Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Addition Reactions Involving Carbonyls and Nitriles
9.10 Mechanism for Addition Reactions Involving Nitriles

Owing to the polarity of the nitrile bond, as shown below, its reaction with an electrophile and a nucleophile is similar to that of carbonyl and imine functionalities. In the case of nitriles, there are two sets of pi (π) electrons, which are not equally distributed between the carbon—nitrogen triple bond, but are closer to the more electronegative nitrogen. As a result, addition reactions involving this functionality are regiospecific as shown in Reaction (9-77).

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You will notice that the product of an initial addition reaction to the nitrile functionality generates another functionality (iminium anion) that we have seen before in the previous section. As shown in the previous section, the iminium anion can undergo a similar addition reaction, owing to the presence of its remaining double bond, as shown in Reaction (9-78).

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Of course, it is possible for the sequence of the addition steps to take place in a different sequence in which the first step involves the addition of the electrophile to the carbon of the nitrile, followed by the addition of the nucleophile as shown in Reaction (9-79).

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In the next step of the reaction, another mole of electrophile is added to the imine and eventually the final product, the amine, is formed as shown in Reaction (9-80).

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9.10.1 Addition of Water to Nitriles

Since nitriles can undergo two successive addition reactions. Nitriles are compounds that are often used as starting materials for the synthesis of a wide variety of other organic compounds. One of the most common transformations that nitriles undergo is the addition of water (hydrolysis) to form the corresponding amide or carboxylic acid. The first reaction that we will examine is the addition of water to cyanobenzene in the presence of an acid catalyst to give an amide functionality. Since the nitrile is a very strong triple bond, these hydrolysis reactions are typically carried out at elevated temperatures and in the presence of an acid catalyst as shown in Reaction (9-81).

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The mechanism for this reaction is shown below. In the initial step, the electrophilic proton adds to the nucleophilic nitrogen of the nitrile triple bond, followed by an attack of the nucleophilic water molecule on the electrophilic carbon of the triple bond as shown in Reaction (9-82).

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After the loss of proton and subsequent protonation of the nitrogen, hydrolysis of the imine (as shown in the previous section) gives the amide as shown in Reaction (9-83).

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Problem 9.14

Complete the following reactions by giving the amide product that results.

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If the hydrolysis is carried out under much more severe conditions, such as prolonged heating and in very strong acidic conditions, the amide that is initially formed is readily transformed into a carboxylic acid. The conversion involved is a combination of addition reaction and a substitution reaction, and we will cover the mechanism for the substitution reaction in Chapter 16, but the overall reaction is shown in Reaction (9-84).

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