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Chapter 34: Problem 782

Explain how use of ultraviolet, infrared, or n.m.r. spectroscopy could be used to distinguish between the following possible structures for civetone. (a) 9-cycloheptadecenone and 2-cycloheptadecenone (b) cis-9-cycloheptadecenone and trans-9-cycloheptadecenone (c) 8-methyl-8-cyclohexadecenone and 9-cycloheptadecenone (d) 8-cycloheptadecenone and 9-cycloheptadecenone

Short Answer

Expert verified
(a) NMR spectroscopy can be used to distinguish 9-cycloheptadecenone and 2-cycloheptadecenone by comparing the chemical shifts of protons near the double bond, which will be different for each compound. (b) IR spectroscopy can be used to distinguish cis-9-cycloheptadecenone and trans-9-cycloheptadecenone by comparing the absorption bands corresponding to the cis-double bond (~1665 cm-\(^{1}\)) and the trans-double bond (~1680 cm-\(^{1}\)). (c) NMR spectroscopy can be used to distinguish 8-methyl-8-cyclohexadecenone and 9-cycloheptadecenone by identifying the unique chemical shift of the methyl protons in 8-methyl-8-cyclohexadecenone compared to the other protons in the molecule. (d) NMR spectroscopy can be used to distinguish 8-cycloheptadecenone and 9-cycloheptadecenone by comparing the chemical shifts of protons near the double bond, which will be different for each compound.

Step by step solution

01

Look for differences in functional groups

In this case, both compounds have the same functional group, a ketone, but the location of the double bond is different. 9-cycloheptadecenone has the double bond at position 9, while 2-cycloheptadecenone has it at position 2.
02

Use NMR spectroscopy

We can use NMR spectroscopy to distinguish between these compounds. The chemical shifts of the protons near the double bond will be different for each compound. For example, the protons adjacent to the double bond in 9-cycloheptadecenone will appear at a lower field than those in 2-cycloheptadecenone. By comparing the NMR spectra of the two compounds, we can determine which one is which based on the characteristic shifts. (b) Distinguishing cis-9-cycloheptadecenone and trans-9-cycloheptadecenone
03

Note the difference in stereochemistry

In this case, the difference between the compounds is the stereochemistry at the double bond. cis-9-cycloheptadecenone has the hydrogen atoms on the same side, while trans-9-cycloheptadecenone has them on opposite sides.
04

Use IR spectroscopy

We can use IR spectroscopy to distinguish between these compounds. The cis isomer will have a characteristic absorption band around 1665 cm-\(^{1}\) due to the presence of the cis-double bond, while the trans isomer will have a band near 1680 cm-\(^{1}\) due to the presence of the trans-double bond. By comparing the IR spectra of the two compounds, we can determine which one is which based on the characteristic absorptions. (c) Distinguishing 8-methyl-8-cyclohexadecenone and 9-cycloheptadecenone
05

Note the difference in functional groups and ring size

In this case, both compounds have a ketone group but the first compound has a methyl group at position 8 and a cyclohexadecene ring, while the second compound has a cycloheptadecene ring.
06

Use NMR spectroscopy

We can use NMR spectroscopy to distinguish between these compounds. The methyl protons in 8-methyl-8-cyclohexadecenone will have a unique chemical shift compared to the other protons in the molecule. By comparing the NMR spectra of the two compounds, we can determine which one is which based on the characteristic chemical shifts. (d) Distinguishing 8-cycloheptadecenone and 9-cycloheptadecenone
07

Note the difference in the position of the double bond

In this case, both compounds have the same functional group, a ketone, but the position of the double bond is different. 8-cycloheptadecenone has the double bond at position 8, while 9-cycloheptadecenone has it at position 9.
08

Use NMR spectroscopy

We can use NMR spectroscopy to distinguish between these compounds. The chemical shifts of the protons near the double bond will be different for each compound. For example, the protons adjacent to the double bond in 8-cycloheptadecenone will appear at a lower field than those in 9-cycloheptadecenone. By comparing the NMR spectra of the two compounds, we can determine which one is which based on the characteristic shifts.

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