What is a functional group?
A carbon chain made only of C and H, such as ethane, is called a hydrocarbon. It reacts very little. If you replace a hydrogen by another atom or small group, you get a new compound. That atom or group is the functional group: it decides the main properties and reactions of the compound.
A monovalent group joins to the carbon chain with one bond. The three groups in this lesson are monovalent: –X (halogen: F, Cl, Br, I), –OH (hydroxyl) and –NH2 (amino). We write the chain as R, so the families are R–X, R–OH and R–NH2.
Halogen compounds (R–X)
Replace H by F, Cl, Br or I and you get a halogen compound, also called a haloalkane. Names: add the halogen as a prefix: CH3Cl is chloromethane, CH3CH2Br is bromoethane, CH3CH2CH2Cl is 1-chloropropane.
Why they react: the halogen pulls the bond electrons towards itself, so carbon is slightly positive (δ+) and the halogen slightly negative (δ−). A negative particle can attack the δ+ carbon and push the halogen out. This is called substitution. For example, R–Cl + OH− gives R–OH + Cl−.
Properties: they boil higher than the alkane with the same chain because of the polar bond. They do not dissolve in water, but they dissolve in organic liquids. Uses: solvents, refrigerants, and starting materials for making other compounds. Some (like CFCs) harm the ozone layer.
Hydroxyl compounds: alcohols
Replace H on a chain by –OH and you get an alcohol. Name: replace the final -e of the alkane by -ol: methanol, ethanol, propan-1-ol, propan-2-ol.
The O–H group is polar, and the H of one molecule is pulled to the O of the next molecule. This is a hydrogen bond (gold beads in the 3D). It holds molecules together, so alcohols have much higher boiling points than other compounds of similar size: ethanol 78 °C, chloroethane 12 °C, ethane −89 °C. Small alcohols mix with water in any amount because they can hydrogen-bond with water too.
Classes: primary (the C holding OH has one carbon neighbour or none), secondary (two), tertiary (three). Alcohols can be oxidised, burnt and dehydrated. They are only very weakly acidic.
Phenols and acidity
In a phenol the –OH is joined straight to a benzene ring, as in C6H5OH. This small change makes a big difference: phenol is a weak acid, much stronger than ethanol. It even reacts with sodium hydroxide, which ethanol does not.
Why? When phenol loses H+, the negative charge left on oxygen spreads into the ring, which makes the particle more stable. In an alcohol the charge stays on one oxygen and that is less stable. A more stable leftover means the acid gives away H+ more easily. Still, phenol is weaker than a carboxylic acid and weaker than carbonic acid, so it does not react with sodium hydrogencarbonate.
Phenol is a solid with a sharp smell. It kills germs but burns skin, so it is used carefully in disinfectants and in making plastics and dyes.
Amines and basic character
Replace H on a chain by –NH2 and you get an amine. Name: methylamine CH3NH2, ethylamine CH3CH2NH2. When a benzene ring holds the group, it is aniline.
Nitrogen has five outer electrons. Three are used in bonds and one pair is left free (the lone pair). This pair can take an H+ from an acid, so amines are bases. Example: CH3CH2NH2 + HCl gives CH3CH2NH3+ Cl−, a salt that dissolves in water.
Chain amines are slightly stronger bases than ammonia because the chain pushes electrons towards N. In aniline the lone pair is shared with the ring, so aniline is a weaker base. Amines can form hydrogen bonds too, but weaker than –OH, so ethylamine (17 °C) boils lower than ethanol (78 °C).
Compare the three groups
| Group | Family | Key idea | Example, boiling point |
|---|---|---|---|
| –Cl | Halogen compound | Polar C–Cl, substitution | Chloroethane, 12 °C |
| –OH | Alcohol / phenol | H-bonds; phenol is weak acid | Ethanol, 78 °C |
| –NH2 | Amine | Lone pair, base | Ethylamine, 17 °C |
Try it: in the 3D, tap each group and compare the bar heights. Then predict which would boil first if you heated all three.
Key formulas and definitions
- R–X: halogen compound R–OH: alcohol or phenol R–NH2: amine
- R–Cl + OH⁻ → R–OH + Cl⁻ (substitution)
- C6H5OH + NaOH → C6H5ONa + H2O (phenol is acidic)
- R–NH2 + HCl → R–NH3⁺ Cl⁻ (amine is a base)
- Boiling point order for similar size: R–OH > R–NH2 > R–X > alkane (roughly)
Worked examples
1. Name CH3CH2CH2Cl and CH3CH2OH.
CH3CH2CH2Cl is 1-chloropropane (a halogen compound). CH3CH2OH is ethanol (an alcohol).
2. Find the molar mass of ethanol, C2H5OH. (C = 12, H = 1, O = 16)
2 x 12 + 6 x 1 + 16 = 24 + 6 + 16 = 46 g/mol.
3. Why does ethanol (46 g/mol) boil at 78 °C but chloroethane (64.5 g/mol) at only 12 °C, although chloroethane is heavier?
Ethanol molecules form hydrogen bonds with each other, and it takes much more heat to pull them apart. Chloroethane has only weaker pulls between molecules.
4. Which reacts with NaOH: ethanol or phenol? Write the reaction for the one that does.
Phenol. C6H5OH + NaOH gives C6H5ONa + H2O, because phenol is a weak acid and its leftover ion is stabilised by the ring. Ethanol is too weakly acidic to react.
5. Ethylamine is added to dilute HCl. What forms and why?
CH3CH2NH2 + HCl gives CH3CH2NH3+ Cl-. The lone pair on nitrogen takes H+ from the acid, so the amine acts as a base. The salt dissolves in water.
6. Arrange ethane, ethanol and ethylamine from lowest to highest boiling point and give the reason.
Ethane (-89 °C) < ethylamine (17 °C) < ethanol (78 °C). Ethane has no polar group; the NH2 group forms weaker hydrogen bonds; the OH group forms the strongest.
Common mistakes
- Saying an alcohol is as acidic as phenol. Phenol is far more acidic because its ion is stabilised by the ring.
- Mixing up -ol (alcohol) and -amine names: ethanol is C2H5OH, ethylamine is C2H5NH2.
- Forgetting that chloroethane has no hydrogen bonds, so its boiling point is lower than ethanol's.
- Thinking all amines are equally basic. Aniline is a weaker base than ethylamine because its lone pair is shared with the ring.