Question

Which of the following would react most readily with a carboxylic acid to form an amide? (A) Methylamine (B) Triethylamine (C) Diphenylamine (D) Ethylmethylamine

Short answer

(A) Methylamine

Step-by-step solution

Understanding the Reaction

When a carboxylic acid reacts with an amine, an amide is formed. Carboxylic acids typically react with primary and secondary amines to form amides.

Analyzing the Amines

Evaluate the given amines to determine if they are primary, secondary, or tertiary: (A) Methylamine is a primary amine. (B) Triethylamine is a tertiary amine. (C) Diphenylamine is a secondary amine. (D) Ethylmethylamine is a secondary amine.

Reactivity of Amines

Primary and secondary amines can react with carboxylic acids to form amides. Tertiary amines generally do not form amides with carboxylic acids because they lack a hydrogen on the nitrogen to form a bond with the carboxylic acid.

Conclusion

Since primary amines react most readily with carboxylic acids to form amides, identify the primary amine among the given options.

Key concepts

Carboxylic Acids

Carboxylic acids are organic compounds containing a carboxyl group $(\text{COOH})$. This functional group consists of a carbonyl $(\text{C=O})$ and a hydroxyl $(\text{OH})$ group attached to the same carbon atom.
Carboxylic acids are known for their acidic properties due to the release of a hydrogen ion from the hydroxyl group. They play a crucial role in organic synthesis, especially in the formation of amides. During amide formation, the carboxylic acid reacts with an amine. The hydrogen from the amine and the hydroxyl from the carboxyl group combine to form water, leaving behind an amide bond $(\text{CONH})$.
The process requires either a primary or secondary amine. Carboxylic acids are often used as precursors in pharmaceuticals, plastics, and various chemical manufacturing processes.

Primary Amines

Primary amines have the general formula $({\text{RNH}}_2)$, where $(\text{R})$ is an alkyl or aryl group. In a primary amine, one hydrogen atom of ammonia $({\text{NH}}_3)$ is replaced by an $(\text{R})$ group.
This structure allows primary amines to readily react with carboxylic acids to form amides. The presence of two hydrogen atoms attached to the nitrogen makes primary amines highly reactive in these types of nucleophilic substitution reactions.
An important example of a primary amine is methylamine $({\text{CH}}_3{\text{NH}}_2)$. It reacts with carboxylic acids to form methylamides, which are significant in various chemical syntheses and industrial applications.

Secondary Amines

Secondary amines follow the general formula $({\text{R}}_2\text{NH})$, where two hydrogen atoms of ammonia are replaced by $(\text{R})$ groups. These can be identical or different alkyl or aryl groups.
Secondary amines can also react with carboxylic acids to form amides, albeit at slightly lower reactivity compared to primary amines. This is due to steric factors that make the nitrogen less accessible.
Examples of secondary amines include diphenylamine $({\text{(C}}_6{\text{H}}_5{\text{)}}_2\text{NH})$ and ethylmethylamine $({\text{C}}_2{\text{H}}_5{\text{NHCH}}_3)$. In the context of amide formation, secondary amines offer a pathway to create more complex amide structures utilized in various organic synthesis routes.

Tertiary Amines

Tertiary amines have the general formula $({\text{R}}_3\text{N})$, where all three hydrogen atoms of ammonia are replaced by $(\text{R})$ groups. These groups can either be the same or different.
Unlike primary and secondary amines, tertiary amines do not have a hydrogen atom attached to the nitrogen. This makes them generally unreactive towards carboxylic acids for amide formation.
An example is triethylamine $({\text{(C}}_2{\text{H}}_5{\text{)}}_3\text{N})$. The lack of a hydrogen on the nitrogen means tertiary amines cannot participate in the nucleophilic substitution required to form amide bonds. Thus, they are typically not used for this type of reaction. Instead, tertiary amines are often used as bases or catalysts in organic reactions.

Organic Reaction Mechanisms

Organic reaction mechanisms describe the step-by-step process by which organic molecules undergo chemical transformations. Understanding these mechanisms is key to mastering organic chemistry.
In the context of amide formation, the mechanism starts with the nucleophilic attack of the amine on the carbonyl carbon of the carboxylic acid. This is followed by the elimination of water, leading to the formation of the amide bond.
Whether the amine is primary, secondary, or tertiary significantly influences its reactivity. Primary and secondary amines, having hydrogens attached to the nitrogen, can form stable intermediates necessary for this reaction. Conversely, tertiary amines usually do not participate as they lack these hydrogens.
Grasping these mechanisms helps predict the behavior and outcomes of reactions, enabling more efficient and targeted synthesis in organic chemistry.