Restricted Rotation

[SaintJude]

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How do you know when the double bond of an alkene like the one below is "not free to rotate" ?

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[chiddler]

How do you know when the double bond of an alkene like the one below is "not free to rotate" ?

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any pi bonds do not allow rotation.

this includes resonanced pi bonds, like amides. the N-C bond doesn't rotate.

 

[MedPR]

How do you know when the double bond of an alkene like the one below is "not free to rotate" ?

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They're all not free to rotate. Any double bond or partial double bond (amide, benzene, etc) will not rotate.

 

[gettheleadout]

Like they said, double/triple bonds don't rotate at all, and this is independent of the substituent groups or anything else. This is why cis/trans alkenes don't interconvert.

I do recall a little blurb in my organic book mentioning that with enough heat you can rotate a C=C bond, but it's so extreme my prof told us to assume its impossible.

 

[MedPR]

Like they said, double/triple bonds don't rotate at all, and this is independent of the substituent groups or anything else. This is why cis/trans alkenes don't interconvert.

I do recall a little blurb in my organic book mentioning that with enough heat you can rotate a C=C bond, but it's so extreme my prof told us to assume its impossible.

Don't forget about the "1.5" bonds 🙂

 

[SaintJude]

Ok, that was simple. To reap the full benefit of the actual question, predict how many absorptions will show up on the proton NMR spectrum.

Answer & explanation in white: 5, b/c the double bond does not rotate, the 2 methyl groups are not identical!

 

[MedPR]

Ok, that was simple. To reap the full benefit of the actual question, predict how many absorptions will show up on the proton NMR spectrum.

Answer & explanation in white: 5, b/c the double bond does not rotate, the 2 methyl groups are not identical!

Yup, one of the methyl's is, and will always be, more sterically hindered because there is no rotation. Had it been a single bond, there would only be 4 because of the constant rotation.

 

[SaintJude]

Wow, good to know. I never thought proton NMR in relation to double bond rotation.

 

[Godric]

They're all not free to rotate. Any double bond or partial double bond (amide, benzene, etc) will not rotate.

Not 100% correct, its just one of those exceptions. Good to know though (showed up on a Molecular Biology exam last semester), this is from my biology notes

"C-N that form peptide bond have a partial double bond character, that gives the O-C-N-H a planar and rigid structure, but there is rotation around C-N bond..."

 

[MedPR]

Not 100% correct, its just one of those exceptions. Good to know though (showed up on a Molecular Biology exam last semester), this is from my biology notes

"C-N that form peptide bond have a partial double bond character, that gives the O-C-N-H a planar and rigid structure, but there is rotation around C-N bond..."

The C-N bond you are talking about is a single bond. My original post is accurate; all non-single bonds (double, triple, partial double, etc) are restricted, while single bonds are not. If the C-N bond was involved in resonance, The bond would be N-CR-C, not N-CHR-C.

 

[pfaction]

Exactly, there's rotations between the alphaC and NH/C=O, but not C=O and C-N.