question about nucleophiles

[IWantH2O]

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k im confused bout the strenght of nucleophiles...

kaplan says that in polar protic solvents the larger the atoms the better the nucleophile....an example would be I- is better than F-

but in my textbook it says that smaller size increases nucleophile strength.
for example, -OCH3 is a better than -O(CH3)3

can someone please explain

 

[211183]

k im confused bout the strenght of nucleophiles...

kaplan says that in polar protic solvents the larger the atoms the better the nucleophile....an example would be I- is better than F-

but in my textbook it says that smaller size increases nucleophile strength.
for example, -OCH3 is a better than -O(CH3)3

can someone please explain

Reaction dependent:

Polar protic solvents are used in SN1 reactions. They stabilize the carbocation. A bigger base cannot directly attack until a carbocation is formed. So when in a polar protic solvent, a hindered base is ideal for SN1 reaction.

Aprotic solvents are used in SN2. This is so they do not interefere with the reaction by protonation of the base. In this case, SN2, the Nu- needs ot attack direclty, so a smaller less hindered base is ideal.

Think of it this way:

In a protic solvent, there are extra protons that are going to want to surround the Base. The fatter the base, the more capable it is of part of it getting to the substrate.

In aprotic, no H bonding exists (ether is an example), so the Nu can be smaller.

So both are sortof right, depends on the reaction and the solvent. Strong base like OCH3 would get protonated by a protic solvent, making it ineffective.

In sn1, you have a weaker base, so the protic solvent's H-bonding is necessary to stabilize the intermediate carbocation.

 

[IWantH2O]

im not sure if i get this yet...so does anybody have a better explanation

 

[211183]

im not sure if i get this yet...so does anybody have a better explanation

sorry🙁

 

[pidbull]

ahh I dont get this now either....so base strength is reaction dependent?