Topic 19 · page 1 of 13

Names and formulae

Amines, amides and amino acids are named from the carbon chain plus one nitrogen group. Learn the three groups, the naming rules, and what a displayed and a skeletal formula must show.

15 min read19.1

Amines 19.1

An amine is a compound in which one or more hydrogen atoms of ammonia, NH₃, have been replaced by alkyl or aryl groups. The nitrogen keeps its lone pair, which is why amines behave as bases.

Naming

There are two accepted ways to name a primary amine. Both use the longest carbon chain that carries the nitrogen.

Formula Name 1: alkyl + amine Name 2: chain + amine
CH₃CH₂CH₂CH₂NH₂ butylamine butan-1-amine
CH₃CH₂CH₂NH₂ propylamine propan-1-amine
CH₃CH(NH₂)CH₃ (1-methylethylamine) propan-2-amine
C₆H₅NH₂ phenylamine (benzenamine, not needed)

For the second method, the number says which carbon carries the –NH₂ group. A substituent on the nitrogen itself is shown with a capital N: CH₃CH₂NHCH₃ is N-methylethanamine. Use the name 1 pattern (alkylamine) in answers unless the question gives you the other.

Primary, secondary and tertiary

The class of an amine is the number of carbon groups attached to the nitrogen.

Skeletal formulae of four amines: butylamine (primary), dimethylamine (secondary), trimethylamine (tertiary) and phenylamine (primary, aromatic). The class is the number of carbon groups on the nitrogen atom.
One carbon group on N is primary, two is secondary, three is tertiary. Phenylamine is a primary aromatic amine.

Amides 19.1

An amide has the group −CONHX2\ce{-CONH2}: a carbonyl carbon bonded to a nitrogen. You name an amide from the carboxylic acid it comes from. Remove "-oic acid" and add "-amide".

Acid Amide Formula
ethanoic acid ethanamide CH₃CONH₂
propanoic acid propanamide CH₃CH₂CONH₂
butanoic acid butanamide CH₃CH₂CH₂CONH₂

If a hydrogen on the nitrogen is replaced by an alkyl or aryl group, you get an N-substituted amide. The capital N shows where the group is.

The carbon count in the name comes from the acid part, so the parent chain in each case is ethan-, from CH₃CO–.

Amino acids 19.1

An amino acid has an amine group and a carboxylic acid group. The amino acids in proteins are 2-amino acids (also called α-amino acids): the –NH₂ is on the carbon next to the carboxylic acid carbon.

RCH(NHX2)COOH \ce{RCH(NH2)COOH}

Number the chain from the carboxylic acid carbon, which is carbon 1. That puts the amino group on carbon 2.

Formula Systematic name Common name
H₂NCH₂COOH aminoethanoic acid glycine
CH₃CH(NH₂)COOH 2-aminopropanoic acid alanine

Three kinds of formula 19.1

Questions ask for a structural, displayed or skeletal formula. They are not interchangeable.

Butylamine Ethanamide 2-Aminopropanoic acid
structural CH₃CH₂CH₂CH₂NH₂ CH₃CONH₂ CH₃CH(NH₂)COOH
Displayed and skeletal formulae of butylamine, ethanamide and 2-aminopropanoic acid. Displayed formulae show every bond, including both N-H bonds and the C=O double bond. Skeletal formulae leave out carbon atoms and the hydrogen atoms on carbon but keep NH2, OH and O.
Displayed (left of each row) shows every atom and bond. Skeletal (right) shows only the bonds between carbons and the groups that are not C or C–H.

Worked exampleName and classify

Name CH₃CH₂CONH₂, CH₃CH₂NHCH₂CH₃ and CH₃CH(NH₂)CH₃, and say what type of compound each is.

  1. CH₃CH₂CONH₂ has a −CONHX2\ce{-CONH2} group and three carbons in the acid part: propanamide, an amide.
  2. CH₃CH₂NHCH₂CH₃ has two ethyl groups on N: diethylamine, a secondary amine.
  3. CH₃CH(NH₂)CH₃: –NH₂ on carbon 2 of a three-carbon chain: propan-2-amine, a primary amine (one carbon group on N).

Check yourself

  1. Name CH₃CH₂CH₂NH₂ and classify (CH₃CH₂)₂NH.
  2. Name CH₃CH₂CH₂CONH₂ and CH₃CONHCH₃.
  3. Draw the displayed formula of ethanamide.
  4. Draw the skeletal formula of butylamine.
  5. Write the structural formula of 2-aminopropanoic acid and state which carbon the amino group is on.
  6. A student calls CH₃CH(NH₂)CH₂CH₃ a secondary amine because the carbon is secondary. What class is it, and what is the error?

Answers cover these until you have tried the questions

  1. CH₃CH₂CH₂NH₂ is propylamine (propan-1-amine). (CH₃CH₂)₂NH has two carbon groups on N, so it is a secondary amine, diethylamine.
  2. CH₃CH₂CH₂CONH₂ is butanamide. CH₃CONHCH₃ is N-methylethanamide.
  3. Displayed: a carbon with three H atoms, single-bonded to a second carbon; the second carbon is double-bonded to O and single-bonded to N; the N carries two H atoms, each shown as an N–H bond. All bonds are drawn. (See the figure above.)
  4. Skeletal: a zigzag of four carbons with NH₂ written at the end of the chain (the three C–C bonds are lines; no C or H on carbon is written).
  5. CH₃CH(NH₂)COOH. The amino group is on carbon 2, counting the −COOH\ce{-COOH} carbon as carbon 1.
  6. It is a primary amine (butan-2-amine). The class depends on the number of carbons attached to the nitrogen, which is one, not on the class of the carbon that carries the –NH₂ group.