9.4.2. Ammonolysis of alkyl halides

You have read (Unit 6, Class XII) that the carbon - halogen bond in alkyl or benzyl halides can be easily cleaved by a nucleophile. Hence, an alkyl or benzyl halide on reaction with an ethanolic solution of ammonia undergoes nucleophilic substitution reaction in which the halogen atom is replaced by an amino (–NH2) group.

Ammonolysis refers to the process where an alkyl or benzyl halide reacts with ammonia (NH3) in the presence of ethanol. In this reaction, the halogen (Cl, Br, or I) attached to the carbon atom in the alkyl or benzyl halide is replaced by an amino group (–NH2). This occurs because the halogen atom is a good leaving group, allowing the nucleophile (ammonia) to take its place.

Imagine a relay race where a runner (the halogen) passes the baton (the carbon atom) to another runner (the ammonia). Since the first runner can easily drop the baton, the second runner seamlessly takes over, showing how the substitution works.

This process of cleavage of the C–X bond by ammonia molecule is known as ammonolysis. The reaction is carried out in a sealed tube at 373 K. The primary amine thus obtained behaves as a nucleophile and can further react with alkyl halide to form secondary and tertiary amines, and finally quaternary ammonium salt.

The ammonolysis reaction is performed in a sealed tube at a temperature of 373 Kelvin. Under these conditions, a primary amine is formed, which has a nucleophilic character—meaning it can donate its electron pair to react with more alkyl halides. As this process continues, it can produce secondary amines, tertiary amines, and even quaternary ammonium compounds.

Think of a series of interviews: the first candidate (the primary amine) gets a job, but later, they continue to refer friends (alkyl groups) for the position. Each new hire can bring in their own network of referrals, showing how one amine can lead to more complex structures.

Ammonolysis has the disadvantage of yielding a mixture of primary, secondary and tertiary amines and also a quaternary ammonium salt. However, primary amine is obtained as a major product by taking large excess of ammonia.

While ammonolysis is a versatile reaction, it can produce a range of products: primary, secondary, tertiary amines, and quaternary ammonium salts. This mixture can complicate the separation and identification of the desired product. To maximize the production of primary amines, it is advantageous to use a large excess of ammonia, which helps shift the reaction balance towards the formation of primary amines.

Imagine cooking a dish where you add too many ingredients. You end up with a meal that’s a mix of flavors instead of a specific taste you intended. By controlling how much you add (using excess ammonia), you can guide the dish towards the flavor you want.

The order of reactivity of halides with amines is RI > RBr >RCl.

In the context of ammonolysis, the reactivity of various halides is important. The reactivity order indicates that iodides (RI) react more readily than bromides (RBr), which in turn are more reactive than chlorides (RCl). This order is due to the varying bond strengths between the carbon and the halogen; weaker bonds break more easily and allow for substitution to occur.

Consider a group of friends trying to get through a door (the carbon bond). The one who’s holding a balloon (iodine) is likely to let go and pass through first, while the person holding a sturdy briefcase (bromine), and then the one holding a heavy backpack (chlorine) will take longer to let go.

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