Acid Amides

Amides, also called organic amides or carboxamides, are organic compounds containing a carbonyl functional group bonded to a nitrogen atom. The simplest amide is derived from ammonia (NH₃), where an acyl group replaces one hydrogen atom.

We will learn the amides in detail, including their definition, types, structures, and properties.

1.0Introduction

Amides are a vital class of organic compounds identified by the presence of a nitrogen atom and a carbonyl functional group. They feature a carbonyl group (C=O) directly bonded to an amine group (-NH₂, -NHR, or -NR₂) and a hydrocarbon group or a hydrogen atom. The carbonyl group consists of a carbon atom double-bonded to an oxygen atom.

2.0Structure of an Amide

The general structure of an amide can be represented as:

                                    R-C(O)-NR'R"

Where:

• R represents a hydrocarbon group or a hydrogen atom.
• R' and R" represent hydrogen atoms or hydrocarbon substituents attached to the nitrogen atom.

Amides are typically represented structurally as follows:

• The carbonyl group (C=O) is central to the structure, with one side bonded to an alkyl or aryl group (R).
• The nitrogen atom (N) is attached to the carbonyl group and further bonded to either hydrogen atoms or additional alkyl/aryl groups (R', R")

Nomenclature: Named by replacing the -oic acid or -ic acid suffix of carboxylic acid with -amide. Example: Ethanoic acid → Ethanamide.

3.0Properties of Amides

4.0Classifications of Amides

Amides are categorised into three types based on the number of carbon atoms bonded to the nitrogen atom. These types are:

1. Primary Amides
2. Secondary Amides
3. Tertiary Amides

1. Primary Amides

In primary amides the nitrogen atom is bonded to only one carbon atom. The general formula for a primary amide is RCONH₂, where R represents either a hydrogen atom or an organic group.

Nomenclature:
Primary amides are named by replacing the suffix -oic acid or -ic acid of the parent carboxylic acid with -amide. Examples: Methanamide, Ethanamide, Propanamide, Acetamide

2. Secondary Amides (2° Amides)

The nitrogen atom is bonded to two carbon atoms in secondary amides. The general formula for a secondary amide is RCONHR', where R and R' can be organic groups or hydrogen atoms.

Nomenclature:
Secondary amides are named using an uppercase N to indicate that the alkyl group is attached to the nitrogen atom. Alkyl groups attached to the nitrogen are treated as substituents.

3. Tertiary Amides (3° Amides)

In tertiary amides, the nitrogen atom is bonded to three carbon atoms. The general formula for a tertiary amide is RCONR'R″, where R, R', and R″ are organic groups or hydrogen atoms.

Nomenclature:
Tertiary amides are named similarly to secondary amides but use two uppercase N's to indicate the two alkyl groups attached to the nitrogen.

5.0Synthesis of Amides

Amides can be synthesised using various methods, two of the most common being the reaction of carboxylic acids with amines and the reaction of acid chlorides with amines or ammonia.

1. Reaction of Carboxylic Acid and Amine

This method involves reacting a carboxylic acid (RCO₂H) directly with an amine (R'NH₂) using a coupling agent like dicyclohexylcarbodiimide (DCC).

Reaction: RCO₂H + R’NH₂ + DCC → RCONHR’ + DCC (byproduct)

2. Reaction of Acid Chloride with Ammonia or Amines

Amides can also be prepared by reacting acid chlorides (RCOCl) with ammonia, a primary amine, or a secondary amine.

Reaction: RCOCl +R’NH₂ → RCONHR’ + HCl

6.0Basicity of Amides

Amides are weak bases compared to amines. While the pKa of an amine’s conjugate acid is approximately 9.5, the pKa of an amide’s conjugate acid is around 0.5. This significant difference arises because the carbonyl group withdraws electrons from the amine, reducing its basicity.

Despite their weak basicity, amides are more substantial bases than carboxylic acids, esters, aldehydes, and ketones, as their conjugate acids have pKa values ranging from 6 to 10. However, due to their diminished basicity, amides exhibit less pronounced acid-base behaviour in water.

7.0Applications of Amides

• Biological Molecules: Amide bonds are key in peptides, proteins, DNA, and RNA, providing stability and structure.
• Pharmaceuticals: Amides are found in many drugs, like paracetamol, and are vital in drug development.
• Polymer Industry: Used in synthetic polymers like nylon and in manufacturing fibers and plastics.
• Solvents: Dimethylformamide (DMF) serves as a versatile solvent in organic synthesis and industry.