GC question

[299678]

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The molecules O=C=N-H (two loan pairs on Oxygen and One loan pair on Nitrogen) has been detected in gas clouds between stars. The predicted C-N-H bond angle is about?

I thought it is trigonal planar with one loan pair = bent or angular, so it should be less than 120 degree. But correct answer is 120

I need to know the reason.

 

[MrBeans]

Correct answer is 120 degrees. Nitrogen is SP2 hybridized (3 equivalent sigma bonds and one set of electrons open for the Pi bond).

According to MO Hybridization, SP2 is 120 degrees.

I think you may be thinking of an exception, like water, who has a less than 109.5 degree bond angle.

 

[299678]

Actually I am looking at VSEPR chart now, but SP2 is trigonal planar with 3 sigma bonds, but bent or angular with two bond with one loan pair which give us <120

 

[JBarr29]

I don't believe there are any free pairs of electrons on the nitrogen. It is stabilized by 5 electrons through the covalent bonding of carbon and hydrogen. Without the repulsion it would just be 120 degrees. Why do you think there's a lone pair on the nitrogen?

 

[UndergradGuy7]

Actually I am looking at VSEPR chart now, but SP2 is trigonal planar with 3 sigma bonds, but bent or angular with two bond with one loan pair which give us <120

(and there is a lone pair on nitrogen Jbarr29) Nitrogen has 5 valence electrons. It usually it makes 3 bonds, like it did here, and the last 2 are a lone pair.

This is in chad's first organic chem video. The molecule has 3 electron domains. 1 from the double bond (we count it as one), a lone pair on N, and the H. 3 electron domains = sp2 = 120 degrees. Think of it as a wheel. A wheel adds up to 360 degrees. If you have 3 things then 360/3 = 120. For other molecules this gets more confusing since they can do 3d structures etc.

Notice the above method uses Molecular orbital theory, but there is another theory which you are using, the VSEPR.

If you are using the VSEPR model. Does this problem ask for the geometry of the electrons or the molecule? Because if it asks for electrons then it is trigonal planar and 120 degrees. If it is asking about the molecule then it is bent and less than 120 degrees and you ignore the lone pair. Here is a table http://intro.chem.okstate.edu/1314f00/lecture/chapter10/vsepr.html

For example look at this taken from wikipedia:
The ammonia molecule (NH3) has three pairs of electrons involved in bonding, but there is a lone pair of electrons on the nitrogen atom. It is not bonded with another atom; however, it influences the overall shape through repulsions. There are four regions of electron density. Therefore, the overall orientation of the regions of electron density is tetrahedral. On the other hand, there are only three outer atoms (the lone pair does not count, count only the H's). This is referred to as an AX3E type molecule because the lone pair is represented by an E. The observed shape of the molecule is a trigonal pyramid, because the lone pair is not "visible" in experimental methods used to determine molecular geometry. The shape of a molecule is found from the relationship of the atoms even though it can be influenced by lone pairs of electrons.

 

[JBarr29]

you're right there is a pair of electrons on N - i didnt bother to count.
However...
if there is a lone pair, i believe it should be less than 120. even from that website you have posted there - 2 bonding and 1 non bonding is less than 120.

thank you for catching my error undergrad =)

 

[UCB05]

It sounds like everyone is in agreement that the molecule is in some way bent in terms of the molecule and trigonal planar in AX2E terms, and considering the lone pair, instead of 120 it should be something less than 120.

Would it make any difference to you guys that the question is asking what "the predicted C-N-H bond angle is about?" I would seriously doubt the question is that picky when it itself just wants a ballpark number to test that you know the answer is closer 120 (trigonal planar) than 109.5 (tetrahedral)

 

[JBarr29]

It sounds like everyone is in agreement that the molecule is in some way bent in terms of the molecule and trigonal planar in AX2E terms, and considering the lone pair, instead of 120 it should be something less than 120.

Would it make any difference to you guys that the question is asking what "the predicted C-N-H bond angle is about?" I would seriously doubt the question is that picky when it itself just wants a ballpark number to test that you know the answer is closer 120 (trigonal planar) than 109.5 (tetrahedral)

I have a protein folding simulation program where you can run insilico mutations. ill plug it in when i get a minute and let you know the expected bond angles. its super accurate.

 

[Simple Jack]

curious as to what the accurate simulator says....

I bet it only bends 1-2 degrees. so maybe 118-119 degrees. perhaps the bending effect isn't pronounced due to the fact that there's only 1 lone pair, plus the C=N double bond is shorter/stronger than a single bond and thus less susceptible to outside electrostatic forces.

 

[299678]

The problem came from ACS book and also came out on real ACS test when I took it long times ago. There were choices 120 and slightly less than 120.

 

[Ibracadabra]

I believe then it should be slightly less than 120 degrees. Lone pairs repel more. However, with only one lone pair, it would be a slight difference.

NH3 has a bond angle of 107.5(H-N-H) because only one lone pair pushes down while water has a bond angle of 104.5(H-O-H) where two lone pair pushes down.. They both do not follow the usual bond angle that a tetrahedral molecule has--109.5, rather slightly smaller (107.5 and 104.5)because of the involvement of the lone pairs 🙂

 

[UCB05]

The problem came from ACS book and also came out on real ACS test when I took it long times ago. There were choices 120 and slightly less than 120.

Well what are the choices, exactly? I googled the question you wrote, and every site that has the same question, word for word, but have the options A) 60 B)90 C)109 D)120. Given these choices, I wouldn't hesitate to go with 120.

If this is the question you are asking, 109 is hardly "slightly less" than 120. When bond angle charts refer to an AX2E molecule as having slightly less than a 120-degree bond angle, they mean 118~119. 109 on a trigonal pyramidal formation represents some significant electrostatic force, enough to repel two groups beyond that normally exhibited in a tetrahedral molecule (which has TWO side groups presenting steric/electrostatic forces on the other two groups). Even the example of water given by Ibracadabra above, which has TWO pairs of electrons, only warps the expected bond angle by two degrees.

 

[299678]

+1 Right it should be slightly less than 120, otherwise it would be tetrahedral ! Thank you