Leaving Group

[cloak25]

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This is a rather simple question but I just can't get over it.

Which of the following is the MOST stable leaving group?

A. HCN
B. CN-
C. H3CCH2S-
D.H3CCH2SH

answer: D

Can someone explain?

 

[MedPR]

This is a rather simple question but I just can't get over it.

Which of the following is the MOST stable leaving group?

A. HCN
B. CN-
C. H3CCH2S-
D.H3CCH2SH

answer: D

Can someone explain?

All else equal, CH3CH2SH is the largest of the four answers so it can delocalize electrons the best and will be the most stable on its own. The best leaving groups are the ones that don't need to be attached to something to be stable. B and C should have been ruled out first, since they have a negative charge. D is better than A because D has more atoms to spread electrons out on.

 

[johnwandering]

What if the choices were between:

H2CN-
SH-


Would SH- still win out because Sulfur is a larger atom?
but 2 things are going for H2CN:
1.) larger molecule size
2.) N is much more electronegative

 

[chiddler]

What if the choices were between:

H2CN-
SH-


Would SH- still win out because Sulfur is a larger atom?
but 2 things are going for H2CN:
1.) larger molecule size
2.) N is much more electronegative

You mean CN-, right? I think what medpr said might be misleading, too. Because you don't just look at the number of atoms attached to a molecule as that may contribute to stability or take away from stability. For example, trifluoroacetic acid is more less due to the extra F's (becomes more acidic), but if you add methyl groups instead of F's then it becomes MORE stable (less acidic). Inductive effect depends on whether the introduced groups are electron withdrawing (F) or donating (alkyl). Withdrawing stabilize negative charges, making stuff more acidic. Donating stabilizes positive charges, making stuff less acidic.

That said, this question sucks because they are really close in acidity. H3CCH2SH is like 10, HCN is like 9. I would like to be corrected if i'm wrong, but I don't think there is any way to figure this out logically unless you know the pKa values.

So to answer your question, we're looking for the species that exists the most stably when it leaves. So the answer is CN-. Comparing this to the original question, CN- exists more stably in solution than H3CCH2S- because H3CCH2S- is more basic. H3CCH2SH exists more stably in solution than HCN because HCN is more acidic.

Also, for contrast, H2S pKa is like 6. So adding that ethyl group to the sulfur makes it much less acidic.

 

[johnwandering]

Just a response, my question was about

HCN- (yes, you were right)
SH-


Not about H3CCH2S-.
Would your explanation still hold?

 

[chiddler]

Just a response, my question was about

HCN- (yes, you were right)
SH-


Not about H3CCH2S-.
Would your explanation still hold?

sorry, i thought you were just being lazy in typing the whole SH molecule out.

no because SH is significantly more acidic than HCN.

 

[johnwandering]

no because SH is significantly more acidic than HCN.

Ok, so for acidity

SIZE is much much more of a determinant than Electronegativity?

 

[chiddler]

Ok, so for acidity

SIZE is much much more of a determinant than Electronegativity?

yeah that usually applies for simple acids though. you can't really apply that to CN because it's polyatomic. like H2CO3. it's larger than CN but a weaker acid. or H2SO4, larger but stronger.

 

[folktale]

Ok, so for acidity

SIZE is much much more of a determinant than Electronegativity?

Factors for acidity depends. For comparing elements on the same row, EN is the more important factor in acidity. On the contrary, the elements of the same column, size is more important in acidity ranking.