Reactivity and Orientation in Electrophilic Aromatic Substitution

It is important to understand why this should happen. In the above examples, the two substituents, the methyl group and the nitro group, exhibit different electronic behaviour. The methyl group is an electron donor and so increases the electron density of the ring. The nitro group is an electron acceptor and withdraws electron density from the ring. 

Both types of substituents affect the electron density at all positions of the ring, but exert their greatest effects at the ortho and para positions, making these sites the most electron rich in the case of donor groups and most electron deficient when electron-withdrawing groups are present. 

Donor groups therefore direct attack of the electrophile to the ortho and para positions and are known as ortholpara directors. Conversely, aromatic compounds containing electron acceptor groups are attacked at the meta position since this is the least electron-deficient site. Such groups are called meta directors. Not all substituents fit exactly into this picture halogens are deactivating but direct attack to the ortho and para positions. 

Substituents exert their influence on a molecule through either the c-bonds or the Jt-bonding system, in other words by inductive and mesomeric (resonance) effects, respectively (see below). The interaction 

Note that some groups can withdraw electrons by one of the two effects but release electrons by the other, although one of the effects usually predominates. 

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Reactivity and orientation in electrophilic aromatic substitution can also be related to the concept of hardness (see Section 1.2.3). Ionization potential is a major factor in determining hardness and is also intimately related to the process of (x-complex formation when an electrophile interacts with the n HOMO to form a new a bond. In MO terms, hardness is related to the gap between the LUMO and HOMO, t] = (sujmo %omo)/2- Thus, the harder a reactant ring system is, the more difficult it is for an electrophile to complete rr-bond formation. 

EFFECT OF SUBSTITUENTS ON REACTIVITY AND ORIENTATION IN ELECTROPHILIC AROMATIC SUBSTITUTION... 

Effect of Substitutents on Reactivity and Orientation in Electrophilic Aromatic Substitution... 

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. 

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. 

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. 

Resonance effects are the primary influence on orientation and reactivity in electrophilic substitution. The common activating groups in electrophilic aromatic substitution, in approximate order of decreasing effectiveness, are —NR2, —NHR, —NH2, —OH, —OR, —NO, —NHCOR, —OCOR, alkyls, —F, —Cl, —Br, —1, aryls, —CH2COOH, and —CH=CH—COOH. Activating groups are ortho- and para-directing. Mixtures of ortho- and para-isomers are frequently produced the exact proportions are usually a function of steric effects and reaction conditions. 

The hardness concepts have recently been used as indices of aromaticity [97] and of the orientation of electrophilic aromatic substitution [98]. The principle of maximum hardness [20] requires higher-order derivatives of the electronic energy with respect to the electron population variables, and especially the hardness derivative Sq/0N [99]. Applications of the EE procedure and the CS concepts to the structural and reactivity problems of solids and clusters are becoming routine [40, 41, 47, 100, 101] and new sensitivity indicators of reactivity, addity/basisity in crystal chemistry are being developed [102]. A novel CS-type approach to the chemical reactivity has recently been proposed by Tachibana and Parr [103]. 

Inductive and resonance effects account not only for reactivity but also for the orientation of electrophilic aromatic substitutions. Take alkyl groups, for instance, which have an electron-donating inductive effect and are ortho and para directors. The results of toluene nitration are shown in Figure 9.17. 

In the discussion of electrophilic aromatic substitution (Chapter 11) equal attention was paid to the effect of substrate structure on reactivity (activation or deactivation) and on orientation. The question of orientation was important because in a typical substitution there are four or five hydrogens that could serve as leaving groups. This type of question is much less important for aromatic nucleophilic substitution, since in most cases there is only one potential leaving group in a molecule. Therefore attention is largely focused on the reactivity of one molecule compared with another and not on the comparison of the reactivity of different positions within the same molecule. 

In this chapter we shall examine the methods that are used to measure these effects on reactivity and orientation, the results of these measurements, and a theory that accounts for these results. The theory is, of course, based on the most likely mechanism for electrophilic aromatic substitution we shall see what this mechanism is, and some of the evidence supporting it. First let us look at the facts. 

However, the presence of certain groups at certain positions of the ring markedly activates the halogen of aryl halides toward displacement. We shall have a look at some of these activation effects, and thei. try to account for them on the basis of the chemical principles we have learned. We shall find a remarkable parallel between the two kinds of aromatic substitution, electrophilic and nucleophilic, with respect both to mechanism and to the ways in which substituent groups affect reactivity and orientation. 

The previous sections leave no doubts that aromatic compounds, react with positively charged electrophiles to form a-complexes-arenium ions. But are they the primary intermediates It is not by accident that the problem of preliminary formation of radical cations has arisen. Its statement is an attempts to explain the orientational peculiarities of electrophilic aromatic substitution of hydrogen. The widespread view that the orientation in the reactions of aromatic compounds with electrophiles is dictated by the relative stabilities of the cr-complexes explains but a part of the accumulated material. In the first place this refers to the meta- and para-orienting effects of electron-releasing substituents in benzene in terms of the QCT -approach and to that of the relative reactivity of various aromatic substrates... 

Electrophilic aromatic substitution reactions (Sec. 15.1) are among the best understood of all organic reactions. The qtuilitative aspects of the reactions that are discussed in textbooks include the effect substituents have on the reactivity of arenes toward electrophiles and the orientation, ortho, meta, or para, of their attack on the ring. However, relatively Httle information is given in textbooks about the quantitative differences in rates and reactivities of substituted aromatic compounds. The experimental procedures of this section provide both semiquantitative and quantitative measures of the differences in reactivity of a series of arenes toward the bromo-nium ion, Br, to produce the corresponding aryl bromides (Eq. 15.20). 

Since the early days of organic chemistry, nitration has been considered to be an important reaction and has been widely used. As early as 1825 Faraday discovered benzene and recorded its reaction with nitric acid. Shortly after, the use of nitric acid sulfuric acid mixtures to effect nitration was reported and was soon quoted in a patent. Nitration figured prominently in the development of ideas of theoretical organic chemistry in the early part of the twentieth century and, as the most widely applicable and most widely used example of electrophilic substitution, it played an important role in the consideration of aromatic stability and reactivity. In 1910 the first report of orientation and deactivation in aromatic electrophilic substitution was published (10MI1). 

Electrophilic Substitution Reactivity Much of the electrophilic reactivity of aromatics is described in great detail in a comprehensive recent book of Taylor [10]. We shall focus attention on the electrophilic substitution reactivity of annelated benzenes and try to interpret the orientational ability of fused small rings. For this purpose we consider here Wheland transition states of the electrophilic substitution reactions. It is also convenient to take the proton as a model of the electrophilic reagent. In order to delineate rehybridization and 7r-electron localization effects, let us consider a series of angularly deformed benzenes (Fig. 21), where two vicinal CH bonds bent toward each other mimick a fused small ring. Angles c of 110° and 94° simulate five and four membered... 

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