Aldehydes and ketones
Aldehydes and ketones
- These are carbonyl compounds — they contain the C=O group.
- In an aldehyde the C=O is on the end of the chain (–CHO).
- In a ketone it is in the middle, between two carbons.
Practice
The difference between an aldehyde and a ketone is that:
Both have the carbonyl C=O; in an aldehyde it is terminal (–CHO), in a ketone it is between two carbons.
Making them
- By oxidation of an alcohol (acidified $\text{K}_2\text{Cr}_2\text{O}_7$ or $\text{KMnO}_4$):
- a primary alcohol, with distillation, gives an aldehyde.
- a secondary alcohol gives a ketone.
Practice
Oxidising a secondary alcohol gives:
Secondary alcohols oxidise to ketones; primary alcohols (with distillation) give aldehydes.
Reactions
- reduction ($\text{NaBH}_4$ or $\text{LiAlH}_4$) → back to an alcohol (aldehyde → primary; ketone → secondary).
- HCN (with KCN catalyst) adds H and CN across the C=O → a hydroxynitrile (adds one carbon).
Practice
Reducing a carbonyl compound (with NaBH₄) gives:
Reduction turns a carbonyl back into an alcohol (aldehyde → primary, ketone → secondary).
Nucleophilic addition
The HCN reaction is nucleophilic addition:
- $\text{CN}^-$ (a nucleophile) attacks the slightly positive carbonyl carbon.
- the C=O double bond breaks, leaving a negative oxygen ($\text{O}^-$).
- the $\text{O}^-$ takes an $\text{H}^+$ to finish the hydroxynitrile.
Practice
In the nucleophilic addition of HCN, the first step is:
The cyanide nucleophile attacks the δ+ carbon, breaking the C=O; the O⁻ then takes an H⁺.
You've got it
Key idea
- carbonyl = C=O; aldehyde (end, –CHO) vs ketone (middle)
- made by oxidising an alcohol: primary + distillation → aldehyde; secondary → ketone
- reduction → alcohol; HCN → hydroxynitrile (+1 carbon)
- the HCN step is nucleophilic addition: $\text{CN}^-$ attacks the $\delta+$ carbon, C=O breaks, $\text{O}^-$ takes $\text{H}^+$