Textbook Question
Identify each of the following compounds from its molecular formula and its IR and 1H NMR spectra:
b. C6H12O2
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15. Analytical Techniques:IR, NMR, Mass Spect
Structure Determination without Mass Spect
6:41 minutesProblem 63d
Textbook Question
Identify each of the following compounds from its molecular formula and its IR and 1H NMR spectra.
d. C4H8O2
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Verified step by step guidance1
Examine the molecular formula C4H8O2. This indicates the compound has 4 carbons, 8 hydrogens, and 2 oxygens. The degree of unsaturation can be calculated using the formula: (2C + 2 - H)/2. For C4H8O2, the degree of unsaturation is (2(4) + 2 - 8)/2 = 1, suggesting one double bond or ring in the structure.
Analyze the IR spectrum. The broad absorption around 3200-3600 cm⁻¹ indicates the presence of an O-H group, likely from an alcohol or carboxylic acid. The sharp peak near 1700 cm⁻¹ corresponds to a C=O stretch, suggesting the presence of a carbonyl group. Together, these features suggest a carboxylic acid functional group.
Interpret the 1H NMR spectrum. The peak at δ ≈ 2.0 ppm corresponds to hydrogens adjacent to a carbonyl group (likely CH3-C=O). The peaks around δ ≈ 1.0 ppm suggest hydrogens in a methyl group (CH3). The peak at δ ≈ 4.0 ppm indicates hydrogens attached to a carbon bonded to an oxygen atom (likely CH-O).
Combine the IR and NMR data. The IR spectrum suggests a carboxylic acid functional group, while the NMR spectrum indicates the presence of methyl and methylene groups. This aligns with a structure containing a carboxylic acid and alkyl groups.
Propose the structure. Based on the molecular formula, IR, and NMR data, the compound is likely butanoic acid (CH3-CH2-CH2-COOH), which satisfies the molecular formula C4H8O2 and matches the spectral data.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Molecular Formula Interpretation
The molecular formula C4H8O2 indicates that the compound contains four carbon atoms, eight hydrogen atoms, and two oxygen atoms. This information is crucial for determining the possible structural isomers and functional groups present in the compound, which can guide the analysis of its IR and NMR spectra.
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Infrared (IR) Spectroscopy
IR spectroscopy is a technique used to identify functional groups in a molecule based on the absorption of infrared light at specific wavelengths. Key peaks in the IR spectrum for C4H8O2 may indicate the presence of hydroxyl (–OH) or carbonyl (C=O) groups, which are essential for deducing the compound's structure.
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Proton Nuclear Magnetic Resonance (1H NMR) Spectroscopy
1H NMR spectroscopy provides information about the hydrogen atoms in a molecule, including their environment and connectivity. The chemical shifts and splitting patterns observed in the NMR spectrum for C4H8O2 can reveal the number of unique hydrogen environments and help confirm the presence of specific functional groups, aiding in the structural identification of the compound.
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