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The characteristic feature -- and functional group -- of the alkenes is clearly the double bond. The characteristic reactions of this functional group fall into two categories.

1) The first of these categories are the addition reactions  which we will be studying in detail in this Unit. These reactions occur at the site of the double bond itself, and in doing so, eliminate the double bond. 

2)  The second of these categories will be examined in the Unit 13: Conjugated Systems. These are systems which include both C=C double bonds and C-C single bonds, often alternating in sequence. In such systems, reactions occur not necessarily at the site of the double bond itself. But rather they occur at certain other positions having special relationships to the double bond. In these reactions, although the double bond remains intact, it might be viewed as something of a catalyst in the sense that -- indirectly --  it plays an instrumental role in the reaction. It not only determines how rapidly a reaction take place, but also by which mechanism it will occur -- if indeed it occurs at all. 

                                            ~ Addition Reactions ~

Since the double bond consists of a strong sigma bond and a weaker pi bond, we might expect that reactions would involve breaking of this weaker bond. In fact, the typical reactions of the double bond are characterized by the breaking of the pi bond, and the formation of two strong sigma bonds in its place.

A reaction in which two molecules combine to yield a single molecule of product is called an addition reaction. The reagent is simply added to the substrate, in contrast to a substitution where part of the reagent is substituted for a part of the substrate. Addition reactions are necessarily limited to compounds that contain atoms sharing more than one pair of electrons (i.e. multiply bonded atoms). Addition is the opposite of elimination. Just as elimination generates a double bond, so addition destroys it.

Because the pi bonds in a double bond are loosely held, they are particularly available to a reagent that is seeking electrons. Thus, the double bond acts a source of electrons, or a base. The compounds with which it reacts are those that are electrophilic reagents. They are deficient in electrons -- that is, are acids. Reagents of another kind, free radicals, also seek electron(s). Thus we find that alkenes also undergo free radical addition. 

Most alkenes contain not only the double bond, but also alkyl groups which have essentially alkane-like structure. Therefore, besides the additions reactions characteristic of the double bond, alkenes may undergo the free radical substitution characteristic of alkanes. The alkyl groups attached to the double bonded carbons modify the reactions of the double bond. In turn, the double bond modifies the reactions of the alkyl groups. We shall observe a number of these modifications and see, where possible, how they can be accounted for.

Several of these reactions, both directly (e.g. hydration, hydrohalin formation, hydroxylation) and indirectly (e.g. addition of sulfuric acid / hydrolysis, oxymercuration / reduction, hydroboration / oxidation) offer convenient routes to the synthesis of alcohols, and it is for this specific purpose that these reactions are outlined here. Some are excellent methods for the large scale manufacture of alcohols, since alkenes are readily obtained by the cracking of petroleum. Certain alcohols may or may not be obtainable by these methods, depending on the adherence of the reaction sequences and corresponding OH positions to Markovnikov's rule.

I. Addition of Hydrogen

We have already encountered hydrogenation as the most useful method for preparing alkanes (from the corresponding alkenes). It is indeed, the most general method for converting double bonds into single bonds in many different compounds. Thus, using an identical apparatus, catalyst and experimental conditions, we can convert an alkene into an alkane, an unsaturated alcohol into a saturated alcohol, or an unsaturated ester into a saturated ester. By varying the catalyst and conditions, we can selectively hydrogenate multiple bonds.

Hydrogenation is of two general kinds: a) heterogeneous (two-phase) and b) homogenous (one-phase). 

Heterogeneous hydrogenation is the classical and most widely used method. The catalyst is a finely divided metal (e.g. platinum, palladium or nickel). A solution of the alkene is shaken under a slight pressure of hydrogen gas in the presence of a small amount of catalyst. Reaction generally takes place rapidly and smoothly. Upon completion, the solution of the saturated product is simply filtered from the insoluble catalyst.

Homogeneous hydrogenation is newer, and offers additional flexibility as well as selectivity. The catalysts consist of organometallic complexes of transition metals like rhodium or iridium. They are soluble in organic solvents, and thus hydrogenation occurs in solution. One inconvenience is the liquid-liquid phase separation afterwards.

Since the reaction is generally quantitative, and since the volume of consumed hydrogen can be easily measured, this method is often used as an analytical tool. It can, for example, tell us the number of double bonds in a compound.

Hydrogenation is exothermic. The two sigma bonds (C - H) being formed are, together, stronger than the sigma bond (H - H) and pi bond being broken. The heat of hydrogenation, D H, is equal to the quantity of heat evolved when one mole of an unsaturated compound is hydrogenated. This heat is approximately 30 kcal for each double bond in an alkene.

Even though it is exothermic, hydrogenation proceeds at a negligible rate in the absence of a catalyst, even at elevated temperatures. The uncatalyzed reaction must therefore have a very large activation energy barrier, which is drastically reduced by introducing a catalyst. The catalyst lowers the energy barrier by permitting the reaction to take place in a different way, i.e. by a different mechanism (in this case, on the surface of a solid).

Like hydrogenation, the addition of other reagents to the double bond is generally exothermic. Thus the energy consumed by the breaking of the Y-Z and pi bonds is almost always less than that liberated by the formation of the C-Y and C-Z bonds.

Heats of hydrogenation can often give valuable information about relative stabilities of unsaturated compounds. E.G. If two isomeric products exhibit a difference in D H values, then the one with the lower value (which evolves less heat per mole) contains less energy, and is therefore more stable. Of the dialkylethylenes, it is usually the trans isomer that is more stable than the cis. The two larger substituents are located farther apart in the trans isomer. Thus there is less crowding and less van der Walls strain. 

D H values also indicate that that the stability of an alkene depends upon the position of the double bond. In general, the greater number of alkyl groups attached to the doubly bonded carbon atoms, the more stable the alkene.

II. Addition of Hydrogen Halides

An alkene is converted by hydrogen halide HX ( X = Cl, Br, or I) into the corresponding alkyl halide.

The reaction is frequently carried out by passing the dry gaseous hydrogen halide directly into the alkene. Acetic acid, a moderately polar solvent which will dissolve both the polar hydrogen halide and the non-polar alkene, is often used. (The familiar aqueous solutions of the hydrogen halides are not generally used, due to the hazards of water addition to the alkene). 

In this way, propylene is converted into isopropyl iodide, the hydrogen becoming attached to one doubly bonded carbon and the halogen to the other. Also, the bromination of propylene yields 2 products: isopropyl bromide or n-propyl bromide, depending on the orientation of addition -- or which carbon atoms the H atom and X atom become attached to. Actually, only the isopropyl halide is formed. Thus, in the addition of an acid to the C=C double bond of an alkene, the H atom of the acid is regioselective. It attaches itself to the C atom already holding the largest number of H atoms. This is Markovnikov's Rule. Thus, the formation of isopropyl iodide (vs. n-propyl iodide) and the formation of tert-butyl iodide (vs. isobutyl iodide) as follows:

A widely observed reversal of orientation caused by the presence of peroxides has come to be known as the peroxide effect. Of the reactions we are studying here, only the addition of hydrogen bromide shows evidence of the peroxide effect. As we shall see later in this Unit, both Markovnikov's Rule and the peroxide effect can readily be accounted for in a manner consistent with our our current understanding of chemical principles.

III. Addition of Sulfuric Acid - Hydrolysis

Alkenes react with cold concentrated sulfuric acid to form compounds of the general formula ROSO3H known as alkyl hydrogen sulfates.

These products are formed by the addition of hydrogen to one carbon of the double bond and a bisulfate ion to the other. Like alkyl sulfonates (esters of sulfonic acids) , these compounds are esters of sulfuric acid.

The reaction is carried out simply by bringing the reactants into contact. A gaseous alkene in bubbled into the acid, and a liquid alkene is stirred or shaken with the acid. Since alkyl hydrogen sulfates are soluble in sulfuric acid, a clear solution results. The sulfates formed are deliquescent solids, and are difficult to isolate. The concentration of sulfuric acid required for reaction depends upon the particular alkene (which we will re-examine later in this Unit).

If the sulfuric acid solution of the alkyl hydrogen sulfate is diluted with water and heated, there is obtained an alcohol bearing the same alkyl group as the original sulfate. The ester has been cleaved by water to form the alcohol and sulfuric acid, and is said to have been hydrolyzed.

IV. Addition of Water (Hydration)

Water adds to the more reactive alkenes in the presence of acids to yield alcohols. Since this addition follows Mark's Rule, the alcohols are the same as those obtained by the two-step synthesis described here. This is the principal industrial source of lower alcohols whose formation is consistent with the rule. 

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Electrophilic Addition: The Mechanism

Before considering other reactions of alkenes, let us examine the mechanism of reactions we have discussed so far. Then we can return to other reactions with a more broad foundation of understanding. Addition of the acidic reagent HZ involves two steps:

Step (1) is the transfer of a hydrogen ion from :Z to the alkene to form a carbocation -- I.E. a transfer of a proton from one base to another. Note that the H atom is transferred as a proton -- that is without its electrons, which are left behind on the base :Z. In order to form the bond to the hydrogen, carbon uses the pi electrons formerly shared with the other C atom. This leaves the other C atom in an electron-deficient condition (6 electrons).

Step (1) is the slow, rate-determining step. It involves electrophilic attack by an acidic, electron-seeking reagent. It is an example of electrophilic addition. The electrophile can be almost any kind of electron-deficient molecule.

Step (2) is the combining of the carbocation with the base :Z.

The general mechanism can be illustrated by specific examples. The addition of hydrogen chloride:

The addition of sulfuric acid.

The addition of water.

Notice that initially the carbocation combines with water to form the protonated alcohol. In a subsequent step of the reaction sequence, the protonated alcohol releases an H+ ion to another base in order to form the alcohol. (Interestingly, this reaction sequence is just the reverse of that proposed for the dehydration of alcohols.)

Evidence for this proposed reaction mechanism includes the following:

a) The rate of reaction depends on the concentration of both the alkene and the reagent HZ. This is consistent with a mechanism that starts (and is limited by) a reaction between these two reagents.

b) The reaction requires an acidic reagent. This agrees with the fact that all these reagents (except water, which requires the presence of strong acid) can readily transfer protons. Thus, an alkene is a weak base, and accepts protons to a significant degree only from strong acids.

c) Where the structure permits, the reaction is accompanied by rearrangements. I.E. The product sometimes contains the group Z attached to a carbon atom that was not doubly bonded in the initial alkene substrate. Sometimes the product even has a carbon skeleton different form that of the substrate. These products are readily accounted for by rearrangements of the intermediate carbocations. Moreover, they follow exactly the same pattern that we have come to expect form previous studies of intermediate carbocations in SN1 substitution and E1 elimination. For example:

In this reaction, since a 1,2-shift of a methyl group can convert the initially formed secondary cation into a more stable tertiary cation, such a rearrangement does occur, and much of the product is derived form this new ion. The occurrence of carbocation rearrangements is the strongest evidence we have to support the proposed mechanism of reaction, since it bears directly on the heart of the mechanism: the formation of the carbocation.

d) The mechanism is consistent with Markovnikov's Rule of orientation of addition of acidic reagents.

Experimental evidence has shown that in the initial step: 1)  a secondary cation is formed faster than a primary cation, 2) a tertiary cation is formed faster than a primary cation, and 3) a tertiary cation is formed faster than a secondary. Thus, in electrophilic addition, the rate of formation of carbocations follows the sequence:

The configuration of the halide which is obtained depends on which carbocation is formed in the first step. The cation most rapidly formed will determine this configuration. Interestingly, we find that in listing the carbocations in order of their relatives rates of formation, we have again listed them on order of their stability: 

Thus, we can reword Markovnikov's rule as follows: Electrophilic addition to a C=C double bond involves the intermediate formation of the more stable carbocation.

Further considerations indicate that the rate of addition of a hydrogen ion to a double bond depends on the stability of the carbocation being formed. This factor determines not only the orientation of addition to a single alkene, but also the relative reactivities of different alkenes. Thus, alkenes generally show th efollowing order of reactivity toward addition of acids:

In an electrophilic addition reaction, these same alkenes will show clear preferences to the most rapid formation of the following (most stable for that compound) carbocations:

As substituents, halogens tend to attract electrons. Just as electron release by alkyl groups disperses the positive charge and stabilizes a carbocation, so electron withdrawal by halogens intensifies the positive charge and destabilizes the carbocation. We saw that this electron withdrawal slows down the formation of carbocations in heterolysis (Unit 7: Alcohols). Similarly, it slows down the formation of carbocations in electrophilic addition. E.G. Vinyl chloride CH2=CHCl is less reactive than ethylene CH2=CH2.

We can begin to see what a powerful weapon we have for attacking the problems that arise in connection with a wide variety of reactions that involve carbocations. We know that the more stable the carbocation, the more rapidly it is formed. We also know that its stability depends upon dispersal of charge, and that dispersal of charge is determined by the electronic effect of the attached groups. This approach has enabled us to deal with such seemingly different matters, including rearrangements as well as :

a) the relative rates of substrate reactivities in SN1 substitution;

b) the relative ease of dehydration of alcohols;

c) the relative reactivities of alkenes toward addition of acids;

d) the orientation of addition of acids to alkenes;

V. Addition of Halogens

Alkenes are readily converted by chlorine or bromine into saturated compounds that contain two atoms of halogen attached to adjacent C atoms.

The reaction is carried out simply by mixing together the two reactants -- usually in an inert (non-polar) solvent like CCl4. The addition proceeds rapidly at room temperature or below, and does not require exposure to UV light. (In fact, we deliberately avoid higher temperatures and undue exposure to light, as well as the presence of excess halogen, since under such conditions substitution might become a significant side reaction.)

This reaction is by far the best method for preparing vicinal dihalides. Also, the addition of bromine is extremely useful for detection of the C=C double bond. A solution of bromine in CCl4 is red, while the dihalide (like the alkene) is colorless. Rapid decolorization of a bromine solution is thus characteristic of compounds containing the C=C double bond.

In the first step, the reaction mechanism for the addition of halogens to alkenes differs distinctly form the previous addition reactions. In short, the cation formed is NOT believed to be a carbocation.

We refer to this cation as halonium ion -- or in this case, a bromonium ion. Thus, in step (1)  bromine is transferred form a bromine molecule to the alkene. In so doing, it is not transferred to just one of the doubly bonded carbons. But rather it attaches itself to both of the doubly bonded C atoms -- forming a cyclic reaction intermediate called a halonium ion.

Step (1) does indeed represent electrophilic addition. Bromine is transferred as positive bromine, without a pair of electrons. The electrons are thus left behind on the newly formed bromide ion. In step (2) this bromide ion, (or more probably another just like it) reacts with the bromonium ion to yield the dibromide product. 

In search of clarity here, consider an alternative viewpoint. From the standpoint of a halogen molecule, the reaction with an alkene is nucleophilic substitution. Acting as a nucleophile, the alkene attaches itself to tone of the bromines and pushes the other bromine out as bromide ion. Bromide ion is the leaving group in this reaction scheme. And as we have seen, the bromide ion is an excellent leaving group.

Evidence includes:

1) The effect of structure of the alkene on reactivity. Thus, alkenes show the same order of reactivity toward halogens as toward the acids already studied. Electron-releasing substituents activate an alkene, and electron-withdrawing substituents deactivate. This fact supports the idea that addition is indeed electrophilic-- that the alkene is acting as the electron source, and that the halogen acts as an acid.

2)  The effect of added nucleophiles on the products obtained. If a halonium ion is the intermediate, and capable of reacting with halide ion, then we might expect it to react with almost any negative ion or basic molecule we care to provide (e.g. fluoride ion, iodide ion, nitrate ion, or water). This is, indeed, the case.

Thus, when ethylene is bubbled into an aqueous solution of bromine and sodium chloride, there is formed not only the dibromo compound, but also the bromochloro compound and the bromoalcohol. Aqueous sodium chloride alone is completely inert toward ethylene. Chloride ion or water can react only after the halonium ion has been formed by the action of bromine. Similarly, bromine and aqueous sodium iodide or sodium nitrate convert ethylene into the dibromoiodo compound or the bromo nitrate, as well as the dibromo compound and the bromoalcohol. Bromine in water with no added ion yields the dibromo compound and the bromoalcohol. 

This work definitely indicates that ethylene reacts with bromine to form something that can react readily with these other nucleophiles. In order to determine more certainly that it is indeed a bromonium ion which forms as the reaction intermediate, we must turn to additional considerations of chirality and stereospecific reactions (see Unit 12: Stereochemistry II) and actual experimental observation.  

VI. Halohydrin Formation

As we have seen, addition of chlorine or bromine in the presence of water can yield compounds containing halogen and hydroxyl on adjacent C atoms. These compounds are thus chloroalcohols or bromoalcohols. They are commonly referred to as halohydrins: chlorohydrins or bromohydrins. Under proper conditions they can be made the major products.

There is considerable evidence that these compounds are not formed by addition of preformed hypohalous acid, HOX, but rather by reaction of the alkene with halogen and water respectively.

Step (1):  Halogens add to form the halonium ion.

Step (2):  The halonium ion reacts with water to yield the protonated alcohol.

Thus, ethylene gives the chlorohydrin in which chlorine is attached to the terminal carbon. This is typical behavior for an asymmetrical alkene. The orientation follows Mark's rule, with positive halogen attaching itself to the same C atom that would be sought after by the hydrogen of a protic reagent. Yet the exclusively anti stereochemistry (experimentally observed) indicates that the intermediate is not an open cation but rather a cyclic halonium ion. Cleavage of this ring must involve attack by the nucleophile (H20) at the more hindered C atom. This is not surprising in view of the relative instability of the halonium ion, where bond-breaking should be fairly easy, and  cleavage should exhibit much SN1 character. In this case, cleavage should occur at the C atom which can best accommodate the positive charge, which is indeed, what is observed experimentally. Thus, the proposed cyclic halonium ion as the reaction intermediate.

VII. Dimerization

Under proper conditions, isobutylene is converted by sulfuric acid or phosphoric acid into a mixture of two isomeric alkenes of molecular formula C8H16.

Hydrogenation of either of these two alkenes produces the same alkane (Isooctane) as follows:

The two alkenes are isomers, and differ only in the position of the double bond. Since the alkenes produced contain exactly twice the number of C and H atoms as the original isobutylene, they are known as dimers of isobutylene, and the reaction is called dimerization.

Since the reaction is catalyzed by acid, we might assume that the first step in the reaction consists of the addition of a hydrogen ion to isobutylene to form the carbocation as follows. (The tertiary cation would, of course, be the preferred ion.) 

An electron-deficient C atom might easily seek out a double bond is an excellent electron source. Let us then write step (2) as the addition of tert-butyl cation to isobutylene. (The orientation of addition is such as to yield the more stable tertiary cation). Step (2) therefore brings about the union of two isobutylene units.

While we might expect this carbocation to bond with yet another molecule of isobutylene, in this particular case the carbocation undergoes the loss of a hydrogen ion in step (3). Since the hydrogen ion can be lost from a C atom on either side of the electron-deficient C atom, two products should be possible.  

We find that the products expected on the basis of our proposed mechanism are indeed the ones which are actually obtained. Thus, form what we have seen here, we can add one more reaction to our list of those which are characteristic of carbocations.

                       A carbocation may add to an alkene in order to form a larger carbocation.

VIII. Addition of Alkanes (Alkylation)

When isobutylene and isobutane are allowed to react in the presence of an acidic catalyst, they form directly 2,2,4-trimethylpentane. This reaction is, in effect, addition of an alkane to an alkene.

The commonly accepted mechanism of this alkylation is quite similar to that of dimerization. In fact, the first 2 steps are identical (see above).  But this reaction involves a 3rd step that we have not previously encountered.

In step (3) a carbocation abstracts a hydrogen atom with its pair of electrons (a hydride ion) form a molecule of alkane. This abstraction yields an alkane of 8 carbons, and anew carbocation to continue the chain. The abstraction occurs in the way that yields the tert-butyl cation rather than the less stable isobutyl cation.

This is not our first encounter with the transfer of hydride ion to an electron-deficient C atom. We saw much the same thing in the 1,2-shifts accompanying the rearrangement of carbocations. In those cases, transfer was intramolecular (within a molecule). Here it is intermolecular (between molecules). There are two important observations here. First of all, this reaction shows us what an extremely strong acids carbocations are. It also illustrates the distinct reactivity (vs. "inertness") of alkanes.

          We might even think of a carbocation as being chemically analogous to an H+ proton.

We conclude by noting that all reactions of a carbocation occur in order to provide a pair of electrons to complete the octet of the positively charged (electron-deficient) carbon atom.

IX. Oxymercuration - Reduction

Alkenes react with mercuric acetate in the presence of water to give hydroxy-mercurial compounds which on reduction yield alcohols.

- OAc    =    CH3COO -

The first stage involves addition to the C=C double bond of -OH and -HgOAc. Them in demercuration (or reduction) the HgOAc is replaced by -H. The reaction sequence amounts to hydration of the alkane, but is much more widely applicable than direct hydration. The alkene is added at room temperature to an aqueous solution of mercuric acetate diluted with the solvent tetrahydrofuran. The reaction is highly regioselective, and gives alcohols corresponding to Markovnikov addition of water to the C=C double bond.

Oxymercuration involves electrophilic addition to the C=C double bond, with the mercuric ion acting as the electrophile. The absence of rearrangement argues against an open carbocation as a reaction intermediate. Instead, it has been proposed that there is formed a cyclic mercurinium ion, analogous to the halonium ions (bromonium and chloronium ions) involved in the addition of halogens.

The mercurinium ion is attacked by the nucleophilic solvent (water) to yield the addition product. Rearrangements can occur, but are not common. Again, as in halohydrin formation, cleavage of the mercurinium ion should occur at the C atom which can best accommodate the positive charge -- which is indeed, what is observed experimentally. It is also worth noting here that mercuration can be carried out in different solvents in order to yield products other than alcohols.

X. Hydroboration - Oxidation

With the reagent diborane (BH3)2 alkenes undergo hydroboration to yield alkylboranes, R3B, which on oxidation give alcohols. The reaction procedure is simple and convenient, the yields are exceedingly high, and the products are difficult to obtain from alkenes in any other way.

For example:

Hydroboration involves the addition of BH3 (or, in following stages, BH2R and BHR2) to the double bond, with hydrogen becoming attached to one doubly bonded C atom, and boron to the other. The alkylborane can then undergo oxidation, in which the boron is replaced by -OH. The net result is the addition the double bond of the elements of H-OH.

Reaction is carried out in an ether (tetrahydrofuran or "diglyme": diethylene glycol methyl ether). The alkylboranes are not isolated, but are simply treated in situ with alkaline hydrogen peroxide.  The addition reaction is highly regioselective. The preferred product here, however, is exactly opposite to the one formed by oxymercuration - reduction or by direct acid-catalyzed hydration. The hydroboration - oxidation process gives products corresponding to anti-Markovnikov addition of water to the C=C double bond.  

The reaction of 3,3-dimethyl-21-butene illustrates a particular advantage of the method. Rearrangement does not occur in hydroboration -- evidently because carbocations are not reaction intermediates. Thus, the method can be utilized without the complications that often accompany other addition reactions.

The orientation of hydration in this reaction appears at first to unusual because hydrogen adds to the opposite end of the double bond from where it adds in ordinary electrophilic addition. But the fundamental idea in electrophilic addition is that the electrophilic part of the reagent -- the acidic part -- becomes attached, using the pi electrons, in such a way that the C atom being deprived of the pi electrons is the one best equipped to accommodate a partial positive charge.

For example, in the addition of HZ to propylene, the proton attaches itself to C1. That way the positive charge develops on C2, where it can be dispersed by the methyl group. Thus, a secondary carbocation is formed instead of a primary one.

 So what is the center of acidity in BH3 ? Clearly it would be the boron atom, with only six electrons. It is therefore not at all surprising that boron should seek out the pi electrons of the double bond and begin to attach itself to carbon. In so doing, it attaches itself in such a way that the partial positive charge can develop on the C atom best suited to accommodate it. Thus:

Unlike other addition reactions, however, the reaction does not proceed to give a carbocation. As the transition state is approached, the carbon that is losing the pi electrons becomes itself increasingly acidic. Now the electron deficient B atom is acidic. But so is the electron-deficient C atom. And not far away is an H atom held to the B atom by a pair of electrons. The C atom begins to take that H atom, with its electron pair. The B atom, as it gains access to the pi electrons, is increasingly willing to release the H atom. The net result:

             Boron and hydrogen both add to the doubly bonded C atoms in the same transition state.   

In view of the basic nature of alkenes and the acidic nature of BH3, the principal driving force for the reaction is most likely the attachment of boron to carbon. In the transition state depicted above, attachment of the B atom to C1 has proceeded to a greater extent than attachment of the H atom to C2. Thus, a loss of pi electrons by C2 to the C1-B bond exceeds its gain of electrons from the H atom. Therefore C2, the carbon atom that can best accommodate the positive charge, has become partially positive.

Thus, orientation of addition in hydroboration is controlled in fundamentally the same way as in two-step electrophilic addition. In this case, the H atom becomes attached to the the opposite end of the double bond (anti Markovnikov positioning) because here, acting as a proton (or acid) the hydrogen adds without electrons. In the other more typical cases of addition, the hydrogen is added as a hydride ion, or a base, with its own electrons.  

In addition to the polar factors discussed here, the orientation of hydroboration is also affected by steric hindrance. Attachment of a boron complex (-BH2, -BHR or -BR2) will take place more readily at the less crowded carbon of the double bond. In general, this leads to the same orientation as described previously. This steric factor will have a stronger influence with bulkier substituent groups. Alternatively, the more polar (either electron-releasing or electron-withdrawing) the substituents, the more important the polar factor.   

XI. Addition of Free Radicals

In the absence of peroxides, HBr adds to alkenes according to Mark's rule. In the presence of peroxides, the direction of addition is reversed. It is generally believed that peroxides initiate a free-radical chain reaction (recall the halogenation of methane in Unit 1) which results in anti Morkovnikov addition.

The essence of the mechanism is that hydrogen and bromine add to the double bond homolytically rather than heterolytically. The reaction intermediate is a free radical rather a carbocation. Like the halogenation of alkanes, this is a chain reaction involving addition (vs. substitution).

Decomposition of the peroxide (step 1) to yield free radicals is a well-known reaction. The free radical thus formed abstracts H from HBr (step 2) to form a bromine atom. In step (3) this bromine atom attaches itself to one of the double bonded C atoms. In so doing, it utilizes its odd electron and one of the pi electrons. The other C atom is left with an odd electron, and the alkene is converted into a free radical.  This free radical, like the one initially generated form the peroxide, abstracts hydrogen from HBr (step 4). Addition is now complete, and a new bromine atom has been generated to continue the propagation of the chain reaction. Occasionally, two free radicals combine, and a chain is terminated.  

Let us attempt to account for the anti Markovnikov addition by comparing the this type of free-radical addition reaction to electrophilic addition. The latter yields isopropyl bromide because the isopropyl cation is formed faster than the n-propyl cation (as it is more stable due to branching). Free-radical addition yields n-propyl bromide because the secondary free radical is formed faster than the primary one. Why is this ? Studies indicate the influence of three factors: a) Stability of the free radical being formed; b) Polar factors; c) Steric factors.

Let's begin by considering the first factor. In the transition state, the bond between bromine and one of the C atoms is partially formed. The pi bond is partially broken and the other C atom has partially gained the odd electron it will carry as an intermediate free radical. To some extent, the organic group possesses the character of the free radical it is to become. Factors that stabilize the free radical also stabilize the incipient free radical in the transition state. Thus, in this example, the secondary free radical is formed faster than the primary one because it is more stable.  

Now, because of its electronegativity we would expect the bromine atom to be electrophilic. In the transition state, bromine holds more than its share of electrons, at he expense of the alkene. Thus, the transition state is a polar one, and the substrate has not only free-radical character but also carbocation character. The stability of the transition state, and hence the rate fo reaction, therefore depends on the ability of the substrate: 1) to accommodate the odd electron, and 2) to accommodate the partial positive charge. The polar factor will thus favor the orientation that places the charge on the C atom that can best accommodate it. In the example, addition of Br to C1 is favored, since in this way positive charge develops on C2 rather than C1, and secondary carbocation character is more stabilizing than primary.  

Finally, addition of a free radical to the terminal carbon C1 is less hindered than addition to C2. The transition state is less crowded and therefore more stable.

Wile all three factors may be at work, orientation in both electrophilic and free-radical substitution of HBr is determined by the preferential formation of the more highly substituted complex. Orientation is reversed simply because the H atom adds first in the electrophilic reaction, and the Br atom adds first in the free-radical reaction. 

XII. Polymerization

When ethylene is heated under pressure with oxygen, there is obtained a compound of high molecular weight (~ 20,000 g / mol) which is essentially an alkane with a very long chain (I.E. a homolog with a very large number of repeating units). This compound consists of many identical ethylene units. Hence it is called polyethylene.

it is familiar to most of us as the plastic material of packaging films. The formation of polyethylene is a simple example of the process known as polymerization : the joining together of many small molecules to form very large molecules with repeating identical structural units or "mers"). Thus, polymers (or n-mers) are made form a number n of monomers. Polymerization of substituted ethylene yields compounds whose structures containing the long chain of polyethylene, with identical substituent groups attached at regular intervals. E.G. vinyl chloride yields poly(vinyl chloride), or PVC, which is used to make phonograph records and plastic pipe. When plasticized with high -boiling esters, it is also used to make raincoats, shower curtains and coatings for metals and upholstery fabrics.   

Many other functional groups (e.g. -COOCH3, -CH3, -C6H5) may be attached to the doubly bonded carbons. These substituted ethylenes polymerize more or loess readily, and yield plastics of widely differing physical properties and uses. But he polymerization process and the polymer structure are basically the same for ethylene or vinyl chloride.

Polymerization requires the presence of a small amount of initiator. Among the commonest of these are peroxides, which (as we have seen) function by breaking down to form a free radical. This radical adds to a molecule of alkene, and in so ding, generates another free radical. This radical adds to another molecule of alkene to generate an even larger radical, which turn adds to another molecule of alkene, and so on.

Eventually the chain is terminated by steps, such as the union of two radicals, that consume but do not generate new radicals. This kind of polymerization, in which each step consumes a reactive complex and produces anotherfresh one in its wake, is an example of chain reaction polymerization.

In Unit 26: Synthetic Polymers we shall encounter step-reaction polymerization, which involves a series of reactions which are all independent of each other.

XIII. Addition of Carbenes (Cycloaddition)

The most important route to cyclic compounds is via the class of reactions called cycloaddition (see also Unit 12: Stereochem II ). The difference between successive members of a homologous series in the CH2 unit, or methylene. Methylene derivatives are known as carbenes. Methylene exists in two different forms.

1)  Singlet methylene --  unshared electrons are paired  --  [ CH2: ]

2)  Triplet methylene -- unshared electrons are not paired  --  [ .CH2. ]

Triplet methylene is thus a free radical. In fact, it is a diradical. Singlet methylene is the less stable form. In the liquid phase, singlet methylene reacts rapidly with the abundant solvent molecules before ti loses energy. In the gas phase (especially inerts, e.g. nitrogen or argon) singlet methylene loses energy through collisions and is converted into triplet methylene, which then reacts.

When methylene is generated in the presence of alkanes, there are obtained cyclopropanes. This is an example 

of the most important reaction of methylene and other carbenes: addition to the C=C double bond. This particular kind of addition, in which a ring is generated, is called cycloaddition. Cycloaddition provides the most important route to rings of various sizes.

The most striking feature of the addition of methylene is that it can occur with two different kinds of stereochemistry. In one case, addition is stereoselective and stereospecific, and syn. In the other case, addition is none of those, but gives both cis and trans isomers.

It is almost certain that it is singlet methylene that undergoes the stereoselective addition. Although neutral, singlet methylene is electron-deficient and hence electrophilic. Like other electrophiles, it can find electrons at the

C=C double bond. the stereochemistry strongly indicates simultaneous attachment to both doubly bonded C atoms. Reaction involves overlap of the pi cloud of the alkene with the empty p orbital of the carbene. Electron density flows into this empty orbital, and the alkene C atoms become relatively positive in the transition state. Electron-releasing substituents in the alkene disperse this developing charge, stabilize the transition state, and speed up reaction. The reactivity pattern of alkenes is similar to that observed for halogen addition. 

It is triplet methylene that undergoes the non-stereoselective addition. Triplet methylene is a diradical, and it adds by a free-radical two-step mechanism: addition followed by combination. The intermediate diradical I last long enough for rotation to occur about the central C-C bond, and both cis and trans products are formed.

XIV. Epoxidation

Epoxides are compounds containing the three-membered ring:

They are ethers, but the three-membered ring gives them unusual properties which make them an exceedingly important class of compounds. Epoxides are commonly made by the oxidation of alkenes by peroxy compounds, such as benzoic acid:

When allowed to stand in ether or chloroform solution, the peroxy acid and the unsaturated compound -- which need not be a simple alkene -- react to yield benzoic acid and the epoxide. For example:

Epoxides owe their importance ot the ease of opening of the highly strained three-membered ring. They undergo acid-catalyzed reactions with extreme ease and -- unlike ordinary ethers -- can even be cleaved by bases.

XV. Hydroxylation (or Glycol formation)

Certain oxidizing agents convert alkenes into 1,2-diols: dihydroxy alcohols containing the two -OH groups on adjacent carbons (aka glycols). The reaction amounts to addition of two hydroxyl groups to the C=C double bond.

Of the numerous oxidizing agents that bring about hydroxylation, two of the most commonly used are: cold alkaline potassium permanganate (KMnO4); and peroxy acids, such as peroxyformic acid (HCO2OH). Hydroxylation with permanganate is carried out in solution at room temperature. Hydroxylation with peroxyformic acid is carried out by allowing the alkene to stand with a mixture of hydrogen peroxide and formic acid (HCOOH) for a few hours, and then heating the product with water to hydrolyze certain intermediate compounds. Hydroxylation of alkenes is the most important method for the synthesis of 1,2-diols, with the special feature of permitting stereochemical control by the proper choice of reagent.

XVI. Halogenation. Allylic Substitution.

Discussed in Unit 13: Conjugated Systems & Dienes.

XVII. Cleavage by Ozonolysis

Cleavage is a reaction in which the double bond is completely broken and the alkene molecule is converted into smaller molecules. The classical reagent for cleaving the C=C double bond is ozone, O3. Ozonolysis (cleavage by ozone) is carried out in two stages.

1) Addition of ozone to the double bond in order to form an ozonide.

2) Hydrolysis of the ozonide to yield the cleavage products.

Ozone gas is passed into a solution of the alkene in some inert solvent like CCl4. Evaporation of he solvent leaves the ozonide as a viscous oil, This unstable, explosive compound is not purified. It is treated directly with water, often in the presence of a reducing agent.

In the cleavage products, a doubly bonded O atom is found attached to each of the originally doubly bonded C atoms. These compounds containing the C=O group are aldehydes and ketones (also the products of oxidation reactions in alcohols). The function of the reducing agent (e.g. zinc dust) is to prevent the formation of hydrogen peroxide, which would otherwise react with the aldehydes and ketones.    

Knowing the number and arrangement of carbon atoms in these aldehydes and ketones, we can work back to the structure of the original alkenes. One general approach to structural analysis of an unknown compound is via the process of degradation: the breaking down into a number of smaller, more easily identifiable fragments. Ozonolysis is a typical means of degradation, and as such, provides a very powerful tool in the science of analytical chemistry.