Functional Groups

The reactive 'handles' bolted onto a carbon skeleton — and why they, not the chain, decide chemistry.

High schoolIntro OrganicUni Year 1
⏱️ About 18 min

Ethane is an unreactive gas. Add one oxygen–hydrogen pair and you get ethanol — a liquid you can drink, burn or turn into vinegar. The carbon skeleton barely changed; the small group you attached changed everything. Those groups are the vocabulary of organic chemistry.

💡
The big idea: A functional group is a specific arrangement of atoms attached to a carbon skeleton that gives a molecule its characteristic reactions. Molecules with the same functional group behave alike — so learning a handful of groups lets you predict the behaviour of millions of compounds.
🎯 By the end, you'll be able to
  • Define a functional group and explain why it governs reactivity
  • Recognise hydroxyl (–OH), carbonyl (C=O), carboxyl (–COOH) and amino (–NH₂) groups
  • Match each group to its family: alcohol, aldehyde/ketone, carboxylic acid, amine
  • Read a structural formula to spot the functional group present
📎 Helpful to know first

Same skeleton, different behaviour

A hydrocarbon chain is fairly inert. The interesting chemistry happens at a functional group — a small cluster of atoms (often containing oxygen or nitrogen) attached to that chain. The group is the reactive site: it is where bonds break and form.

The power of this idea is generalisation. Every alcohol reacts like an alcohol because every alcohol carries the same –OH group. So instead of memorising millions of molecules, you learn the behaviour of about a dozen groups.

🔑 R stands for 'the rest of the molecule'
Chemists write a functional group with an R for the carbon skeleton it hangs off: R–OH is any alcohol, R–COOH is any carboxylic acid. The R can be short or huge — the group is what sets the chemistry.

The hydroxyl group: alcohols

The hydroxyl group is –OH bonded to carbon, and it defines the alcohols. Ethanol, CH3CH2OH, is the alcohol in beverages and hand sanitiser. The polar –OH makes small alcohols mix with water and boil far higher than the matching alkane.

\[ \ce{CH3CH2OH} \]
Ethanol — an ethane skeleton carrying a hydroxyl (–OH) group. That one group makes it an alcohol.

The carbonyl group: aldehydes and ketones

A carbonyl group is a carbon double-bonded to oxygen, C=O. Where it sits on the chain names the family:

  • Aldehyde — the carbonyl is at the end of the chain (R–CHO), as in methanal (formaldehyde).
  • Ketone — the carbonyl is in the middle, between two carbons (R–CO–R), as in propanone (acetone).

The carboxyl group: carboxylic acids

Combine a carbonyl and a hydroxyl on the same carbon and you get the carboxyl group, –COOH, which defines the carboxylic acids. Ethanoic acid (CH3COOH) is the acid in vinegar. The –COOH group can donate its hydrogen as an H+, which is why these molecules are acidic.

\[ \ce{CH3COOH -> CH3COO^- + H^+} \]
The carboxyl group makes ethanoic acid acidic: it releases a proton (H⁺) in water.
✨ The amino group: amines and life
The amino group is –NH2, and it defines the amines. Amines are basic (the nitrogen accepts a proton), the opposite tendency to a carboxyl group. Crucially, every amino acid carries both an amino group and a carboxyl group — that pairing is what lets amino acids link into proteins.
⚠️ The group rules, not the chain length
Do not judge a molecule by its size or its carbon count. A two-carbon alcohol and a twenty-carbon alcohol both react as alcohols, because both carry –OH. Change the functional group and the chemistry changes; lengthen the chain and it mostly does not.
📝 Worked example: A molecule has the structure CH₃–CO–CH₃. Which functional group does it contain, and what family is it?
  1. Look for the reactive cluster: there is a carbon double-bonded to oxygen (C=O) — a carbonyl group.
  2. Locate the carbonyl: it sits between two carbon atoms (CH3 on each side), i.e. in the middle of the chain.
  3. A carbonyl in the middle of the chain defines a ketone (this molecule is propanone/acetone).
✓ A carbonyl group; the molecule is a ketone (propanone).
✏️ Practice: How many carbon atoms are in the carboxylic acid ethanoic acid, CH₃COOH? (Count every C in the formula.)
carbons
Solution
  1. Write out the carbons: the CH3 group has 1 carbon.
  2. The COOH (carboxyl) group has 1 carbon.
  3. 1 + 1 = 2 carbons — which is why it is named on the 'ethan-' (2-carbon) root.

Check your understanding

1. Why do all alcohols share similar chemical behaviour?
The functional group governs reactivity. Every alcohol carries –OH, so they react in the same characteristic ways regardless of chain length.
2. Which functional group makes a molecule a carboxylic acid?
The carboxyl group –COOH defines carboxylic acids and can donate an H⁺, making them acidic. Hydroxyl gives alcohols; amino gives basic amines.
3. Ethanol (2 carbons) and a 16-carbon alcohol are compared. What is true?
Both share the –OH functional group, so both behave as alcohols. Chain length changes physical properties like boiling point, but the group sets the chemistry.
🔎 Related lessons
Previous: Hydrocarbons Next: IUPAC Nomenclature Biomolecules
✅ Key takeaways
  • A functional group is a reactive cluster of atoms that gives a molecule its characteristic chemistry.
  • Hydroxyl (–OH) → alcohols; carbonyl (C=O) → aldehydes (end) and ketones (middle).
  • Carboxyl (–COOH) → acidic carboxylic acids; amino (–NH₂) → basic amines.
  • Molecules sharing a group react alike, so a few groups predict millions of compounds.
  • The group, not the chain length, controls reactivity.
➡️ Now that you can spot the chain and the group, you can give any molecule a precise, unambiguous name. Next up is the IUPAC system — the grammar that turns a structure into a name and back again.
Want to test yourself on this? Try the Chemistry practice test →