johnwandering

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May 18, 2009
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In a question in BR:

Rank the nucleophiles by their relative strengths:
Answer: CN->CH3S->NH3>HOCH3


what? this is very confusing because, BR lists in that exact same chapter that
S is less electronegative than N, and hence CH3S- should be a better nucleophile than CN-. To confirm this process, NH3 is listed >HOCH3 because N is less electronegative than O.

so................ I'm not really sure what to think here.
 

MedPR

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In a question in BR:

Rank the nucleophiles by their relative strengths:
Answer: CN->CH3S->NH3>HOCH3


what? this is very confusing because, BR lists in that exact same chapter that


S is less electronegative than N, and hence CH3S- should be a better nucleophile than CN-. To confirm this process, NH3 is listed >HOCH3 because N is less electronegative than O.

so................ I'm not really sure what to think here.

That's an error. CH3S- is a better nucleophile than CN- for the reasons you state.
 
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J

johnwandering

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Wait, nevermind, BR just stated AGAIN in a different chapter verbatim:

"In general, Nitrogen compounds are more basic than oxygen- and sulfur- containing compounds of equal hybridization."
This is from a question that noted "Secondary Amine is more basic than Tertiary Thiol" (neither being ions). I answered thiol, but no.


This is the second such notation from BR, and I believe it may warrant more discussion?
 

MedPR

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basicity and nucleophilicity aren't the same and don't always follow the same trends.
 

ipodtouch

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Is this common knowledge?
I don't have it written anywhere in my notes that S < N in basicity.

Also, there's no way to "figure this out" on the fly because they are diagonally displace on the periodic table and relationships are difficult to decipher (we know N is smaller, but is also more electronegative).


??? How do we figure this out?
 

Bean Counter

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In a question in BR:

Rank the nucleophiles by their relative strengths:
Answer: CN->CH3S->NH3>HOCH3


what? this is very confusing because, BR lists in that exact same chapter that


S is less electronegative than N, and hence CH3S- should be a better nucleophile than CN-. To confirm this process, NH3 is listed >HOCH3 because N is less electronegative than O.

so................ I'm not really sure what to think here.
CN- is indeed a better nucleophile than CH3S-.

The reason is because both molecules have a negative charge but in CH3S- the negative charge is located on the Sulfur but on the CN- atom the negative charge is not on the nitrogen it is on the Carbon!

Take a look at the structure of CN you will see that it is C-triple bond-N. Both atoms have lone pairs on them which is fine for nitrogen because the formal charge is 5 but the formal charge on Carbon is 4 making it negative 4-(3 bonds and 2 electrons) = -1.

You understand the concept of the more electronegative atom being able to stabilize the negative charge better. Carbon never likes negative charges so it is highly nucleophilic. This is why Grignard reagents work in so many reactions. HTH
 

gettheleadout

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Wait so CH3S- is a weaker nucleophile than CN-??

So the order in basicity goes
C->N->S->O-

Confirm?
No, S- is more nucleophilic than O- but less basic.