Nucleophile, Base, Leaving Group

[studbud]

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Hi, I'm wondering if someone could explain to me the differences/correlations between nucleophicity, basicity, and leaving groups. In some places I've read that a good Nu is a strong Base while others contradict that. And what constitutes a good leaving group?

Thanks!

 

[PiBond]

Hi, I'm wondering if someone could explain to me the differences/correlations between nucleophicity, basicity, and leaving groups. In some places I've read that a good Nu is a strong Base while others contradict that. And what constitutes a good leaving group?

Thanks!

A nucleophile has a lone pair of electrons available to donate--either a pi bond or a lone pair. By definition, it is also a lewis base

A leaving good (a good one) is one that can stabilize negative charge well in solution and is thus a weak (or very weak) base. The best leaving groups are thus the conjugate bases of strong acids--like I, Cl, Br, etc. The pka of the conjugate acid of a good leaving group is very low and the pkb of the conjugate base of high

Basicity is a thermodynamic term pertaining to an equilibrium. In essence, a good base is a bad leaving group but a good base could act as a nucleophile depending on the compound it's acting on. There's a lot of grey area here

Bases abstract protons, nucleophilicty is a kinetic term, and good leaving groups are very weak bases

 

[G1SG2]

Hi, I'm wondering if someone could explain to me the differences/correlations between nucleophicity, basicity, and leaving groups. In some places I've read that a good Nu is a strong Base while others contradict that. And what constitutes a good leaving group?

Thanks!

Another thing to keep in mind is that strong bases can be good nucleophiles, but not if they are bulky. For example, t-butoxide is a strong base but it is a poor nucleophile because it is so bulky.

Bases want protons, and nucleophiles (usually) want the delta +/partial positive charge on the backside of the carbon they are going to attack.

 

[lesset01]

All good nucleophiles are strong bases, but the opposite is not always true because nucleophilicity and basicity aren't always the same. As the steric hindrance around a base increases, its nucleophilicity decreases, but its basicity is not affected. So OH- is a strong base and good nucleophile, but OC(CH3)3- is a strong base but a bad nucleophile.

In the end, bases attack H atoms and nucleophiles attack carbon atoms. So you can see why OH- is a strong base: it can very easily attack H atoms. It is also a strong Nu because it can easily displace a good LG and attach onto a C atom. Looking at OC(CH3)3-, it is strong base and can take H atoms, but its too big (sterically hindered) to attach onto a C atom and therefore is not a good Nu. (a base like this would lead to elimination, not substitution)

You also have to look at the solvent you are using. The trends for better LG, better Nu and strong base are all for aprotic solvents. The trends reverse for protic solvents due to interactions of the base/Nu with the solvent.

I hope that makes sense. 🙂

ps G1SG2 responded while I was typing my response, so sorry if its redundant!

 

[studbud]

Wow, very helpful. Thank you all!