Chapter-11
Electrophilic Aromatic Substitution
Introduction
We have already seen that the characteristic reactions of benzene involve substitution, in which the resonance-stabilized ring system is preserved. What kind of reagents bring about this substitution? What is the mechanism by which these reactions take place?
Above and below the plane of the benzene ring there is a cloud of TT electrons. Because of resonance, these TT electrons are more involved in holding together carbon nuclei than are the TT electrons of a carbon-carbon double bond. Still, in comparison with a electron, these n electrons are loosely held and are available to a reagent that is seeking electrons.
It is not surprising that in its typical reactions the benzene ring serves as a source of electrons, that is, as a base. The compounds with which it reacts are deficient in elections, that is, are electrophilic reagents or acids. Just as the typical reactions of the alkenes are electrophilic addition reactions, so the typical reactions of the benzene ring are electrophilic substitution reactions.
These reactions are characteristic not only of benzene itself, but of the benzene ring wherever it is found and, indeed, of many aromatic rings, benzenoid and non-benzenoid.
Electrophilic aromatic substitution includes a wide variety of reactions: nitration, halogenation, sulfonation, and Friedel-Crafts reactions, undergone by nearly all aromatic rings; reactions like nitridation and diazo coupling, undergone only by rings of high reactivity; and reactions like dislocations, isotopic exchange, and many ring closures which, although apparently unrelated, are found on closer examination to be properly and profitably viewed as reactions of this kind. In synthetic importance electrophilic aromatic substitution is probably unequaled by any other class of organic reactions. It is the initial route of access to nearly all aromatic compounds: it permits the direct introduction of certain substituent groups which can then be converted, by replacement or by transformation, into other substituents, including even additional aromatic rings.
ELECTROPHILIC AROMATIC SUBSTITUTION
AR s aryl, any aromatic group with attachment directly to ring carbon
Effect of substituent groups
Ike benzene, toluene undergoes electrophilic aromatic substitution: sulfonating, for example. Although there are three possible monosulfonation products, this reaction actually yields appreciable amounts of only two of them: the o- and /Moser's.
Benzene and toluene are insoluble in sulfuric acid, whereas the sulfonic acids are readily soluble; completion of reaction is indicated simply by disappearance of the hydrocarbon layer. When shaken with fuming sulfuric acid at room temperature, benzene reacts completely within 20 to 30 minutes, whereas toluene is found to react within only a minute or two.
Studies of nitration, halogenation, and Friedel-Craffs alkylation of toluene give analogous results. In some way the methyl group makes the ring more reactive than unsubstituted benzene and directs the attacking reagent to the ortho and para positions of the ring.
On the other hand, nitrobenzene, to take a different example, has been found to undergo substitution more slowly than benzene, and to yield chiefly the meta isomer.
Like methyl or nitro, any group attached to a benzene ring affects the reactivity of the ring and determines the orientation of substitution. When an electrophilic reagent attacks an aromatic ring, it is the group already attached to the ring that determines how readily the attack occurs and where it occurs.
Determination of orientation
To determine the effect of a group on orientation is, in principle, quite simple: the compound containing this group attached to benzene is allowed to undergo substitution and the product is analyzed for the proportions of the three isomers. Identification of each isomer as ortho, meta, or para generally involves comparison with an authentic sample of that isomer prepared by some other method from a compound whose structure is known. In the last analysis, of course, all these identifications go back to absolute determinations of the Kerner type (Problem 10.8, p.
In this way it has been found that every group can be put into one of two classes: orthoptera directors or meta directors. Table 11.1 summarizes the orientation of nitration in a number of substituted benzenes. Of the five positions open to attack, three (60%) are ortho and para to the substituent group, and two (40%) are meta to the group; if there were no selectivity in the substitution reaction, we.
would expect the ortho and para isomers to make up 60% of the product, and the meta isomer to make up 40%. We see that seven of the groups direct 96-100% of nitration to the ortho and para positions; the other six direct 72-94% to the meta positions.
A given group causes the same general kind of orientation predominantly orthoptera or predominantly meta whatever the electrophilic reagent involved. The actual distribution of isomers may vary, however, from reaction to reaction. In Table 11.2, for example, compare the distribution of isomers obtained from toluene by sulfonation or bromination with that obtained by nitration.
Determination of relative reactivity
A group is classified as activating if the ring it is attached to is more reactive than benzene and is classified as deactivating if the ring it is attached to is less reactive than benzene. The reactivities of benzene and a substituted benzene are compared in one of the following ways.
The time required for reactions to occur under identical conditions can be measured. Thus, as we just saw, toluene is found to react with fuming sulfuric acid in about one-tenth to one-twentieth the time required by benzene. Toluene is more reactive than benzene, and -zCH^js therefore an activating group.
The severity of conditions required for comparable reaction to occur within the same period of time can be observed. For example, benzene is nitrated in less than an hour at 60 by a mixture of concentrated sulfuric acid and concentrated nitric acid; comparable nitration of nitrobenzene requires treatment at 90 with fuming nitric acid and concentrated sulfuric acid. Nitrobenzene is evidently less reactive than benzene, and the nitro group, NO2, is a deactivating group.
For an exact, quantitative comparison under identical reaction conditions, competitive reactions can be carried out, in which the compounds to be compared are allowed to compete for a limited amount of a reagent (Sec. 3.22). For example, if equimolar amounts of benzene and toluene are treated with a small amount of nitric acid (in a solvent like nitromethane or acetic acid, which will dissolve both
organic and inorganic reactants), about 25 times as much nitrotoluene as nitrobenzene is obtained, showing that toluene is 25 times as reactive as benzene. On the other hand, a mixture of benzene and chlorobenzene yields a product in which nitrobenzene exceeds the nitro chlorobenzenes by 30: 1, showing that chlorobenzene is only one-thirtieth as reactive as benzene. The chloral group is therefore classified as deactivating, the methyl group as activating. The activation or deactivation caused by some groups is extremely powerful: aniline, C6H5 NH2, is roughly one million times as reactive as benzene, and nitrobenzene, C6H5 NO2, is roughly one-millionth as reactive as benzene.
Classification of substituent group
The methods described in the last two sections have been used to determine the effects of a great number of groups on electrophilic substitution. As shown in Table 1 1.3, nearly all groups fall into one of two glasses: activating and ortho. Para directing or deactivating and wet directing. The halogens are in a class by themselves, being deactivating but 0r//70, /? <7ra-direct
Just by knowing the effects summarized in these short lists, we can now predict fairly accurately the course of hundreds of aromatic substitution reactions. We now know, for example, that bromination of nitrobenzene will yield chiefly the Aw-isomer and that the reaction will go more slowly than the bromination of benzene itself; indeed, it will probably require severe conditions to go at all. We now know that nitration of C6 H5 NHCOCHj, (acetanilide) will yield chiefly the 0- and /Mesomeres and will take place more rapidly than nitration of benzene.
Orientation in disubstituted benzenes
The presence of two substituents on a ring makes the problem of orientation more complicated, but even here we can frequently make very definite predictions. First of all, the two substituents may be located so that the directive influence of e reinforces that of the other; for example, in I, II, and III the orientation clearly it be that indicated by the arrows.
On the other hand, when the directive effect of one group opposes that of the icr, it may be difficult to predict the major product; in such cases complicated trues of several products are often obtained.
Orientation and synthesis
As we discussed earlier (Sec. 3.14), a laboratory synthesis is generally aimed at obtaining a single, pure compound. Whenever possible we should avoid use of a reaction that produces a mixture, since this lowers the yield of the compound we want and causes difficult problems of purification. With this in mind, let us see some of the ways in which we can apply our knowledge of orientation to the synthesis of pure aromatic compounds.
First of all, we must consider the order in which we introduce these various substituents into the ring. In the preparation of the trinitrobenzene's, for example, it is obvious that if we nitrate first and then brominate, we will obtain the w-isomer; whereas if we brominate first and then nitrate, we will obtain a mixture of the o- and p-isomers* The order in which we decide to carry out the two steps, then, depends upon which isomer we want.
Next, if our synthesis involves conversion of one group into another, we must consider the proper time for this conversion. For example, oxidation of a methyl group yields a carboxyl group (Sec. 12.10). In the preparation of nitrobenzoic acids from toluene, the particular product obtained depends upon whether oxidation or nitration is carried out first.
Substitution controlled by an activating group yields a mixture of ortho and para isomers; nevertheless, we must often make use of such reactions, as in the examples just shown. It is usually possible to obtain the pure para isomer from Jhe mixture by fractional crystallization. As the more symmetrical isomer, it is take less soluble (Sec. 12.3), and crystallizes while the solvent still retains the soluble ortho isomer. Some para isomer, of course, remains in solution to contaminate the ortho isomer, which is therefore difficult to purify. As we shall see, special approaches are often used to prepare ortho isomers.
A goal of aromatic synthesis is control of orientation: the preparation, at will and from the same substrate, of a pure ortho, a pure meta, or a pure para isomer. Steps toward this goal have been taken very recently by Edward C. Taylor (Princeton University) and Alexander McKillop (University of East Anglia), chiefly through the chemistry of thallium: thallium as the cation in organic salts; thallium salts as Lewis's acids; aryl thallium compounds (Sec. 11.13) as reactive organometallic intermediates. One approach to regiospecific substitution involves complexing attachment through a Lewis acid-base reaction of the attacking reagent by some other molecule. Complexing of the reagent by the substituent group prior to reaction tends to favor attack at the nearest position: ortho. Complexing of the reagent by a bulky molecule tends to favor attack at the least crowded position: para. If reaction can be carried out so that orientation is governed, not by relative rates of reaction as it usually is but by position of equilibrium, then the most stable isomer is favored: often the meta isomer. We shall see examples of all these ways of controlling orientation.
Mechanism of nitration
Now that we have seen the effects that substituent groups exert on (dentation and reactivity in electrophilic aromatic substitution, let us see how we can account for these effects. The first step in doing this is to examine the mechanism for the reaction. Let us begin with nitration, using benzene as the aromatic substrate.The commonly accepted mechanism for nitration with a mixture of nitric and sulfuric acids (the widely used "mixed acid" of the organic chemist) involves the following sequence of reactions:
Step (1) generates the nitrenium ion, cNO2, which is the electrophilic particle that actually attacks the benzene ring. This reaction is simply an acid-base equilibrium in which sulfuric acid serves as the acid and the much weaker nitric acid serves as a base. We may consider that the very strong acid, sulfuric acid, causes nitric acid to ionize in the sense, HO% ~ . . . + NO2, rather than in the usual way, H+ . . . ~ONO2 . The nitrenium ion is well known, existing in salts such as nitrenium perchlorate, NO2 + C1O4 ~, and nitrenium fluoborite, NO2 +BF4 ~. Indeed, solutions of these stable nitrenium salts in solvents like nitromethane or acetic acid have been found by George Olah (of Case Western Reserve University) to nitrate aromatic compounds smoothly and in high yield at room temperature.
Electrophilic substitution, then, like electrophilic addition, is a stepwise process involving an intermediate carbonium ion. The two reactions differ, how- -ever, in the fate of the carbonium ion. While the mechanism of nitration is, perhaps, better established than the mechanisms for other aromatic substitution reactions, it seems clear that all these reactions follow the same course.
Mechanism of sulfonation
Again, the first step, which generates the electrophilic sulfur trioxide, is simply an, 'iced-base equilibrium, this time between molecules of sulfuric acid. For olefination we commonly use sulfuric acid containing an excess of SO3; even if this is not done, it appears that SO3 formed in step (1) can be the electrophile.
Mechanism of Friedel-Crafts alkylation
In certain cases, there is no free carbonium ion involved. Instead, the alkyl group is transferred without a pair of electrons directly to the aromatic ring from the polar complex, I, between A1C13 and the alkyl halide:
The electrophile is thus either (a) R + or (b) a molecule like I that can readily transfer R + to the aromatic ring. This duality of mechanism is common In electrophilic aromatic substitution. In either case, the Lewis acid R4 " is displaced from RC1 by the other Lewis acid,
Mechanism of halogenation
The key step (2) is the attachment of positive chlorine to the aromatic ring, ft seems unlikely, though, that an actually free Cl + ion is involved. Instead, ferric chloride combines with C12 to form complex II, from which chlorine is transferred, without its electrons, directly to the ring.
Addition of halogens to alkenes, we have seen (Sec. 6.13), similarly involves attack by positive halogen to form an intermediate carbonium ion. The loosely held TT electrons of an alkene make it more reactive, however, and positive halogen is transferred from the halogen molecule itself, X2, with loss of Cl~. The less reactive benzene molecule needs the assistance of a Lewis acid; reaction occurs with the loss of the better leaving group, FeCl4 ~. Indeed, more highly reactive aromatic compounds, i.e., those whose it electrons are more available, do react with halogens in the absence of any added Lewis acid.
Sulfonating. Mechanism of protonation
When an aromatic sulfonic acid is heated to 100-175 with aqueous acid, it is converted into sulfuric acid and an aromatic hydrocarbon. This sulfonating is the exact reverse of the sulfonation process by which the sulfonic acid was originally made.
By applying the usual equilibrium principles, we can select conditions that will drive the reaction in the direction we want it to go. To sulfonate we use a large excess of concentrated or fuming sulfuric acid; high concentration of sulfonating agent and low concentration of water (or its removal by reaction with So) shift the equilibrium toward sulfonic acid. To desulphonated we use dilute acid and often pass superheated steam through the reaction mixture; high concentration of water and removal of the relatively volatile hydrocarbon by steam distillation shift the equilibrium toward hydrocarbon.
According to the principle of microscopic reversibility (p. 170), the mechanisms of sulfonating must be the exact reverse of the mechanism of sulfonation.
The reaction is simply another example of electrophilic aromatic substitution. The electrophile is the proton, H+, and the reaction is protonation or, more specifically, protodesulfonation.
Sulfonation is unusual among electrophilic aromatic substitution reactions in its reversibility. It is also unusual in another way: in sulfonation, ordinary hydrogen (protium) is displaced from an aromatic ring about twice as fast as deuterium. These two facts are related to each other and, as we shall see in Sec. 11.16, give us a more detailed picture of sulfonation and of electrophilic aromatic substitution in general.
Thallation
Treatment of aromatic compounds with thallium trifluoroacetate, T1(OOCCF3)3, dissolved in trifluoroacetic acid (CF3COOH) gives rapidly and in high yield aryl thallium Di trifluoroacetates, stable crystalline compounds. Reaction is believed by Taylor and McKillop (p. 345) to involve electrophilic attack on the aromatic ring by the (Lewis) acidic thallium.
Thallium compounds are very poisonous and must be handled with extreme care. Although substituent groups affect the reactivity of the aromatic substrate as expected for electrophilic substitution, orientation is unusual in a number of ways, and it is here that much of the usefulness of thallation lies. Thallation is almost exclusively para to R, Cl, and -OCH3, and this is attributed to the bulk of the electrophile, thallium trifluoroacetate, which seeks out the uncrowded para position.
Thallation is almost exclusively ortho to certain substituents like COOH, COOCH3 , and CH2OCH3 (even though some of these are normally meta directing), and this is attributed to prior complexing of the electrophile with the substituent; thallium is held at just the right distance for easy intramolecular delivery to the ortho position. For example:
(In C6H5CH2CH2 COOH, however, it is evidently held too far from the ring, and must leave the substituent before attacking the ring intermolecularly at the para position.)
Thallation is reversible, and when carried out at a higher temperature (73 instead of room temperature) yields the more stable isomer: usually the meta (compare 1,2- and 1,4-addition, Sec. 8.22). For example:
Now, these aryl thallium compounds are useful, not in themselves, but as intermediates in the synthesis of a variety of other aromatic compounds. Thallium can be replaced by other atoms or groups which cannot themselves be introduced directly into the aromatic ring - or at least not with the same regiospecificity. In this way one can prepare phenols (Aaroh, Sec. 24.5) and aryl iodides (Sec. 25.3). Direct iodination of most aromatic rings does not work very well, but the process of thallation followed by treatment with iodide ion gives aryl iodides in high yields.
Mechanism of electrophilic aromatic substitution: a summary
Electrophilic aromatic substitution reactions seem, then, to proceed by a single mechanism, whatever the particular reagent involved. This can be summarized for the reagent YZ as follows:
But this is only part of the mechanism. Granting that substitution is electrophilic, how do we know that it involves two steps, as we have shown, and not just one! And how do we know that, of the two steps, the first is much slower than the second? To understand the answer to these questions, we must first learn something about isotope effects.
Isotope effects
Different isotopes of the same element have, by definition, the same electronic configuration, and hence similar chemical properties. This similarity is the basis of the isotopic tracer technique (Sec. 3.29): one isotope does pretty much what another will do, but, from its radioactivity or unusual mass, can be traced through a chemical sequence.
Yet different isotopes have, also by definition, different masses, and because of this their chemical properties are not identical: the same reactions can occur but at somewhat Different rates (or, for reversible reactions, with different positions of equilibrium)/^ difference in rate (or position of equilibrium) due to a difference in the isotope present in the reaction system is called an isotope effect. \
In intermolecular competition, a mixture of labeled and unlabeled reactants compete for a limited amount of reagent; reactions (1) and (2) thus go on in the same mixture, and we measure the relative amounts of H Z and D Z r .educed. (Sometimes, larger amounts of the reagent Z are used, and the relative amounts of the two reactants ordinary and labeled left unconsented are measured; the less reactive will have been used up more slowly and will predominate. The relative rates of reaction can be calculated without much difficulty.)
In intramolecular competition, a single reactant is used which contains several equivalent positions, some labeled and some not:
One can then measure either the relative amounts of H Z and D Z, or the relative amounts of the D-containing product formed by reaction (3) and the H-containing product formed by reaction (4)
Mechanism of electrophilic aromatic substitution: the two step
Now that we know what isotope effects are and, in a general way, how they arise, we are ready to see why they are of interest to the organic chemist. Let us return to the questions we asked before : how do we know that electrophilic aromatic substitution involves two steps,
We can see why nitration and reactions like it are not reversible. In the reverse of nitration, nitrobenzene is protonated (the reverse of reaction 2) to form carbonium ion I; but this is, of course, no different from the ion I formed in the nitration process, and it does the same thing: (re)forms nitrobenzene.
Nitration. Formation of carbonium ion is rate-controlling step; occurs equally rapidly whether protium (H) or deuterium (D) at point of attack. All carbonium ions go on to product. There is no isotope effect, and nitration is irreversible.
Unlike most other electrophilic substitution reactions, sulfonation shows a moderate isotope effect: ordinary hydrogen (protium) is displaced from an aromatic ring about twice as fast as deuterium. Docs this mean that sulfonation takes place by a different mechanism than nitration, one involving a single step? Almost certainly not.
Unlike most other electrophilic substitution reactions, sulfonation is reversible, and this fact gives us our clue. Reversibility means that carbonium ion II can lose SO3 to form the hydrocarbon. Evidently here reaction (2) is not much
Sulfonation. Some carbonium ions go on to product, some revert to starting material. There is an isotope effect, and suffocation is reversible.
faster than the reverse of reaction (1). In sulfonation, the energy barriers on either side of the carbonium ion II must be roughly the same height; some ions go one way, some go the other (Fig. 1 1.3). Now, whether the carbonium ion is II(D) or II(H), the barrier to the left (behind it) is the same height. But to climb the barrier to the right (ahead), a carbon-hydrogen bond must be broken, so this barrier is higher for carbonium ion II(D) than for carbonium ion II(H). More deuterated ions than ordinary ions revert to starting material, and so overall sulfonation is slower for the deuterated benzene. Thus, the particular shape of potential energy curve that makes sulfonation reversible also permits an isotope effect to be observed.
By use of especially selected aromatic substrates highly hindered ones isotope effects can be detected in other kinds of electrophilic aromatic substitution, even in nitration. In certain reactions the size of the isotope can be deliberately varied by changes in experimental conditions- and in a way that shows dependence on the relative rates of (2) and the reverse of (1). There can be little doubt that all these reactions follow the same two-step mechanism, but with differences in the shape of potential energy curves. In isotope effects the chemist has an exceedingly delicate probe for the examination of organic reaction mechanisms.
Reactivity and orientation
We have seen that certain groups activate the benzene ring and direct substitution to 0/7//0 and para positions, and that other groups deactivate the ring and (except halogens) direct substitution to Meia positions. Let us sec if we can account for these effects on the basis of principles we have already learned.
For closely related reactions, a difference in rate of formation of carbonium ions is largely determined by a difference in aft t , that is, by a difference in stability of transition states. As with other carbonium ion reactions we have studied, factors that stabilize the ion by dispersing the positive charge should for the same rea c on stabilize the incipient carbonium ion of the transition state. Here again we expect the more stable carbonium ion to be formed more rapidly. We shall therefore concentrate on the relative stabilities of the carbonium ions.
In electrophilic aromatic substitution the intermediate carbonium ion is a hybrid of structures I, II, and III, in which the positive charge is distributed about the ring, being strongest as the positions ortho and para to the carbon atom being attacked.
A group already attached to the benzene ring should affect the stability of the carbonium ion by dispersing or intensifying the positive charge, depending upon its electron-releasing or electron-withdrawing nature. It is evident from the structure of the ion (I-III) that this stabilizing or destabilizing effect should be especially important when the group is attached ortho or para to the carbon being attacked.
Theory of reactivity
To compare rates of substitution in benzene, toluene, and nitrobenzene, we compare the structures of the carbonium ions formed from the three compounds:
By releasing electrons, the methyl group (II) tends to neutralize the positive charge of the ring and so become more positive itself; this dispersal of the charge stabilizes the carbonium ion. In the same way the inductive effect stabilizes the developing positive charge in the transition state and thus leads to a faster reaction.
The NO2 group, on the other hand, has an electron-withdrawing inductive effect (III); this tends to intensify .the positive charge, destabilizes the carbonium ion, and thus causes a slower reaction.
Like CH3 , other alkyl groups release electrons, and like CH3 they activate the ring. For example, ferf-butylbenzene is 16 times as reactive as benzene toward nitration. Electron release by NH2 and OH, and by their derivatives OCH3 and NHCOCH3 , is due not to their inductive effect but to resonance, and is' discussed later (Sec. 11.20).
We are already familiar with the electron-withdrawing effect of the halogens (Sec. 6.11). The full-fledged positive charge of the N(CH3) 3 + group has, of course, a powerful attraction for electrons. In the other deactivating groups (e.g., NO2 , CN, COOH), the atom next to the ring is attached by a multiple bond to oxygen or nitrogen. These electronegative atoms attract the mobile n electrons, making the atom next to the ring electron-deficient; to make up this deficiency, the atom next to the ring withdraws electrons from the ring.
We might expect replacement of hydrogen in CH3 by halogen to decrease the electron-releasing tendency of the group, and perhaps to convert it into an electron-withdrawing group. This is found to be the case. Toward nitration,
Theory of orientation
Before we try to account for orientation in electrophilic substitution, let us look more closely at the facts.
An activating group activates all positions of the benzene ring; even the positions metgj.o it arenite reactive than any single position in benzene itself. It directs ortho and para simply because it activates the ortho and para positions much more than it does the meta.
A deactivating group deactivates all positions in the ring, even the positions meta to it. It directs meta simply because it deactivates the ortho and para positions even more than it does the meta.
In nitrobenzene, orthoptera substitution is thus slower than meta substitution because electron withdrawal by NO2 is more effective during attack at the positions ortho and para to it.
Thus, we see that both orthoptera orientation by activating groups and meta orientation by deactivating groups follow logically from the structure of the intermediate carbonium ion. The charge of the carbonium ion is strongest at the positions ortho and para to the point of attack, and hence a group attached to one of these positions can exert the strongest effect, whether activating or deactivating.
The unusual behavior of the halogens, which direct ortho and para although deactivating, results from a combination of two opposing factors, and will be taken up in Sec.
Electron release via resonance
We have seen that a substituent group affects both reactivity and orientation in electrophilic aromatic substitution by its tendency to release or withdraw electrons. So far, we have considered electron release and electron withdrawal only as inductive effects, that is, as effects due to the electronegativity of the group concerned. believed to do this by a resonance effect. But before we discuss this, let us review a little of what we know about nitrogen and oxygen
Effect of halogen on electrophilic aromatic substitution
Halogens are unusual in their effect on electrophilic aromatic substitution: they are deactivating yet 0rf/io,/>ara-directing. Deactivation is characteristic of electron withdrawal, whereas orthro para orientation is characteristic of electron release. Can halogen both withdraw and release electrons
The answer is yes. Halogen withdraws electrons through its inductive effect, and releases electrons through its resonance effect. So, presumably, can the NH2 and OH groups, but there the much stronger resonance effect greatly .
Next, to understand orientation, let us compare the structures of the carbonium ions formed by attack at the para and Netta positions of chlorobenzene. Each of
these is a hybrid of three structures, Hl-V for para, VI-VII I for meta. In one of these six structures, IV, the positive charge is located on the carbon atom to which chlorine is attached. Through its inductive effect chlorine withdraws electrons most from the carbon to which it is joined, and thus makes structure IV especially unstable. As before, we expect IV to make little contribution to the hybrid, which should therefore be less stable than the hybrid ion resulting from attack at the meta positions. If only the inductive effect were involved, then, we would expect not only deactivation but also meta orientation.
But the existence of halonium ions (Sec. 7.12) has shown us that halogen can share more than a pair of electrons and can accommodate a positive charge. If we apply that idea to the present problem, what do we find? The ion resulting from para-attack is a hybrid not only of structures III III V, but also of structure IX, in which chlorine bears a positive charge and is joined to the ring by a double bond. This structure should be comparatively stable, since in it every atom (except hydrogen, of course) has a complete octet of electrons. (Structure IX is exactly analogous to those proposed to account for activation and orthoptera direction by NH2 and OH.) No such structure is possible for the ion resulting from meta-attack. To the extent that structure IX contributes to the hybrid, it makes the ion resulting from para-attack more stable than the ion resulting from meta .
Thus, we find that a single structural concept partial double-bond formation between halogen and carbon helps to account for unusual chemical properties of such seemingly different compounds as aryl halides and vinyl halides. The structures involving doubly bonded halogen, which probably make important contribution not only to benzenediol ions but to the parent aryl halides as well (Sec. 25.6), certainly do not seem to meet our usual standard of reasonableness (Sec. 6.27). The sheer weight of evidence forces us to accept the idea that certain carbon-halogen bonds possess double-bond character. If this idea at first appears strange to us, it simply shows how little, after all, we really know about molecular structure.
Relation to other carbonium ion reactions
In summary, we can say that both reactivity and orientation in electrophilic aromatic substitution are determined by the rates of formation of the intermediate carbonium ions concerned. These rates parallel the stabilities of the carbonium ions, which are determined by the electron-releasing or electron-withdrawing tendencies of the substituent groups.
A group may release or withdraw electrons by an inductive effect, a resonance effect, or both. These effects oppose each other only for the NH2 and OH groups (and their derivatives) and for the halogens, X. For NH2 and OH the resonance effect is much the more important; for X the effects are more evenly matched. It is because of this that the halogens occupy the unusual position of being deactivating groups but orthoptera directors.
We have accounted for the facts of electrophilic aromatic substitution in exactly the way that we accounted for the relative ease of dehydration of alcohols, and for reactivity and orientation in electrophilic addition to alkenes: the more stable the carbonium ion, the faster it is formed; the faster the carbonium ion is formed, the faster the reaction goes.
In al! this \\e have estimated the stability of a carbonium ion on the same basis: the dispersal or concentration of the charge due to electron release or electron withdrawal by the substituent groups. As we shall see, the approach that has worked so well for elimination, for addition, and for electrophilic aromatic substitution works for still another important class of organic reactions in which a positive charge develops nucleophilic aliphatic substitution by the S^I mechanism (Sec. 14.14). It works equally well for nucleophilic aromatic substitution (Sec. 25.9), in which a negative charge develops. Finally, we shall find that this approach will help us to understand acidity or basicity of such compounds as carboxylic acids, sulfonic acids, amines, and phenols.
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