Question

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

(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

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.

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

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.

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

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.

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

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.

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.

Key concepts

NMR Spectroscopy

NMR (Nuclear Magnetic Resonance) spectroscopy is a powerful tool in organic chemistry, especially for identifying the structure of molecules. It operates by detecting the magnetic properties of atomic nuclei. When a sample is placed in a magnetic field and exposed to radio waves, nuclei with an odd number of protons or neutrons resonate at a specific frequency. This resonance depends on the local magnetic environment. For example, in the case of 9-cycloheptadecenone and 2-cycloheptadecenone, the chemical shifts of protons near the double bond vary, revealing differences in the structure, as protons adjacent to double bonds have distinct shielding characteristics. NMR not only distinguishes different hydrogen environments but can also interpret complex carbon skeletons using techniques like

• Proton NMR (\(^1\)H NMR)
• Carbon-13 NMR (\(^{13}\)C NMR)

IR Spectroscopy

Infrared (IR) spectroscopy measures the vibrations of molecules to identify functional groups. Different bonds absorb IR radiation at different frequencies due to how they vibrate, bending and stretching. This technique is instrumental in determining double bond stereochemistry, such as in cis- and trans-9-cycloheptadecenone. Cis-trans isomers show unique absorption bands in the IR spectrum.

• Cis-double bonds exhibit absorption near 1665 cm\(^{-1}\)
• Trans-double bonds show peaks around 1680 cm\(^{-1}\)

Detecting these peaks enables chemists to distinguish isomers, aiding in the identification of compounds based on double bond placement and orientation.

Ketone Functional Group

The ketone functional group is a carbonyl group (C=O) bonded to two carbon atoms. Renowned for its sharp C=O stretching frequency in IR spectroscopy, usually around 1700 cm\(^{-1}\), the ketone is pivotal in organic chemistry for identifying compounds. Ketones like those in 9-cycloheptadecenone and 2-cycloheptadecenone have this carbonyl band, but subtle differences arise based on the molecule's environment. This shift can be attributed to:

• Electronic effects from neighboring groups
• Conjugation with double bonds

Understanding these impacts helps chemists determine the specific identity and position of ketones within compounds.

Double Bond Stereochemistry

Stereochemistry focuses on the spatial arrangement of atoms within molecules, crucial for understanding molecular behavior and reactions. Double bonds can lead to forms known as cis and trans isomers. These isomers differ based on the relative positioning of substituents around the double bond:

• Cis isomers: Substituents are on the same side. This geometry often results in molecules with polar characteristics.
• Trans isomers: Substituents are on opposite sides. These typically have lower polarity and may show differing physical properties compared to their cis counterparts.

Stereochemical knowledge, especially regarding double bonds, enriches understanding of molecular interactions and properties.

Chemical Shift Analysis

Chemical shift is a key concept in NMR spectroscopy, whereby the resonance frequency of a nucleus is influenced by its electronic environment. In organic molecules, protons or carbons shifted from their typical positions indicate structural variations. These shifts are measured in parts per million (ppm) and can provide deep insights:

• Shielded nuclei resonate at lower ppm values
• Deshielded nuclei resonate at higher ppm values

For example, in 8-cycloheptadecenone versus 9-cycloheptadecenone, the different placement of the double bond influences nearby hydrogen shifts, thus revealing structural differences. Analyzing chemical shifts comprehensively aids in elucidating precise molecular structures and interactions.