Hydrocarbons and Their Names: Quick Revision

A brief note on Hydrocarbons and Their Names. Read it before a test. Try each question at the end before you open its answer.

Three families of hydrocarbons

A ring also takes away two hydrogens. Cyclohexane, $\text{C}_6\text{H}_{12}$, is saturated.

Isomers

Isomers have the same molecular formula and different structures. Butane and 2-methylpropane are both $\text{C}_4\text{H}_{10}$.

Functional groups and names

Stems: meth $1$, eth $2$, prop $3$, but $4$, pent $5$, hex $6$. Count the carbon of $-\text{CHO}$ and $-\text{COOH}$ in the chain.

Homologous series

The same functional group runs through the series. Each member has one more $\text{CH}_2$ than the one before. The chemical properties stay alike, and the boiling points rise steadily.

A worked example

Name $\text{CH}_3\text{CH}_2\text{CH}_2\text{Cl}$ and find its molar mass. Use C $= 12$, H $= 1$ and Cl $= 35.5$, all in g/mol.

1. Name. Three carbons give prop-, and the chlorine gives the prefix chloro-: chloropropane.
2. Molar mass. $3 \times 12 + 7 \times 1 + 35.5 = 78.5\ \text{g/mol}$.

Practice with new numbers

A program to try

The program writes the formula of each family for one number of carbons.

Mistakes to avoid

• Forgetting the carbon of $-\text{COOH}$. $\text{CH}_3\text{COOH}$ has two carbons, so it is ethanoic acid.
• Assuming $\text{C}_n\text{H}_{2n}$ is always an alkene. It may be a ring.
• Looking for methene. A double bond needs at least two carbon atoms.
• Mixing up an aldehyde and a ketone. $-\text{CHO}$ ends the chain; C=O inside it makes a ketone.

Check yourself

1. Write the formula of the alkyne with $10$ carbon atoms.

2. Which functional group is in ethanal?

3. Are butane and 2-methylpropane isomers? Why?

4. What is the general formula of the alkenes?