MORE REACTIONS OF ALKENES

TOP

TABLE OF CONTENTS FOR THIS PAGE

• ACID CATALYZED HYDRATION
• MECHANISM OF ACID CATALYZED HYDRATION
• TS MODEL FOR ACID CATALYZED HYDRATION
• RELATIVE RATES OF HYDRATION OF ALKENES
• ELECTROPHILES AND ELECTROPHILIC ADDITION
• CARBOCATION REARRANGEMENTS
• STEREOCHEMISTRY OF ADDITIONS INVOLVING CARBOCATION INTERMEDIATES
• ELECTROPHILIC ADDITION OF BROMINE AND OTHER HALOGENS
• ANTI STEREOSPECIFIC ADDITION OF BROMINE
• HYDROBORATION/OXIDATION
• TS MODEL FOR HYDROBORATION
• MORE COMPLEX EXAMPLES OF HYDROBORATION/OXIDATION
• CATALYTIC HYDROGENATION

  ---

  ADDITION OF WATER TO ALKENES (HYDRATION)

  THE OVERALL REACTION

  The acid catalyzed addition of water to a general alkene and also to isobutene is illustrated below:

  ---

  

  ---

  • Note that this is not a reaction mechanism, but an equation for the overall reaction.
  • 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.
  • Note also that the product is an alcohol.
  • 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 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

  ---

  

  ---

  • In the addition of HCl, the acid which transfers the proton 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.

   

  • 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 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 the conjugate acid of the product alcohol.

   

  • 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 isolated by 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.

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 (including a 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.

 

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 this oxygen 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 passive carbon, 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 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.

 

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 WITH NUCLEOPHILES, 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 Ad_E.

 

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

We have already seen that the order of carbocation stability of 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 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 hydride migrations 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 OF REARRANGEMENTS, WHICH MAY OR MAY NOT BE DESIRABLE FROM A STANDPOINT OF ORGANIC SYNTHESIS

STEREOCHEMICAL CONSEQUENCES OF CARBOCATION INTERMEDIATES

The scheme shown below illustrates another consequence of the involvement of carbocation intermediates in organic reactions, in particular addition reactions:

Carbocations are planar, sp² hybridized, and have a vacant 2p orbital, the latter being the specific site of electrophilic reactivity--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 mixture of products. A standard way of expressing this is to say that the carbocation can react equally from either face (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 TO ALKENES

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 that an extra bond (a carbon-halogen bond) is formed in this intermediate, which is called an EPIHALONIUM ION.

 

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.

THE EPIBROMONIUM ION IS A RESONANCE HYBRID OF BROMONIUM

AND CARBOCATION STRUCTURES

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

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 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 such a 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 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:

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 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 an organoborane, 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 the Nobel 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

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, we can 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 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 the oxidation 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.

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 GIVE ALKANES. THIS PROCESS IS CALLED CATALYTIC HYDROGENATION.

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 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 calculate the 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 is important 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 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 for this 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.



---



 

---

BACK TO THE TOP OF THIS PAGE

ON TO ALKYL HALIDES

BACK TO THE PREVIOUS SECTION OF ALKENES

BACK TO THE BAULD HOME PAGE