AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Students should be able to outline the electrophilic substitution…

    Full specification wording

    Electrophilic substitution (A-level only): Students should be able to outline the electrophilic substitution mechanisms of: nitration, including the generation of the nitronium ion acylation using AlCl as a catalyst.

    Both mechanisms begin by making the electrophile.

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

    • Point 4 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Both mechanisms begin by making the electrophile. For nitration the two concentrated acids react, HNO₃ + H₂SO₄ giving NO₂⁺ + HSO₄⁻ + H₂O, and the positive charge in the nitronium ion sits on the nitrogen atom. For acylation, RCOCl + AlCl₃ gives RCO⁺ + AlCl₄⁻. Then the ring attacks: a curly arrow runs from inside the hexagon to the nitrogen or to the acylium carbon, giving the horseshoe intermediate with its positive charge. A second arrow runs from the C-H bond at that carbon back into the hexagon, releasing H⁺. The negative ion mops up that proton, regenerating H₂SO₄ in the nitration and AlCl₃ with HCl in the acylation.

    What earns the marks

    • The equation generating the nitronium ion from the two concentrated acids.
    • The electrophile drawn with the positive charge on the nitrogen atom of NO₂⁺.
    • The curly arrow from inside the hexagon to the nitrogen or to the positive charge.
    • The curly arrow from the C-H bond into the hexagon.
    • The equation that regenerates the catalyst using the H⁺ released.

    Common mistakes and how to put them right

    • Putting the positive charge of the nitronium ion on an oxygen atom.

    • Drawing the attacking arrow from a Kekulé double bond instead of from inside the ring.

    • Omitting the hydrogen from the attacked carbon of the intermediate, leaving no C-H bond for the second arrow.

    • Regenerating the catalyst without accounting for the H⁺ that was released.

    Exam tips

    • Every curly arrow starts from a lone pair or a bond; an arrow drawn from a charge symbol is penalised each time.
    • If you use skeletal formulae in a mechanism and a C-H bond breaks, you must draw in both that bond and the H to earn the mark.

    Try the lesson

    The start of the Electrophilic substitution (A-level only): Students should be able to outline the electrophilic substitution mechanisms of: nitration, including the generation of the nitronium ion acylation using AlCl as a catalyst. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which reagent mixture is used to generate the nitronium ion for the nitration of benzene?

    Listen

    Both mechanisms begin by making the electrophile. For nitration the two concentrated acids react, HNO₃ + H₂SO₄ giving NO₂⁺ + HSO₄⁻ + H₂O, and the positive charge in the nitronium ion sits on the nitrogen atom. For acylation, RCOCl + AlCl₃ gives RCO⁺ + AlCl₄⁻. Then the ring attacks: a curly arrow runs from inside the hexagon to the nitrogen or to the acylium carbon, giving the horseshoe intermediate with its positive charge. A second arrow runs from the C-H bond at that carbon back into the hexagon, releasing H⁺. The negative ion mops up that proton, regenerating H₂SO₄ in the nitration and AlCl₃ with HCl in the acylation. A strong answer earns marks for these points. the equation generating the nitronium ion from the two concentrated acids. the electrophile drawn with the positive charge on the nitrogen atom of NO₂⁺. the curly arrow from inside the hexagon to the nitrogen or to the positive charge. the curly arrow from the C-H bond into the hexagon. Exam tip: Every curly arrow starts from a lone pair or a bond; an arrow drawn from a charge symbol is penalised each time.

    See it

    A top-down flowchart. A single node, Generate the electrophile, branches into two nodes: Nitration, where HNO₃ plus H₂SO₄ gives NO₂⁺ plus HSO₄⁻ plus H₂O, and Acylation, where RCOCl plus AlCl₃ gives RCO⁺ plus AlCl₄⁻. Both branch nodes lead into one node describing the ring attack, with an arrow from inside the hexagon to the nitrogen or the acylium carbon. This leads to the horseshoe intermediate with its positive charge, then to a node where an arrow from the C-H bond goes into the hexagon, releasing H⁺. The next node is the negative ion mopping up the proton, which branches into two final nodes: for nitration, H₂SO₄ regenerated, and for acylation, AlCl₃ regenerated with HCl.
    The shared sequence of electrophilic substitution, showing how the electrophile is made for nitration and acylation, the ring attack, the intermediate, and catalyst regeneration.

    Key words

    Say what each word means before you turn the card.

    nitronium ion generation

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    acylium ion

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    curly arrow mechanism

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    horseshoe intermediate

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    See every definition
    nitronium ion generation
    The two concentrated acids react, HNO₃ + H₂SO₄ giving NO₂⁺ + HSO₄⁻ + H₂O, with the positive charge sitting on the nitrogen atom.
    acylium ion
    Formed when RCOCl + AlCl₃ gives RCO⁺ + AlCl₄⁻, with AlCl₃ acting as a catalyst.
    curly arrow mechanism
    An arrow runs from inside the hexagon to the nitrogen or to the acylium carbon, then a second arrow runs from the C-H bond at that carbon back into the hexagon, releasing H⁺.
    horseshoe intermediate
    The species formed when the ring attacks the electrophile, carrying its positive charge.

    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 top-down flowchart. A single node, Generate the electrophile, branches into two nodes: Nitration, where HNO₃ plus H₂SO₄ gives NO₂⁺ plus HSO₄⁻ plus H₂O, and Acylation, where RCOCl plus AlCl₃ gives RCO⁺ plus AlCl₄⁻. Both branch nodes lead into one node describing the ring attack, with an arrow from inside the hexagon to the nitrogen or the acylium carbon. This leads to the horseshoe intermediate with its positive charge, then to a node where an arrow from the C-H bond goes into the hexagon, releasing H⁺. The next node is the negative ion mopping up the proton, which branches into two final nodes: for nitration, H₂SO₄ regenerated, and for acylation, AlCl₃ regenerated with HCl.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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