Question

Anethole, \(\mathrm{C}_{10} \mathrm{H}_{12} \mathrm{O},\) a major constituent of the oil of anise, has the \({ }^{1} \mathrm{H}\) NMR spectrum shown. On oxidation with \(\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7},\) anethole yields \(p\) -methoxybenzoic acid. What is the structure of anethole? Assign all peaks in the NMR spectrum, and account for the observed splitting patterns.

Short answer

Anethole has a methoxy group on a benzene ring and a vinyl group (\(\mathrm{CH}_2=\mathrm{CH}-\) attached.)

Step-by-step solution

Analyze the Chemical Formula

Anethole has the molecular formula \( \mathrm{C}_{10} \mathrm{H}_{12} \mathrm{O} \). This implies that it contains a benzene ring due to the presence of six hydrogen atoms less than the number of carbons and a potential functional group involving oxygen.

Consider the Oxidation Product

Anethole oxidizes to \(p\)-methoxybenzoic acid with \(\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\). This indicates the presence of a methoxy \((\mathrm{-OCH}_3)\) group attached para to a carboxylic acid functionality on a benzene ring. Hence, anethole must have a corresponding methoxy group.

Interpret the NMR Spectrum

The \(^1 \mathrm{H}\) NMR spectrum of anethole is analyzed for different peaks. A total of ten hydrogen atoms are involved. Identify signals that correlate with the methoxy group (typically a singlet around 3.7 ppm) and the \(\mathrm{H}_2\)C=CH- (vinyl group) that appears around 5-6 ppm. The aromatic protons should display signals between 6.5-8 ppm.

Assign the Splitting Patterns

The NMR spectrum should reveal splitting patterns corresponding to adjacent hydrogen atoms. The methoxy group is a singlet, the aromatic protons usually appear as multiplets due to neighboring protons, and the vinyl group will show coupling with their direct neighbors.

Propose the Structure

With all information, anethole can be proposed to have a structure where the methoxy group is directly connected to a benzene ring. The structure includes a vinyl (\(\mathrm{-CH=CH}_2\)) group attached orthogonally on the benzene ring, suggesting the structural formula as: \(\mathrm{CH}_2\mathrm{=CH-}\underset{\cdot}{\mathrm{C}}_6\mathrm{H}_4\mathrm{-OCH}_3\).

Key concepts

NMR Spectrum Analysis

Nuclear Magnetic Resonance (NMR) Spectroscopy is a powerful tool for analyzing molecular structures, particularly in organic chemistry. By measuring how atomic nuclei absorb electromagnetic radiation, it provides insights into the chemical environment surrounding the nuclei. In the case of anethole,

• It helps to determine the distinct environments of hydrogen atoms.
• Peaks in the spectrum provide information about the number of hydrogen atoms in a particular environment.
• The position of these peaks (in ppm) indicates different chemical environments, like aromatic and aliphatic regions.

Understanding these peaks allows for mapping out complicated molecules and understanding their functional groups. For anethole, it includes signals that align with known functional groups such as methoxy and vinyl groups.

Methoxy Group

The methoxy group \((\text{-OCH}_3)\) is a common functional group in organic compounds. It consists of an oxygen atom bonded to both a methyl group and a carbon skeleton. In NMR spectroscopy,

• The methoxy group's protons typically appear as a singlet due to their isolation in terms of neighboring protons.
• Its signal generally appears around 3.7 ppm due to the electronegative oxygen atom.

This recognizable signature in the spectrum is useful for confirming the presence of a methoxy group, helping to piece together the full molecular structure.

Vinyl Group

A vinyl group (\(\text{-CH=CH}_2\)) is an alkenyl group, featuring a carbon-carbon double bond. This functional group is prevalent in many organic compounds. The presence of a vinyl group in anethole can be identified by specific signals in the NMR spectrum:

• Signals corresponding to vinyl protons usually appear in the 5-6 ppm range.
• The peaks may show splitting due to interactions with adjacent hydrogen atoms.

The distinctive chemical shift and splitting patterns provide evidence of this structural element within the molecule, facilitating the assignment of specific paths in complex spectra.

Benzene Ring

The benzene ring is a fundamental structure in organic chemistry, characterized by its stability and unique electron configuration. In NMR analysis, the aromatic protons of a benzene ring show distinct features:

• They typically appear in the chemical shift range of 6.5-8 ppm.
• The splitting patterns can appear as multiplets due to the interactions with neighboring protons on the ring.

These characteristics are crucial when analyzing aromatic compounds like anethole, helping to confirm the presence of the benzene ring as part of the overall molecular structure.

Oxidation Reaction

Oxidation reactions in organic chemistry involve the increase in oxidation state, often through the addition of oxygen or removal of hydrogen. Anethole's oxidation to \(p\)-methoxybenzoic acid provides insights into its structure:

• It indicates the conversion of the vinyl group into a corresponding carboxylic acid group.
• Using an oxidizing agent like \(\text{Na}_2\text{Cr}_2\text{O}_7\) highlights how functional groups can transform.

This transformation supports the proposed structure of anethole and demonstrates the methodology behind structural determination using reactivity.