Best Nucleophile

[ekam]

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Which is best nuc?
H2O
AlCl3
C2H5OH
CH3SH
CH3COOH

Answer is CH3SH
because its less electronegative and S larger in size than O


According to Chad, nuc strength increases going up and left in aprotic solvent. Nuc strength increase going left and down in protic solvent.

I am confused because solvent was not mentioned in the question. How do we approach such questions?

 

[I Brush My Teeth]

I just stumbled upon this problem while doing Destroyer..
when there is no solvent given, don't worry about that rule, just simply see what nucleophiles you have once you deprotonate each molecule. The best nucleophile is basically the MOST UNSTABLE... since O is more electronegative, it will be more stable carrying the negative charge than S.. hope this helps!

 

[510586]

I just stumbled upon this problem while doing Destroyer..
when there is no solvent given, don't worry about that rule, just simply see what nucleophiles you have once you deprotonate each molecule. The best nucleophile is basically the MOST UNSTABLE... since O is more electronegative, it will be more stable carrying the negative charge than S.. hope this helps!

So if it goes left it's a better nucleopihle as well right? Since it's less electronegative?

 

[LolaSF]

If you are not given a solvent - you choose the strongest nucleophile based on charge and polarizability. So that's down the column (the electron density of the larger atoms is more polarizable) and to the left (across the row = less electronegative to the left, and the electron density is then less stable)

If you are given a solvent - you have to consider protic vs. aprotic because of the ability of a nucleophile to participate in hydrogen bonding.
- Polar Protic solvent = nucleophilicity increases going down
- Polar Aprotic solvent = nucleophilicity increases going up

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

 

[100%permwinner]

So if it goes left it's a better nucleopihle as well right? Since it's less electronegative?

Electronegativity increases as you move from left to right. If an atom is less likely to share electrons with other atoms, it is less likely to attack an electrophile. C is more nucleophilic than N, which is more than O and F.

 

[100%permwinner]

I remember in one of KhanAcademy videos, Sal makes the point that nucleophilicity has to do kinetics, or how likely is the molecule attacking the electrophile with its lone-pair electrons. He also mentions basicity has to do with thermodynamics, or the equilibrium position.

 

[ekam]

Thank you everyone. It definitely cleared things up.