OChem Acidity Question

[DrHouse579]

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Hullo,
So...
which is the more acidic/stable compound?

Never done this before so please bear with me.

C-N=C-C^-

OR

H2N^-

I thought it was the amine because the charge is on a more EN atom (N vs. C - 1st rule, no need to go further). But, now I see that the first compound does share the charge with the N in a resonance structure. Is this the reason I got the problem wrong? If it is wouldn't that mean you should probably always check resonance structures before checking the atoms when determining acidity? Thanks!

 

[PingPongPro]

In general you should try to consider resonance when considering stability/acidity. A lot of times these kinds of questions are kind of random/arbitrary. One rule might seem to favor a certain answer, but there are always exceptions that seem impossible to predict to me. Just try to make your best judgement. In this case, you can see that the resonance structure can move the - sign to the N so its likely to be more stable than the molecule with just the N holding the charge.

 

[DrHouse579]

In general you should try to consider resonance when considering stability/acidity. A lot of times these kinds of questions are kind of random/arbitrary. One rule might seem to favor a certain answer, but there are always exceptions that seem impossible to predict to me. Just try to make your best judgement. In this case, you can see that the resonance structure can move the - sign to the N so its likely to be more stable than the molecule with just the N holding the charge.

Thanks a lot! Yeah, so I think from now on I'm going to necessarily draw all res structures first, but quickly scan for all possible atoms that could hold the charge. If the first rule doesn't apply, I'll then go draw the resonance structures to determine acidity that way. A little twist on the method given in the book but I think it should help to get rid of some ambiguity.

 

[jsp132]

make sure to also look at the s percentage and the geometry

and also inductive effect