Nucleophilicity and Basicity

[matrix570]

Ok this is what the Blue Kaplan book says...

In a protic solvent, large atoms tend to be better nucleophiles as they can shed their solvent molecules and are more polarizable.

In aprotic solvents however, the nucleophiles are naked; they are not solvated. In this situation, nucleophilic strength is related to basicity

Can somebody please explain the logic here?

What does larger size and polarizability have to do with protic solvent molecules?

and why does aprotic solvent have to do with nucleophilic strength being related to basicity?

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[BaylorDDS]

Well this is basically explaining the difference between nucleophilicity and basicity. Obviously you know things can act as bases or nucleophiles. Nucleophilicity is a measure of how fast the reagent attacks a compound. Basicity is a measure of base strength, and doesn't involve how fast something attacks. It is basically a thermodynamics (basicity) vs. kinetics (nucleophilicity).

A good nucleophile is polarizable. Like iodine, it is a very large atom, so one area of its spherical volume may be quite more negatively/positively charged than another area. However flouride is not a good nucleophile because its so small, it can't have those partial charges. In a protic solvent, the larger atom is polarized which lets it momentarily break free of the solvent shell (H2O's surrounding it). But flouride is not able to that or break its solvent shell.

In aprotic solvents there is no solvent shell, so polarizing part of the atom doesn't serve a purpose of breaking free from the solvent shell. So the nucleophilicty would correspond to the basicity (the most stable molecule is formed).

This is a horribly confusing explanation b/c its hard to explain. But to understand this problem learn how nucleophilicity relates to polarizability. And also the difference between nucleophilicity vs. basicity.

 

[hopeful786]

Well this is basically explaining the difference between nucleophilicity and basicity. Obviously you know things can act as bases or nucleophiles. Nucleophilicity is a measure of how fast the reagent attacks a compound. Basicity is a measure of base strength, and doesn't involve how fast something attacks. It is basically a thermodynamics (basicity) vs. kinetics (nucleophilicity).

A good nucleophile is polarizable. Like iodine, it is a very large atom, so one area of its spherical volume may be quite more negatively/positively charged than another area. However flouride is not a good nucleophile because its so small, it can't have those partial charges. In a protic solvent, the larger atom is polarized which lets it momentarily break free of the solvent shell (H2O's surrounding it). But flouride is not able to that or break its solvent shell.

In aprotic solvents there is no solvent shell, so polarizing part of the atom doesn't serve a purpose of breaking free from the solvent shell. So the nucleophilicty would correspond to the basicity (the most stable molecule is formed).

This is a horribly confusing explanation b/c its hard to explain. But to understand this problem learn how nucleophilicity relates to polarizability. And also the difference between nucleophilicity vs. basicity.

I need some clarity:
protic = polarized solvent
aprotic = non-poliarized solvent

Right?

And also, am I correct in thinking that the charge you refer to is a negative charge which builts up on the halogens due to their high electron affinity?

 

[Sublimation]

I need some clarity:
protic = polarized solvent
aprotic = non-poliarized solvent

Right?

And also, am I correct in thinking that the charge you refer to is a negative charge which builts up on the halogens due to their high electron affinity?

Yes. yes halogens are EWG. In regards to Nuc and Base. definition says it all. Bases usually have an affinity, or liking if u wanna say that, for protons. So a base will pick up a proton. Nuc. Well these little suckers like E+ (electrophiles) more commanly carbocations< there are many exceptions, however a large majority are carbocations. So remember that Bases are hunting for Protons, and Nuc are hunting for anyhting with a strong positive charge, more often then not a carbocation.

 

[BaylorDDS]

I need some clarity:
protic = polarized solvent
aprotic = non-poliarized solvent

Right?

And also, am I correct in thinking that the charge you refer to is a negative charge which builts up on the halogens due to their high electron affinity?

No, protic and aprotic solvents can both be polar. The protic refers to hydrogen being bound to an electronegative atom. Like H2O, hydrogen bound to an electronegative atom can participate in hydrogen bonding, and will form solvent shells. Protic is a solvent in which this occurs. Aprotic is a solvent with no hydrogen's bound to electronegative atoms, so no solvent shells form.

 

[NeoAkira]

Well this is basically explaining the difference between nucleophilicity and basicity. Obviously you know things can act as bases or nucleophiles. Nucleophilicity is a measure of how fast the reagent attacks a compound. Basicity is a measure of base strength, and doesn't involve how fast something attacks. It is basically a thermodynamics (basicity) vs. kinetics (nucleophilicity).

A good nucleophile is polarizable. Like iodine, it is a very large atom, so one area of its spherical volume may be quite more negatively/positively charged than another area. However flouride is not a good nucleophile because its so small, it can't have those partial charges. In a protic solvent, the larger atom is polarized which lets it momentarily break free of the solvent shell (H2O's surrounding it). But flouride is not able to that or break its solvent shell.

In aprotic solvents there is no solvent shell, so polarizing part of the atom doesn't serve a purpose of breaking free from the solvent shell. So the nucleophilicty would correspond to the basicity (the most stable molecule is formed).

This is a horribly confusing explanation b/c its hard to explain. But to understand this problem learn how nucleophilicity relates to polarizability. And also the difference between nucleophilicity vs. basicity.

I would like to clarify something about your post that some might find confusing.

a favorable property of nucleophilicity is being smaller/less bulky.
a disfavorable property of nucleophilicity is being larger/more bulky.

Just for general knowledge's sake. This has naught to do with the problem posted at the beginning of the thread.