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I am confused between option (2) and (4). Both lone pairs of nitrogen are localized, so next, I looked at the hybridization of $\ce{N}$ in each compound. As in (2), the $\ce{N}$ is $\mathrm{sp^2}$ hybridized and therefore is more electronegative, I assumed it would be a weaker base compared to (4).

But the answer is (2). I drew the protonated structures and found that there is resonance in the compound (2).

I was taught that electronegativity is more important in questions like this. Is this a wrong concept? Any help is appreciated!

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  • $\begingroup$ chemistry.stackexchange.com/questions/96513/… $\endgroup$ – Mithoron Jan 7 at 18:11
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    $\begingroup$ You are comparing nitrogen with nitrogen; how can electronegativity be an issue? $\endgroup$ – Martin - マーチン Jan 7 at 18:32
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    $\begingroup$ This is also closely related to: chemistry.stackexchange.com/q/125449/61578. That link has an answer why guanidine is stronger base. $\endgroup$ – Mathew Mahindaratne Jan 7 at 19:04
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    $\begingroup$ No that is a myth propagated by some organic chemists. Electronegativity is a property of an element, nothing more, nothing less. Hybridisation is also just a mathematical concept, it doesn't change what the atom or molecule is. $\endgroup$ – Martin - マーチン Jan 7 at 23:58
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    $\begingroup$ @RahulVerma M.S. Chouhan elementary problems in organic chemistry for JEE. I'm using this for NEET but if you're preparing for JEE, my teachers recommend using the advanced version. $\endgroup$ – laksheya Jan 17 at 15:43
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A good way to compare basicity of basic compounds is to draw their corresponding conjugate acids and compare their stability. As you might figured out from drawing resonance forms of compound (2)'s conjugate acid (its protonated form ), the positive charge is going to be shared by the 3 nitrogens in the compound. On the other hand, in compound (4)'s conjugate acid (protonated form) the positive charge is on a single nitrogen. Of course, this gives compound (2)'s conjugate acid higher stability.

Your reasoning about the nitrogen being sp2 makes it less basic in comparison with sp3 nitrogen is correct,however; it only holds true if you were comparing two nitrogens with the only significant difference being the hybridization state, but in some cases, resonance will stabilize or destabilize the conjugate conjugate acid of the base, this is usually have greater impact than hybridization with regard to ranking baisicity of the compounds as in the question presented. Electronegativity is an important factor in ranking acids/bases,but as I said, it should be used with compounds where the only significant different in the structures is about it (for example, 4 compounds with same structure but only difference is Cl, I, Br, OH at a specific position), in other cases, other factors such as resonance stabilization and aromaticity should be also considered alongside electronegativity.

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See the stability of conjugate acid is very important. In case of secondary amine the basicity is appreciable but in guanidine system the conjugate acid is remarkably stable due to stabilizing conjugation that is occurring there.enter image description here

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