Amine Reactions: Videos & Practice Problems

Amines, acting as weak bases, undergo acid-base reactions by accepting a proton (H⁺) from an acid, resulting in the formation of an ammonium ion. This process is well-illustrated by the reaction between methylamine and hydrochloric acid. According to the Brønsted-Lowry definition, the acid (hydrochloric acid) donates a proton to the base (methylamine), transforming it into a positively charged ammonium ion, specifically methylammonium ion. This reaction highlights the change in the amine's structure as the nitrogen atom now forms four bonds, carrying a positive charge, indicative of the ammonium ion's formation.

Acid-Base Reaction

Amine reactions can be understood through the lens of acid-base chemistry, where an acid interacts with an amine, which acts as a weak base. In this context, when a base accepts a proton (H⁺), it is converted into its conjugate acid. For amines, this conjugate acid is referred to as the ammonium ion.

To illustrate this, consider the reaction of methylamine with hydrochloric acid (HCl). According to the Brønsted-Lowry definition, an acid donates a proton to a base. In this case, when methylamine (a generic amine) accepts an H⁺ from HCl, the nitrogen atom in the amine forms four bonds instead of three, resulting in a positive charge. This transformation leads to the formation of the methylammonium ion.

Thus, the reaction can be summarized as follows:

Methylamine + HCl → Methylammonium ion + Cl^-

In this reaction, the methylamine is converted into the methylammonium ion, demonstrating the process of protonation and the formation of a positively charged ammonium ion.

Amine Reactions Example

When nitric acid (HNO₃) reacts with ammonia (NH₃), a neutral amine and a weak base, the reaction can be understood through the Bronsted-Lowry acid-base theory. In this context, nitric acid acts as a strong acid, donating a proton (H⁺) to ammonia. This process results in the formation of two key products: the ammonium ion (NH₄⁺) and the nitrate ion (NO₃⁻).The reaction can be summarized as follows:\[\text{HNO}_3 + \text{NH}_3 \rightarrow \text{NH}_4^+ + \text{NO}_3^-\]In this equation, the nitric acid donates a proton to ammonia, which transforms into the ammonium ion, while the nitric acid itself becomes the nitrate ion. The presence of a strong acid (nitric acid) reacting with a weak base (ammonia) indicates that the reaction proceeds completely to the right, as denoted by a solid arrow. This complete conversion underscores the strength of the acid in driving the reaction to form the stable products, ammonium and nitrate ions.

Determine the ammonium ion formed in the following reaction between diethylamine and hydrobromic acid.

• A.
• B.
• C.
• D.

Answer: (D)

Here’s what students ask on this topic:

Why is it so difficult to control the alkylation of primary amines in an SN2-type reaction manifold? What is the best answer?

Alkylation of primary amines in an SN2-type reaction is difficult to control due to the high reactivity of amines. Primary amines are nucleophilic, meaning they readily donate a pair of electrons to an electrophile. When a primary amine is alkylated, it transforms into a secondary amine, which is still nucleophilic and can further react to form a tertiary amine, and so on, often leading to a mixture of products with varying degrees of alkylation.

Moreover, the alkylated amines are often stronger bases than the starting primary amine. As the basicity increases, so does the nucleophilicity, which enhances the rate of further alkylation. This phenomenon is known as the "alkylation cascade" or "over-alkylation."

To control the alkylation of primary amines, chemists often use protecting groups to temporarily mask the amine functionality, steric hindrance to limit the access of the electrophile, or employ more selective reagents and reaction conditions that favor the formation of only the desired monoalkylated product.