a saturated fat.
HEATS OF HYDROGENATION AND RELATIVE ALKENE
STABILITIES
The heat of hydrogenation of an alkene is the heat of
reaction per mole of alkene when it is hydrogenated to the
corresponding alkane.
The hydrogenation of an alkene is always exothermic, i.e.,
heat is released. This is because the hydrogenation
process involves the breaking of two bonds which are
weaker than the two bonds which are formed. Most
important in this is the relatively weak pi bond which is
broken.
The heat of hydrogenation is given approximately by the
sums of the dissociation energies of the bonds broken
(which heat must be supplied as energy, and therefore is
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positive), less the sums of the dissociation energies of the
bonds formed in the product(s). This heat is released
(decrease in energy of the products relative to the
reactants), so it is taken with a negative sign. If we are
given a table of such D's, we should be able to calculatethe approximate heat of hydrogenation. We will also use
this method, in general, to calculate approximate heats of
other reactions. It is useful to within about 5 kcal/mol.
The heat of hydrogenation of ethene is -32.8 kcal/mol, and
this heat decreases to -30.1 for propene, to 27.6 for trans-
2-butene, and in general decreases with each substitution
of an alkyl group for a hydrogen atom of ethene. It isimportant to note that a decrease in the amount of heat
given off implies an increase in stability (lower energy) of
the reactant alkene. Thus, alkyl groups stabilize a pi
bond, just as they do carbocations and radicals. The
approximate amount of the stabilization per alkyl
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group,however, is only about 2.6 kcal/mol, much less than
the amount of stabilization of a carbocation center by an
attached alkyl group.
We will not presently concern ourselves with the basis forthis stabilization effect.
The heat of hydrogenation of cis-2-butene is 1.0 kcal more
negative than that of trans-2-butene. Since they both give
the same product, namely butane, this must mean that
cis-2-butene is 1.0 kcal/mole higher in energy than trans-
2-butene. This is considered to be caused by a steric
repulsion between the two methyl groups in cis-2-butene
which are closer than the sums of the van der Waals radii
of two hydrogen atoms. No such interaction exists in the
trans isomer.
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