White Yeti
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Amine groups are trigonal planar?!
This is just a quick question, and it might be the stupidest question on the forum, but I really need to know (I'm building a stereochemical modelling
kit).
I was under the impression that amine groups did not have trigonal planar geometry due to the lone pair of electrons on the nitrogen. As instead, I
thought that the molecular geometry was trigonal pyramidal...
But in some pictures of amines like acetamide, 1-Naphthylamine, and urea, the amine groups appear to be planar.
Is this functional group displayed like this, so as to improve clarity? Is there something I'm missing?
"Ja, Kalzium, das ist alles!" -Otto Loewi
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AndersHoveland
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When the amine is electron-donating, such as acetamide, urea, and 2-nitroaniline, it is trigonal planar.
Think of it this way. The structure of actamide could be seen as:
CH3-C(-O[-])=NH2[+]
The amine in methylamine is, like you suspected, trigonal pyramidal.
see this picture from wikipedia:
http://upload.wikimedia.org/wikipedia/commons/6/6d/Methylami...
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fledarmus
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Nitrogen in an all single-bonded, neutral configuration (primary, secondary, or tertiary amine) is sp3 hybridized, and therefore pyramidal, with the
lone pair occupying one of the tetragonal sp3 orbitals. The pyramidal structure rapidly inverts itself at room temperature, meaning the lone pair can
be seen as going back and forth through the molecule, while the other three groups move side to side like an inverting umbrella. This prevents the
amine from having a fixed stereochemical conformation in the way that a chiral carbon would. A quaternary ammonium salt, on the other hand, ties that
lone pair into a bond and fixes the conformation - no more umbrella inversions without actually making and breaking bonds.
If there is a good energetic incentive, however, the nitrogen will form sp2 hybrid orbitals, and leave the lone pair in a p orbital. This occurs in
examples like pyridine and indole, where the lone pair can form part of an aromatic system. In these cases, the nitrogen is trigonal planar, and the
lone pair occupies the p orbital above and below the plane.
Resonance structures in compounds like amides and ureas, where the amine is adjacent to a double bond, can lead to a structure that is somewhere
between sp3 and sp2. In these cases, bond rotation may be restricted due to partial double bond character of the bond from the resonance structure
AndersHoveland showed for acetamide, and the pyramidal structure of an sp3 nitrogen will be flattened because it has some sp2 character as well.
Changes in the electronics can increase or decrease the stability of different resonance forms, leading to subtle changes in the shape of the nitrogen
bonds.
Variable temperature 1H NMR can be used to show the significance of the sp2 resonance structure by measuring the barrier to rotation of the N-C bond.
At low temperatures, the two methyl groups of N,N-dimethyl formamide (for example) are different, but as the temperature increased and there ismore
energy for bond rotation, the two peaks collapse into one.
Incidentally, at room temperature, there are two peaks for the two methyl groups.
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White Yeti
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That make a whole lot more sense, thanks for explaining! So following this trend, amine compounds without an electron withdrawing group will have a
pyramidal amine group. Those that have a carbonyl, nitrile, a halogen or a nitro group will be trigonal planar.
"Ja, Kalzium, das ist alles!" -Otto Loewi
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fledarmus
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Not an electron withdrawing group - you need some sort of pi system for the lone pair to interact with.
Of course, all the ones you listed have both 
Edited: Oops, missed the halogens. Those won't be planar.
[Edited on 21-12-2011 by fledarmus]
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