Question

The red fox (Vulpes vulpes) uses a chemical communication system based on scent marks in urine. One component of fox urine is a sulfide. Mass spectral analysis of the pure scent-mark component shows \(\mathrm{M}^{+}=116,\) IR spectroscopy shows an intense band at \(890 \mathrm{~cm}^{-1},\) and 1H NMR spectroscopy reveals the following peaks. Propose a structure for the molecule. [Note: \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{~S}\) absorbs at \(\left.2.1 \mathrm{\delta} .\right]\) \(1.74 \delta(3 \mathrm{H},\) singlet \() ; 2.11 \delta(3 \mathrm{H},\) singlet \() ; 2.27 \delta(2 \mathrm{H},\) triplet \(, J=4.2 \mathrm{~Hz})\) \(2.57 \delta(2 \mathrm{H},\) triplet \(, J=4.2 \mathrm{~Hz}) ; 4.73 \delta(2 \mathrm{H},\) broad \()\)

Short answer

The molecule is dimethyl disulfide (CH₃-S-(CH₂)₂-S-CH₃).

Step-by-step solution

Determine Molecular Formula from Mass Spectrum

The mass spectrometry data shows an M⁺ peak at 116, indicating the molecular mass of the compound is 116. This is our starting point for determining the molecular formula.

Interpret the IR Spectroscopy

The IR spectrum shows an intense band at 890 cm⁻¹, which is characteristic of C-S bond stretching, supporting the presence of sulfur in the molecule (as expected because it's a sulfide).

Analyze the 1H NMR spectrum

The NMR spectrum provides several important clues: - A singlet at 1.74 ppm indicating a methyl group, likely bonded to a non-carbon atom. - A singlet at 2.11 ppm akin to the signal for dimethyl sulfide ((CH₃)₂S), suggesting the methyls are bonded to sulfur. - Triplet signals at 2.27 ppm and 2.57 ppm (J = 4.2 Hz) suggesting two sets of -CH₂- groups that are part of an ethyl chain. - A broad singlet at 4.73 ppm likely indicates a hydrogen bonded to a sulfur.

Assemble the Structure

Given these details: - The direct bonding of CH₃ groups to sulfur. - The presence of an ethyl (-CH₂-CH₂-) structure. - The sulfur bonded to the hydrogen seen in the IR band and a broad signal in NMR. The structure is likely CH₃-S-(CH₂)₂-S-CH₃, known as dimethyl disulfide.

Key concepts

Mass Spectrometry

Mass spectrometry is a powerful analytical technique used to determine the mass of molecules. This is particularly helpful for understanding molecular structures and compositions. In the exercise, the mass spectrometry data showed an M⁺ peak at 116, indicating the molecular weight of the compound was 116 atomic mass units (amu). This initial step is crucial as it establishes the molecular mass and provides a foundation for hypothesizing the molecular formula.

To break it down further for clarity:

• The M⁺ peak reflects the mass of the molecule in its ionized form.
• By identifying this peak, we confirm not just the mass but infer potential components within the molecule.
• This particular mass suggests that we're dealing with a small organic molecule, characteristic of sulfur-containing compounds like sulfides.

By understanding this mass spectrometry analysis, one can effectively infer crucial information about the molecular structure that complements the findings from other spectroscopic techniques.

IR Spectroscopy

IR (Infrared) spectroscopy is a technique that provides information on the types of chemical bonds present in a molecule. It does so by measuring the absorption of infrared light, which causes molecular vibrations. In this case, the IR spectrum showed an intense band at 890 cm⁻¹, a key indicator of a C-S (carbon-sulfur) bond within the molecule, which supports the presence of a sulfide.

This is what you need to note about IR spectroscopy:

• Each bond absorbs infrared light at a unique frequency, creating a distinctive pattern.
• The absorption band at 890 cm⁻¹ is specifically indicative of C-S stretching.
• Recognizing this signal helps us confirm sulfur's involvement, a significant clue given the context of the exercise.

Thus, IR spectroscopy acts as a confirmatory test post-mass spectrometry, ensuring sulfur is indeed a constituent of the compound being analyzed.

1H NMR Spectroscopy

1H NMR (nuclear magnetic resonance) spectroscopy is used to explore the environment of hydrogen atoms within a molecule. It provides insights into how these atoms are arranged and what atoms are nearby, thus playing a crucial role in structure elucidation. In this exercise, the 1H NMR data reveals several significant peaks.

Let's delve into the breakdown:

• A singlet at 1.74 ppm points to a methyl group not neighboring any other hydrogens, suggesting it's attached to a non-carbon element, likely sulfur.
• A singlet at 2.11 ppm is similar to that in dimethyl sulfide ((CH₃)₂S), reaffirming the methyl groups are indeed bonded to sulfur.
• Two triplets at 2.27 ppm and 2.57 ppm (split by a coupling constant J = 4.2 Hz) signify adjacent methylene groups (–CH₃-CH₂-CH₂-CH₃–), characteristic of an ethyl chain.
• A broad singlet at 4.73 ppm indicates a hydrogen associated with sulfur, possibly due to proximity to a sulfur bridge.

By analyzing this NMR data, we conclude a structure containing terminal methyl groups and an internal ethyl group, leading us to propose dimethyl disulfide as the molecular structure, corroborating what was suggested by mass spectrometry and IR spectroscopy insights.