Topic 10 · page 22 of 27
Distil to stop, reflux to go on
A primary alcohol gives an aldehyde if the product is distilled out as it forms, and a carboxylic acid if the mixture is heated under reflux with excess oxidising agent. The boiling temperatures explain why.
Read first: Oxidation of alcohols, Heating under reflux and distillation
Why the conditions matter 10.1810.18(i)10.18(ii)10.19(i)10.19(iii)
The aldehyde made in stage 1 can be oxidised again to the acid in stage 2. So the conditions decide how far a primary alcohol is oxidised.
| Distil as it forms | Heat under reflux | |
|---|---|---|
| Apparatus | distillation | reflux condenser |
| Oxidising agent | added gradually; limited | excess |
| What happens to the aldehyde | escapes from the mixture at once | stays in the flask |
| Product | aldehyde (propanal) | carboxylic acid (propanoic acid) |
| Positive test | Benedict's or Fehling's | sodium carbonate or sodium hydrogencarbonate |
The boiling temperatures explain it 10.1810.19
The aldehyde has a lower boiling temperature than the alcohol, so it distils off first.
| Compound | Boiling temperature / °C | Intermolecular forces |
|---|---|---|
| propanal | 49 | permanent dipole–dipole and London forces. No O–H, so no hydrogen bonds between its molecules |
| propan-1-ol | 97 | hydrogen bonds between its molecules |
| propanoic acid | 141 | hydrogen bonds, and a larger (74.0 against 60.0) |
The temperature on the thermometer in the distillation stays near 49 °C, because only propanal is boiling. The propan-1-ol (97 °C) stays in the flask. If the temperature starts to rise towards 97 °C, unreacted alcohol is distilling too.
Distil gently, with a dropping funnel for the oxidising agent, and collect the product in a receiver kept in ice: propanal is volatile and flammable. The orange mixture turns green as the dichromate is reduced.
Secondary alcohols 10.1810.18(iii)
A secondary alcohol gives a ketone however you heat it. The ketone has no hydrogen on its C=O carbon, so it cannot be oxidised further. Reflux or distillation makes no difference to the product, only to how fast the reaction goes and whether the ketone stays in the flask.
Worked exampleWhy propanal is collected at 49 °C
Explain why a thermometer reading of about 49 °C shows that propanal is being collected, and not propan-1-ol or propanoic acid.
- Propanal has no O–H, so it cannot form hydrogen bonds with itself. Its boiling temperature, 49 °C, is the lowest of the three.
- Propan-1-ol has hydrogen bonds and boils at 97 °C.
- Propanoic acid has hydrogen bonds and a larger and boils at 141 °C.
- A steady reading near 49 °C means only the lowest-boiling compound, propanal, is leaving the flask.
Check yourself
- State the conditions used to make an aldehyde from a primary alcohol, and explain why they work.
- State the conditions used to make a carboxylic acid from a primary alcohol.
- Explain why propanal has a lower boiling temperature than propan-1-ol.
- A student heats propan-1-ol with acidified dichromate under reflux. Name the product.
- Explain why a secondary alcohol gives the same product by distillation or by reflux.
- State the colour change in the flask, and say what it shows.
Answers cover these until you have tried the questions
- Warm with acidified potassium dichromate(VI) and distil off the aldehyde as it forms. The aldehyde boils at a lower temperature than the alcohol, so it leaves the mixture before it can be oxidised again.
- Heat under reflux with excess acidified potassium dichromate(VI). The aldehyde stays in the flask and is oxidised to the acid.
- Propan-1-ol has an O–H group and forms hydrogen bonds between its molecules. Propanal has no O–H group, so it cannot, and it needs less energy to separate its molecules.
- Propanoic acid, CH₃CH₂COOH.
- The product is a ketone, which has no H on the carbonyl carbon and cannot be oxidised further by dichromate, so the extra heating changes nothing.
- Orange to green: dichromate(VI) is reduced to chromium(III), so something has been oxidised.