Nucleophilic substitution reactions

A nucleophile is a species which donates electrons, so needs to have high electron density. As polarity increases, the more electronegative atom in the bond will become more negatively charged, making a better nucleophile


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Yeah I get that, but according to what I quoted, if we look at the halides for example, iodine would be the best nucleophile as apparentely: 'nucleophilicity increases going down the periodic table as polarizability increases'. Which I'm confused with as isn't fluorine more electron dense so wouldn't it be the best nucleophile?

Fluorine is the most electronegative element in the periodic table, therefore its electrons are very tightly held around the nucleus and hence it cannot easily give them up.

Iodine on the other hand is a large atom with multiple shells of electrons round it, therefore the outer ones are loosely held and can be donated more easily.

Thank you so much! I can't believe I didn't think of that, so does the fact that fluorine is more electron dense get outweighed by the electronegativity?

What exactly do you mean by electron dense? Because Iodine has more electrons than fluorine anyway.

In any case, it's basically governed by the size of the atom and how tightly held the valence electrons are.

I mean that outer shell electrons are spread over a larger area in iodine, so I was told it's less electron dense than fluorine (I never really understood this concept)

I guess so, then yes, fluorine's electronegativity would counteract that.

I guess so, then yes, fluorine's electronegativity would counteract that.

Can I ask you something else, what does the bit in the red box mean:



how can something be a good base yet poor nucleophile?

It's usually down to steric bulk which makes a good base a poor nucleophile

http://en.wikipedia.org/wiki/Non-nucleophilic_base