What has a higher melting/boiling point Aldehyde or Ketone?

If two molecules (one an aldehyde one a ketone) of similar Mr are compared which would have a higher boiling point and why?

Reply 1

TSR Jessica

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Reply 2

R T

Original post by adam32900

If two molecules (one an aldehyde one a ketone) of similar Mr are compared which would have a higher boiling point and why?

I doubt there is any consistent trend in BP when you compare the two.

Perhaps ketone's may have a slightly higher BP for low-Mr compounds. This is because the intermolecular forces are mostly made up of the polar interactions between C=O groups of different molecules. Ketone's have alkyl donating groups on both sides of the C=O bond (aldehydes only have one) and so its dipole will be greater, making the polar interaction stronger, the intermolecular force stronger, and the boiling point higher.

However obviously larger compounds will run into complications from london dispersion forces and steric effects preventing polar interactions in certain setups (ketones suffering more than aldehydes for sure). So long term I doubt there will be any simple trend that can easily be explained without a lot of detail.

Reply 3

charco

Study Forum Helper

Original post by Oli.Cich

Ketones have a higher boiling points than aldehydes (with a similar Mr). This is because, with ketones, there are two alkyl groups around the C=O bond which donate electrons, making ketones more polar than aldehydes.

This has to be incorrect.

We know that aldehydes are more reactive than ketones as there is only one alkyl group inducing electrons towards the partially positive carbonyl carbon. This makes the aldehyde MORE polarised than the ketone (stabilised by two alkyl groups).

The variation in bp of ketones and aldehydes with similar Mr is not a general phenomenon. The lower members has ketones with higher bp and the higher members have aldehydes with higher bp.

pentanal and pentanone both have the same bp (102ºC)