Functional Groups in Organic Chemistry: Chart and Names
A functional group is a specific group of atoms in an organic molecule that gives it its characteristic reactions. Common ones are alkenes (C=C), alcohols (–OH), aldehydes (–CHO), ketones (C=O), carboxylic acids (–COOH), esters (–COO–) and amines (–NH₂). Molecules with the same group react in similar ways.
What is a functional group?
A functional group is an atom or group of atoms with a particular structure that gives the molecule its characteristic chemical properties. The rest of the molecule, usually a chain of carbon and hydrogen, is the carbon skeleton. Ethane (C₂H₆) has no functional group and is quite unreactive. Add an –OH to it and you get ethanol, which has very different properties.
All compounds that share a functional group and differ only in chain length form a homologous series, with a general formula and similar chemistry.
Functional groups chart
| Functional group | Structure | Name ending | Example |
|---|---|---|---|
| Alkene | C=C | -ene | Ethene (C₂H₄) |
| Alkyne | C≡C | -yne | Ethyne (C₂H₂) |
| Haloalkane | C–F, C–Cl, C–Br, C–I | chloro-, bromo-, iodo- | Chloromethane (CH₃Cl) |
| Alcohol | –OH | -ol | Ethanol (C₂H₅OH) |
| Ether | C–O–C | alkoxy- | Methoxymethane (CH₃OCH₃) |
| Aldehyde | –CHO at the end of a chain | -al | Ethanal (CH₃CHO) |
| Ketone | C=O inside the chain | -one | Propanone (CH₃COCH₃) |
| Carboxylic acid | –COOH | -oic acid | Ethanoic acid (CH₃COOH) |
| Ester | –COO– | -yl …-oate | Ethyl ethanoate |
| Amine | –NH₂ | -amine | Methylamine (CH₃NH₂) |
| Amide | –CONH₂ | -amide | Ethanamide (CH₃CONH₂) |
These functional group names follow a pattern: the ending tells you the family, and the stem tells you the number of carbons (meth-, eth-, prop-, but-, pent-).
How to identify functional groups
Follow these steps for any structure:
- Draw the full structure, including every double bond and every atom that is not carbon or hydrogen.
- Look for double or triple bonds between carbons. C=C is an alkene and C≡C is an alkyne.
- Find oxygen. –OH is an alcohol. C=O is a carbonyl: at the end of a chain it is an aldehyde, inside the chain a ketone. If the C=O also has an –OH on the same carbon, it is a carboxylic acid. If it has an –O–C beside it, it is an ester.
- Find nitrogen. –NH₂ on a carbon chain is an amine, and if a C=O is next to it, it is an amide.
- Find halogens. Fluorine, chlorine, bromine or iodine attached to carbon makes a haloalkane.
- Check the name ending to confirm.
Some molecules have more than one functional group. Amino acids, for example, contain both an amine and a carboxylic acid.
What do the main functional groups do?
- Alkenes decolourise orange bromine water, which is the standard test for a double bond.
- Alcohols burn and can be oxidised: ethanol oxidises to ethanoic acid.
- Carboxylic acids are weak acids. They react with carbonates to give carbon dioxide, which fizzes.
- Esters are formed when an alcohol reacts with a carboxylic acid, and they have sweet, fruity smells.
- Amines are weak bases, and they have a fishy smell.
- Haloalkanes can be broken down by water or alkali, replacing the halogen with an –OH.
Worked example: ethanol, ethanal and ethanoic acid
Three molecules, all with two carbons, but different groups:
- Ethanol, CH₃CH₂OH: contains –OH, so it is an alcohol.
- Ethanal, CH₃CHO: contains C=O at the end of the chain, so it is an aldehyde.
- Ethanoic acid, CH₃COOH: contains –COOH, so it is a carboxylic acid.
Heating ethanol with acidified potassium dichromate oxidises it to ethanal, then to ethanoic acid. The carbon skeleton does not change, but the functional group does, and so do the properties.
Aldehyde vs ketone
Both contain C=O, but in an aldehyde the carbonyl is at the end of the chain, with a hydrogen attached, while in a ketone it sits in the middle between two carbons. The smallest ketone is propanone, because a carbonyl needs a carbon on each side.
Common mistakes
- Calling C=O a hydroxyl. The hydroxyl group is –OH.
- Mixing up esters (–COO– with a carbon on both sides) and carboxylic acids (–COOH with a hydrogen).
- Forgetting that alcohol names end in -ol and ketones in -one.
- Counting a C=C in a benzene ring as an alkene. Benzene's structure behaves differently.
Frequently asked questions
What is a functional group in chemistry?
A functional group is a specific arrangement of atoms in a molecule that is responsible for its characteristic reactions. For example, the –OH group makes a compound an alcohol, and the –COOH group makes it a carboxylic acid.
What are the main functional groups to learn?
The ones most courses cover are alkenes, haloalkanes, alcohols, aldehydes, ketones, carboxylic acids, esters, amines and amides. Learn the structure, the name ending and one example for each.
How do you tell an aldehyde from a ketone?
Look at where the C=O is. In an aldehyde it is at the end of the carbon chain and carries a hydrogen (–CHO). In a ketone it is inside the chain, joined to carbon atoms on both sides.
Why are functional groups important?
They allow chemists to predict how a molecule will react from its structure. Compounds with the same functional group show similar reactions, so one set of rules covers whole families of molecules.
Try it yourself
Open the Molecule Builder and build ethanol, then ethanoic acid. In each, circle the functional group and say what its name ending should be. Then look at how simple molecules are drawn on the Lewis structure pages.
See it for yourself, in 3D and free.
Try it in the Molecule Builder