Question

What are some uses of Nucleophilic Aromatic Substitution reactions in industry? Why is the reaction important?

Answer 1

Answer,

Use of Nucleophilic Aromatic Substitution reaction ,

Substituted benzene molecules have tremendous importance in industrial chemical applications. They are commonly used as solvents and they are often important intermediates in many types of syntheses, including those of powerful pharmaceutical agents.One class of compounds produced in this way serves as intermediates in the synthesis of antidegradants compounds that are very important in the chemical rubber industry.

Importance,

This module deals with a recently developed method for the production of one of these intermediates, 4-aminodiphenylamine (4-ADPA). The chemistry presented in exemplifies an innovative approach to the problem of introducing nucleophiles onto aromatic ring systems in such a way that greatly reduces the negative environmental problems associated with the more well known synthetic procedures.

The most known common method of producing substituted benzene rings involves the well-known reaction of electrophilic aromatic substitution reaction (EAS).This allows facile substitution for hydrogen of nitro groups, halogens, sulfonic acid, alkyl and acyl groups.While the applications of this reaction for multiple substitutions are limited by the ortho/para or meta directing group properties of groups on the ring as well as their activating or deactivating properties, it is usually possible to prepare the desired compound as long as the group to be introduced to the ring is an electrophile.This means that a whole family of substituents, which are nucleophilic in nature, cannot be introduced to the ring by EAS. In general, reactions involving the substitution of nucleophiles are difficult. Under the usual conditions for nucleophilic substitution reactions they produce vanishingly small amounts of products.Recalling the two mechanisms for nucleophilic substitution, SN1 and SN2 demonstrates the reason: with an aromatic ring there is no available backside approach to the carbon, hydrogen is a very poor leaving group, and the aromatic system will not readily ionize by loss of hydrogen.Therefore, in order to introduce the important nucleophiles onto the aromatic ring some tricks have to played on the ring itself. Under the special conditions which then result it is possible to incorporate nucleophiles onto the ring. It is this reaction, Nucleophilic Aromatic Substitution (NAS) that is the topic of this module.We will examine the traditional chemistry for NAS along with the elucidated mechanisms.In doing this we will see that this chemistry has several environmental problems that make it undesirable as a large scale industrial process.In response to these problems Flexsys America L.P. (1) developed a method for eliminating these problems in the production of the industrially important intermediate, 4-aminodiphenylamine.

The mechanism.

There are at least four mechanisms (2), operating under different conditions, for the introduction of a nucleophile onto an aromatic ring.They are the SNAr, Benzyne, SN1, and SNR1 mechanism.Although these four have been identified and studied most nucleophilic aromatic substitution reactions which are commonly performed are accomplished by either the SNAr or Benzyne mechanisms so we will concentrate on those and only briefly consider the other two.

SNAr mechanism [addition/elimination]

It is the most important of the mechanisms for nucleophilic aromatic substitution reactions. The experimental evidence indicates that there are two steps and so it is often referred to as the addition or elimination mechanism.reaction begins with attack by the nucleophile on the ring carbon haveing the leaving group. This is the addition step of the reaction. This produces an anionic intermediate. This ion, known as the Meisenheimer complex, is resonance stabilized.

" ADDITION ADDITION" ELIMINATION ELIMINATION RDS Nu slow fast NU: Meisenheimer Complex Resonance Stabilized Nu -NU Resonance Stabilization of the Intermediate Anion [The Meisenheimer Complex]

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