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In methyl isocyanate, $\ce{CH3CNO}$, shown below,

enter image description here it seems that the nitrogen lone pair can resonate with the carbon oxygen double bond, generating a resonance structure something like acetonitrile oxide:

enter image description here

In that case, why is the geometry around the nitrogen in the methyl isocyanate compound bent and not linear? Can the lone pair even resonate like that?

I remember in Klein's organic chemistry, it is stated that

Whenever an atom possesses both a π bond and a lone pair, they will not both participate in resonance

In methyl isocyanate, $\ce{CH3CNO}$, shown above, it seems that the nitrogen atom has both a π bond and a lone pair. So perhaps the sp2 lone pair on nitrogen atom cannot resonate with the carbon oxygen double bond? I am not sure.

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  • $\begingroup$ I don't understand the title. Vinyl is CH2=CH-. It has no lone pair ! $\endgroup$ – Maurice Dec 29 '19 at 16:48
  • $\begingroup$ @Maurice I mean vinyl as in lone pair on a double bond (such as in N) $\endgroup$ – Cyclopropane Dec 29 '19 at 16:49
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Whenever an atom possesses both a π bond and a lone pair, they will not both participate in resonance

The sentence means that only one, either the 𝜋 bond or the lone pair will participate in resonance if the atom has both.


For example, in a pyridine, the nitrogen has a lone pair and it is also attached to a 𝜋 bond.

Here, however, only the 𝜋 bond participates in resonance because it is in the plane perpendicular to the molecule, while the lone pair is not.

enter image description here


Methyl isocyanate is best represented by a group of three resonance structures:

enter image description here

Here is the 3D structure of the most significant resonance structure of methyl isocyanate:

enter image description here

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  • $\begingroup$ So C and O are both sp2 hybridized? $\endgroup$ – Cyclopropane Dec 29 '19 at 18:53
  • $\begingroup$ The first Carbon is sp3, the second sp. The oxygen atom must have either sp2 or sp hybridization, because it needs a p orbital to participate in the C–O π bond. This oxygen atom has three attachments (the carbon and two lone pairs), so we use sp2 hybridization. $\endgroup$ – Shootforthemoon Dec 29 '19 at 19:01
  • $\begingroup$ Did this answer ur question? @DrPepper $\endgroup$ – Shootforthemoon Dec 29 '19 at 19:25
  • $\begingroup$ kind of? I know in amides the N looks sp3 but it is actually sp2 because it is conjugated to the carbonyl, so I was wondering why that doesn't happen here. The N looks like it should be sp2, but if this resonance works, it would be sp because of conjugation to the C=O in H3CNCO $\endgroup$ – Cyclopropane Dec 29 '19 at 20:27
  • $\begingroup$ @DrPepper Well, look at the most significant structure, N is hybridized sp2. When an atom that appears to be sp3 hybridized can participate in resonance, that atom is in fact sp2 hybridized. This is because resonance requires an open p-orbital to occur. Sp3 atoms have 4 sp3-hybridized orbitals but no open p-orbitals, and therefore cannot participate in resonance, while sp2-hybridized atoms can. $\endgroup$ – Shootforthemoon Dec 29 '19 at 20:37

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