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Oxygen attracts the carbonyl bonding electrons, leaving carbon partially positive. A nucleophile can attack carbon while the pi electron pair moves onto oxygen. The intermediate oxygen is negatively charged and is then protonated. This is addition, not substitution, because there is no ordinary leaving group expelled from the carbonyl carbon. Aldehydes have at least one hydrogen at the carbonyl carbon, while ketones have two carbon groups.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Distinguish aldehyde and ketone reactions
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Where does a nucleophile attack a carbonyl?
The partially positive carbon. Carbonyl polarity makes the carbon electrophilic.
After watching 2: What does sodium borohydride produce from a ketone?
A secondary alcohol. Reduction converts C=O into C–OH.
What you'll be able to do
Oxygen attracts the carbonyl bonding electrons, leaving carbon partially positive. A nucleophile can attack carbon while the pi electron pair moves onto oxygen. The intermediate oxygen is negatively charged and is then protonated. This is addition, not substitution, because there is no ordinary leaving group expelled from the carbonyl carbon. Aldehydes have at least one hydrogen at the carbonyl carbon, while ketones have two carbon groups.
A suitable hydride reagent such as sodium borohydride reduces an aldehyde to a primary alcohol and a ketone to a secondary alcohol. Use [H] in the overall reduction equation when requested, but show the electron movement appropriate to the mechanism. Aldehydes are readily oxidised to carboxylic acids under suitable conditions, while ordinary ketones resist mild oxidation. This difference supports Tollens’ or Fehling’s tests. A positive DNPH test supports a carbonyl but does not by itself separate the two families.
Cyanide attacks a carbonyl to make a hydroxynitrile after protonation. The incoming cyanide adds one carbon to the chain. A planar carbonyl permits attack from either side, so a racemate may form if the product carbon carries four different groups. State the specified reagent system rather than suggesting uncontrolled use of hydrogen cyanide. When deducing the product, retain the original carbonyl oxygen as OH and attach the complete CN group to the same carbon.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Predict the product of cyanide addition followed by protonation to ethanal.
CH3CH(OH)CN, 2-hydroxypropanenitrile; it can form as a racemate.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.