AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    The nucleophilic addition reactions of carbonyl compounds with KCN,…

    Full specification wording

    Aldehydes and ketones (A-level only): The nucleophilic addition reactions of carbonyl compounds with KCN, followed by dilute acid, to produce hydroxynitriles.

    Potassium cyanide provides the cyanide ion, a nucleophile with a lone pair on its carbon.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 4 of 7
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Potassium cyanide provides the cyanide ion, a nucleophile with a lone pair on its carbon. It attacks the δ⁺ carbonyl carbon, the π electrons moving onto the oxygen to give a negatively charged intermediate; dilute acid added afterwards protonates that oxygen to a hydroxyl group. The product is a hydroxynitrile, with the OH and the CN on the same carbon, and it has one more carbon than the starting material, which makes the reaction useful for lengthening a chain. Ethanal gives 2-hydroxypropanenitrile; propanone gives 2-hydroxy-2-methylpropanenitrile. Note the naming rule: the nitrile carbon is carbon one, so the hydroxyl group ends up on carbon two.

    What earns the marks

    • Naming the reagent as KCN, or NaCN, followed by dilute acid.
    • The curly arrow from the lone pair on the carbon of the cyanide ion to the δ⁺ carbonyl carbon.
    • The arrow from the C=O bond to the oxygen, giving the negatively charged intermediate.
    • The correct hydroxynitrile, with the hydroxyl and nitrile groups on the same carbon.
    • Naming the product with the nitrile carbon numbered as carbon one.

    Common mistakes and how to put them right

    • Giving HCN when the question asks for the reagent for the mechanism, where AQA wants KCN (or NaCN) followed by dilute acid.

      Put it right: HCN is correct only in the overall equation.

    • Starting the curly arrow at the negative charge rather than at the lone pair on carbon.

    • Bonding the cyanide through its nitrogen atom instead of its carbon.

    • Numbering the chain from the hydroxyl end, so the product is named as a 1-hydroxy compound.

    Exam tips

    • Write reagent and condition as a pair: KCN followed by dilute acid, warmed.
    • Show the charge on the intermediate oxygen; it is often a separate mark.
    • Count carbons before and after; the product must have one more than the carbonyl compound.

    Try the lesson

    The start of the Aldehydes and ketones (A-level only): The nucleophilic addition reactions of carbonyl compounds with KCN, followed by dilute acid, to produce hydroxynitriles. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which reagent supplies the nucleophile that attacks the carbonyl carbon when ethanal is converted into a hydroxynitrile?

    Listen

    Potassium cyanide provides the cyanide ion, a nucleophile with a lone pair on its carbon. It attacks the δ⁺ carbonyl carbon, the π electrons moving onto the oxygen to give a negatively charged intermediate; dilute acid added afterwards protonates that oxygen to a hydroxyl group. The product is a hydroxynitrile, with the OH and the CN on the same carbon, and it has one more carbon than the starting material, which makes the reaction useful for lengthening a chain. Ethanal gives 2-hydroxypropanenitrile; propanone gives 2-hydroxy-2-methylpropanenitrile. Note the naming rule: the nitrile carbon is carbon one, so the hydroxyl group ends up on carbon two. A strong answer earns marks for these points. naming the reagent as KCN, or NaCN, followed by dilute acid. the curly arrow from the lone pair on the carbon of the cyanide ion to the δ⁺ carbonyl carbon. the arrow from the C=O bond to the oxygen, giving the negatively charged intermediate. the correct hydroxynitrile, with the hydroxyl and nitrile groups on the same carbon. Exam tip: Write reagent and condition as a pair: KCN followed by dilute acid, warmed.

    See it

    A vertical flowchart with nine boxes connected by arrows. The first box says KCN provides cyanide ion, leading to a box saying the cyanide ion has a lone pair on carbon. An arrow leads to attack on the δ⁺ carbonyl carbon, then to the π electrons moving onto oxygen, then to a negatively charged intermediate. An arrow leads to dilute acid added, then to oxygen protonated to a hydroxyl group, then to the hydroxynitrile with OH and CN on the same carbon, and finally to one more carbon than the starting material.
    The sequence of steps from KCN and a carbonyl compound, through the negatively charged intermediate, to a hydroxynitrile after dilute acid.

    Key words

    Say what each word means before you turn the card.

    cyanide as nucleophile

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    hydroxynitrile product

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    chain lengthened by one carbon

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    naming from the nitrile carbon

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    See every definition
    cyanide as nucleophile
    The cyanide ion has a lone pair on its carbon and attacks the δ⁺ carbonyl carbon.
    hydroxynitrile product
    The product has the OH and the CN on the same carbon.
    chain lengthened by one carbon
    The product has one more carbon than the starting material, which makes the reaction useful for lengthening a chain.
    naming from the nitrile carbon
    The nitrile carbon is carbon one, so the hydroxyl group ends up on carbon two.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A vertical flowchart with nine boxes connected by arrows. The first box says KCN provides cyanide ion, leading to a box saying the cyanide ion has a lone pair on carbon. An arrow leads to attack on the δ⁺ carbonyl carbon, then to the π electrons moving onto oxygen, then to a negatively charged intermediate. An arrow leads to dilute acid added, then to oxygen protonated to a hydroxyl group, then to the hydroxynitrile with OH and CN on the same carbon, and finally to one more carbon than the starting material.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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