Coaching Centre
Functional group compounds
The functional groups in the HSC course and how to name compounds that contain them: primary, secondary and tertiary alcohols, aldehydes, ketones, carboxylic acids, amines and amides, with a priority order for choosing the suffix and worked naming examples.
Smell a ripe banana, a bottle of nail-polish remover and a jar of vinegar and you have smelled three different functional groups: an ester, a ketone and a carboxylic acid. Change one group of atoms on a carbon chain and the compound behaves like a completely different substance. The carbon chain is the skeleton; the functional group is the part that decides what the molecule does.
The functional groups
A functional group is an atom or group of atoms that gives a homologous series its characteristic reactions. The rest of the molecule (the alkyl part) is often written as R.
| Series | Group | Name ending | Example |
|---|---|---|---|
| Haloalkane | –X (F, Cl, Br, I) | prefix: fluoro-, chloro-… | 2-chloropropane |
| Alcohol | –OH, hydroxyl | -ol | propan-2-ol, CH₃CH(OH)CH₃ |
| Aldehyde | –CHO, carbonyl at the end of the chain | -al | propanal, CH₃CH₂CHO |
| Ketone | C=O, carbonyl inside the chain | -one | propanone, CH₃COCH₃ |
| Carboxylic acid | –COOH, carboxyl | -oic acid | propanoic acid, CH₃CH₂COOH |
| Ester | –COO– | alkyl …-oate | methyl ethanoate, CH₃COOCH₃ (7I) |
| Amine | –NH₂, amino | -amine | propan-1-amine, CH₃CH₂CH₂NH₂ |
| Amide | –CONH₂ | -amide | propanamide, CH₃CH₂CONH₂ |
Alcohols: primary, secondary and tertiary
Alcohols are named like alkanes with -e replaced by -ol, and the chain is numbered to give the OH the lowest number: propan-1-ol, butan-2-ol. The OH outranks branches and multiple bonds when numbering. Two OH groups make a diol: ethane-1,2-diol is the antifreeze in car radiators.
Alcohols are classified by the carbon that carries the OH:
- Primary (1°): the C–OH carbon is bonded to one other carbon (or none, in methanol). Examples: ethanol, butan-1-ol.
- Secondary (2°): bonded to two carbons. Example: butan-2-ol.
- Tertiary (3°): bonded to three carbons. Example: 2-methylpropan-2-ol.
The class is not just a label. It decides what happens when the alcohol is oxidised (7H): primary alcohols give aldehydes and then acids, secondary give ketones, and tertiary barely react.
Aldehydes and ketones
Both contain a carbonyl group, C=O. In an aldehyde it is at the end of the chain (–CHO), so that carbon is always C1 and needs no number: butanal, 2-methylpropanal. In a ketone it is inside the chain, joined to two carbons, and gets a number: pentan-2-one, pentan-3-one. Propanone (acetone, nail-polish remover) needs no number because C2 is the only place the C=O can go.
Carboxylic acids
The carboxyl group, –COOH, combines a carbonyl and a hydroxyl on the same carbon. Like the aldehyde group it is always on C1: ethanoic acid (vinegar), butanoic acid (the smell of rancid butter), 3-methylbutanoic acid. Carboxylic acids are weak acids (7I).
Amines and amides
Amines are organic relatives of ammonia, with one, two or three of its H atoms replaced by carbon groups (primary, secondary and tertiary amines). Name a primary amine from the longest chain attached to the N, with -amine and a number: butan-2-amine. Groups attached directly to the nitrogen get an N- locant: CH₃NHCH₂CH₃ is N-methylethanamine, and (CH₃)₂NCH₂CH₃ is N,N-dimethylethanamine. Amines smell fishy: the smell of fish going off comes partly from trimethylamine.
Amides have a nitrogen bonded to a carbonyl carbon, –CONH₂. The C=O carbon is C1, and the name ends in -amide: ethanamide, CH₃CONH₂. Substituents on the nitrogen again take N-: CH₃CONHCH₃ is N-methylethanamide. The C–N bond in an amide is the same link that joins amino acids in proteins and holds nylon together (7M).
Choosing the suffix when there are two groups
When a molecule has more than one group, the one with the highest priority becomes the suffix and sets the numbering; the others become prefixes (hydroxy-, amino-, oxo-). For the HSC, this order is enough:
carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > C=C / C≡C > halogen and alkyl branches
For example, HOCH₂CH₂COOH is 3-hydroxypropanoic acid, and CH₃CH(OH)COOH, the acid that builds up in tired muscles and sour milk, is 2-hydroxypropanoic acid (lactic acid). CH₂=CHCH₂OH is prop-2-en-1-ol: the OH gets the lowest number first, then the double bond.
Name (a) CH₃CH(CH₃)CH₂CHO, (b) CH₃CH₂C(CH₃)(OH)CH₃, (c) CH₃CH₂CH(CH₃)CONH₂.
- Build butan-2-ol. Then move the OH to C3. Why doesn't the name change?
- Switch the group to a ketone on a five-carbon chain. Try the C=O on C2 and on C3. Are these isomers, and what kind?
- Choose a carboxylic acid on a four-carbon chain and add a methyl to C2. Now move it to C4. What does the sim say, and why?
- Build propan-1-amine and propan-2-amine. Both are primary amines. Why, even though the nitrogen in propan-2-amine is on a secondary carbon?
- Switch to Quiz me and get five in a row right.
Test yourself
Which compound is a secondary alcohol?
In pentan-3-ol the C–OH carbon is bonded to two other carbons.
The IUPAC name of CH₃CH₂COCH₂CH₂CH₃ is
Six carbons; numbering from the left puts the carbonyl on C3.
CH₃CH₂NHCH₃ is
Two carbon groups on the nitrogen; the longer (ethyl) is the parent, and the methyl on N gets N-.
Which name is correct?
The COOH carbon is C1 and takes no number; a methyl on C1 would extend the chain.
Name each functional group.
Classify each alcohol.
The C=O group is called a group. When it is at the end of a chain the compound is an ; inside the chain it is a . The –COOH group is a group. A compound with –CONH₂ is an . An alcohol whose C–OH carbon is bonded to two other carbons is .
Three compounds each contain three carbon atoms and one oxygen atom: propan-1-ol, propan-2-ol and propanone. Write a condensed structural formula for each, classify each alcohol, and explain why propanone has no number in its name.
Propan-1-ol is CH₃CH₂CH₂OH. Its C–OH carbon is bonded to one other carbon, so it is a primary alcohol. Propan-2-ol is CH₃CH(OH)CH₃. Its C–OH carbon is bonded to two other carbons, so it is a secondary alcohol. Propanone is CH₃COCH₃, a ketone. A ketone's carbonyl group must be on a carbon bonded to two other carbons, because on an end carbon it would be an aldehyde. In a three-carbon chain the only such carbon is C2, so there is only one possible propanone and the locant is not needed.