Question

Propose structures for molecules that meet the following descriptions. Assume that the kinds of carbons \(\left(1^{\circ}, 2^{\circ}, 3^{\circ},\right.\) or \(\left.4^{\circ}\right)\) have been assigned by DEPT-NMR. (a) \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O} ; \mathrm{IR}: 1715 \mathrm{~cm}^{-1} ;{ }^{13} \mathrm{C} \mathrm{NMR}: 8.0 \delta\left(1^{\circ}\right), 18.5 \delta\left(1^{\circ}\right), 33.5 \delta\left(2^{\circ}\right)\) \(40.6 \delta\left(3^{\circ}\right), 214.0 \delta\left(4^{\circ}\right)\) (b) \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O} ; \mathrm{IR}: 1730 \mathrm{~cm}^{-1} ;{ }^{13} \mathrm{C} \mathrm{NMR}: 22.6 \delta\left(1^{\circ}\right), 23.6 \delta\left(3^{\circ}\right), 52.8 \delta\left(2^{\circ}\right)\) \(202.4 \delta\left(3^{\circ}\right)\) (c) \(\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O} ; \mathrm{IR}: 1680 \mathrm{~cm}^{-1} ;{ }^{13} \mathrm{C} \mathrm{NMR}: 22.9 \delta\left(2^{\circ}\right), 25.8 \delta\left(2^{\circ}\right), 38.2 \delta\left(2^{\circ}\right)\) \(129.8 \delta\left(3^{\circ}\right), 150.6 \delta\left(3^{\circ}\right), 198.7 \delta\left(4^{\circ}\right)\)

Short answer

(a) Hexan-2-one; (b) Cyclopentanone; (c) Cyclohexenone.

Step-by-step solution

Analyze the Molecular Formula for (a)

The molecular formula is \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}\). This suggests an unsaturation degree indicating either a ring or a double bond. The formula is saturated except for one double bond, indicating a carbonyl group presence.

Interpret the IR and NMR Data for (a)

The IR absorption at \(1715 \mathrm{~cm}^{-1}\) is typical for a carbonyl group such as a ketone. The \(^{13}\mathrm{C}\) NMR gives signals indicative of primary, secondary, tertiary, and quaternary carbons: \(8.0 \delta\ (1^{\circ}), 18.5 \delta\ (1^{\circ}), 33.5 \delta\ (2^{\circ}), 40.6 \delta\ (3^{\circ}), 214.0 \delta\ (4^{\circ})\). The \(4^{\circ}\) carbon signal at 214.0 indicates a carbonyl carbon, likely in a ketone.

Propose a Structure for (a)

Combining the molecular formula and spectroscopic data, one possible structure is hexan-2-one. It follows from a structure where the carbonyl group (=O) is at the second carbon position, with the remaining carbons forming an alkyl chain: \(\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{C(O)-CH}_2-\mathrm{CH}_2-\mathrm{CH}_3\).

Analyze the Molecular Formula for (b)

The molecular formula is \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}\), indicating potential presence of a saturation. The IR absorption at \(1730 \mathrm{~cm}^{-1}\) is characteristic for a carbonyl functional group, possibly ester, aldehyde, or ketone.

Interpret the NMR Data for (b)

The \(^{13}\mathrm{C}\) NMR includes signals: \(22.6 \delta\ (1^{\circ}), 23.6 \delta\ (3^{\circ}), 52.8 \delta\ (2^{\circ}), 202.4 \delta\ (3^{\circ})\). The \(3^{\circ}\) carbon at 202.4 likely corresponds to a carbonyl carbon. The chemical shifts and environment suggest a ketone, likely a cyclopentanone.

Propose a Structure for (b)

The predominant structure fitting the conditions is cyclopentanone. It matches the unsaturation degree (ring plus carbonyl), carbon signals (four carbons forming the ring, with a carbonyl at one), and molecular formula \(\mathrm{C}_5\mathrm{H}_{10}\mathrm{O}\).

Analyze the Molecular Formula for (c)

The molecular formula is \(\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}\), indicating multiple degrees of unsaturation. This could imply a cyclic system and a carbonyl.

Interpret the IR and NMR Data for (c)

The IR peak at \(1680 \mathrm{~cm}^{-1}\) suggests a conjugated carbonyl. The \(^{13}\mathrm{C}\) NMR data shows secondary and tertiary carbon signals and a quaternary carbon at 198.7 indicative of a carbonyl.

Propose a Structure for (c)

Considering conjugation and multiplicity of carbons, one possible structure is cyclohexenone with a conjugated double bond, supporting signals for tertiary carbons and extended π-system: \(\mathrm{O=C-CH=CH-CH=CH-CH}_2-\mathrm{CH}_2\).

Key concepts

DEPT-NMR

DEPT-NMR (Distortionless Enhancement by Polarization Transfer) is a helpful technique in nuclear magnetic resonance spectroscopy that aids in distinguishing different types of carbon atoms. It classifies carbon atoms based on the number of hydrogen atoms attached to them.

• Primary Carbons (1°): These carbons are attached to one hydrogen atom.
• Secondary Carbons (2°): These have two hydrogen atoms attached.
• Tertiary Carbons (3°): Three hydrogen atoms are attached here.
• Quaternary Carbons (4°): No hydrogen atoms are attached to these carbons.

In the context of molecular structure determination, DEPT-NMR helps identify structural details by highlighting carbon types, which can be correlated with chemical environments provided by other spectroscopy data. This makes it easier to map out the molecular structure accurately.

Carbonyl Group

The carbonyl group, represented as C=O, is a key functional group in organic chemistry. It's characterized by a carbon atom double-bonded to an oxygen atom. This group is prevalent in several types of compounds, such as ketones, aldehydes, and carboxylic acids.

• Ketones: The carbonyl group is flanked by carbon atoms.
• Aldehydes: One side of the carbonyl group is bonded with at least one hydrogen atom.
• Esters, Amides, and Anhydrides: Variations occur with additional bonds to oxygen or nitrogen.

The presence of the carbonyl group results in distinct signals in both IR and NMR spectra. For instance, the chemical environment around the carbonyl influences its position in the spectra, helping chemists identify its presence in unknown samples.

IR Spectroscopy

Infrared (IR) spectroscopy is a powerful tool for identifying functional groups within a molecule. It provides information based on how molecular vibrations absorb different frequencies of infrared light.

The frequencies at which peaks appear in an IR spectrum correspond to the vibrational frequencies of bonds within the molecule.

• Carbonyl Stretch: A sharp, strong peak around 1700 cm^-1 signifies the presence of a carbonyl group.
• Hydrogen Bonded O-H: Broad peaks between 3200-3500 cm^-1 indicate hydrogen-bonded hydroxyl groups.

By analyzing these peaks, chemists can identify key functional groups in a sample and infer details about their bonding environment and molecular structure.

Unsaturation Degree

The degree of unsaturation in a molecule provides insights into its structure, indicating the presence of rings or double bonds. It's determined by the molecular formula, revealing how many π-bonds or ring structures may be present.

To calculate the degree of unsaturation:
\[\text{UD} = \frac{2C + 2 + N - X - H}{2}\] Where:

• C: Number of carbon atoms
• N: Number of nitrogen atoms
• X: Number of halogen atoms
• H: Number of hydrogen atoms

Each degree of unsaturation corresponds to a potential double bond or ring, providing clues about molecular structure beyond simple connectivity.

Chemical Shifts

Chemical shifts in NMR spectroscopy denote the position of a signal relative to a standard, often given in parts per million (ppm). They reveal the electronic environment surrounding nuclei in a molecule, primarily hydrogen or carbon atoms.

• Upfield Shifts: High electron density environments result in lower chemical shifts.
• Downfield Shifts: Low electron density (like near electron-withdrawing groups) leads to higher shifts.

In ¹³C NMR, different carbon environments result in varying shifts, aiding in the identification of functional groups and structural features. For example, a quaternary carbon (4°) in a carbonyl group would appear significantly downfield due to the electron-withdrawing properties of the oxygen.