More Reactions of Alkenes

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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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