Oxidation of Primary Alcohol to Aldehyde

In this JC2 webinar we want to discuss the oxidation of primary alcohol to aldehyde.

Let's take a look at the reagents and conditions.

oxidation of primary alcohol to aldehyde reagents and conditions

Note that the alcohol carbon and oxygen will lose 1 hydrogen each.

This is consistent with the definition of oxidation via loss of hydrogen.

The product aldehyde is not stable to oxidation and can be further oxidised to carboxylic acid.

Hence we need to control the oxidation in order to get aldehyde.

We cannot use potassium manganate (VII) or KMnO4 in this reaction.

KMnO4 is too strong and will oxidise primary alcohol to acid directly.

Immediate distillation is also required, or the product will also be carboxylic acid.

Since we can use reflux with immediate distillation to remove aldehyde from this reaction mixture, does it mean we can always remove the product from any reaction via distillation?

To answer this question we need to understand the basis of separation.

Separation of mixture is usually based on the difference in physical properties of the components in the mixture.

Examples:

1. If mixture is solid and liquid with different physical states, we can separate them via filtration.
2. If mixture is 2 miscible liquids with different boiling points, we can separate them via distillation.
3. If mixture has different solubility in organic solvent and water, we can separate them via extraction via organic solvent or water.

For distillation, the liquid with the lower boiling point will be distilled out first.

oxidation of primary alcohol to aldehyde reflux with distillation setup

So for the oxidation of primary alcohol to aldehyde, the aldehyde will be distilled first due to its lower boiling point as compared to alcohol.

oxidation of primary alcohol to aldehyde compare bp alcohol aldehyde acid

Using concepts from Chemical Bonding (Intermolecular Forces), we can deduce that:

- Alcohol can form hydrogen bonds
- Aldehyde cannot form hydrogen bonds so will have weaker permanent dipole - permanent dipole attractions, hence lower boiling point
- Acid has more extensive hydrogen bonds due to additional acyl C=O group, hence higher boiling point than alcohol

If we were to oxidise primary alcohol to acid, we cannot remove the acid via distillation since it has a higher boiling point than alcohol.

Therefore in order to remove product via distillation, we have to ensure that the product has a lower boiling point than its reactant.

Topic: Alcohols, Organic Chemistry, A Level Chemistry, Singapore

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