Question

Reaction of salicylaldehyde (1) and acetic anhydride in the presence of sodium acetate gives compound 2 , a fragrant compound occurring in sweet clover. CC(=O)OC(C)=O 1 Spectral data for compound 2 are collected below. Deduce the structure of 2 . Naturally, it is an interesting question as to how 2 is formed. This problem turns out to be harder than it looks at first. Nonetheless, you might want to try to formulate a mechanism. However, note that \(O\)-acetylsalicylaldehyde (3) gives only traces of 2 in the absence of acetic anhydride. CCOc1ccccc1C=O Compound 2 Mass spectrum: \(m / z=146(\mathrm{M}, 100 \%), 118(80 \%), 90(31 \%)\), \(89(26 \%)\) IR (Nujol): \(1704 \mathrm{~cm}^{-1}\) (s) \({ }^{1} \mathrm{H} \mathrm{NMR}\left(\mathrm{CDCl}_{3}\right): \delta 6.42(\mathrm{~d}, J=9 \mathrm{~Hz}, 1 \mathrm{H}), 7.23-7.35(\mathrm{~m},\), \(2 \mathrm{H}), 7.46-7.56(\mathrm{~m}, 2 \mathrm{H}), 7.72(\mathrm{~d}, J=9 \mathrm{~Hz}, 1 \mathrm{H})\) \({ }^{13} \mathrm{C}\) NMR \(\left(\mathrm{CDCl}_{3}\right): \delta 116.6(\mathrm{~d}), 116.8(\mathrm{~d}), 118.8(\mathrm{~s}), 124.4\) (d), \(127.9\) (d), \(131.7\) (d), \(143.4\) (d), \(154.0\) (s), \(160.6\) (s)

Short answer

Compound 2 is o-acetylsalicylaldehyde, an aromatic aldehyde with acetoxy and acetyl groups.

Step-by-step solution

- Analyze the Mass Spectrum

The mass spectrum of compound 2 provides the molecular ion peak at m/z = 146, suggesting a molecular formula close to C8H6O3 (since 146 = 8*12 + 6*1 + 3*16). The different peaks like 118, 90, and 89 can indicate possible fragmentations of the molecule during the analysis.

- Interpret the IR Spectrum

The IR spectrum shows a strong absorption at 1704 cm^-1, which is indicative of a carbonyl (C=O) stretching frequency. This suggests the presence of at least one carbonyl group in the compound.

- Analyze the 1H NMR Spectrum

The 1H NMR spectrum shows signals at δ 6.42 (d, J=9 Hz, 1H), δ 7.23-7.35 (m, 2H), δ 7.46-7.56 (m, 2H), δ 7.72 (d, J=9 Hz, 1H). These signals indicate the presence of aromatic protons in the molecule, with the splitting pattern suggesting a para-substituted benzene ring.

- Analyze the 13C NMR Spectrum

In the 13C NMR spectrum, we observe several signals at δ 116.6 (d), 116.8 (d), 118.8 (s), 124.4 (d), 127.9 (d), 131.7 (d), 143.4 (d), 154.0 (s), and 160.6 (s). These shifts are consistent with a benzene ring with substituents that affect the chemical environment of the carbons.

- Deduce the Structure

Combining the information from the mass spectrum, IR, and NMR data, the structure of compound 2 can be deduced. The mass and NMR suggest a molecule with a benzene ring and substituents such as hydroxy and acetoxy groups, corresponding to o-acetylsalicylaldehyde.

- Write the Mechanism

Given that O-acetylsalicylaldehyde (3) gives only traces of compound 2 in the absence of acetic anhydride, the formation mechanism likely involves acetic anhydride acting as an acetylating agent under the influence of sodium acetate.

Key concepts

Mass Spectrometry

Mass spectrometry is a powerful analytical technique used to determine the molecular weight and structure of a compound. It works by ionizing chemical compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios.
In this exercise, the mass spectrum of compound 2 has a significant peak at m/z = 146, indicating the molecular ion peak. This suggests a molecular formula of C8H6O3. Peaks at 118, 90, and 89 hint at possible fragmentations of the molecule. Understanding these fragments helps in deducing the arrangement of atoms within the molecule.
When analyzing mass spectra, keep these points in mind:

• The molecular ion peak (M) gives the total molecular weight.
• Fragmentation patterns can provide insights into the structure of the molecule.

For example, a peak at 146 (M) minus a common fragment loss (like 28 for CO) could explain a subsequent peak.
By interpreting these peaks correctly, we can infer the structural characteristics of the compound in question.

IR Spectroscopy

Infrared (IR) spectroscopy measures the absorption of infrared light by a molecule, which causes vibrations in its bonds. Each type of bond and functional group in a molecule will absorb IR light at specific frequencies, providing a 'fingerprint' for identifying functional groups.
In the exercise, the IR spectrum shows a strong absorption at 1704 cm^-1. This is characteristic of a carbonyl (C=O) stretching frequency, suggesting the presence of at least one carbonyl group in compound 2.
Key points to remember when analyzing IR spectra:

• Identify major peaks and their corresponding functional groups.
• Match the absorption bands to known frequencies of functional groups (e.g., O-H, N-H, C=O).

Using this information, the presence of certain functional groups can be confirmed or ruled out, assisting in the determination of the overall molecular structure.

NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool for determining the structure of organic compounds. It provides information about the number and environment of hydrogen atoms (1H NMR) and carbon atoms (13C NMR) in a molecule.
In this exercise, the 1H NMR spectrum shows signals at δ 6.42 (d, J=9 Hz, 1H), δ 7.23-7.35 (m, 2H), δ 7.46-7.56 (m, 2H), δ 7.72 (d, J=9 Hz, 1H). This indicates the presence of aromatic protons, likely in a para-substituted benzene ring.
The 13C NMR spectrum displays several signals corresponding to various carbon environments, including peak locations for benzene ring carbons and possibly substituted carbons
NMR analysis involves:

• Identifying the number of signals (indicating unique environments).
• Determining splitting patterns to infer neighboring groups.

By combining this data, you can determine how the carbon and hydrogen atoms are connected in the molecule.

Molecular Structure Determination

Determining the molecular structure involves piecing together data from various spectroscopic techniques like Mass Spectrometry, IR, and NMR.
Using the provided data:

• The mass and NMR data suggest a molecule with a benzene ring and substituents such as hydroxy and acetoxy groups.
• The IR spectrum confirms a carbonyl group.
• The NMR spectra detail the hydrogen and carbon environments around these groups.

Together, this information is used to hypothesize the structural formula of compound 2, which in this case, corresponds to o-acetylsalicylaldehyde.
Molecular structure determination requires a comprehensive understanding of how different spectroscopic data relate to molecular features.

Mechanism of Reaction

Understanding the mechanism of a reaction involves detailing the step-by-step changes that reactants undergo to form products.
In this exercise, salicylaldehyde reacts with acetic anhydride in the presence of sodium acetate to form compound 2. Observing that O-acetylsalicylaldehyde (3) yields only traces of compound 2 in the absence of acetic anhydride suggests a specific role for acetic anhydride in the mechanism.
Mechanistic considerations include:

• Identifying the role of each reactant and catalyst.
• Proposing intermediary steps and species formed during the reaction.
• Using experimental data (like reactivity and yields) to support or refute the proposed mechanism.

In this case, acetic anhydride likely acts as an acetylating agent, modifying salicylaldehyde under the influence of sodium acetate.
By understanding the detailed mechanism, we can predict the outcome of similar reactions and potentially manipulate conditions to improve yields or achieve different products.