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Thread: Nucleophilic substitution & Electrophilic substitution

  1. #1 Nucleophilic substitution & Electrophilic substitution 
    Forum Junior ArezList's Avatar
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    I wonder how we tell what knid of reaction it is....I mean I don't know how to distinguish Electrophilies and Nucleophilie?

    For a Sn2 reaction, there're two reactant , if course one is a Nucleophilie but is the other not a Electrophilie? then how can you call it a Sn2....? Just not clear about those "charade explaination on the book"


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  3. #2  
    Forum Masters Degree organic god's Avatar
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    usually you have your starting product lets call it Y.

    you then add your reacting species which we call X.

    depending on what X is the reaction is nucleophilic,electrophilic or something else but we are only dealing with these two cases.

    The basic definition of a nucleophile is a species which donates a pair of electrons. so if X is donating a bonding pair of electrons to a part of Y the reaction is nucleophilic.

    Electophiles are electron pair acceptors so if X accepts a pair of electrons from a part of Y then the reaction is electrophilic.

    Basic hint for organic chemistry when you can't figure it out.
    1) draw what happens. i.e reactants, intermediates, products
    2) label up the positive and negative parts of the molecules.

    this is usually enough to figure out what is going on.


    everything is mathematical.
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  4. #3  
    Forum Junior ArezList's Avatar
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    I'm afraid I still don't understand.

    I'm clear about all your reply...except that how you define the reactant X. That's the essence of my misunderstanding.....

    I mean if X donate a pair of electrons to Y, if you call it a nucleophilic reaction, then, in turn Y is the electrons acceptor, why don't you call it a electrophilic reaction. Yean I really got a terrible ability of expressing myself.

    Thanks anyway.....
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  5. #4  
    Forum Masters Degree organic god's Avatar
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    yeh i see what you mean about how if one is a nucleophile the other is technically an electrophile.

    But usually you are trying to turn your reactant into another product.

    so you are going from the previously mentioned Y to some molecule Z.

    the reactant (X) you use to turn from Y to Z determines how you classify the reaction.

    this common organic reaction as an example.

    2-bromopropane ---> propan-2-ol.

    you need OH- to make this reaction happen. which is a nucleophile.
    everything is mathematical.
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  6. #5  
    Forum Junior ArezList's Avatar
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    Oh .....Kind of understand...

    But still....what happens to OH,if you use water as a nucleophile....I mean the reaction, please write a integral formula...
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  7. #6  
    Forum Masters Degree organic god's Avatar
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    water isn't really a good nucleophile because the amount of dissociation into ions is low.

    the equlibrium H<sub>2</sub>O == OH<sup>-</sup> + H<sup>+</sup>

    is way over to the the left hand side.


    for this reaction you typically use aqeuos pottasium or sodium hydroxide as this produces lots of OH- ions.

    But still....what happens to OH
    well the reaction is nucleophilic substitution.

    So the bromide group is substituted by the OH group

    CH3CH(Br)CH3 + OH- -----------> CH3CH(OH)CH3 + Br-

    incidentally does anyone know if this is primarily Sn1 of Sn2.
    i would imagine it is primarily Sn1 but i'm not sure.
    everything is mathematical.
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  8. #7  
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    Yeah I agree that this is a Sn1, if an appropriate solvent si used, sicnce the substrate is on a secondary carbon....am I correct/
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  9. #8  
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    yeh the carbocation intermediate formed would be pretty stable
    everything is mathematical.
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