DAT Destroyer 2009, #73 Ochem sect.

[roquer2]

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Going thru the 09 DAT Destroyer, I come across this simple little gem:

Answer was d) however. I deprotonated each one, and thought OH- would be the best nucleophile.

The explanation in the book stated, "S is larger than O," then yadda yadda yadda. I stopped reading after that very first statement because since S is bigger than O, it's more stable. More stable = less reactive= weaker base and weaker nucleophilicity.

I hope I'm not butchering this concept. Maybe I'm getting it confused with nucleophilicity differing from aprotic and protic solvents. Lol any clarification? 🙄

 

[tillerybass]

The larger sulfur ion has greater polarizability allowing it to interact with the partial (+) on the carbonyl C better

 

[roquer2]

The larger sulfur ion has greater polarizability allowing it to interact with the partial (+) on the carbonyl C better

We aren't talking about "polarity" right? Polarizability is completely difference?

So is it right to say that greater london dispersion = greater nucleophilicity?

 

[cmF]

A good nucleophile has a number of various characteristics but most notably, a negative charge and lone pairs. Once CH3SH and C2H5OH have been deprotonated, they're very similar. However, the oxygen is more electronegative meaning it wants to hold onto its lone pairs more than sulfur. Thus, sulfur is the better nucleophile and more likely to attack an electrophile like partial positive charge or carbocation. That's the way I think of it anyway.

 

[roquer2]

A good nucleophile has a number of various characteristics but most notably, a negative charge and lone pairs. Once CH3SH and C2H5OH have been deprotonated, they're very similar. However, the oxygen is more electronegative meaning it wants to hold onto its lone pairs more than sulfur. Thus, sulfur is the better nucleophile and more likely to attack an electrophile like partial positive charge or carbocation. That's the way I think of it anyway.

I went thru Chad's and his explanation of CH3SH is that it is TECHNICALLY a better nuc: in protic solvent but all professors will say it is a better nuc: period.

I guess the main, simplfied concept is that the less electronegative = better nucleophile.

Thanks for chippin in

 

[tillerybass]

The electronic sphere around the S is polarizable. Electron density will have a greater "range of motion" and be able to donate electron density into the pi* anti bonding orbitals in the carbonyl better.

 

[bluemoon27]

I may be wrong but I was thinking that for the since O is more electroneg, it can better stabilize a negatve charge . And S being bigger has the unstable electron density spread out over bigger area maybe? So because it stabilizes less, it will be better, stronger nucleophile and reactive. not sure if my reasoning is right but wow thats a tough question hehe

 

[roquer2]

Thanks everyone for the breakdown. I'm pretty sure I'm just getting my nucleophilicity confused with the concepts of acids and bases.

My original train of thinking was that the bigger an atom is = more stable = less reactive = more acidic (concept of Acids). However, I think i was trying to apply this concept to nucleophilicity. The only trend I took away from Chad's is that nucleophilicity increases going left of the perioidic table. I just didn't know about the trend going downwards