One group, one personality: the eight organic families

How to use this simulation

1. It opens on “Alcohol” with two carbons. Ethanol appears atom by atom, and its –OH glows amber.
2. Switch family and watch the amber part change each time. The carbon chain stays grey; only the group changes.
3. Slide the carbon count. The name, formula and molar mass climb by exactly 14 each step, because each step adds one CH₂.
4. Slide below the minimum for a ketone, ester or ether and nothing breaks: the smallest member of that family is drawn, and a line under the table says so.
5. Read the name, general formula, suffix or prefix and the lab test in the readouts. The current member is highlighted in the table.

Same carbons, different personality

Picture a short chain of carbons. Put an –OH on the end and you have an alcohol: flammable, with that hospital smell. Put –COOH on the same end and you get a sour acid, like vinegar. Put –NH₂ there and it turns into a fishy-smelling base. Same backbone, three very different substances.

Nobody can memorise thousands of organic compounds. But learn eight or ten groups and you can predict most of the rest, because a group behaves much the same wherever it sits. This page is practice at spotting them.

What a functional group actually is

A functional group is the atom or cluster of atoms in a compound where most of the reactions happen. The hydrocarbon chain around it is comparatively inert. Think of the group as the handle on the molecule: reagents grab that.

All compounds sharing a group form a family, or homologous series. Neighbouring members differ by exactly one CH₂, which is 14 g/mol. The masses on this page use C = 12, H = 1, O = 16, N = 14 and Cl = 35.5.

Eight families at a glance

Here is the three-carbon member of each family side by side. Match the group, the suffix and the example.

How to read the animation

The IUPAC name sits at the top. In the middle is the structure, every atom written with its hydrogens, such as CH₃, CH₂ or OH. The hydrogen counts are not typed in: each atom is given enough hydrogens to reach its valence, which is 4 for carbon, 2 for oxygen, 3 for nitrogen and 1 for chlorine.

The amber atoms and bonds are the functional group. A double bond is drawn as two parallel lines. The bottom line names the group and writes its condensed formula.

Atoms appear one after another from left to right, as if someone were drawing the molecule. Change the family or the carbon count and the drawing starts again.

Naming a compound, step by step

A name has three parts: a stem for the number of carbons (meth, eth, prop, but, pent), “an” if the chain has single bonds only, and an ending for the group. So prop + an + ol is propanol. From three carbons up you also say where the group sits: propan-1-ol.

An ester is named in two pieces: the alkyl part from the alcohol first (methyl), then the acid part (-oate). The smallest is methyl methanoate, C₂H₄O₂. Halogens and ethers use prefixes instead of suffixes: chloro-, methoxy-. So CH₃Cl is chloromethane and C₂H₆O is methoxymethane.

• Step 1: find the longest carbon chain that carries the group.
• Step 2: count the carbons and pick the stem.
• Step 3: add the suffix or prefix, with a number if the position matters.

Homologous series: one CH₂ per step

Look at the alcohol table. Each extra carbon adds exactly 14 to the molar mass. That regular climb is what we call a homologous series.

Members share chemical behaviour because the group is the same. Physical properties drift: boiling points rise and solubility in water falls as the chain lengthens. That is why methanol mixes with water freely while long-chain alcohols barely do.

Isomers: one formula, different families

Ethanol, C₂H₆O, and methoxymethane, C₂H₆O, have the same formula and the same molar mass (46 and 46). One has an –OH, the other a C–O–C. One is a liquid that releases H₂ with sodium; the other is a gas that ignores sodium.

Propanal (an aldehyde) and propanone (a ketone) are both C₃H₆O, with masses 58 and 58. Ethanoic acid and methyl methanoate are both C₂H₄O₂, with masses 60 and 60. Equal masses, different families, different tests. These pairs are called functional group isomers.

Lab tests, family by family

Groups are usually identified with one or two simple tests. Here they are in one table.

Try it yourself in the simulation

Go in order. Write your guess down first, then check the canvas.

• Experiment 1: choose Alcohol and 4 carbons and note the name, formula and mass. Switch to Carboxylic acid. What changed?
• Experiment 2: set Aldehyde and Ketone to 3 carbons each. The formula and the mass are identical (58); only the position of the amber part differs.
• Experiment 3: choose Ketone and slide down to 1. A ketone needs at least three carbons, so the drawing stays at 3. Explain why.
• Experiment 4: put Ester and Ether at 2 carbons. Both have an –O–, so why is one an ester? Find the extra atom in the amber part.
• Experiment 5: run 1 to 5 carbons in every family and write the masses. Check that each step is exactly 14.

Worked problems

Every number here comes from the same counting the animation does.

Mistakes everyone makes

These cost marks every year.

• Mixing up aldehyde –CHO and acid –COOH. An acid has both a =O and an –OH on the same carbon.
• Drawing a ketone with fewer than three carbons. The C=O has to sit between two carbons, so a ketone needs at least three.
• Reading an ester as an ether. An ester has a C=O next to the –O–; an ether does not.
• Treating ethanol and methoxymethane as the same because the formula matches. Different group, different behaviour.
• Putting the suffix in the wrong place: alcohol “-ol”, aldehyde “-al”, ketone “-one”, ester “-oate”.
• Miscounting hydrogens: every carbon makes four bonds in total, and a double bond counts as two.

Functional groups all around you

These eight groups are in your kitchen and your medicine cabinet.

• Alcohol: the ethanol in hand sanitiser kills germs, and it is blended into some fuels too.
• Carboxylic acid: vinegar is ethanoic acid. Other acids give lemons their tang.
• Ester: much of the smell of mango, banana and pineapple is ester, which is why artificial fruit essences contain them.
• Amine: the smell of fish going off is amines. Lemon juice cuts it because acid turns the amine into a salt.
• Haloalkane: found in some solvents and pesticides; their environmental cost is why use is restricted.
• Ketone: nail-polish remover is propanone (acetone), a ketone.

Exam corner

Try these without looking, then check.

• Give one test to separate an aldehyde from a ketone. Answer: Tollens' reagent; only the aldehyde gives a silver mirror.
• A compound CₙH₂ₙO₂ has molar mass 74. Find n. Answer: 3.
• A compound has formula C₂H₆O. Draw two isomers and name the families. Answer: ethanol and methoxymethane.
• Why does NaHCO₃ identify a carboxylic acid? (The acid must be stronger than carbonic acid to release CO₂.)
• What is the reaction that makes an ester called? Answer: esterification, acid plus alcohol giving ester and water.

Revision: the whole page in a minute

Read this just before the exam.

• Functional group = the part that decides the behaviour.
• Eight families: alcohol –OH, aldehyde –CHO, ketone >C=O, acid –COOH, ester –COO–, amine –NH₂, halo –Cl, ether C–O–C.
• A homologous series climbs by one CH₂ per step, 14 in mass.
• Name = stem + an + ending; esters read alkyl first, then -oate.
• Same formula, different group = functional group isomers.
• Tollens': aldehyde. NaHCO₃: acid. Red litmus turns blue: amine. AgNO₃: halide.

Frequently asked questions