strong bases vs. strong nucleophiles

[ysk]

Hey guys,

In general, a strong base is also a strong nucleophile and the two are pretty directly related. But in some cases there are strong bases that are not as strong nucleophiles and vice versa. Does anyone know how to determine what makes a strong nucleophile vs. strong base in these cases when you need to consider just basicity or nucleophilicity by itself?

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[dat_student]

Hey guys,

In general, a strong base is also a strong nucleophile and the two are pretty directly related. But in some cases there are strong bases that are not as strong nucleophiles and vice versa. Does anyone know how to determine what makes a strong nucleophile vs. strong base in these cases when you need to consider just basicity or nucleophilicity by itself?

Strong nucleophiles can better attack electrophiles and strong bases can better donate electrons. A molecule/atom, that can better donate electrons, is NOT necessarily good at attacking electrophilic centers. You have to consider many factors (e.g. steric hindrance). Some patterns can help you remember ... but ultimately, in each case, you have to make independent judgements.

 

[ysk]

Strong nucleophiles can better attack electrophiles and strong bases can better donate electrons. A molecule/atom, that can better donate electrons, is NOT necessarily good at attacking electrophilic centers. You have to consider many factors (e.g. steric hindrance). Some patterns can help you remember ... but ultimately, in each case, you have to make independent judgements.

Ok, so that's the definition of the two, but what actually makes an atom/molecule better able to attack electrophiles thus making it a good nucleophile and what makes an atom/molecule better able to donate electrons making it a good base? Can you elaborate a little more and also use examples of real molecules? Thanks!

 

[HBomb]

Ok, so that's the definition of the two, but what actually makes an atom/molecule better able to attack electrophiles thus making it a good nucleophile and what makes an atom/molecule better able to donate electrons making it a good base? Can you elaborate a little more and also use examples of real molecules? Thanks!

__Less steric hindrance__ makes for a good nucleophile, e.g. OH-

Similar charge, but a bad nucleophile is something like t-butoxide (hard to draw, but it's a tertiary carbon attached to 3 methyl groups and an O-)