Reactions of Alcohols and Phenols: Quick Revision

A brief note on Reactions of Alcohols and Phenols. Read it before a test. Try each question at the end before you open its answer.

Acidity

• pKa: ethanol 15.9, water 15.7, phenol 10.0, ethanoic acid 4.76, carbonic acid 6.35. A smaller pKa is a stronger acid.
• Phenol dissolves in sodium hydroxide, not in sodium hydrogencarbonate. A carboxylic acid does both.
• The phenoxide ion is stabilised by resonance. A nitro group at ortho or para raises the acidity most. A methyl group lowers it.
• Acidity of alcohols: methanol, then primary, then secondary, then tertiary.

Reactions of alcohols

• With HX: tertiary, then secondary, then primary. HI is faster than HBr, and HBr is faster than HCl.
• Lucas test: cloudy at once (tertiary), in about 5 minutes (secondary), not at room temperature (primary).
• Dehydration: concentrated $\text{H}_{2}\text{SO}_{4}$ at 443 K gives ethene from ethanol. At 413 K with excess ethanol it gives ethoxyethane. Saytzeff's rule gives the more substituted alkene.
• Esterification: acid and alcohol with concentrated $\text{H}_{2}\text{SO}_{4}$ is reversible, and $K_c$ is about 4 for ethanoic acid and ethanol.
• Oxidation: primary gives an aldehyde (PCC) or an acid (strong agent), secondary gives a ketone, tertiary is not oxidised.

Reactions of phenol

• Preparation: cumene process (phenol and propanone), chlorobenzene with NaOH at 623 K and 300 atm, benzenesulphonic acid fusion, diazonium salt with water.
• Ring reactions: bromine water gives white 2,4,6-tribromophenol. Dilute nitric acid gives o- and p-nitrophenol. Concentrated nitric acid gives picric acid in low yield.
• Kolbe: phenoxide, $\text{CO}_{2}$, then acid gives salicylic acid. Reimer-Tiemann: $\text{CHCl}_{3}$ and NaOH, then acid gives salicylaldehyde.
• Zinc dust gives benzene. Chromic acid gives p-benzoquinone. Neutral ferric chloride gives violet.

A worked example

Find the share of p-nitrophenol (pKa 7.2) present as its anion at pH 8.2.

1. The ratio. $\dfrac{[\text{A}^-]}{[\text{HA}]} = 10^{\,8.2 - 7.2} = 10$.
2. The share. $\dfrac{10}{1 + 10} \times 100 = 90.9\ \%$.

Practice with new numbers

A program to try

The program predicts what three tests show for a primary, a secondary and a tertiary alcohol.

Mistakes to avoid

• Expecting a tertiary alcohol to oxidise.
• Using hydrogencarbonate to detect a phenol.
• Forgetting that esterification is reversible.
• Putting the Kolbe acid at the para position.
• Forgetting the acid step at the end of the Kolbe and Reimer-Tiemann reactions.

Check yourself

1. Which of phenol and ethanol is the stronger acid, and why?

2. What do 2-methylpropan-2-ol and butan-1-ol each show with the Lucas reagent at room temperature?

3. What does phenol give with excess bromine water?

4. Name the reaction that gives salicylic acid from sodium phenoxide.

Sources

• NCERT Chemistry Part II (Class 12), Chapter 7.
• Lide (ed.), CRC Handbook of Chemistry and Physics, dissociation constants of organic acids.
• Morrison and Boyd, Organic Chemistry, the chapters on alcohols and phenols.