Oxidation of alcohols and tests
Introduced| English | Meaning |
|---|---|
| oxidation/ˌɒksɪˈdeɪʃn/ | A chemical change involving a loss of electrons or an increase in oxidation state. |
| aldehyde/ˈældɪhaɪd/ | An organic compound containing a carbonyl group bonded to at least one hydrogen atom. |
| distillation/dɪstɪˈleɪʃn/ | Separating substances by vaporising and condensing them, using differences in volatility or boiling point. |
| carboxylic acid/ˌkɑːbəkˈsɪlɪk ˈæsɪd/ | An organic compound containing the COOH functional group. |
| reflux/ˈriːflʌks/ | Heating a reaction mixture while condensing its vapour and returning the condensate to the same vessel. |
| ketone/ˈketəʊn/ | An organic compound with a carbonyl carbon bonded to two carbon groups. |
| iodoform test/ˈaɪədɒfɔːm test/ | iodoform test |
Oxidation 氧化 and tests
- Alcohols come in three classes: primary, secondary, tertiary.
- Oxidation tells them apart.
- A couple of tests confirm structure.
Alcohol oxidation test lab
Classify alcohols by oxidation product and observation.
Oxidising a primary alcohol and distilling off the product gives an:
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Distilling removes the aldehyde before further oxidation.
Oxidation by class
| Class | Oxidation product |
|---|---|
| primary | aldehyde 醛 (by distillation 蒸馏), then carboxylic acid 羧酸 (by reflux 回流) |
| secondary | a ketone 酮 |
| 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.
Oxidation products of primary, secondary and tertiary alcohols
A primary alcohol can be oxidised to:
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Primary → aldehyde (by distillation) → carboxylic acid (by reflux); secondary → ketone; tertiary → not oxidised.
With acidified dichromate, a tertiary alcohol:
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Tertiary alcohols are not oxidised, so the dichromate stays orange; primary/secondary turn it green.
Match each oxidation fact.
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Each item links the term to its correct meaning.
To stop the oxidation of a primary alcohol at the aldehyde you distil the product off as it forms; refluxing instead gives the carboxylic acid.
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Distilling removes the volatile aldehyde before it can be oxidised again; reflux keeps it in the mixture for full oxidation.
Distillation, reflux and the iodoform test 碘仿试验
- 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.
Ethanol, the alcohol in hand sanitiser, kills microbes.
To stop oxidation of a primary alcohol at the aldehyde, you use:
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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 $\text{RO}^-$.
Distillation removes the aldehyde; reflux fully oxidises to the acid
Compared with water, alcohols are:
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The electron-pushing alkyl group makes RO⁻ less stable, so alcohols hold their H⁺ more tightly than water.
You've got it
- 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