September 23, 2019, 05:26:19 AM
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Topic: Calculating the percentage of isomers from integration values  (Read 365 times)

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Offline aat1124

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Calculating the percentage of isomers from integration values
« on: August 30, 2019, 04:17:02 PM »
We were given the H1 NMR spectrum of the reduction of 4-t-butylcyclohexanone (4-t-butylcyclohexanol), which produces a mixture of diastereomers. The lab question is:

"In the expanded region between 3.2 and 4.5 ppm, the peak at 4.05 integrates to 1.0, while the peak at 3.52 ppm integrates to 2.52. which protons give rise to these peaks? Calculate the percentage of each isomer in the sample by using these values."

I understand (I think) that the singlet at 4.05 ppm shows the hydrogen on the alcohol and the septet at 3.52 ppm shows the hydrogen on the carbon that is alpha to the alcohol but I have no idea how to calculate the percentages of the cis/trans mixture based on the integration values. If someone could show me how to work that out that would be amazing.

Thanks in advance!!
« Last Edit: August 30, 2019, 05:48:06 PM by aat1124 »

Offline AWK

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Re: Calculating the percentage of isomers from integration values
« Reply #1 on: August 30, 2019, 05:33:57 PM »
cis/trans isomers
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Offline hollytara

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Re: Calculating the percentage of isomers from integration values
« Reply #2 on: August 30, 2019, 11:40:16 PM »
What effect does the t-butyl group have on the chairs for the cyclohexanol product?  How would that alter the chemical shift and coupling of the products? 

Offline kriggy

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Re: Calculating the percentage of isomers from integration values
« Reply #3 on: September 01, 2019, 06:54:06 PM »
aat1124: you need to compare the same signals within the molecule, you cant compare the alpha proton and the hydroxyl proton. You need to compare the alpha proton to alpha proton from another isomer

Offline aat1124

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Re: Calculating the percentage of isomers from integration values
« Reply #4 on: September 02, 2019, 10:58:03 PM »
aat1124: you need to compare the same signals within the molecule, you cant compare the alpha proton and the hydroxyl proton. You need to compare the alpha proton to alpha proton from another isomer

Okay see that’s what I would have though but they only give me the one spectrum saying that it’s the spectrum of the mixture of cis/trans and that the percentage can be determined from the two absorption scale farthest downfield so I don’t really know what to do with that.

Offline hollytara

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Re: Calculating the percentage of isomers from integration values
« Reply #5 on: September 02, 2019, 11:48:38 PM »
So the reductant can come from either side, generating cis or trans 4-t-butyl cyclohexanol. 

The t-butyl group "locks" the cyclohexane into the chair with the t-butyl equatorial. 

If cis, the -OH is axial and adjacent -H equatorial.
If trans, the -OH is equatorial and adjacent -H axial.

There is a difference in chemical shift for these - the axial H is upfield (lower ppm) of the equatorial H. 

There are different coupling constants for axial and equatorial H's. 

Axial H's have a large (12 Hz) coupling to axial neighbors, smaller couplings (5 Hz) to equatorial neighbors.

Equatorial H's have small coupling constants (5 Hz) to both axial and equatorial neighbors. 

Does this help? 

Offline aat1124

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Re: Calculating the percentage of isomers from integration values
« Reply #6 on: September 03, 2019, 09:26:27 AM »
So the reductant can come from either side, generating cis or trans 4-t-butyl cyclohexanol. 

The t-butyl group "locks" the cyclohexane into the chair with the t-butyl equatorial. 

If cis, the -OH is axial and adjacent -H equatorial.
If trans, the -OH is equatorial and adjacent -H axial.

There is a difference in chemical shift for these - the axial H is upfield (lower ppm) of the equatorial H. 

There are different coupling constants for axial and equatorial H's. 

Axial H's have a large (12 Hz) coupling to axial neighbors, smaller couplings (5 Hz) to equatorial neighbors.

Equatorial H's have small coupling constants (5 Hz) to both axial and equatorial neighbors. 

Does this help?

It does help me understand what’s going on in the reaction but I’m still being asked to calculate a quantitative number using the two different absorption integrations from an NMR that supposedly shows a mixture of cis and trans and here it seems the consensus is I can’t compare the two peaks so I still don’t understand how to calculate the individual percentages of cis and trans in the mixture I’ve been given

Offline hollytara

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Re: Calculating the percentage of isomers from integration values
« Reply #7 on: September 03, 2019, 11:35:46 AM »
If both are present, the axial (trans) H is upfield of the equatorial (cis) H.

so...

trans is 3.52
cis is 4.05


Offline AWK

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Re: Calculating the percentage of isomers from integration values
« Reply #8 on: September 03, 2019, 12:50:15 PM »
Assume that integration intensity is proportional to isomer contents. You need 2 sec. without any calculator.
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