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    can someone explain to me why I is considered a weak base and F is a strong base? , with regard to how fast/slow they leave in an Sn2 reaction.

    I know I leaves quicker than F would in a Sn2 but i dont know why, surely as F is more electronegative it would attract the electrons quicker and leave faster ?
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    (Original post by Tori1607)
    can someone explain to me why I is considered a weak base and F is a strong base? , with regard to how fast/slow they leave in an Sn2 reaction.

    I know I leaves quicker than F would in a Sn2 but i dont know why, surely as F is more electronegative it would attract the electrons quicker and leave faster ?
    substitution tends to compete with elimination.

    a nucleophile could sometimes be regarded as a base or the other way round.

    it all depends on the conditions, the type of substrates you have.

    When you talk about substitution rxn, you want to compare nucleophilicity and not basicity.

    HI is a stronger acid than HF, hence its conjugate base I- would be a poorer base than F- simply because it abstracts H+ rather less favourably.

    However, due to its massive size and relatively lower Zeff, so its overall charge density is more delocalised and we said I- is more polarizable than F-. I- hence is a better nucleophile because of this criteria.

    In sn2 rxn, rate = k [Nucleophile] [ substrate]

    stronger nucleophile => faster rate

    What you are thinking are also equivalent to determining acidity of hydrogen fluorides. Despite F being more electronegative than I and electron density is more distorted towards F in HF than in HI, but the MUCH WEAKER BOND in HI means it is more readily broken/dissociated into H+ and I- and hence it is a stronger acid.

    Yes, F is more electronegative, but C-I bond is weaker relative to C-F. and breaking weaker bond to form new stronger bond is favoured!
 
 
 
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