DAT Destroyer Ochem 105 - 2011

[Demo0710]

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Which is the best nucleophile?

A) H2O
B) AlCl3
C) C2H5OH
D) CH3SH
E) CH3COOH

I thought it would be C, but apparently it's D since S is larger and less electronegative, making it more polarized.

Can someone explain this?

 

[Happyfaace]

When I came over this problem, the only time the explanation makes sense to me is if we are in protic solvent. (however, there is nothing mentioned about the solvent) Then the larger the atom of Nucleophile, the easier it is for this atom to cope with all the Hs floating around. that's why also e.g. if you compare I, Br, Cl; I will be best Nu in protic solvent, while Cl will be best in aprotic.
But I am not sure what Electronegativity has to do here. Let's see what others think.

 

[smiley00]

I justified it the same way as you did Happyface, assuming that the solvent would be protic.

 

[coheedutk]

The EN of sulfur compared to oxygen allows the electrons on sulfur to have more room to attack than the oxygen, which is more EN and pulls the electrons closer allowing less reaction, becoming a weaker nucleophile than sulfur.

 

[Miley13]

Nucleophiles (being electron rich) are Lewis bases (electron donors). In C the lone pairs are on carbon and in D the lone pairs are on sulfur. Sulfur is larger than carbon and being larger, sulfur holds its outer electrons not as tight as carbon.

Sulfur is more willing to donate its electrons because it's bigger than carbon and therefore holds them less tight. This makes sulfur a better Lewis base and Nucleophile.

This is how I think of it. Hope this helps!

 

[Happyfaace]

Miley 13,
Why do you think it is Carbon and not O in choice C that is attacking. I would think C is out of competition here. Just making sure you realize it is O that is attacking in choice C, not Carbon.

 

[Miley13]

Sorry I meant to say oxygen instead of carbon in my explanation! Thanks for catching that

 

[kalviinn]

Nucleophilicity is = to basicity🙂

 

[Demo0710]

Can you explain why CH3SH is more basic than C2H5OH?

 

[ScarletKnight24]

Can you explain why CH3SH is more basic than C2H5OH?

using CARDIO, you can rank the conjugate bases of the answer choices.

Charge= both have no charge
Atom= oxygen vs sulfur

when you are comparing atoms that are on the same column, you are supposed to look at their size and the bigger the atom, the more stable it is, i.e weaker base. However, the answer choice says that Sulfur is the better nucleophile which means that they must be comparing them in protic solvent. But again, the question mentioned NOTHING about solvent so I don't think you will see this type of question.

 

[biomolecmed]

S is less electronegative than O. Therefore can donate electrons better than O. If so, better nucleophile. Remember, nucleophile must donate electrons. Hope this helps.