Reactions of Aromatic Compounds:

Trait reactions of benzene involve substitution, in that the resonance-stabilized ring organization is conserved. Why is this so?  We might answer via saying that this is due to the resonance stabilization of the benzene. But then the question occurs, why then benzene goes in into reactions at all, why is it not inert? This dual behaviour, the coexistence of stability and reactivity is due to the occurrence of the circulating π electrons in the benzene ring that, on one hand, remains the carbon nuclei within bonding distance and, on the other, presents a site of attack to positively charged species. This explains the electrophilic reactivity of benzene.

Electrophilic substitution contains a wide variety of reactions, these as nitration, halogenation, sulphonation and the Friedel-Crafts reactions undergone through nearly all aromatic rings. Several significant reactions are summarized in Table.

Table: Electrophilic aromatic substitution reactions

Nitration

Replacement of hydrogen via the nitro-group is recognized as 'Nitration'.

Nitration of benzene can be carried out via the reaction of benzene through a blend of concentrated nitric and sulphuric acids. The electrophile in this reaction is the nitronium ion, NO₂⁺. It is produced through the reaction of H₂SO₄ by HNO₂.

Additional verification for the participation of the nitronium ion move towards from the fact that other species proficient of producing nitronium ion, these as NO⁺₂BF^-₄, NO⁺₂NO₃⁺ and NO⁺₂NO^-₃ and NO⁺₂ClO^-₄ as well nitrate benzenoid compounds.

Nitration of benzene is a significant reaction since the nitro group can be transferred into other functional groups.

Halogenation

Benzene reacts through halogens in the presence of a catalyst (FeBr₃, FeCl₃) to yield halogen substituted products, that is, aryl halides.

The major function of the catalyst is to moderately or completely polarize the halogen-halogen bond, for example:

A typical reaction of aromatic halogenation is the bromination of benzene. As a general rule, fluoride is too reactive and a poor yield of fluorobenzene is obtained. Chlorine reacts smoothly and provides a tremendous give way of chlorobenzene. Iodine itself is unreactive; though, iodination of benzene is carried out in the presence of oxidizing agent such as hydrogen peroxide, H2O2, or copper salt such as CuCl₁. This oxidizing agent oxidizes molecular iodine to an electrophiles I⁺

Halogenation can as well be influenced through other reagents, these as hypochlorous or hypobromous acids in occurrence of strong acids.

Sulphonation

Aromatic compounds in that the sulphonic group (^-SO₃H) is directly connected to the benzene ring are termed aromatic sulphonic acids. Replacement of hydrogen from benzene through the sulphonic group is termed sulphonation. It is another instance of electrophilic substitution reaction. Sulphonation is usually accomplished using sulphuric acid or fuming sulphuric acid (H₂SO₄ + SO₃) having fluctuating proportions of sulphur trioxide.

The reactive electrophile is neutral SO₃, as is evident from its structure following below:

Dissimilar the other electrophilic substitution reaction reaction of benzene, sulphonation is a extremely reversible reaction and the direction based on the reaction conditions. Sulphonation is favored in the existence of concentrated or fuming sulphuric acid, desulphonation in hot, dilute aqueous acids.

Friedel-crafts alkylation

Complex substituted aromatic compounds are always synthesized from the simpler, readily obtainable aromatic compounds. Because benzene is extremely general and simply available, chemists utilize it as beginning material and introduce the desired substituents. We have already learned the substitution of halogen, nitrogen and sulphur-based functional groups to the aromatic ring. Now we will learn another significant reaction, that is, alkylation of aromatic ring.

Alkylation of benzene is the substitution of ring hydrogen by an alkyl group in the ring. Reaction of the aromatic compounds through alkyl halides in the presence of anhydrous AlCl3, as the catalyst provides alkylated products. This reaction is recognized as Friedel-Crafts alkylation.

The reaction of 2-chloropropane by benzene in the presence of AlCl₃ to yield (1-methyl ethyl) benzene is a representative Friedel-Crafts alkylation reaction.

The electrophile in the Friedel-Crafts reaction is R+. This ion is shaped when an alkyl halide reacts through a Lewis acid. Lewis acids these as AlCl₃, FeCl₃, ZnCl₂, AlBr₃, BF₃ etc. are utilized in Friedel-Crafts alkylation. In case of alkylation by tertiary alkyl halides, the electrophilic species is a free carbonations, the electrophilic species is an alkyl halide-Lewis acid complex through positively polarized carbon.

Although the Friedel-Crafts reaction is extensively appropriate in organic synthesis, it has several limitations as following below:

i). The main complexity through the Friedel-Crafts alkylation is that the substitution of the 1^st alkyl group activates the ring towards further substitution. We will talk about activation and deactivation in Sec 3.3.1.

The best way of avoiding this 2^nd reaction is to utilize an excess of aromatic compound.

ii) Friedel-Crafts alkylations are bounded to alkyl halides, aryl halides and alkenyl halides don't react. Aryl and alkenyl carbocations are as well unstable to form under Friedel-Crafts reaction conditions.

iii) If the aromatic compound has an electron withdrawing substituent, it doesn't undergo Friedel-Crafts alkylation; the deactivated ring is not reactive to attack carbocations.

iv) Aromatic amines fail to undergo alkylation, probably since amino group forms a complex through Lewis acid. Because this complex has a positive charge on nitrogen, it deactivates the aromatic ring for electrophilic substitution.

v) Sometimes during alkylation, the bothering electrophile undergoes reshuffle by 1, 2 shift of H or R. For instance, the alkylation of benzene by chloropropane leads to a mixture of propylbenzene and (1-methylethyl) benzene. The mechanism is alike to alkylation through an alkyl halide and this reaction continues throughout the more stable carbocation intermediate.

The mechanism is similar to alkylation with an alkyl halide and this reaction proceeds through the more stable carbocation intermediate.

Friedel-crafts acylation

        O                  O

        ||                  || 

The RC-group or ArC-group is termed an acyl group. Substitution of an acyl group into an aromatic ring via the reaction through an acid chloride in the existence of Lewis acid as catalyst is termed an aromatic acylation reaction or Friedel-Crafts acylation. For instance, the reaction of benzene through ethanoylchloride (acetyl chloride) gives the ketone, phenylethanone (acetophenone).

The mechanism of Friedel-Crafts acylation is similar to that of the other electrophilic aromatic substitution reactions. The electrophile in this reaction is the resonance - stabilized carbocation, acylium ion. This ion is formed when the acid chloride reacts with the Lewis acid, AlCl₂.

Carboxylic acid anhydride can be used as alternative to acid chloride for the Friedel-Crafts acylation reaction.

Friedel-Crafts acylation reaction is a synthetically helpful reaction. For instance, the carbonyl group of the ketone created via Friedel-Crafts acylation can be reduced to > CH₂ group through using zinc amalgam and hydrochloric acid. This process of reduction is identified as Clemmensen reduction. Through the combination of Friedel-Crafts acylation and Clemmensen reduction, an alkyl benzene might be prepared.

Unlike Friedel-Crafts alkylation, Friedel-Crafts acylation aren't accompanied via rearrangements within the acyl group. Furthermore, there is no polysubstitution as the aromatic ring is deactivated after the introduction of the first acyl group.

Mechanism of Electrophilic Substitution

There are causes to believe that the electrophilic substitutions occur through alike mechanism. It is needed to comprehend the principles of this mechanism. Therefore, we will discuss the common electrophilic substitution mechanism via using E* for electrophiles. Before learning detailed mechanism, let us in brief remember what we have learnt about electrophilic addition to alkenes. Electrophilic bother on C=C provides carbocation intermediate which is then attacked via nucleophile to yield addition product.

An electrophilic aromatic substitution reaction starts in an alike way. The π electrons of the ring attack on electrophile E*, forming an σ bond with electrophile. In this procedure, the positive charge of the electrophile is to be shifted to the adjacent ring carbon atom that is termed carbocation. This is a slow step and is, hence, the rate measuring step.

If a reaction occurs through a series of steps, the slowest step is termed the rate-determining step (RDS).This carbocation is stabilized via resonance as given below:

Such 3 resonance structure of the intermediate are often joined and symbolized as follows:

In the case of alkenes, we have seen that nucleophile attacks the carbocation to yield the addition product. Because, in the present case, the addition of the nucleophile would demolish the aromatic stabilization of the benzene ring; this kind of addition doesn't occur in aromatic carbocation. Instead, nucleophile acts as base and abstracts a ring proton yielding substituted aromatic product.

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