Sn1 vs Sn2 question

[thanotoriousfob]

So I am going over my Kaplan OAT practice exam questions and on the 2nd exam question #77 ochem it states:

Which of the following condition(s) favor(s) Sn2 reactions over Sn1 reactions?
a.) Nonpolar solvent
b.) Low temperature
c.) Weak nucleophile
d.) High concentration of nucleophile
e.) All the above


The answer is C but that does not make sense to me. I thought a strong nucleophile favored Sn2 rxns while weak nucleophiles favor Sn1. Can someone explain this to me?

---

Members don't see this ad.

 

[Absolute Vision]

I agree with your logic...SN2 reactions are favorable when there's a strong nucleophile (usually strong bases; except, when the strong bases are sterically hindered, which then makes them weak nucleophiles...for example, t-butoxide is a strong base but a poor nucleophile since it contains 3 bulky methyl groups that sterically hinder the approach of the nucleophile to the substrate)...also negatively charged nucleophiles are stronger nucleophiles than neutral nucleophiles (HS- is a stronger nucleophile than H2S...OH- is a stronger nucleophile than H2O), less sterically hindered substrates (methyl and primary substrates), polar aprotic solvents (solvents containing no N-H or O-H bonds...good polar aprotic solvents include dichoromethane, ethers, dimethyl sulfoxide (DMSO), and finally SN2 reactions require a good leaving group (typically weak bases, which are conjugate bases from strong acids)...for instance, the conjugate bases of the strong acids (HI, HBr, HCl) are excellent leaving groups because they are weak conjugate bases (I- > Br- > Cl-).

Summary of Sn2 reactions:
1) prefers strong/good nucleophile (nucleophilicity refers to the ability of a lone pair on the nucleophile to attack a carbon on an electrophile/substrate)
2) Methyl and primary substrates are preferred, since there's less steric hindrance/crowding.
3) prefer polar aprotic solvents
4) Reaction rate is dependent on both substrate and nucleophile concentration.
5) The product exhibits inverted stereochemistry

Again, based on the above criteria for Sn2 reactions, the answer choice provided by Kaplan seems incorrect...it seems like answer choice D is the correct answer choice. I hope this helps.

 

[KHE]

I agree with your logic...SN2 reactions are favorable when there's a strong nucleophile (usually strong bases; except, when the strong bases are sterically hindered, which then makes them weak nucleophiles...for example, t-butoxide is a strong base but a poor nucleophile since it contains 3 bulky methyl groups that sterically hinder the approach of the nucleophile to the substrate)...also negatively charged nucleophiles are stronger nucleophiles than neutral nucleophiles (HS- is a stronger nucleophile than H2S...OH- is a stronger nucleophile than H2O), less sterically hindered substrates (methyl and primary substrates), polar aprotic solvents (solvents containing no N-H or O-H bonds...good polar aprotic solvents include dichoromethane, ethers, dimethyl sulfoxide (DMSO), and finally SN2 reactions require a good leaving group (typically weak bases, which are conjugate bases from strong acids)...for instance, the conjugate bases of the strong acids (HI, HBr, HCl) are excellent leaving groups because they are weak conjugate bases (I- > Br- > Cl-).

Summary of Sn2 reactions:
1) prefers strong/good nucleophile (nucleophilicity refers to the ability of a lone pair on the nucleophile to attack a carbon on an electrophile/substrate)
2) Methyl and primary substrates are preferred, since there's less steric hindrance/crowding.
3) prefer polar aprotic solvents
4) Reaction rate is dependent on both substrate and nucleophile concentration.
5) The product exhibits inverted stereochemistry

Again, based on the above criteria for Sn2 reactions, the answer choice provided by Kaplan seems incorrect...it seems like answer choice D is the correct answer choice. I hope this helps.