Resonance forms electrophilic aromatic substitution

The partial rate factors af and /3f for the a- and /3-positions of thiophene have been calculated for a wide range of electrophilic reactions these have been tabulated (71 AHC(13)235, 72IJS(C)(7)6l). Some side-chain reactions in which resonance-stabilized car-benium ions are formed in the transition states have also been included in this study. A correspondence between solvolytic reactivity and reactivity in electrophilic aromatic substitution is expected because of the similar electron-deficiency developed in the aromatic system in the two types of reactions. The plot of log a or log /3f against the p-values of the respective reaction determined for benzene derivatives, under the same reaction conditions, has shown a linear relationship. Only two major deviations are observed mercuration and protodemercuration. This is understandable since the mechanism of these two reactions might differ in the thiophene series from the benzene case. 

Resonance stabilization is important in electrophilic aromatic substitution as well. While each of the canonical forms of the Wheland intermediate has a sextet carbon atom, the charge is distributed over the remaining five atoms of the ring by resonance and is thus greatly stabilized. 

Although aromatic compounds have multiple double bonds, these compounds do not undergo addition reactions. Their lack of reactivity toward addition reactions is due to the great stability of the ring systems that result from complete n electron delocalization (resonance). Aromatic compounds react by electrophilic aromatic substitution reactions, in which the aromaticity of the ring system is preserved. For example, benzene reacts with bromine to form bromobenzene. 

Polycyclic aromatic compounds also undergo electrophilic aromatic substitution reactions. Because the aromatic resonance energy that is lost in forming the arenium ion is lower, these compounds tend to be more reactive than benzene. For example, the brotni-nation of naphthalene, like that of other reactive aromatic compounds, does not require a Lewis acid catalyst ... 

Styrene (vinylbenzene) undergoes electrophilic aromatic substitution much faster than benzene, and the products are found to be primarily ortho- and para-substituted styrenes. Use resonance forms of the intermediates to explain these results. 

The mechanism of Friedel-Crafts acylation (shown next) resembles that for alkylation, except that the electrophile is a resonance-stabilized acylium ion. The acylium ion reacts with benzene or an activated benzene derivative via an electrophilic aromatic substitution to form an acylbenzene. 

Naphthalene undergoes electrophilic aromatic substitution at C-1 more easily than at C-2. There is a smaller loss of resonance energy in forming the intermediate for reaction at C-1 and reaction takes place more rapidly at this centre. However, the products of aromatic substitution at C-1 suffer interactions with C-8 (peri interactions) and are less stable than the corresponding products of substitution at C-2. Hence those aromatic substitution reactions that are carried out under conditions that allow equilibration between isomers (thermodynamic control) lead to substitution ai C-2, but reactions that are carried out under conditions... 

No matter what electrophile is used, all electrophilic aromatic substitution reactions occur via a two-step mechanism addition of the electrophile to form a resonance-stabilized carboca-tion, followed by deprotonation with base, as shown in Mechanism 18.1. 

The first step in electrophilic aromatic substitution forms a carbocation, for which three resonance structures can be drawn. To help keep track of the location of the positive charge ... 

In bromination (Mechanism 18.2), the Lewis acid FeBr3 reacts with Br2 to form a Lewis acid-base complex that weakens and polarizes the Br- Br bond, making it more electrophilic. This reaction is Step [1] of the mechanism for the bromination of benzene. The remaining two steps follow directly from the general mechanism for electrophilic aromatic substitution addition of the electrophile (Br in this case) forms a resonance-stabilized carbocation, and loss of a proton regenerates the aromatic ring. 

In Friedel-Crafts acylation, the Lewis acid AICI3 ionizes the carbon-halogen bond of the acid chloride, thus forming a positively charged carbon electrophile called an acylium ion, which is resonance stabilized (Mechanism 18.7). The po.sitively charged carbon atom of the acylium ion then goes on to react with benzene in the two-step mechanism of electrophilic aromatic substitution. 

To understand why some substituents make a benzene ring react faster than benzene itself (activators), whereas others make it react slower (deactivators), we must evaluate the rate-determining step (the first step) of the mechanism. Recall from Section 18.2 that the first step in electrophilic aromatic substitution is the addition of an electrophile (E ) to form a resonance-stabilized carbo-cation. The Hammond postulate (Section 7.15) makes it pos.sible to predict the relative rate of the reaction by looking at the stability of the carbocation intermediate. 

Although two products (A and B) are possible when naphthalene undergoes electrophilic aromatic substitution, only A is formed. Draw resonance structures for the intermediate carbocation to explain why this is observed. 

This reaction is another example of electrophilic aromatic substitution, with the diazonium salt acting as the electrophile. Like all electrophilic substitutions (Section 18.2), the mechanism has two steps addition of the electrophile (the diazonium ion) to form a resonance-stabilized carbocation, followed by deprotonation, as shown in Mechanism 25.4. 

An electrophilic aromatic substitution reaction begins in a similar way, but there are a number of differences. One difference is that aromatic rings are less reactive toward electrophiles than alkenes are. For example, Bi in CH2CI2 solution reacts instantly with most alkenes but does not react with benzene at room temperature. For bromination of benzene to take place, a catalyst such as FeBr is needed. The catalyst makes the Br2 molecule more electrophilic by polarizing it to give an FeBi " Br species that reacts as if it were Br. The polarized Br2 molecule then reacts with the nucleophilic benzene ring to yield a nonaromatic carbocation intermediate that is doubly allylic (Section 11.5) and has three resonance forms. 

All of these electrophilic aromatic substitution reactions take place by the same two-step mechanism. In the first step, benzene reacts with an electrophile (Y ), forming a carbocation intermediate. The structure of the carbocation intermediate can be approximated by three resonance contributors. In the second step of the reaction, a base in the reaction mixture pulls off a proton from the carbocation intermediate, and the electrons that held the proton move into the ring to reestablish its aromaticity. Notice that the proton is always removed from the carbon that has formed the new bond with the electrophile. 

We can understand how the trifluoromethyl group affects orientation in electrophilic aromatic substitution if we examine the resonance structures for the arenium ion that would be formed when an electrophile attacks the ortho, meta, and para positions of trifluoromethylbenzene. 

The intermediate first formed is somewhat stabilized by resonance and does not rapidly undergo reaction with a nucleophile in this behavior, it is different from fhe unsfabilized carbocation formed from cyclohexene plus an electrophile. In fact, aromaticity can be restored to the ring if elimination occurs instead. (Recall that elimination is often a reaction of carbocations.) Removal of a proton, probably by HS04, from the sp -ring carbon restores the aromatic system and yields a net substitution wherein a hydrogen has been replaced by a nitro group. Many similar reactions are known, and they are called electrophilic aromatic substitution reactions. 

Polynuclear aromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene undergo electrophilic aromatic substitution reactions in the same manner as benzene. A significant difference is that there are more carbon atoms, more potential sites for substitution, and more resonance structures to consider. In naphthalene, it is important to recognize that there are only two different positions Cl and C2 (see 122). This means that Cl, C4, C5, and C8 are chemically identical and that C2, C3, C6, and C7 are chemically identical. In other words, if substitution occurs at Cl, C4, C5, and C8 as labeled in 122, only one product is formed 1-chloronaphthalene (121), which is the actual product isolated from the chlorination reaction. Chlorination of naphthalene at Cl leads to the five resonance structures shown for arenium ion intermediate 127. 

In anthracene (123), there are three different positions (Cl, C2, and C9) and there are five different positions (Cl, C2, C4, C5, and C9) in phenanthrene (124). Electrophilic aromatic substitution of anthracene leads to substitution primarily at C9 because that gives an intermediate with the most resonance forms and the most intact benzene rings. A comparison of attack at Cl and at C2 in anthracene will show that there are more resonance forms for attack at Cl and more fully aromatic rings. Attack at C9 leads to an intermediate with even more resonance, and electrophilic substitution of anthracene leads to C9 and Cl products, with little reaction at C2. 

A variety of other electrophilic aromatic substitution reactions involve very strong electrophiles reacting with the weakly nudeophilic aromatic tt cloud to form an intermediate resonance-stabilized cation on the ring that loses a proton to give the substituted arene. 

Draw a contributing structure for the resonance-stabilized cation formed during electrophilic aromatic substitution that shows the role of each group in stabilizing the intermediate by further delocalizing its positive charge. 

Electrophilic aromatic substitution occurs primarily at C2, because the intermediate formed during attack at C2 is stabilized by resonance. 

Both meta and para substitution give an intermediate with three resonance forms, so there is no gross difference here. Note that we are introducing a second positive charge when we do any electrophilic aromatic substitution reaction on this molecule. 

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