Reactions of the Carbon Carbon Double Bond Electrophilic and Free-Radical Addition

Chapter 6

Reactions of the Carbon Carbon Double Bond Electrophilic and Free-Radical Addition

The functional group

• The characteristic feature of the alkene structure is the carbon-carbon double bond. The characteristic reactions of an alkene are those that take place at the double bond. The atom or group of atoms that defines the structure of a particular family of organic compounds and, at the same time, determines their properties is called the functional group.

• In alkyl halides the functional group is the halogen atom, and in alcohols the OH group; in alkcnes it is the carbon- carbon double bond. We must not forget that an alkyl halide, alcohol, or alkene has alkyl groups attached to these functional groups; under the proper conditions, the alkyl portions of these molecules undergo the reactions typical of alkanes. However, the reactions that are characteristic of each of these compounds are those that occur at the halogen atom or the hydroxyl group or the carbon-carbon double bond.

• A large part of organic chemistry is therefore the chemistry of the various functional groups. We shall learn to associate a particular set of properties with a particular group wherever we may find it. When we encounter a complicated molecule, which contains a number of different functional groups, we may expect the properties of this molecule to be roughly a composite of the properties of the various functional groups. The properties of a particular group may be modified, of course, by the presence of another group and it is important for us to understand these modifications, but our point of departure is the chemistry of individual functional groups.

Reactions of the carbon-carbon double bond: addition

• Alkene chemistry is the chemistry of the carbon -carbon double bond. What kind of reaction may we expect of the double bond. The double* bond consists of a strong or bond and a weak IT bond; we might expect, therefore, that reaction would involve the breaking of this weaker bond. This expectation is correct; the typical reactions of the double bond are of the sort where the -n bond is broken, and two strong bonds are formed in its place. 

Hydrogenation. Heat of hydrogenation

• We have already encountered hydrogenation as the most useful method for preparing alkanes. It is not limited to the synthesis of alkanes but is a general method for the conversion of a carbon-carbon double bond into a carbon carbon single bond: using the same apparatus, the same catalyst, and the same conditions, we can convert an alkene into an alkane. an unsaturated alcohol into a saturated alcohol, or an unsaturated ester into a saturated ester. Since the reaction is generally_quantitative,_and since tritium of hydrogen consumed can be easily measured, hydrogenation is frequently used as an analytical tool; it can, for example, tell us the number of double bonds in a compound.

• Hydrogenation is exothermic: the two a bond (C H) being formed are, together, stronger than a bond (H H) and -n bond being broken. The quantity of heat evolved when one mole of an unsaturated compound is hydrogenated is called the heat of hydrogenation; it is simply A// of the reaction, but the minus sign is not included. The heat of hydrogenation of nearly every alkene is fairly close to an approximate value of 30 kcal for each double bond in the compound.

• Although hydrogenation is an exothermic reaction, it proceeds at a negligible rate in the absence of a catalyst, even at elevated temperatures. The uncatalyzed reaction must have, therefore, a very large energy of activation. The function of the catalysfis to lower the energy of activation ( act) so that the reaction can proceed rapidly at room temperature. The catalyst does not, of course, affect the net energy change of the overall reaction; it simply lowers the energy hill between the reactants and products

• Lowering the energy hill, as we can see, decreases the energy of activation of the reverse reaction as well, and thus increases the rate of e/dehydrogenation. We might expect, therefore, that platinum, palladium, and nickel, under the proper conditions, should serve as dehydrogenation catalysts; this is indeed the case.

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

Heat of hydrogenation and stability of alkenes

• Heats of hydrogenation can often give us valuable information about the relative stabilities of unsaturated compounds. For example, of the isomeric 2-butenes, the cw-isomer has a heat of hydrogenation of 28.6 kcal, the trans homer one of 27.6 kcal. Both reactions consume one mole of hydrogen and yield the same product, n-butane.

• Of simple disubstituted ethylene's, it is usually the flaws-isomer that is the more stable. The two larger substituents are located farther apart than in the cafe-isomer; there is less crowding, and less van der Waals strain.

• CH3CH2CH=CH2 CH3CH=CHCH3 30.3 kcal cis 28.6; trans 27.6 CH3CH2CH2CH=CH2 CH3CH2CH=CHCH3 30. 1 kcal cis 28.6 ; trans 27.6 CH3 CH3 CH3 CH3CHCH=CH2 CH2=CCH2CH3 CH3C=CHCH3 30.3 kcal 28.5 26.9.

• Each set of isomeric alkenes yields the same alkane. The differences in heat of hydrogenation must therefore be due to differences in stability. In each case, the greater the number of alkyl groups attached to the doubly bonded carbon atoms, the more stable the 

Addition of halogen

• Alkenes are readily converted by chlorine or bromine into saturated compounds that contain two atoms of halogen attached to adjacent carbons; iodine generally fails to react.

• C=C + X2 > C-C Alkene (X2 = Cl2 ,Br2) ^ ^

• The reaction is carried out simply by mixing together the two reactants, usually in an inert solvent like carbon tetrachloride. The addition proceeds rapidly at room temperature or below, and does nofrequire exposure to ultraviolet light; in fact, we deliberately avoid higher temperatures and undue exposure to light, as well as the presence of excess halogen, since under those conditions substitution might become an important side reaction.

• Addition of bromine is extremely useful for detection of the carbon-carbon double bond. A solution of bromine in carbon tetrachloride is red; the dihalide, like the alkene, is colorless. Rapid decolorization of a bromine solution is characteristic of compounds containing the carbon-carbon double bond.  

• A common method of naming alkene derivatives is illustrated here. As we see, the product of the reaction between ethylene and bromine has the IUPAC name of 1,2-dibromoethane. It is also frequently called ethylene bromide, the word ethylene forming part of the name even though the compound is actually  saturated. This is an old-fashioned name, and is meant to indicate the product of the reaction between ethylene and bromine, just as, for example, sodium bromide would indicate the product of the reaction between sodium and bromine. It should not be confused with the different compound, 1,2-dibromoethene, BrCH=CHBr. In a similar way, we have propylene bromide, isobutylene bromide, and so on.

• We shall shortly encounter other saturated compounds that are named in a similar way, as, for example, ethylene bromohydrin and ethylene glycoL These names have in common the use of two words, the first of which is the name of the alkene; in this way they can be recognized as applying to compounds no longer containing the double 

Addition of hydrogen halidcs. Markovnikov's rule

• An alkene is converted by hydrogen chloride, hydrogen bromide, or hydrogen iodide into the corresponding alkyl halide.

• The reaction is frequently carried out by passing the dry gaseous hydrogen halide directly into the alkene. Sometimes the moderately polar solvent, acetic acid, which will dissolve both the polar hydrogen halide and the non-polar alkene, is used. The familiar aqueous solutions of the hydrogen hatides are not generally used; in part, this is to avoid the addition of water to the alken.

• Propylene could yield either of two products, the //-propyl halide or the isopropyl halide, depending upon the orientation of addition, that is, depending upon which carbon atoms the hydrogen and halogen become attached to. Actually, it is found that the isopropyl halide greatly predominates.

• CH3-~CH=CH2 ~X-> CH3 CH CH2 -Sw .'' I I H-l H I n-Propyl iodide CH3-CH=CH2 - CH3 ~CH-CH2 Actual product I-H I H

• In the same way, isobutylene could yield either of two products, isobutyl halide or ter/-butyl halide; here the orientation of addition is such that the tert-butyl halide greatly predominates.

• Orientation in alkane substitutions (Sec. 3.21) depends upon which hydrogen is replaced; orientation in alkene additions depends upon which doubly-bonded carbon accepts Y and which accepts Z of a reagent YZ.

• Examination of a large number of such additions showed the Russian chemist Vladimir Markovnikov (of the University of Kazan) that where two isomeric products are possible, one product usually predominates. He pointed out in 1869 that the orientation of addition follows a pattern which we can summarize as: In the ionic addition of an acid to the carbon-carbon double bond of an alkene, the hydrogen of the acid attaches itself to the carbon atom that already holds the greater number of hydrogens. This statement is generally known as Markovnikov's rules.

• hus, in the addition to propylene we see that the hydrogen goes to the carbon bearing two hydrogen atoms rather than to the carbon bearing one. In the addition to isobutylene, the hydrogen goes to the carbon bearing two hydrogens rather than to the carbon bearing none.

 Addition of hydrogen bromide. Peroxide effect

• Addition of hydrogen chloride and hydrogen iodide to alkenes follows Markovnikov's rule. Until 1933 the situation with respect to hydrogen bromide Was exceedingly confused. It had been reported by some workers that addition of hydrogen bromide to a particular alkene yields a product in agreement with Markovnikov's rule; by others, a product in contradiction to Markovnikov's rule; and by still others, a mixture of both products. It had been variously reported that the product obtained depended upon the presence or absence of water, or of light, or of certain metallic halides; it had been reported that the product obtained depended upon the solvent used, or upon the nature of the surface of the reaction vessel.

• Organic peroxides are compounds containing the O linkage. They are encountered, generally in only very small amounts, as impurities in many organic compounds, where they have been slowly formed by the action of oxygen. Certain peroxides are deliberately synthesized and used as reagents.

• This reversal of the orientation of addition caused by the presence of peroxides is known as the peroxide effect. Of the reactions we are studying, only the addition of hydrogen bromide shows the peroxide effect. The presence or absence of peroxides has no effect on the orientation of addition of hydrogen chloride, hydrogen iodide, sulfuric acid, water, etc. As we shall see (Sees. 6.11 and 6.17), both Markovnikov's rule and the peroxide effect can readily be accounted for in ways that are quite consistent with the chemistry we have learned so far.

Addition of sulfuric acid

• Alkenes react with cold, concentrated sulfuric acid to form compounds of the genera] formula ROSO3 H, known as alkyl hydrogen sulfates. These products are formed by addition of hydrogen ion to one side of the double bond and basculation to the other. It is important to notice that carbon is bonded to oxygen and not to sulfur.

• Reaction is carried out simply by bringing the reactants into contact: a gaseous alkene is bubbled through 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 alkyl hydrogen sulfates are deliquescent solids, and are difficult to isolate. As the examples below show, the concentration of sulfuric acid required for reaction depends upon the particular alkene involved.

• 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 alkyl hydrogen sulfate. The alkyl hydrogen sulfate has been cleaved by water to form the alcohol and sulfuric acid, and is said to have been hydrolyzed. This sequence of reactions affords a route to the alcohols, and it is for this purpose that addition of sulfuric acid to alkenes is generally carried out. This is an excellent method for the large-scale manufacture of alcohols, since alkenes are readil obtained by the cracking of petroleum. Because the addition of sulfuric acid follows Markovnikov's rule, certain alcohols cannot be obtained by this method. For example, isopropyl alcohol can be made but not w-propyl alcohol; ferf-butyl alcohol, but not isobutyl alcohol.

• The fact that alkenes dissolve in cold, concentrated sulfuric acid to form the alkyl hydrogen sulfates is made use of in the purification of certain other kinds of compounds. Alkanes or alkyl halides, for example, which are insoluble in sulfuric acid, can be freed from alkene impurities by washing with sulfuric acid. A gaseous alkane is bubbled through several bottles of sulfuric acid, and a liquid alkane is shaken with sulfuric acid in a separatory funnel. 

Addition of water. Hydration

• Water adds to the more reactive alkenes in the presence of acids to yield alcohols. Since this addition, too, follows Markovnikov's rule, the alcohols are the same as those obtained by the two-step synthesis just described; this direct hydration is, of course, the simpler and cheaper of the two processes. Hydration of alkenes is the principal industrial source of those lower alcohols whose formation is consistent with Markovnikov's rule.

• CH3 CH3 CH3-C=CH2 H2 * H *> CH3-C-CH3 Isobutylene I OH

Electrophilic addition: mechanism

• Before we consider other reactions of alkenes, it will be helpful to examine the mechanism of some of the reactions we have already discussed. After we have done this, we shall return to our systematic consideration of alkene reactions, prepared to understand them better in terms of these earlier reaction
• Ared to understand them better in terms of these earlier reactions. We shall take up first the addition of those reagents which contain ionizable hydrogen: the hydrogen halides, sulfuric acid, and water. The generally accepted mechanism will be outlined, and then we shall see how this mechanism accounts for certain facts. Like dehydration of alcohols, addition is pictured as involving carbonium ions. We shall notice certain resemblances between these two kinds of reaction; these resemblances are evidence that a common intermediate is involved.

• 0=C-- + H: Z > -C-C- +: Z HZ - HC1, HBr, HI, e H2S04, H30+ -0=C-- + H: Z > -C-C- + ^ e (2) -C--C--+: Z > -C-C-: Z C1-, Br~, I-, i i i HS04.

• We notice that the carbonium ion combines with water to form not the alcohol but the protonated alcohol; in a subsequent reaction this protonated alcohol releases a hydrogen ion to another base to form the alcohol. This sequence of reactions, we can see, is just the reverse of that proposed for the dehydration of alcohols (Sec. 5.20). In dehydration, the equilibria are shifted in favor of the alkene chiefly by the removal of the alkene from the reaction mixture by distillation: in hydration, the equilibria are shifted in favor of the alcohol partly by the high concentration of water.
• Let us see how this mechanism accounts for some of the facts. First, the mechanism is consistent with (a) the acidic nature of the reagents. According to the mechanism, the first step in all these reactions is the transfer of a hydrogen ion to the alkene. This agrees with the fact that all these reagents except water are strong acids in the classical sense ; that is, they can readily supply hydrogen ions. The exception, water, requires the presence of a strong acid for reaction to occur.

Electrophilic addition: orientadon and reactivity

• The mechanism is consistent with the orientation of addition of acidic reagents, and with the effect of structure on relative reactivities.

• Addition of hydrogen chloride to three typical alkenes is outlined below, with the two steps of the mechanism shown. In accord with Markovnikov's rule, propylene yields isopropyl chloride, isobutylene yields terf-butyl chloride, and 2-methyl-2-butene yields /er/-pentyl chloride.

• Which alkyl halide is obtained depends upon which intermediate carbonium ion is formed. This in turn depends upon the alkene and upon which carbon of the double bond hydrogen goes to. Propylene, for example, could yield an /i-propyl cation if hydrogen went to C-2 or an isopropyl cation if hydrogen went to C-l.

• In each of the examples given above, the product obtained shows that in the initial step a secondary cation is formed faster than* a primary, or a tertiary faster than a primary, or a tertiary faster than a secondary. Examination of many cases of addition of acids to alkenes shows that this is a general rule: orientation is governed by the ease of formation of carbonium ions, which follows the sequence.

• In addition reactions, the carbonium ion is formed by attachment of hydrogen ion to one of the doubly-bonded carbons. In the reactant the positive charge is entirely on the hydrogen ion; in the product it is on the carbon atom. In the transition state, the C H bond must be partly formed, and the double bond partly broken. As a result the positive charge is divided between hydrogen and carbon.

• Electron-releasing groups tend to disperse the partial positive charge (8+) developing on carbon and in this way stabilize the transition state. Stabilization of the transition state lowers act and permits a faster reaction (see Fig. 6.4). As before, the electron release that stabilizes the carbonium ion also stabilizes the incipient carbonium ion in the transition state. The more stable carbonium ion is formed faste.

Electrophilic addition: rearrangement

• The mechanism of electrophilic addition is consistent with the occurrence of rearrangements.

• If carbonium ions are intermediates in electrophilic addition, then we should expect the reaction to be accompanied by the kind of rearrangement that we said earlier is highly characteristic of carbonium ions (Sec. 5.22). Rearrangements are not only observed, but they occur according, to just the pattern that would be predicted.

• For example, addition of hydrogen chloride to 3,3-dimethyl-l-butene yields not only 2-chloro-3,3-dimethylbutane, but also 2-chloro-2,3-dimethylbutane:

• CH3 CH3 CH3CHj-C: CH==CH2 2!E> CH3-C CH-CH3 2^> CH3-C CH-CH3<!:H3 CH3 CH3 Cl

• Since a 1,2-shift of a methyl group can convert the initially formed secondary cation into the more stable tertiary cation, such a rearrangement does occur, and much of the product is derived from this new ion. (If we compare this change in carbon skeleton with the one accompanying dehydration of 3,3-dimethyl-2-butanol (p. 171), we can begin to see how the idea arose that these apparently unrelated reactions proceed through the same intermediate.)

• From the structure of the double bond we might expect that here again it is an electron source, a base, and hence that the halogen acts as an electrophilic reagent, an acid. This idea is supported by the fact that alkenes usually 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 an alkene.

• It is true that a halogen molecule is non-polar, since the two identical atoms share electrons equally. This is certainly not true, however, for a halogen molecule while it is under the influence of the powerful electric field of a nearby carbon carbon double bond. The dense electron cloud of the double bond tends to repel the similarly charged electron cloud of the halogen molecule; this repulsion makes the halogen atom that is nearer the double bond relatively positive and its partner.

• relatively negative. The distortion of the electron distribution in one molecule caused by another molecule is called polarization. Here, we would say that the alkene has polarized the halogen molecule.

• The facts are in complete agreement with this expectation. When ethylene is bubbled into an aqueous solution of bromine and sodium chloride, there is formed not only the dibromo compound but also the bromchlorenone compound and the Bromo alcohol. Aqueous sodium chloride alone is completely inert toward ethylene; chloride ion or water can react only after the carbonium ion has been formed by the action of bromine. In a similar way bromine and aqueous sodium iodide or sodium nitrate convert ethylene into the bromoiodism compound or the Bromo nitrate, as well as into the dibromo compound and the Bromo alcohol. 

Halohydrin formation 

• As we hav* just seen, addition of chlorine or bromine in the presence of water can yield compounds containing halogen and hydroxyl groups on adjacent carbon atoms. These compounds are commonly referred to as halohydrins. Under proper conditions, they can be made the major products. For example:

• There is evidence, of a kind we are not prepared to go into here, that these compounds are formed by reaction of halogen and water rather than by addition of preformed hypohalous acid, HOX. Whatever the mechanism, the result is addition of the elements of hypohalous acid (HO and X), and the reaction is often referred to in that way.

• The reaction is often referred to in that way. We notice that in propylene chlorohydrin chlorine is attached to the terminal carbon. This orientation is, we say, quite understandable in light of the mechanism and what we know about formation of carbonium ions: the initial addition of chlorine occurs in the way that yields the more stable secondary cation. However, we shall have to modify this interpretation of the orientation to fit the modified mechanism.

 Free-radical addition. Mechanism of the peroxide-initiated addition of HBr

• To account for this peroxide effect, Kharasch and Mayo proposed that addition can take place by two entirely different mechanisms: Markovnikov addition by the ionic mechanism that we have just discussed, and anti-Markovnikov addition by a free-radical mechanism. Peroxides initiate the free-radical reaction; in their absence (or if an inhibitor, p. 189, is added), addition follows the usual ionic path.

• This free radical, like the free radical initially generated from the peroxide, abstracts hydrogen from hydrogen bromide (step 4). Addition is now complete, and a new bromine atom has been generated to continue the chain. As in halogenation of alkanes, every so often a reactive particle combines with another one, or is captured by the wall of the1 reaction vessel, and a chain is terminated.

• The mechanism is well supported by the facts. The fact that a very few molecules of peroxide can change the orientation of addition of many molecules of hydrogen bromide strongly indicates a chain reaction. So, too, does the fact that a very few molecules of inhibitor can prevent this change in orientation. It is not surprising to find that these same compounds are efficient inhibitors of many other chain reactions. Although their exact mode of action is not understood, it seems clear that they break the chain, presumably by forming unreactive radicals.

• The mechanism involves addition of a bromine atom to the double bond. It is supported, therefore, by the fact that anti-Markovnikov addition is caused not only by the presence of peroxides but also by irradiation with light of a wavelength known to dissociate hydrogen bromide into hydrogen and bromine atoms.

• Recently, the light-catalyzed addition of hydrogen bromide to several alkenes was studied by means of ear (electron spin resonance) spectroscopy, which not only can detect the presence of free radicals at extremely low concentrations, but also can tell something about their structure. Organic free radicals were shown to be present at appreciable concentration, in agreement with the mechanism. 

• Thus we find the chemistry of free radicals and the chemistry of carbonium ions following much the same pattern: the more stable particle is formed more easily, whether by abstraction or dissociation, or by addition to a double bortd. Even the order of stability of the two kinds of particle is the same: 3 > 2 > 1 > CH3 . In this particular case orientation is reversed simply because the hydrogen adds first in the ionic reaction, and bromine adds first in the radical reaction.

Other free-radical additions

• In the years since the discovery of the peroxide effect, dozens of reagents besides HBr have been found (mostly by Kharasch) to add to alkenes in the presence of peroxides or light. Exactly analogous free-radical mechanisms are generally accepted for these reactions, too.

• In the dark at room temperature, a solution of chlorine in tetrachloroethylene can be kept for long periods with no sign of reaction. When irradiated with ultraviolet light, however, the chlorine is rapidly consumed, with the formation of hexachloroethane; many molecules of product are formed for each photon of light absorbed; this reaction is slowed down markedly when oxygen is bubbled through the solution.

• Free-radical addition is probably even commoner than has been suspected. Recent work indicates that free-radical chains do not always require light or decomposition of highly unstable compounds like peroxides for their initiation. Sometimes a change from a polar solvent which can stabilize a polar transition state to a non-polar solvent causes a change from a heterolytic reaction to a homolytic one. In some cases, it may even be that chains are started by concerted homolysis, in which cleavage of comparatively stable molecules (halogens, for example) is aided by the simultaneous breaking and making of other bonds. In the absence of the clue usually given by the method of initiation, the free-radical nature of such Reactions is harder to detect; one depends upon inhibition by oxygen, detailed analysis of reaction kinetics, or a change in orientation or stereochemistry.

Free-radical polymerization of alkenes

• When ethylene is heated under pressure with oxygen, there is obtained a" compound of high molecular weight (about 20,000), which is essentially an alkane with a very long chain. This compound is made up of many ethylene units and hence is called polyethylene (poly = many). It is familiar to most of us as the plastic material of packaging films.

• Many other groups (e.g., COOCH3 , CN, C6H5) may be attached to the doubly bonded carbons. This substituted ethylene polymerizes more or less readily, and yield plastics of widely differing physical properties and uses, but the polymerization process and the structure of the polymer are basically the same as for ethylene or vinyl chloride.

• Polymerization requires the presence of a small amount of an initiator. Among the commonest of these initiators are peroxides, which function by breaking down to form a free radical. This radical adds to a molecule of alkene, and in doing so generates another free radical. This radical adds to another molecule of alkene to generate a still larger radical, which in corn adds to another molecule of alkene, and so on. Eventually the chain is terminated by steps, such as union of two radicals, that consume but do not generate radicals.

• This kind of polymerization, each step of which consumes a reactive particle and produces another, similar particle, is an example of chain-reaction polymerization. In Chap. 32, we shall encounter chain-reaction polymerization that takes place, not by way of free radicals, but by way of organic ions. We shall also encounter step-reaction polymerization, which involves a series of reactions each of which is essentially independent of the others.

Hydroxylation. Glycol formation

• Hydroxylation with permanganate is carried out by stirring together at room temperature the alkene and the aqueous permanganate solution: either neutral the reaction produces OH~ or, better, slightly alkaline. Heat and the addition of acid are avoided, since these more vigorous conditions promote further oxidation of the glycol, with cleavage of the carbon-carbon double bond.

Substitution by halogen. Allylic hydrogen

• So far in our discussion of alkenes, we have concentrated on the carbon Carbon double bond, and on the addition reactions that take place there. Now let us turn to the alkyl groups that are present in most alkene molecules. Since these alkyl groups have the alkane structure, they should undergo alkane Reactions, for example, substitution by halogen. But an alkene molecule presents two sites where halogen can attack, the double bond and the alkyl groups. Can we direct the attack to just one of these sites? The answer is yes, by our choice of experimental conditions.

• We know that alkanes undergo substitution by halogen at high temperatures or under the influence of ultraviolet light, and generally in the gas phase: conditions that favor formation of free radicals. We know that alkenes undergo addition of halogen at low temperatures and in the absence of light, and generally in the liquid phase: conditions that favor ionic reactions, or at least do not aid formation of radicals.

• If we wish to direct the attack of halogen to the alkyl portion of an alkene molecule, then, we choose conditions that are favorable for the free-radical reaction and unfavorable for the ionic reaction. Chemists of the Shell Development Company found that, at a temperature of 500-600, a mixture of gaseous propylene and chlorine yields chiefly the substitution product, 3-chloro-l-propene, known as allyl chloride (CH2=CH CH2 = allyl). Bromine behaves similarly.

• Consistent with Brown's explanation is the finding that low concentration of halogen can be used instead of high temperature to favor substitution over (free radical) addition. Addition of the halogen atom gives radical I, which falls apart (to regenerate the starting material) if the temperature is high or if it does not soon encounter a halogen molecule to complete the addition. The allyl radical, on the other hand, once formed, has little option but to wait for a halogen molecule, whatever the temperature or however low the halogen concentration.

Orientation and reactivity in substitution

• Thus alkenes undergo substitution by halogen in exactly the same way as do alkanes. Furthermore, just as the alkyl groups affect the reactivity of the double bond toward addition, so the double bond affects the reactivity of the alkyl groups toward- substitution..

• Halogenation of many alkenes has shown that: (a) hydrogens attached to doubly bonded carbons undergo very little substitution; and (b) hydrogens attached to carbons adjacent to doubly bonded carbons are particularly reactive toward substitution. Examination of reactions which involve attack not only by halogen atoms but by other free radicals as well has shown that this is a general rule: hydrogens attached to doubly bonded carbons, known as vinylic hydrogens, are harder to abstract than ordinary primary hydrogens; hydrogens attached to a carbon atom adjacent to a double bond, known as allylic hydrogens, are even easier to abstract than tertiary hydrogens.

• The bond dissociation energies in Table show that 104 kcal of energy is needed to form vinyl radicals from a mole of ethylene, as compared with 98 kcal for formation of ethyl radicals from ethane. Relative to the hydrocarbon from which each is formed, then, the vinyl radical contains more energy and is less stable than a primary radical, and about the same as a methyl radical. On the other hand, bond dissociation energies show that only 88 kcal is needed for formation of allyl radicals from propylene, as compared with 91 kcal for formation of tert-ouiyi radicals. Relative to the hydrocarbon from which each is formed, the allyl radical contains less energy and is more stable than the tertbutyl radical.

Stability of the allyl radical

• A further, most important outcome of the resonance theory is this: as a resonance hybrid, the allyl radical is more stable (i.e., contains less energy) than either of the contributing structures. This additional stability possessed by the molecule is referred to as resonance energy. Since these particular contributing structures are exactly equivalent and hence of the same stability, we expect stabilization due to resonance to be large.

• Just how large is the resonance energy of the allyl radical. To know the exact value, we would have to compare the actual, hybrid allyl radical with a non-existent radical of structure I or II something we cannot do, experimentally. We can, however, estimate the resonance energy by comparing two reactions: dissociation of propane to form a w-propyl radical, and dissociation of propylene to form an allyl radical.

• CH3 CH2 CHj > CH3 CH2CHr + H- A// = + 98 kcal Propane w-Propyl radical CH2^CH -CH3 > CH2=CH -CH2 - + H- A// = +85

Orbital picture of the allyl radical

• To get a clearer picture of what a resonance hybrid is and, especially, to understand how resonance stabilization arises let us consider the bond orbitals in the allyl radical. Since each barbital.

• As in the case of ethylene, the p orbital of one carbon can overlap the p orbital of an adjacent carbon atom, permitting the electrons to pair and a bond to be formed. In this way we would arrive at either of the contributing structures, I or II, with the odd electron occupying the/? orbital of the remaining carbon atom. But the overlap is not limited to a pair of p orbitals as it was in ethylene; the p orbital of the middle carbon atom overlaps equally well the p orbitals of both the carbon atoms to which it is bonded. The result is two continuous IT electron clouds, one lying above and one lying below the plane of the atoms.

• Since no more than two electrons may occupy the same orbital (Pauli exclusion principle), these ir clouds are actually made up of two orbitals. One of these, containing two n electrons, encompasses all three carbon atoms; the other, containing the third (odd) -n electron, is divided equally between the terminal carbons.

• The overlap of the p orbitals in both directions, and the resulting participation of each electron in two bonds, is equivalent to our earlier description of the allyl radical as a resonance hybrid of two structures. These two methods of representation, the drawing of several resonance structures and the drawing of an electron cloud, are merely our crude attempts to convey by means of pictures the idea that a given pair of electrons may serve to bind together more than two nuclei.

• We saw earlier that the methyl radical may not be quite flat: that hybridization of carbon may be intermediate between sp 2 and sp*. For the allyl radical, on the other hand and for many other free radicals flatness is clearly required to permit the overlap of/? orbitals that leads to stabilization of the radical.

• In terms of the conventional valence-bond structures we employ, it is difficult to visualize a single structure that is intermediate between the two structures, and the orbital approach, on the other hand, gives us a rather clear picture of the allyl radical: the density of electrons holding the central carbon to each of the others is intermediate between that of a single bond and that of a double bond.