Classes, names and structure of amines

The smell of fish

Open a packet of fish that has been left out in the sun. The sharp smell is mostly trimethylamine, $(\text{CH}_{3})_{3}\text{N}$. Bacteria break down proteins and one product is this gas. Ammonia, $\text{NH}_{3}$, has a similar sharp smell. The two are close relatives.

An amine is a compound in which one or more hydrogen atoms of ammonia are replaced by alkyl groups or aryl groups. An alkyl group is a chain such as $\text{CH}_{3}\text{-}$. An aryl group is a ring such as $\text{C}_{6}\text{H}_{5}\text{-}$. The nitrogen atom keeps its lone pair of electrons. That lone pair decides almost everything an amine does.

Three classes

The class of an amine depends on how many carbon groups sit on the nitrogen atom.

• A primary amine has one: $\text{R}\text{-}\text{NH}_{2}$. Nitrogen also holds two hydrogen atoms.
• A secondary amine has two: $\text{R}_{2}\text{NH}$. Nitrogen holds one hydrogen atom.
• A tertiary amine has three: $\text{R}_{3}\text{N}$. Nitrogen holds no hydrogen atom.

Here is a trap. In alcohols, the class depends on the carbon that carries the OH group. In amines it depends on the nitrogen. So $(\text{CH}_{3})_{3}\text{C}\text{-}\text{NH}_{2}$, tert-butylamine, is a primary amine. Its nitrogen has only one carbon group, even though that group is a branched one.

When the groups on nitrogen are all the same, the amine is simple, as in $(\text{CH}_{3})_{2}\text{NH}$. When they differ, the amine is mixed, as in ethylmethylamine, $\text{CH}_{3}\text{-}\text{NH}\text{-}\text{C}_{2}\text{H}_{5}$.

Look next at what is joined to the nitrogen. If the nitrogen is joined to an alkyl group only, the amine is aliphatic. If it is joined straight to a benzene ring, the amine is aromatic. Aniline, $\text{C}_{6}\text{H}_{5}\text{NH}_{2}$, is aromatic. Benzylamine, $\text{C}_{6}\text{H}_{5}\text{CH}_{2}\text{NH}_{2}$, has a CH2 group between the ring and the nitrogen, so it is an aliphatic amine that happens to carry a ring.

A nitrogen atom with four carbon groups is a quaternary ammonium ion, $\text{R}_{4}\text{N}^{+}$. It has no lone pair left, so it is not a base. Its salts, such as tetramethylammonium chloride, are named like any other salt.

Names of amines

There are two ways to name an amine.

Common names. Name the groups on the nitrogen in alphabetical order and add amine, all as one word. So $\text{CH}_{3}\text{NH}_{2}$ is methylamine, $(\text{C}_{2}\text{H}_{5})_{2}\text{NH}$ is diethylamine and $\text{CH}_{3}\text{-}\text{NH}\text{-}\text{C}_{2}\text{H}_{5}$ is ethylmethylamine. Use di and tri when the groups repeat.

IUPAC names. Take the longest carbon chain that holds the nitrogen. Drop the final e of the alkane and add amine. Number the chain so the carbon with the nitrogen gets the lowest number.

• $\text{CH}_{3}\text{NH}_{2}$ is methanamine.
• $\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{NH}_{2}$ is propan-1-amine, and $\text{CH}_{3}\text{CH}(\text{NH}_{2})\text{CH}_{3}$ is propan-2-amine.
• Every other carbon group on nitrogen is a prefix with the letter N. So $(\text{CH}_{3})_{2}\text{NH}$ is N-methylmethanamine and $(\text{C}_{2}\text{H}_{5})_{2}\text{NH}$ is N-ethylethanamine.
• $\text{CH}_{3}\text{-}\text{NH}\text{-}\text{C}_{2}\text{H}_{5}$ is N-methylethanamine. The longer chain is the parent.
• $(\text{CH}_{3})_{2}\text{N}\text{-}\text{C}_{2}\text{H}_{5}$ is N,N-dimethylethanamine.
• $\text{C}_{6}\text{H}_{5}\text{NH}_{2}$ is benzenamine. The name aniline is also accepted by IUPAC and used everywhere.
• $\text{C}_{6}\text{H}_{5}\text{N}(\text{CH}_{3})_{2}$ is N,N-dimethylaniline, or N,N-dimethylbenzenamine.
• A methyl group on the ring of aniline gives toluidine. The three isomers are o-, m- and p-toluidine, and p-toluidine is 4-methylaniline.

When another group has higher priority, the NH2 group is named as the prefix amino, as in 2-aminoethanol or 4-aminobenzoic acid.

The shape of the nitrogen atom

Nitrogen in an amine has three bonds and one lone pair, so it has four electron pairs. They point to the corners of a tetrahedron. The nitrogen is $sp^3$ hybridised. The three bonds form a pyramid and the lone pair sits at the top. The bond angles are close to the tetrahedral angle of $109.5^\circ$. In ammonia the H-N-H angle is $107^\circ$. NCERT draws the C-N-C angle of trimethylamine as $108^\circ$.

The lone pair is the key. It takes up a proton, so amines are bases. It attacks carbon atoms, so amines are nucleophiles. In aniline, the lone pair is partly spread into the ring, and that is why aniline is a much weaker base. Course a2 follows that idea.

A nitrogen with three different groups looks chiral. Yet the pyramid turns inside out billions of times each second, like an umbrella in the wind, and the two mirror images cannot be separated at room temperature. So simple amines are not optically active.

Amine isomers

The formula of a saturated open-chain amine with $n$ carbon atoms is $\text{C}_n\text{H}_{2n+3}\text{N}$. It has one hydrogen atom more than the alkane $\text{C}_n\text{H}_{2n+2}$, because an NH2 group takes the place of one H. Take $\text{C}_{3}\text{H}_{9}\text{N}$. It has four isomers:

• two primary amines: propan-1-amine and propan-2-amine;
• one secondary amine: N-methylethanamine;
• one tertiary amine: N,N-dimethylmethanamine, also called trimethylamine.

$\text{C}_{4}\text{H}_{11}\text{N}$ has eight: four primary, three secondary and one tertiary. To count them, use the number of alkyl groups. There are 1, 1, 2, 4 and 8 different alkyl groups with 1, 2, 3, 4 and 5 carbon atoms. A primary amine is one alkyl group on $\text{-}\text{NH}_{2}$. A secondary amine is a pair of alkyl groups. A tertiary amine is a trio.

Raise $n$ from 4 to 5 and watch the three counts. The secondary amines overtake the tertiary ones, and the total grows by about the same factor as the number of alkyl groups does.

Practise with new numbers

Try it in Python

The program counts the amine isomers for each size. It picks every way to split the carbon atoms between one, two or three groups, and it multiplies the numbers of alkyl groups.

Common mistakes

• Naming the class from the carbon, not the nitrogen. tert-Butylamine is a primary amine.
• Calling benzylamine aromatic. Its nitrogen is joined to a CH2 group, not to the ring.
• Forgetting the N. In $(\text{CH}_{3})_{2}\text{NH}$ the second methyl group is N-methyl, not a number on the chain.
• Choosing the shorter chain as the parent. In $\text{CH}_{3}\text{-}\text{NH}\text{-}\text{C}_{2}\text{H}_{5}$ the parent is ethane, so the name is N-methylethanamine.
• Calling an amine with four groups on nitrogen a quaternary amine. It is an ammonium ion with no lone pair.

Remember these

• Primary, secondary and tertiary: one, two or three carbon groups on the nitrogen.
• Aromatic means nitrogen joined straight to a ring.
• IUPAC: alkane name, drop the e, add amine; other groups on nitrogen are N-prefixes.
• The nitrogen atom is a pyramid with a lone pair, and the lone pair makes an amine a base and a nucleophile.
• $\text{C}_{3}\text{H}_{9}\text{N}$ has 4 amine isomers and $\text{C}_{4}\text{H}_{11}\text{N}$ has 8.

References

• NCERT Chemistry Class 12, the chapter Amines: classification, nomenclature and structure.
• OpenStax Chemistry 2e, Chapter 20, section 20.4 Amines and Amides.
• OpenStax Organic Chemistry, Chapter 24 Amines and Heterocycles, section on naming amines.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Classify $(\text{CH}_{3})_{2}\text{CH}\text{-}\text{NH}_{2}$ and give its IUPAC name.

2. Give the IUPAC name of $\text{CH}_{3}\text{-}\text{N}(\text{C}_{2}\text{H}_{5})\text{-}\text{CH}_{2}\text{CH}_{2}\text{CH}_{3}$.

3. Is benzylamine, $\text{C}_{6}\text{H}_{5}\text{CH}_{2}\text{NH}_{2}$, a primary aromatic amine?

4. Why can the tetramethylammonium ion not act as a base?