Nucleophiles

[DrStraggler]

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What makes one nucleophile better than another?

So here's teh question: Which Nu would give greatest amount of product

a) I-
b) CH3OH
c) CH3O-
d) F-
e) NH3

So, first off, I know charged nuc's are stronger than the ones with unshared elec pairs, so that narrows my decision to A, D, and C.

The answer is C but I do not know why.

 

[Jepstein30]

What makes one nucleophile better than another?

So here's teh question: Which Nu would give greatest amount of product

a) I-
b) CH3OH
c) CH3O-
d) F-
e) NH3

So, first off, I know charged nuc's are stronger than the ones with unshared elec pairs, so that narrows my decision to A, D, and C.

The answer is C but I do not know why.

I'd type out a whole answer but I've always found that this page explains things better than I can. Post again if you aren't exactly sure how to get the answer.

http://www.masterorganicchemistry.com/2012/06/18/what-makes-a-good-nucleophile/

Also, is this the exact question? Best to post a picture rather than paraphrase because you may be leaving out vital information that you mistakenly thought was irrelevant! Passage based? What reaction exactly is occurring? In what solvent?

 

[1TB4RKSB4CK]

I just though conjugate base and inductive effect

 

[icedragon]

I just though conjugate base and inductive effect

My thoughts exactly, CH3O- is shaped like a bullet and its electro-negativity will allow it to attack electrophiles more readily than those ions because the ions already have full shells and act relatively like noble gases.

 

[sps27]

What makes one nucleophile better than another?

So here's teh question: Which Nu would give greatest amount of product

a) I-
b) CH3OH
c) CH3O-
d) F-
e) NH3

So, first off, I know charged nuc's are stronger than the ones with unshared elec pairs, so that narrows my decision to A, D, and C.

The answer is C but I do not know why.

As per TBR, I- is a better nucleophile than RO-. If you look at their list it goes something like SH- > CN- > I- > RO- etc....

 

[1TB4RKSB4CK]

I think you are forgetting something in the question. Under protic conditions it is to my knowledge that I- would be a better nucleophile but under aprotic conditions, ch3o- would be the stronger nucleophile and further boost it's already strengthened ion utility, thus making it a better nucleophile than I-.

So applicably, let's examine n Sn2 reaction under an aprotic solvent. The CH3O- would start off as CH3OH. The aprotic solvent converts this into it's conjugate base, the alkoxide ion.

 

[Jepstein30]

I think you are forgetting something in the question. Under protic conditions it is to my knowledge that I- would be a better nucleophile but under aprotic conditions, ch3o- would be the stronger nucleophile and further boost it's already strengthened ion utility, thus making it a better nucleophile than I-.

So applicably, let's examine n Sn2 reaction under an aprotic solvent. The CH3O- would start off as CH3OH. The aprotic solvent converts this into it's conjugate base, the alkoxide ion.

this is why I told the OP to post the whole question.. the solvent makes a difference here.

 

[sps27]

I think you are forgetting something in the question. Under protic conditions it is to my knowledge that I- would be a better nucleophile but under aprotic conditions, ch3o- would be the stronger nucleophile and further boost it's already strengthened ion utility, thus making it a better nucleophile than I-.

So applicably, let's examine n Sn2 reaction under an aprotic solvent. The CH3O- would start off as CH3OH. The aprotic solvent converts this into it's conjugate base, the alkoxide ion.

yup that is correct. The solvent does make a difference......

 

[Czarcasm]

this is why I told the OP to post the whole question.. the solvent makes a difference here.

My thoughts exactly, but how are we suppose to know ch3o- makes a better nucleophile than flouride ion under aprotic conditions. I know ch3o- is a very strong base and fluoride is a weak base, but I was under the impression bascity and nucleophilicity strength have little relation to one another. If anything I'd assume fluoride, being a smaller atom overall would be a better nucleophile, but then again, it's really electronegative and so CH3O- seems like it'd be more willing to share it's electrons. In these types of scenarios, where different factors compete, how do you guys decide on an answer -- is this something you commit to memory, or just know based on experience? Or is there something I'm not understanding here.

 

[Schenker]

My thoughts exactly, but how are we suppose to know ch3o- makes a better nucleophile than flouride ion under aprotic conditions. I know ch3o- is a very strong base and fluoride is a weak base, but I was under the impression bascity and nucleophilicity strength have little relation to one another. If anything I'd assume fluoride, being a smaller atom overall would be a better nucleophile, but then again, it's really electronegative and so CH3O- seems like it'd be more willing to share it's electrons. In these types of scenarios, where different factors compete, how do you guys decide on an answer -- is this something you commit to memory, or just know based on experience? Or is there something I'm not understanding here.

Sterically unhindered bases are generally good nucleophiles. Nucleophilicity is something that you learn to evaluate through experience, after having seen a bunch of reactions. There are a few rules that generally work, like:

1. More electronegative atoms are less nucleophilic, as you said. Thus, alkoxides are more nucleophilic than flouride.
2. In most cases, in polar protic solvents, ions with larger radii are better nucleophiles, because smaller ions are more strongly solvated. Thus, thiolates are better nucleophiles than alkoxides. In polar aprotic solvents, ions with smaller radii are sometimes better nucleophiles.​

but these rules are general extrapolations from observations on many different nucleophiles.

If the MCAT were to test you on your nucleophile skills, I have a hunch that they would isolate one quality of good nucleophiles. For example:

Which is the best nucleophile?

A. CH3CH2O-
B. (CH3)2CHO-
C. (CH3)3CO-
D. CH3CH2OH​

On the other hand, it's conceivable that the MCAT asks you to identify the major product of a reaction where two nucleophiles directly compete. For example:

When CH3CH2Br is in the presence of equal amounts of F- and CH3O-, what is the major product?

A. no reaction
B. CH3CH2F
C. CH3CH2OCH3
D. CH2=CH2​

In this case, you'd evaluate, among other things, which nucleophile was the better leaving group.



Answers: A and C

 

[Czarcasm]

Sterically unhindered bases are generally good nucleophiles. Nucleophilicity is something that you learn to evaluate through experience, after having seen a bunch of reactions. There are a few rules that generally work, like:

1. More electronegative atoms are less nucleophilic, as you said. Thus, alkoxides are more nucleophilic than flouride.
2. In most cases, in polar protic solvents, ions with larger radii are better nucleophiles, because smaller ions are more strongly solvated. Thus, thiolates are better nucleophiles than alkoxides. In polar aprotic solvents, ions with smaller radii are sometimes better nucleophiles.​

but these rules are general extrapolations from observations on many different nucleophiles.

If the MCAT were to test you on your nucleophile skills, I have a hunch that they would isolate one quality of good nucleophiles. For example:

Which is the best nucleophile?

A. CH3CH2O-
B. (CH3)2CHO-
C. (CH3)3CO-
D. CH3CH2OH​

On the other hand, it's conceivable that the MCAT asks you to identify the major product of a reaction where two nucleophiles directly compete. For example:

When CH3CH2Br is in the presence of equal amounts of F- and CH3O-, what is the major product?

A. no reaction
B. CH3CH2F
C. CH3CH2OCH3
D. CH2=CH2​

In this case, you'd evaluate, among other things, which nucleophile was the better leaving group.



Answers: A and C

Sweet, very helpful explanation. Thank you for taking the time to write that. Helped clear a lot of things up.

 

[BerkReviewTeach]

Excellent post Schenker! I've seen quite a few insightful explanations from you around here.

 

[Schenker]

Thank you! Glad I could help.