Aldehydes and ketones
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 practice questions, 4 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Three bottles, one test
- The carbonyl group
- Nucleophilic addition with HCN
Read the transcript of the teaching part
Last time
Before we start: last time, in Alcohols, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Three bottles, one test
Three colourless liquids: ethanal, propanal and propanone. They look and smell alike, yet three simple tests can name each one. All three contain the carbonyl group, C double bond O, which is polar, and which controls every reaction in this lesson.
The carbonyl group
An aldehyde has the C double bond O at the end of the chain, with a hydrogen on the carbonyl carbon: RCHO. A ketone has it inside the chain: RCOR. The C double bond O is polar. The carbon is delta plus, the oxygen delta minus. So the carbonyl carbon is attacked by nucleophiles. The reaction type is nucleophilic addition.
Nucleophilic addition with HCN
Reagent: hydrogen cyanide with a little potassium cyanide as catalyst. The cyanide ion is the nucleophile. Step one: the lone pair on carbon of the cyanide ion attacks the delta plus carbonyl carbon. The pi bond breaks, and the electrons move onto oxygen, giving an anion. Step two: the oxygen anion takes a proton from HCN, regenerating the cyanide ion. The product is a hydroxynitrile. Ethanal gives 2-hydroxypropanenitrile: CH three CHO plus HCN goes to CH three CH(OH)CN. One more carbon in the chain.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Nucleophilic addition · Tests for aldehydes and ketones.
The whole chapter: Carbonyl Compounds, Carbonyl Compounds.
Chapter 17: Carbonyl compounds
- Aldehydes and ketones 7 min
Also for A Level Chemistry 9701
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