Classifying and naming alcohols

A label on a bottle

Read the label on a bottle of hand sanitiser. It says ethanol or propan-2-ol. The same small group of atoms sits in both names, and in the sugar of a cake. That group is -OH, the hydroxyl group.

A compound with an OH group on a saturated carbon (a carbon with single bonds only) is an alcohol. The simple ones with one OH group have the formula $\text{C}_n\text{H}_{2n+1}\text{OH}$. Two other kinds of OH compound look similar and are different.

• Phenol: the OH is on a carbon of a benzene ring. Course 2 of this chapter treats phenols.
• Enol: the OH is on a carbon of a double bond, as in $\text{CH}_{2}=\text{CHOH}$. It is unstable and turns into an aldehyde or a ketone.

Count the OH groups too. A monohydric alcohol has one, such as ethanol. A dihydric alcohol has two, such as ethane-1,2-diol (also called ethylene glycol, the antifreeze of a car). A trihydric alcohol has three, such as propane-1,2,3-triol (glycerol or glycerine, in soaps and creams).

Primary, secondary and tertiary

Take the carbon that holds the OH. Count the other carbon atoms bonded to it. The count gives the class.

• Primary (1°): one carbon neighbour. Methanol, with none, is put with the primary alcohols.
• Secondary (2°): two carbon neighbours.
• Tertiary (3°): three carbon neighbours.

Two more words are useful later. In an allylic alcohol the carbon with OH is next to a C=C double bond, as in prop-2-en-1-ol. In a benzylic alcohol it is next to a benzene ring, as in phenylmethanol (benzyl alcohol). Both react faster than a plain alcohol, because the carbocation that can form is stabilised.

Naming by the IUPAC rules

Follow these steps in order.

1. Find the longest carbon chain that contains the carbon with OH. A longer chain that misses that carbon does not count.
2. Number the chain from the end that gives the OH carbon the lowest number. The OH decides, before any branch does.
3. Take the alkane name, drop the final e and add ol, with the number of the OH carbon in front: propan-2-ol. Older books write 2-propanol. Both mean the same.
4. Name each branch and put its number in front. Branches go in alphabetical order, before the main name: 3-methylbutan-2-ol.

More cases. With two or three OH groups keep the e and add the multiplier: ethane-1,2-diol, propane-1,2,3-triol. A double bond comes with the OH still lowest: $\text{CH}_{2}=\text{CHCH}_{2}\text{OH}$ is prop-2-en-1-ol. On a ring, the OH carbon is number 1: cyclohexanol, 2-methylcyclohexanol.

Short names are still used in laboratories. The alkyl group comes first: methyl alcohol (methanol), ethyl alcohol (ethanol), isopropyl alcohol (propan-2-ol), tert-butyl alcohol (2-methylpropan-2-ol). Learn both forms.

The shape at the oxygen

The oxygen of an alcohol is $sp^3$. Two of its four orbitals hold lone pairs, and two make the bonds to carbon and to hydrogen. The lone pairs push the two bonds a little closer than the ideal tetrahedral angle of $109.5^\circ$. The C-O-H angle is about $109^\circ$, and the C-O bond is about $142\ \text{pm}$ long, shorter than a C-C bond of $154\ \text{pm}$.

Because oxygen attracts the shared electrons strongly, both the C-O bond and the O-H bond are polar. Lesson 3 uses this.

Slide to $n = 4$. Four alcohols share the formula $\text{C}_4\text{H}_9\text{OH}$: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol. The number of isomers grows quickly: 17 for six carbons and 39 for seven.

Practise with new numbers

Try it in Python

The program sorts four alcohols into primary, secondary and tertiary by counting the carbon neighbours of the carbon that holds the OH.

Common mistakes

• Taking the longest chain without the OH carbon. The chain must include the carbon that holds OH, even when another chain is longer.
• Numbering from the wrong end. The OH gets the lowest number, not the first branch.
• Counting the OH when classifying. Count only carbon neighbours of the carbon with OH.
• Calling propan-2-ol primary. Its OH carbon has two carbon neighbours, so it is secondary.
• Dropping the e too early. Keep the e in a diol or triol: propane-1,2-diol.

Remember these

• An alcohol has OH on a saturated carbon: $\text{C}_n\text{H}_{2n+1}\text{OH}$ for the simple ones.
• The class depends on the carbon neighbours of the OH carbon: 1 is primary, 2 is secondary, 3 is tertiary.
• Name: longest chain with the OH carbon, lowest number to OH, ending -ol.
• $\text{C}_4\text{H}_9\text{OH}$ has 4 alcohols, $\text{C}_5\text{H}_{11}\text{OH}$ has 8.

Check yourself

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

1. Name $\text{CH}_{3}\text{CH}(\text{CH}_{3})\text{CH}_{2}\text{CH}_{2}\text{OH}$ and give its class.

2. Name $\text{CH}_{3}\text{CH}(\text{CH}_{3})\text{CH}(\text{OH})\text{CH}_{2}\text{CH}_{3}$.

3. How many carbon neighbours does the OH carbon of 2-methylbutan-2-ol have, and what class is the alcohol?

4. Write the formula of ethane-1,2-diol and say why the name keeps the e.

References

• NCERT Chemistry Part II (Class 12), Chapter 7, the sections on classification and nomenclature.
• OpenStax Chemistry 2e, Chapter 20, section 20.2 on alcohols and ethers.