Amide

What are amides?

In organic chemistry, you might have previously come across Amines. These are organic molecules with the amine functional group, -NH₂. Amides are molecules that are similar to amines. They contain the amine group, -NH₂, bonded to the carbonyl group, C=O. This is known as the amide functional group.

Amide general formula

We now know that amides contain a carbonyl group, C=O, bonded to an amine group, -NH₂. This gives amides the general formula RCONH₂. Here, R represents an organic group joined to the other side of the carbonyl group.

Amide structure

Let's use our new knowledge of amides to draw their structure. Here is an example of an amide.

The general structure of an amide. StudySmarter Originals

Note the carbonyl group on the left, with its C=O double bond, and the amine group on the right. Because this is a primary amide, the nitrogen atom is bonded to two hydrogen atoms and no other R groups.

Amide polarity

We can expand on the structure of amides by showing their polarity. You might know that both the carbonyl and the amine group are polar. This makes amides polar as well. The carbon atom in the carbonyl group is always partially positively charged, whilst the oxygen atom is partially negatively charged. Meanwhile, the nitrogen atom in the amine group is partially negatively charged, whilst the hydrogen atoms are partially positively charged.

A diagram showing the polarity of amides. StudySmarter Originals

Naming amides

Moving on, let's look at amide nomenclature.

Primary amides

Naming primary amides is fairly simple. It all depends on the R group attached to the carbonyl group. In fact, it is very similar to naming carboxylic acids.

To name primary amides, we follow these steps.

1. Taking the carbon atom in the carbonyl group as carbon 1, find the length of the longest carbon chain. This gives you the molecule's root name.
2. Show any side chains or additional functional groups using prefixes and numbers.
3. Finish it all off with the suffix -amide.

Let's look at an example.

Secondary and tertiary amides

You should remember from earlier in the article that secondary and tertiary amides have additional R groups attached to their nitrogen atom. To indicate these R groups, we use additional prefixes, indicated by the letter N-. Here's an example.

Production of amides

Next, let's move on to look at the production of amides. You need to know about two similar reactions:

• The nucleophilic addition-elimination reaction between an acyl chloride and ammonia.
• The nucleophilic addition-elimination reaction between an acyl chloride and a primary amine.

Amide production: acyl chloride and ammonia

Reacting an acyl chloride with ammonia (NH₃) produces a primary amide and ammonium chloride. This is a nucleophilic addition-elimination reaction. It is also a condensation reaction, as it releases a small molecule in the process. Here, that small molecule is hydrochloric acid (HCl). The hydrochloric acid then reacts with another molecule of ammonia to form ammonium chloride (NH₄Cl).

For example, reacting ethanoyl chloride (CH₃COCl) with ammonia (NH₃) produces ethanamide (CH₃CONH₂)and hydrochloric acid, which further reacts with another molecule of ammonia to form ammonium chloride (NH₄Cl).

A diagram showing the reaction between ethanoyl chloride and ammonia, producing ethanamide and ammonium chloride.StudySmarter Originals

Amide production: acyl chloride and primary amine

Reacting an acyl chloride with a primary amine produces a secondary amide, also known as an N-substituted amide. Once again, this is an example of a nucleophilic addition-elimination reaction. It is also a condensation reaction, releasing hydrochloric acid in the process. The hydrochloric acid reacts with another molecule of the primary amine to form an ammonium salt.

For example, reacting ethanoyl chloride (CH₃COCl) with methylamine (CH₃NH₂) produces N-methylethanamide (CH₃CONHCH₃) and methylammonium chloride (CH₃NH₃Cl):

A diagram showing the reaction between ethanoyl chloride and methylamine, which produces N-methylethanamide and methylammonium chloride.StudySmarter Originals

Similarly, reacting an acyl chloride with a tertiary amine produces an amide with two N-substitutes.

Reactions of amides

Wondering how amides react? Let's explore that next. You need to know about two different reactions:

• Hydrolysis with an aqueous acid or alkali.
• Reduction with LiAlH₄.

We'll also touch on amide basicity.

Reactions of amides: hydrolysis with aqueous acid or alkali

First up, let's look at what happens when you react an amide with an aqueous acid or alkali. You actually produce a carboxylic acid and either ammonia or an amine, depending on whether your amide is primary, secondary, or tertiary. This is a hydrolysis reaction and requires heating. The acid or alkali then reacts with the products formed.

• If you use an acid, the acid reacts with the ammonia or amine formed to produce an ammonium salt.
• If you use an alkali, the alkali reacts with the carboxylic acid formed to produce a carboxylate salt.

Here are a couple of examples. Heating ethanamide (CH₃CONH₂) with aqueous hydrochloric acid (HCl) produces ethanoic acid (CH₃COOH) and ammonia (NH₃), which further reacts to form ammonium chloride (NH₄Cl):

A diagram showing the reaction between ethanamide, water, and hydrochloric acid, which produces ethanoic acid and ammonium chloride.StudySmarter Originals

Heating ethanamide with aqueous sodium hydroxide (NaOH) also produces ethanoic acid and ammonia. The ethanoic acid further reacts to form sodium ethanoate (CH₃COONa):

A diagram showing the reaction between ethanamide and sodium hydroxide, which produces sodium ethanoate and ammonia.StudySmarter Originals

Reactions of amides: reduction with LiAlH₄

Next up, let's consider what happens when you reduce an amide using a strong reducing agent such as lithium tetrahydridoaluminate, LiAlH₄. The reaction gets rid of the oxygen atom in the amide's carbonyl group and replaces it with two hydrogen atoms. This reaction takes place at room temperature in dry ether and also produces water.

For example, reducing methanamide (HCONH₂) with LiAlH₄ produces methylamine (CH₃NH₂) and water:

A diagram showing the reaction between methanamide and a reducing agent, which produces methylamine and water.StudySmarter Originals

Reactions of amides: basicity

You might know that amines act as weak bases. This is because the nitrogen atom in their amine group is able to pick up a hydrogen ion from solution using its lone pair of electrons. However, despite also containing an amine group, amides aren't basic. This is because they contain a carbonyl group, C=O. The carbonyl group is extremely electronegative and draws electron density towards it, reducing the attractive strength of nitrogen's lone pair of electrons. Therefore, amides don't act as bases.

Examples and uses of amides

Knowing what amides are and how they react is all well and good, but how does that apply to real life? Here are some examples of amides and their uses.

• Proteins, from the keratin in your hair and nails to the enzymes catalysing your cellular reactions, are all polyamides. They are made up of lots of smaller monomer units, called amino acids, joined together by amide linkage groups.
• Plastics and synthetic fibres such as nylon and Kevlar are also types of polyamides. So too are natural fibres like silk and wool.
• They play a role in the pharmaceutical industry - paracetamol, penicillin, and LSD are all examples of amides.
• The organic molecule urea, a natural waste product that we excrete in urine, is also an amide. It is produced industrially for use in fertilisers and animal feeds.

You should now feel confident at defining amides and giving their general formula and structure. You should be able to describe how they are formed, as well as how they react. Finally, you should be able to name some common examples of amides.