EasyChemistryHigh School (11–12)Organic Compounds with Functional Groups › Aldehydes, Ketones and Carboxylic AcidsConceptual
Arrange propanone, methanal, ethanal and butanone in order of increasing reactivity towards…
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Arrange propanone, methanal, ethanal and butanone in order of increasing reactivity towards nucleophilic addition. Give two reasons for the order. Then say which of benzaldehyde and ethanal is the more reactive, and why.
Need a reminder? The key ideaShow
A nucleophile attacks the carbonyl carbon, the carbon changes from to , and the oxygen then takes a proton. Alkyl groups lower the positive charge and crowd the carbon, so methanal > other aldehydes > ketones. HCN gives a cyanohydrin, NaHSO a bisulphite compound, alcohols with dry HCl an acetal, and compounds a C=N compound and water.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The order. Methanal has two hydrogens on the carbonyl carbon. Ethanal has one methyl group. Propanone has two methyl groups. Butanone has a methyl and an ethyl group. So the order of increasing reactivity is butanone, propanone, ethanal, methanal.
Step 2. Reason one. Alkyl groups push electrons towards the carbon, so it is less positive. More alkyl groups mean less positive charge.
Step 3. Reason two. The carbon is crowded by large groups, and the tetrahedral product is more crowded still.
Step 4. Benzaldehyde. The ring shares electrons with the carbonyl group by resonance, so the carbon is less positive. Ethanal is the more reactive.
Check. The same order appears in the hydration constants: methanal is nearly all hydrate, and ketones are hardly hydrated.
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