Recognise the common functional groups in organic molecules (alcohol, aldehyde, ketone, carboxylic acid, ester, amine, amide) and what each one tells you.
Why this matters
Organic chemistry, the chemistry of carbon compounds, covers fuels, plastics, medicines, solvents and every molecule in a living cell. There are millions of organic compounds, but you do not need to memorise them. Chemists sort them by functional groups: small arrangements of atoms that react the same way wherever they appear. Spot the group and you can predict how a molecule behaves, how it might be named, and how it will show up on an instrument.
Carbon's backbone
Carbon forms four bonds. It bonds readily to other carbons, making chains and rings, and to hydrogen, oxygen and nitrogen. A molecule made only of carbon and hydrogen is a hydrocarbon:
| family | bonds between carbons | example |
|---|---|---|
| alkane | single only (C–C) | ethane, CH₃CH₃ |
| alkene | at least one double (C=C) | ethene, CH₂=CH₂ |
| alkyne | at least one triple (C≡C) | ethyne (acetylene), HC≡CH |
| aromatic | a benzene ring | benzene, C₆H₆ |
The C–H backbone is nonpolar and fairly unreactive. The interesting chemistry happens at the functional groups attached to it.
The groups to know
Chemists write R for "the rest of the molecule" (a carbon chain).
| group | structure | name ending | example |
|---|---|---|---|
| alcohol | R–OH | -ol | ethanol, CH₃CH₂OH (drinks, hand sanitiser) |
| ether | R–O–R | ether | diethyl ether, CH₃CH₂OCH₂CH₃ |
| aldehyde | R–CHO (C=O at the end of a chain) | -al | methanal (formaldehyde), HCHO |
| ketone | R–CO–R (C=O inside a chain) | -one | propanone (acetone), CH₃COCH₃ |
| carboxylic acid | R–COOH | -oic acid | ethanoic (acetic) acid, CH₃COOH |
| ester | R–COO–R | -oate | ethyl ethanoate (ethyl acetate), CH₃COOCH₂CH₃ |
| amine | R–NH₂ | -amine | methylamine, CH₃NH₂ |
| amide | R–CONH₂ | -amide | ethanamide (acetamide), CH₃CONH₂ |
| haloalkane | R–X (X = F, Cl, Br, I) | chloro-, bromo- | chloromethane, CH₃Cl |
The carbonyl group, C=O, appears in aldehydes, ketones, acids, esters and amides. What is attached to the carbonyl carbon decides which one it is:
H on the carbonyl carbon -> aldehyde R-CHO
two carbons on it -> ketone R-CO-R
an -OH on it -> carboxylic acid R-COOH
an -O-R on it -> ester R-COO-R
an -N on it -> amide R-CONH2
Worked examples: name the group
- CH₃CH₂CH₂OH. An –OH on a carbon chain: alcohol (propan-1-ol).
- CH₃COCH₂CH₃. A C=O with carbon on both sides: ketone (butanone).
- CH₃CH₂COOH. –COOH at the end: carboxylic acid (propanoic acid).
- HCOOCH₃. C=O bonded to an O that leads to another carbon: ester (methyl methanoate).
- CH₃CH₂NH₂. Nitrogen bonded to carbon and hydrogens: amine (ethylamine).
- Aspirin contains a benzene ring, a carboxylic acid and an ester, all in one molecule. Real molecules often have several groups.
What groups tell you about properties
- –OH, –COOH, –NH₂ can hydrogen-bond, so small alcohols, acids and amines mix with water and have higher boiling points than hydrocarbons of similar size. Ethanol boils at 78 °C; propane, of similar mass, boils at −42 °C.
- Carboxylic acids are weak acids (vinegar is about 4–8% acetic acid in water). Amines are weak bases, like ammonia.
- Esters often smell fruity and are used in flavourings and solvents. Many nail-polish removers are acetone, a ketone.
- An acid plus an alcohol can form an ester and water (esterification); an acid plus an amine can form an amide. The amide link is the peptide bond that joins amino acids in proteins, which you will see in the next lesson.
Where you will see this at work
Functional groups are how chemists read infrared spectra (a C=O shows a strong peak near 1700 cm⁻¹), how pharmacologists think about how a drug behaves in the body, and how materials scientists describe polymers (polyesters, polyamides such as nylon). Lab and instrument work is learned in supervised lab courses; this lesson is the vocabulary.
Common mistakes
- Mixing up aldehyde and ketone. Aldehyde: the C=O carbon carries a hydrogen (chain end). Ketone: C=O between two carbons.
- Calling an ester an ether. An ester has a C=O next to the O; an ether has only C–O–C.
- Calling every –OH an alcohol. In –COOH the –OH is part of a carboxylic acid, which behaves very differently.
- Amine vs amide. Amide has a C=O attached to the nitrogen; amine does not.
Quick recap
- Carbon makes four bonds and forms chains and rings; hydrocarbons are the backbone.
- Functional groups set the chemistry: alcohol, ether, aldehyde, ketone, acid, ester, amine, amide.
- Look at what is attached to a C=O to tell its group apart.
- Polar groups raise boiling points and water solubility.
Try it
Open the Do tab and take the quiz: you will identify the functional group in several formulas.