Question

A compound of unknown structure gave the following spectroscopic data: Mass spectrum: \(\quad \mathrm{M}^{+}=88.1\) \(\mathrm{IR}:\) \(3600 \mathrm{~cm}^{-1}\) \(\begin{array}{ll}1 \mathrm{H} \mathrm{NMR}: & 1.4 \delta(2 \mathrm{H}, \text { quartet, } J=7 \mathrm{~Hz}) ; 1.2 \delta(6 \mathrm{H}, \text { singlet }) ; \\\ & 1.0 \delta(1 \mathrm{H}, \text { singlet }) ; 0.9 \delta(3 \mathrm{H}, \text { triplet }, J=7 \mathrm{~Hz}) \\ { }^{13} \mathrm{C} \text { NMR: } & 74,35,27,9 \delta\end{array}\) (a) Assuming that the compound contains \(\mathrm{C}\) and \(\mathrm{H}\) but may or may not contain O, give three possible molecular formulas. (b) How many protons (H) does the compound contain? (c) What functional group(s) does the compound contain? (d) How many carbons does the compound contain? (e) What is the molecular formula of the compound? (f) What is the structure of the compound? (g) Assign the peaks in the \({ }^{1} \mathrm{H}\) NMR spectrum of the molecule to specific protons.

Short answer

The compound is likely \( \mathrm{C}_4\mathrm{H}_{10}\mathrm{O} \), a secondary alcohol (e.g., butanol). It contains an -OH group, 4 carbons, and 12 protons.

Step-by-step solution

Analyze the Mass Spectrum

The mass spectrum indicates a molecular ion peak (\( \mathrm{M}^{+} = 88.1 \)), suggesting a molecular weight of roughly 88. This will help narrow down potential molecular formulas.

Deduce Possible Molecular Formulas

Considering possible elements, common choices for such mass are hydrocarbons with potential inclusion of oxygen. Three possible formulas that fit the molar mass are:1. \( \mathrm{C}_5\mathrm{H}_{12}\)2. \( \mathrm{C}_4\mathrm{H}_{8}\mathrm{O} \)3. \( \mathrm{C}_3\mathrm{H}_8\mathrm{O}_2 \)

Determine the Number of Protons

Using the NMR data, the integration values suggest a total of 12 protons from the following peaks: - Quartet: 2 H - Singlet: 6 H - Singlet: 1 H - Triplet: 3 H

Identify Functional Groups from IR Data

The IR spectrum shows a peak at 3600 cm⁻¹, indicative of an -OH group (likely an alcohol). This suggests the presence of oxygen in the molecule.

Count Carbons from NMR Data

The \( {}^{13} \mathrm{C} \) NMR data suggests four distinct carbon environments, indicating the presence of 4 carbon atoms.

Confirm the Molecular Formula

Based on the mass, IR, NMR data, and given options, the formula \( \mathrm{C}_4\mathrm{H}_{10}\mathrm{O} \) is consistent with the evidence. This compound has 4 carbons and 1 oxygen, matching the spectral data.

Propose the Structure of the Compound

Given the -OH group, the structure of the compound is consistent with a secondary alcohol, specifically butanol, as it fits the formula and spectroscopy data.

Assign NMR Peaks to Protons

- 1.4 \( \delta \) (2H, quartet): Methylene protons (\( \mathrm{CH}_2 \)) adjacent to the alcohol group.- 1.2 \( \delta \) (6H, singlet): Two methyl groups (\( \mathrm{CH}_3 \)) not adjacent to the alcohol group.- 1.0 \( \delta \) (1H, singlet): Hydroxyl proton (\( \mathrm{OH} \) group).- 0.9 \( \delta \) (3H, triplet): Methyl protons (\( \mathrm{CH}_3 \)) adjacent to the methylene.

Key concepts

Mass Spectrum Analysis

Mass spectrum analysis is a method used to determine the molecular weight and composition of a compound by measuring the masses of its ionized fragments. When a compound is analyzed using mass spectrometry, it is vaporized and ionized, often by an electron impact, to break it into charged particles (ions). A mass spectrometer sorts these ions based on their mass-to-charge ratio (m/z). The tallest peak on a mass spectrum is known as the "base peak," which represents the most stable ion, while other peaks represent fragments of the molecule.
In this exercise, the molecular ion peak ( \( \mathrm{M}^{+} = 88.1 \) ) suggests that the molar mass of the compound is about 88 g/mol. This information helps narrow down possible molecular formulas for the compound, as seen in solutions like considering hydrocarbons or oxygen-containing compounds. Understanding mass spectrometry helps chemists deduce the elements present and propose possible molecular structures based on known molecular weights.

NMR Spectroscopy

NMR (Nuclear Magnetic Resonance) Spectroscopy is a powerful tool for elucidating the structure of organic compounds. It primarily focuses on the magnetic properties of certain atomic nuclei. When placed in a strong magnetic field, nuclei such as \( ^1\mathrm{H} \) protons absorb electromagnetic radiation at characteristic frequencies.
The \( ^1\mathrm{H} \) NMR spectrum provides information about the different types of hydrogen atoms in a molecule, their environments, and how many there are, based on peak areas (also called integration). The chemical shift, observed in parts per million (ppm), indicates the hydrogen's environment — with shielding or deshielding affecting its resonance.
In our example, we see peaks included: a quartet at 1.4 \( \delta \) ppm for 2 hydrogens, singlets at 1.2 (6 H) and 1.0 (1 H) ppm, and a triplet at 0.9 ppm (3 H). This configuration suggests different types of proton environments, such as methyl \( \mathrm{CH}_3 \) and methylene \( \mathrm{CH}_2 \) groups, providing clues for constructing the compound's structure.

IR Spectroscopy

IR (Infrared) Spectroscopy is used to identify functional groups within a compound by measuring absorption of infrared light, which causes molecular vibrations such as bending or stretching. Each functional group absorbs energy at characteristic frequencies, allowing chemists to determine what groups are present in the molecule.
In this exercise, the notable IR absorption at \( 3600 \mathrm{~cm}^{-1} \) is indicative of an -OH group, a common marker for alcohols. Such strong and broad peaks correspond to O-H stretching, hinting at the presence of hydrogen bonding as in alcohols or phenols. Additionally, IR spectra can indicate other functional groups such as carbonyls (C=O) and nitro (NO2) groups by their unique absorption frequencies.

Molecular Structure Determination

Molecular structure determination is a comprehensive approach combining data from various spectroscopic techniques to deduce the arrangement of atoms within a compound. In our example, by using mass spectra to determine molecular mass and elemental composition, NMR for structural details of carbon and hydrogen frameworks, and IR for identifying functional groups, we can propose a complete molecular formula and structure.
The combination of four methylene and methyl groups, as indicated by the \( ^1\mathrm{H} \) NMR, along with the molecular ion peak from mass spectrometry and the identification of an alcohol group from IR spectroscopy, leads to the determination of a probable structure for the compound, such as butanol. Structure elucidation involves piecing together the insights gained from each technique to provide a comprehensive understanding of the compound's layout.