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Is the mechanism for propanamine and hydrogen bromide electrophilic addition or nucle Watch

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    Is the mechanism for propanamine and hydrogen bromide electrophilic addition or nucleophilic substitution?


    I think it is meant to be nucleophilic substitution, but I'm not sure how to draw this mechanism using these two. I have drawn it electrophilic addition so which is correct?
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    (Original post by AHungryCaterpillar)
    Is the mechanism for propanamine and hydrogen bromide electrophilic addition or nucleophilic substitution?


    I think it is meant to be nucleophilic substitution, but I'm not sure how to draw this mechanism using these two. I have drawn it electrophilic addition so which is correct?
    Neither, it is an acid base reaction:

    R-NH2 + HBr --> R-NH3+Br-
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    (Original post by charco)
    Neither, it is an acid base reaction:

    R-NH2 + HBr --> R-NH3+Br-
    the question is 'name and draw out the mechanism for this reaction'


    so surely it cant just be an equation.
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    If HBr is a diatomic molecule (rather than a dissolved ionic species) nominally it is a nuclophilic substitution, the amine will attack the H-Br and kick off the Bromide ion which is a stable leaving group. Probably SN2.

    Amines are nucleophiles (has a lone pair, so lots of electrons), it won't make a good electrophile.
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    (Original post by gingerbreadman85)
    If HBr is a diatomic molecule (rather than a dissolved ionic species) nominally it is a nuclophilic substitution, the amine will attack the H-Br and kick off the Bromide ion which is a stable leaving group. Probably SN2.

    Amines are nucleophiles (has a lone pair, so lots of electrons), it won't make a good electrophile.
    This is a bit confused...

    If the bromide ion is a leaving group then it will leave. But leave what? The Hydrogen as an H+ ion?

    So are you suggesting that the hydrogen ion substitutes the NH2 group?

    The HBr is an acid, it does not need to be in aqueous solution to behave as a lewis acid. The hydrogen accepts a pair of electrons from the nitrogen of the amine and the H-Br bond breaks heterolytically leaving the bromide ion.



    EDIT: Mechanisms are always named from the point of view of the reagent, which is initiating the change in the organic species. In this case the reagent is HBr.

    I think the confusion arises because you are trying to name the mechanism from the point of view of the amine.

    The second part of the name of the mechanism is the overall effect.

    If (and I say if) you were to name this reaction from the point of view of the amine then you could argue that the amine is behaving as a nucleophile and that it is substituting the bromine from the HBr.

    However, that flies in the face of conventional terminology. In this case the HBr is behaving as a Lewis acid and initiating an acid-base reaction forming a salt.
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    (Original post by AHungryCaterpillar)
    the question is 'name and draw out the mechanism for this reaction'


    so surely it cant just be an equation.
    Name is acid-base reaction

    Mechanism is above:
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    (Original post by charco)
    This is a bit confused...

    If the bromide ion is a leaving group then it will leave. But leave what? The Hydrogen as an H+ ion?


    EDIT: Mechanisms are always named from the point of view of the reagent, which is initiating the change in the organic species. In this case the reagent is HBr.

    I think the confusion arises because you are trying to name the mechanism from the point of view of the amine.
    Agreed, the point of view determines the answer. Looking at that diagram you could call it SN2 attack on HBr by the amine (if you considered the amine to be the reagent). As it's SN2 you wont get H+ at any point (that would be SN1) however you could still say Br is a leaving group under that context.
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    (Original post by gingerbreadman85)
    Agreed, the point of view determines the answer. Looking at that diagram you could call it SN2 attack on HBr by the amine (if you considered the amine to be the reagent). As it's SN2 you wont get H+ at any point (that would be SN1) however you could still say Br is a leaving group under that context.
    This is why it's important to stick to conventions.

    The HBr is the reagent by all definitions of 'reagent' as it is acting on an organic structure and is itself inorganic. It is therefore the focus for naming the mechanism.

    If you try to deal with it the other way round it's going to cause the OP confusion and give the wrong answer.
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    (Original post by charco)
    This is a bit confused...

    If the bromide ion is a leaving group then it will leave. But leave what? The Hydrogen as an H+ ion?

    So are you suggesting that the hydrogen ion substitutes the NH2 group?

    The HBr is an acid, it does not need to be in aqueous solution to behave as a lewis acid. The hydrogen accepts a pair of electrons from the nitrogen of the amine and the H-Br bond breaks heterolytically leaving the bromide ion.



    EDIT: Mechanisms are always named from the point of view of the reagent, which is initiating the change in the organic species. In this case the reagent is HBr.

    I think the confusion arises because you are trying to name the mechanism from the point of view of the amine.

    The second part of the name of the mechanism is the overall effect.

    If (and I say if) you were to name this reaction from the point of view of the amine then you could argue that the amine is behaving as a nucleophile and that it is substituting the bromine from the HBr.

    However, that flies in the face of conventional terminology. In this case the HBr is behaving as a Lewis acid and initiating an acid-base reaction forming a salt.
    Thankyou your explanation is very helpful. I would think the mechanism was nucleophilic addition because a nucleophile donates a pair of electrons to the hydrogen, and addition because it is joined onto the amine group. I have never come across an acid-base reaction mechanism before, this is why I am unsure. I'm not saying you're wrong.. it is just I think we would have been taught it if it were a question we were given. (Unless of course it has more than one name).

    Thanks for the help!
 
 
 
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