Alcohols and Phenols Compared: MDCAT Chemistry notes
Alcohols and Phenols Compared for MDCAT: acidity order, reaction with NaOH, Lucas, ferric chloride and bromine water tests, and esterification.
What they share
Both alcohols and phenols contain the hydroxyl group, $-\mathrm{OH}$. In an alcohol it sits on an sp$^3$ carbon of an alkyl group; in a phenol it sits on an sp$^2$ carbon of a benzene ring. Because of the O–H bond, both form hydrogen bonds, both react with sodium metal to release hydrogen, and both form esters (phenols usually need an acid chloride or anhydride rather than the free acid).
Acidity: the key difference
When phenol loses $\mathrm{H^+}$, the negative charge on the phenoxide ion is delocalized over the benzene ring by resonance. The alkoxide ion from an alcohol has no such stabilization; the electron-releasing alkyl group actually concentrates the charge on oxygen. So phenol is the stronger acid.
$$\text{Carboxylic acid} > \text{Phenol} > \text{Water} > \text{Alcohol}$$
Consequences:
- Phenol dissolves in aqueous NaOH, forming sodium phenoxide. Alcohols are too weakly acidic to react with NaOH.
- Phenol is still weaker than a carboxylic acid and carbonic acid, so it does not react with $\mathrm{NaHCO_3}$.
- Phenol is more acidic than water and ethanol, but less acidic than a carboxylic acid.
Comparison table
| Property / reagent | Alcohol (e.g. ethanol) | Phenol |
|---|---|---|
| Carbon bearing OH | sp$^3$ | sp$^2$ (ring) |
| Sodium metal | $\mathrm{H_2}$ + alkoxide | $\mathrm{H_2}$ + phenoxide |
| Aqueous NaOH | No reaction | Forms sodium phenoxide |
| Lucas reagent (HCl/$\mathrm{ZnCl_2}$) | Gives alkyl chloride (turbidity) | No reaction |
| C–O bond cleavage by HX | Yes | No (C–O has partial double-bond character) |
| Neutral $\mathrm{FeCl_3}$ | No colour | Violet colour |
| Bromine water | No reaction | White ppt of 2,4,6-tribromophenol |
| Nitration of ring | Not applicable | Easy (o/p-nitrophenol, picric acid) |
| Esterification | Yes (acid + conc. $\mathrm{H_2SO_4}$) | Yes (acid chloride or anhydride) |
| Physical state (room temp.) | Ethanol: liquid | Solid |
Distinguishing tests
- Lucas test identifies alcohols and classifies them as 1$^\circ$, 2$^\circ$ or 3$^\circ$ by how fast turbidity appears; phenol does not respond. It is therefore a test that differentiates alcohols from phenol.
- Ferric chloride test and bromine water test identify phenol.
- Solubility in NaOH separates phenol from an alcohol.
- Halogenation and nitration are reactions of the phenol ring; the iodoform test depends on the $\mathrm{CH_3CH(OH)-}$ group, not on the phenol/alcohol difference.
Worked comparison
Why does phenol react with NaOH but ethanol does not? Because the phenoxide ion is resonance-stabilized, the equilibrium $\mathrm{C_6H_5OH + OH^- \rightleftharpoons C_6H_5O^- + H_2O}$ lies to the right. For ethanol the ethoxide ion is a stronger base than hydroxide, so the equilibrium lies to the left. Boiling points and hydrogen bonding are not the reason.
Common MDCAT traps
- Phenol's greater acidity comes from a resonance-stabilized conjugate base, not from stronger hydrogen bonding.
- Phenol is not completely insoluble in water; it is moderately soluble.
- Phenol does not undergo nucleophilic substitution at its OH easily; alcohols do.
- Reaction given by both: esterification. Lucas reaction is given by alcohols only.
Quick revision
- Acidity: RCOOH > phenol > water > alcohol.
- Phenol + NaOH → sodium phenoxide; alcohol does not react.
- Lucas test is for alcohols.
- $\mathrm{FeCl_3}$ violet colour indicates phenol.