Help concerning Octet Rule/Double bonds

6

Hi.

I'm doing a Biology degree but am taking an Intro chemistry module as I did not do A level chemistry.

I have a question where I have to give the geometry of the bonds involved in CH3NCHOH.

I have drawn the lewis structure and have satisfied the octet rule for all but the second carbon (the one next to O and N), so one of either O or N will have to share a lone pair and form a double bond with the carbon. My question is, how do you know which one will do this? Is it the case of the most electronegative having a tendency to form the double bond? Its a multiple choice answer I have but I don't know wether to select the double bond on N or O.

Any help would be greatly appreciated.

Thanks!!

Reply 1

marcusfox

16

H₃C-N=CH-OH. There is no double bond on the O, as it forms one bond to the hydrogen and one to the carbon.

H-C-N 109.5°
C-N=C 180°
H-C-O(-H) 120°
C-O-H 104.5° (this is close to a tetrahedral angle, but there are two lone pairs on the oxygen atom, so it's called bent).

Carbon - four single bonds, two double bonds or a triple bond and a single bond.

Nitrogen - three single bonds, a double bond and a single bond, or one triple bond.

Oxygen - two single bonds or one double bond.

Oxygen, because of it's electronegativity can sometimes form a triple bond as in C ≡ O. In CO, carbon contributes 2 electrons to the bonding, while oxygen contributes 4. That way we finish up with 6 shared electrons -- a triple bond. The oxygen atom has 2 of its own valence electrons left, and a share of the 6 bonding electrons, to make 8; The carbon atom also has 2 of its own valence electrons and a share of the 6 bonding electrons. Because the oxygen is more electronegative than the carbon, it will grab the lion's share of the 6 shared electrons. In practice it gets close to a 2/3 share. Seeing that it contributed 4 of the 6 shared electrons in the first place, the whole arrangement is a neutral one. Carbon monoxide has a triple bond.

Hope that's what you mean.

Marcus

molecule.JPG8.2KB

Reply 2

Olivia22

OP

6

Here is an attempt to include the question I am confused about. I understand what you say in your last post, but you have a hydrogen bonded to the second carbon but that isn't one of the optional answers (its multiple choice), all have carbon bonding with 2 nitrogens and an oxygen. There has to be a double bond somewhere as otherwise carbon only has 6 e-s in its outer shell.

I'll try and put it in an attachment.

chem2q.doc39.9KB

Reply 3

marcusfox

16

But you asked me about CH₃NCHOH, not NH₂CONH₂. I would say (e), as 2 is a valid resonance structure, and 1 has an extra lone pair on the oxygen. Note if you're assuming you have to satisfy the octet rule, this would give oxygen 10 electrons - count them.

H₂N-C=O.
.......|
.......NH₂

Marcus

molecule2.jpg8.9KB

Reply 4

Olivia22

OP

6

marcusfox

But you asked me about CH₃NCHOH, not NH₂CONH₂. I would say (e), as 2 is a valid resonance structure, and 1 has an extra lone pair on the oxygen. Note if you're assuming you have to satisfy the octet rule, this would give oxygen 10 electrons - count them.

H₂N-C=O.
.......|
.......NH₂

Marcus

God, I'm so not with it tonight, there are a few questions that I was unsure of because of a double bond on either the O or N. Sorry - I posted the wrong one in the attachment.

However, you have actually helped me on both so thanks a lot :smile:

Thanks again, I appreciate it.

Reply 5

marcusfox

16

No problem. Very refreshing to know you've helped out. It's very common on these boards that you'll offer help to someone and you'll never hear from them again.

Marcus

Help concerning Octet Rule/Double bonds

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