Oxidation of alcohols and tests
Oxidation and tests
- Alcohols come in three classes: primary, secondary, tertiary.
- Oxidation tells them apart.
- A couple of tests confirm structure.
Oxidation by class
| Class | Oxidation product |
|---|---|
| primary | aldehyde (by distillation), then carboxylic acid (by reflux) |
| secondary | a ketone |
| tertiary | not oxidised |
- Acidified $\text{K}_2\text{Cr}_2\text{O}_7$ turns orange → green with a primary or secondary alcohol, but stays orange with a tertiary one.
Practice
A primary alcohol can be oxidised to:
Primary → aldehyde (by distillation) → carboxylic acid (by reflux); secondary → ketone; tertiary → not oxidised.
Practice
With acidified dichromate, a tertiary alcohol:
Tertiary alcohols are not oxidised, so the dichromate stays orange; primary/secondary turn it green.
Distillation, reflux and the iodoform test
- distillation removes the aldehyde as it forms (before further oxidation).
- reflux keeps boiling and returning the vapour, so the alcohol is fully oxidised to the acid.
- iodoform test: an alcohol with the $\text{CH}_3\text{CH(OH)}-$ group, warmed with alkaline aqueous iodine, gives a pale-yellow precipitate of CHI₃.
Practice
To stop oxidation of a primary alcohol at the aldehyde, you use:
Distillation removes the aldehyde before it oxidises further; reflux instead gives the carboxylic acid.
Acidity of alcohols
- Alcohols are very weakly acidic (they can lose $\text{H}^+$ to form $\text{RO}^-$).
- But they are less acidic than water: the alkyl group pushes electrons onto the oxygen, destabilising $\text{RO}^-$.
Practice
Compared with water, alcohols are:
The electron-pushing alkyl group makes RO⁻ less stable, so alcohols hold their H⁺ more tightly than water.
You've got it
Key idea
- primary → aldehyde (distil) → carboxylic acid (reflux); secondary → ketone; tertiary → not oxidised
- acidified dichromate goes orange → green (primary/secondary), stays orange (tertiary)
- distillation stops at the aldehyde; reflux gives the acid; iodoform tests for $\text{CH}_3\text{CH(OH)}-$
- alcohols are less acidic than water