Alcohols: Naming, Preparation and Physical Properties: Quick Revision

A brief note on Alcohols: Naming, Preparation and Physical Properties. Read it before a test. Try each question at the end before you open its answer.

Classes and names

• An alcohol has OH on a saturated carbon. Simple ones: $\text{C}_n\text{H}_{2n+1}\text{OH}$.
• Primary: the OH carbon has 1 carbon neighbour. Secondary: 2. Tertiary: 3.
• Name: longest chain that includes the OH carbon, lowest number to OH, ending -ol. Diols and triols keep the e: ethane-1,2-diol.
• Isomers: $\text{C}_3\text{H}_7\text{OH}$ has 2 alcohols, $\text{C}_4\text{H}_9\text{OH}$ has 4, $\text{C}_5\text{H}_{11}\text{OH}$ has 8, $\text{C}_6\text{H}_{13}\text{OH}$ has 17.

Preparation

• Acid hydration of an alkene: Markovnikov, OH on the more substituted carbon, may rearrange. Only ethene gives a primary alcohol.
• Hydroboration then oxidation (borane, then hydrogen peroxide with sodium hydroxide): OH on the less substituted carbon, no rearrangement.
• Reduction: aldehyde gives primary, ketone gives secondary ($\text{NaBH}_{4}$ or $\text{LiAlH}_{4}$). A carboxylic acid or ester needs $\text{LiAlH}_{4}$.
• Grignard reagent $\text{RMgBr}$ in dry ether, then water: methanal gives primary, another aldehyde secondary, a ketone tertiary. Ethylene oxide adds two carbons.

Physical properties

• Hydrogen bonds (about $20\ \text{kJ/mol}$) make alcohols boil far above alkanes and ethers of the same molar mass.
• Boiling point rises with chain length and falls with branching.
• The first alcohols mix with water. Solubility falls as the chain grows. Alcohols are less dense than water.

A worked example

Give the product when but-1-ene reacts by (i) acid hydration and (ii) hydroboration then oxidation.

1. Carbons of the double bond. C1 is $\text{CH}_{2}$ with no alkyl group. C2 has one ethyl group.
2. Route (i). OH goes to C2: butan-2-ol.
3. Route (ii). OH goes to C1: butan-1-ol.

Practice with new numbers

A program to try

The program prints the oxygen share of the first six straight-chain alcohols.

Mistakes to avoid

• Numbering the chain from the wrong end.
• Expecting a primary alcohol from acid hydration of propene.
• Using sodium borohydride on a carboxylic acid.
• Making a Grignard reagent in the presence of water.
• Comparing boiling points of compounds with different molar masses.

Check yourself

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

2. Which alkene gives 2-methylpropan-2-ol by acid hydration?

3. Why does ethanol boil higher than methoxymethane?

4. Which reagent turns butanone into butan-2-ol?

Sources

• NCERT Chemistry Part II (Class 12), Chapter 7.
• OpenStax Chemistry 2e, Chapter 20, section 20.2.
• Lide (ed.), CRC Handbook of Chemistry and Physics, physical constants of organic compounds.