2011-02-26, 09:55 PM
The first to be protonated in a solution (i.e. the more basic nitrogen) is the nitrogen in the imidazole group (the 5 membered ring with a 1,3-nitrogen) that is double-bonded to carbon, although in reality the two nitrogens in the ring are really equivalent.
Simply put, the protonated nitrogen on the ring can be stabilised by resonance, whereas a protonated amino group at the alpha-carbon down there (not the R-group) does not possess any stabilisation mechanisms. Equilibrium shifts to the right (getting protonated i.e. more basic) the better your conjugate acid (the protonated stuff) can be stabilised. Resonance is a very good way of stabilising an ion because the positive charge is no longer localised over 1 atom, but in this case 3 atoms (N -- C --N).
The two nitrogens on the imidazole ring are (almost) equally basic because both takes part in the resonance structures of each others' protonation stabilisation, and from there you can draw a side step to show that the double bond is equally likely to land on either nitrogens, so either nitrogens are equally basic.
BUT why don't we use that nitrogen that's single-bonded only? Because at that instance, the lone pairs on that nitrogen are oriented perpendicular to the plane in a p-orbital (not sp3), and this allows the lone pairs to interact with the two double bonds to form an aromatic pi-system (4n + 2 = 6, n = 1). Conversely, the lone pair becomes unavailable for protonation, because that would mean breaking aromaticity (and it's associated resonance stabilisation energy like for benzene).
By the same arguments, the double-bonded nitrogen's lone pair is pointing out of the ring i.e. it's in the sp2 orbital. This is very clearly not going to have an effect on aromaticity because protonation of this nitrogen doesn't disturb the p-orbital.
Hanabira - DRAW THE RESONANCE STRUCTURES AND THE ARROW PUSHING OR YOU DON'T GET ALL YOUR MARKS. It should look like this one:
![[Image: hisH-resonance.gif]](http://guweb2.gonzaga.edu/faculty/cronk/CHEM440/images/hisH-resonance.gif)
Of course remember to draw the arrow pushing for the two structures on the right.
Curious why you didn't ask this in the Backyard. No funky heteroaromatic chemistry here. Just simple resonance stabilisation.
Hadriel
Simply put, the protonated nitrogen on the ring can be stabilised by resonance, whereas a protonated amino group at the alpha-carbon down there (not the R-group) does not possess any stabilisation mechanisms. Equilibrium shifts to the right (getting protonated i.e. more basic) the better your conjugate acid (the protonated stuff) can be stabilised. Resonance is a very good way of stabilising an ion because the positive charge is no longer localised over 1 atom, but in this case 3 atoms (N -- C --N).
The two nitrogens on the imidazole ring are (almost) equally basic because both takes part in the resonance structures of each others' protonation stabilisation, and from there you can draw a side step to show that the double bond is equally likely to land on either nitrogens, so either nitrogens are equally basic.
BUT why don't we use that nitrogen that's single-bonded only? Because at that instance, the lone pairs on that nitrogen are oriented perpendicular to the plane in a p-orbital (not sp3), and this allows the lone pairs to interact with the two double bonds to form an aromatic pi-system (4n + 2 = 6, n = 1). Conversely, the lone pair becomes unavailable for protonation, because that would mean breaking aromaticity (and it's associated resonance stabilisation energy like for benzene).
By the same arguments, the double-bonded nitrogen's lone pair is pointing out of the ring i.e. it's in the sp2 orbital. This is very clearly not going to have an effect on aromaticity because protonation of this nitrogen doesn't disturb the p-orbital.
Hanabira - DRAW THE RESONANCE STRUCTURES AND THE ARROW PUSHING OR YOU DON'T GET ALL YOUR MARKS. It should look like this one:
Of course remember to draw the arrow pushing for the two structures on the right.
Curious why you didn't ask this in the Backyard. No funky heteroaromatic chemistry here. Just simple resonance stabilisation.
Hadriel