Wolff-Kischner

[MedPR]

Not sure if I spelled that right, oh well.

In the mechanism (shown below), there is resonance between two nitrogens and a carbon. Part of the mechanism is that one of the resonance forms involve the double bond between two nitrogens, leaving the carbon with a partial negative. Two question about this.

1. Nitrogen is more electronegative than carbon, so why does the carbon have the negative? I understand resonance structures are constantly switching, but still. Why doesn't a reaction occur at the Nitrogen when it had the negative?

2. Wouldn't the electron donating R groups (on the carbon) make the carbon even more angry about taking a negative from the nitrogens?

Thanks!

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[chiddler]

1. If the nitrogen does eat up a proton, it would just move back a step. So then that part of the mechanism occurs again until the carbonyl finally grabs a proton. Not sure. I'd think that nitrogen would hold the (-) better, too. It doesn't help that the carbon anion is tertiary, either.

2. :x i agree! i'm not sure.

 

[mcloaf]

I'm with Chiddler on Q1. As for Q2, well, yeah, but where does that leave you? You're still just waiting for the negative charge to be on a carbon so that the mechanism can proceed, regardless of the R-groups. Because you're making an alkane, you don't really have any leeway about what you're attaching to that carbon. So the mechanism definitely isn't optimal for getting a negative charge on that carbon, but however unhappy that carbon may be, it's not so unhappy as to preclude the reaction--or we wouldn't be studying it.

 

[milski]

1. Nitrogen is more electronegative than carbon, so why does the carbon have the negative? I understand resonance structures are constantly switching, but still. Why doesn't a reaction occur at the Nitrogen when it had the negative?

There is no constant switching between the resonance structures - the state of the atoms participating is something like the average of all the resonances that you can draw. That means that you never get to an N with a -1 charge - the negative charge is spread along the two N atoms and the C atom. Yes, Ν is more electronegative and will have slightly higher δ- than the C but like the other two guys said, that does just means that only part of the molecules involved proceed with the reaction.

Yes, the EDGs will decrease the negative charge over the C - that has the same consequences as N being more negative - lower partially negative charge on C and less molecules proceeding with the reaction.

 

[MedPR]

So basically I shouldn't be so picky about where the partial negative really is. If a reaction occured at Nitrogen, the reaction wouldn't move forward, so we must be looking at the Carbon.

 

[milski]

So basically I shouldn't be so picky about where the partial negative really is. If a reaction occured at Nitrogen, the reaction wouldn't move forward, so we must be looking at the Carbon.

Pretty much yes. Of course, if you start being less picky, a reaction where this matters will show up immediately. OChem explanations can be so made up sometimes.

 

[MedPR]

Pretty much yes. Of course, if you start being less picky, a reaction where this matters will show up immediately. OChem explanations can be so made up sometimes.

Yea, you're not kidding. Good thing MCAT is probably more about the rules than the exceptions.