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Acylation and ammonia/primary amines

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    Hi

    I am reading about acylation and something struck me as odd:

    "The following nucleophiles all react with acid chlorides and acid anhydrides. They are listed in order of reactivity:

    ammonia > primary amines > alcohols > water"

    Why is ammonia listed as more reactive than primary amines? Surely ammonia is a worse nucleophile because the primary amines have alkyl groups which release electrons nearer the nitrogen, increasing the d- charge?

    Could anyone clear this up and explain why this is written?

    Many thanks
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    (Original post by l4ith)
    Hi

    I am reading about acylation and something struck me as odd:

    "The following nucleophiles all react with acid chlorides and acid anhydrides. They are listed in order of reactivity:

    ammonia > primary amines > alcohols > water"

    Why is ammonia listed as more reactive than primary amines? Surely ammonia is a worse nucleophile because the primary amines have alkyl groups which release electrons nearer the nitrogen, increasing the d- charge?

    Could anyone clear this up and explain why this is written?

    Many thanks
    Hey dude how the application process going? When does the cambridge offer arrive?

    Anyways, in answer to your question: Ammonia is a primary amine with an R group of H, and as you said other primary amines have akyl R groups. I'm not 100% sure but i would assume that as Hydrogen is less electronegative than Carbon, nitrogen has a greater d- when ammonia than with any other primary amine.
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    Ammonia is smaller, which makes it a better nucleophile - sterics as well as electronics need to be considered
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    (Original post by EierVonSatan)
    Ammonia is smaller, which makes it a better nucleophile - sterics as well as electronics need to be considered
    Ahh thanks so much! I thought it might be due to stearic hindrance because I remembered the thing about tertiary haloalkanes not undergoing substitution as well as primary ones... thank you so much!
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    later on in the book it says that primary amines are better bases. How can ammonia be a better nucleophile and the primary amine a better base?
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    (Original post by moonie9995)
    later on in the book it says that primary amines are better bases. How can ammonia be a better nucleophile and the primary amine a better base?
    As sterics are not relevant here - the nitrogen only needs to get up to an H+ which is tiny, therefore it is only about the electron donating properties of the alkyl group making the lone pair more available in the primary amine.
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    (Original post by clownfish)
    As sterics are not relevant here - the nitrogen only needs to get up to an H+ which is tiny, therefore it is only about the electron donating properties of the alkyl group making the lone pair more available in the primary amine.
    Thanks!
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    Sorry for raising the dead here, but chemguide.co.uk states:

    You can think of the primary amine as a slightly modified ammonia molecule. It has a lone pair on the nitrogen atom and an even bigger delta- charge than in ammonia.

    That means that the primary amine is going to be a better nucleophile than ammonia is. You can therefore get a reaction between it and a molecule of the halogenoalkane.
    I'd trust Jim Clark with my life (and am currently trusting him with my chances of getting an A* in Chemistry), and my teachers also agree with him on this. I've heard a lot of conflicting information about which is the better nucleophile, so what the hell am I supposed to write down if they ask about it in six days? Is ammonia being listed as a better nucleophile a mistake in the textbook or is this what AQA actually believe?

    edit: the reason given for primary amines being better nucleophiles is the +ve inductive effect
Updated: June 7, 2012
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