Functional groups, homologous series and naming

Atoms other than hydrogen

Carbon atoms also bond to oxygen, chlorine and other atoms. An atom or group that replaces a hydrogen, and decides the chemical properties of the compound, is a functional group.

Homologous series

A homologous series is a family of compounds with the same functional group and similar chemical properties, in which the formula of each member differs from the next by $\text{CH}_2$.

For the alcohols:

• Methanol, $\text{CH}_3\text{OH}$: $32\ \text{u}$.
• Ethanol, $\text{C}_2\text{H}_5\text{OH}$: $46\ \text{u}$.
• Propanol, $\text{C}_3\text{H}_7\text{OH}$: $60\ \text{u}$.

The mass goes up by $14\ \text{u}$ at each step. The chemical properties are the same for all the members. The physical properties, such as the boiling point, change steadily with the number of carbon atoms.

You should find that the three lines run side by side. Each step adds the same $14\ \text{g/mol}$, whatever the group.

Naming carbon compounds

A name has three parts.

1. A stem for the number of carbon atoms in the longest chain: meth (1), eth (2), prop (3), but (4), pent (5), hex (6).
2. An ending for the kind of bond or the functional group:

3. A prefix for a halogen atom, such as chloro or bromo: $\text{CH}_3\text{Cl}$ is chloromethane.

Example. $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$ has four carbon atoms (but) and a –COOH group (-oic acid). It is butanoic acid.

If the group ends with a carbon, such as –COOH and –CHO, that carbon is counted in the chain. If the ending starts with a vowel, the final "e" of the alkane is dropped: propane and -ol give propanol.

Practise with new numbers

Try it in Python

This program builds the name from the number of carbon atoms and the group.

Remember these

• A functional group decides the chemical properties. The groups: halo, –OH, –CHO, C=O, –COOH.
• A homologous series has the same functional group, and each member has one more $\text{CH}_2$ ($14\ \text{u}$).
• Name = stem (meth, eth, prop, but...) + ending (-ane, -ene, -yne, -ol, -al, -one, -oic acid).

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the name of $\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$?

2. Which functional group does propanone have?

3. Ethanol has a molar mass of $46\ \text{g/mol}$. What is the molar mass of propanol?

4. Do methanol and ethanol react in similar ways?