Textbook Question
a. How could you use IR spectroscopy to determine whether the following reaction had occurred?
b. After purifying the product, how could you determine whether all the NH2NH2 had been removed?
15. Analytical Techniques:IR, NMR, Mass Spect
Infrared Spectroscopy Table
2:36 minutesProblem 29e,f
Textbook Question
How can IR spectroscopy be used to distinguish between the following compounds?
e. cyclohexene and cyclohexane
f. a primary amine and a tertiary amine
Verified step by step guidance1
Step 1: Understand the functional groups present in each compound. Cyclohexene contains a carbon-carbon double bond (C=C), while cyclohexane is a saturated hydrocarbon with only single bonds. A primary amine contains an -NH2 group, while a tertiary amine has no N-H bonds, as the nitrogen is bonded to three carbon atoms.
Step 2: Identify the characteristic IR absorption regions for the functional groups. For cyclohexene, the C=C bond typically absorbs in the range of 1600-1680 cm⁻¹. Cyclohexane, lacking a C=C bond, will not show this absorption. For primary amines, the N-H stretching vibrations appear as two peaks in the range of 3300-3500 cm⁻¹ due to symmetric and asymmetric stretching. Tertiary amines, lacking N-H bonds, will not show these peaks.
Step 3: Analyze the IR spectrum of cyclohexene and cyclohexane. Look for the presence of a peak in the 1600-1680 cm⁻¹ range for cyclohexene, which indicates the C=C bond. Cyclohexane will not have this peak, but both compounds will show C-H stretching vibrations in the range of 2800-3000 cm⁻¹.
Step 4: Analyze the IR spectrum of the primary amine and tertiary amine. For the primary amine, confirm the presence of two distinct N-H stretching peaks in the 3300-3500 cm⁻¹ range. For the tertiary amine, confirm the absence of these N-H stretching peaks, as it lacks N-H bonds.
Step 5: Use the absence or presence of these characteristic peaks to distinguish between the compounds. Cyclohexene and cyclohexane can be differentiated by the C=C absorption, while primary and tertiary amines can be distinguished by the N-H stretching peaks.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Infrared (IR) Spectroscopy
IR spectroscopy is an analytical technique used to identify functional groups in organic compounds by measuring the absorption of infrared light. Different bonds and functional groups absorb characteristic wavelengths of IR radiation, resulting in a spectrum that can be analyzed to determine the molecular structure of a compound.
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General Features of IR Spect
Functional Group Identification
Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In the context of IR spectroscopy, the presence of certain functional groups, such as alkenes or amines, can be identified by their unique absorption peaks in the IR spectrum, allowing for differentiation between compounds.
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Comparison of Spectra
To distinguish between compounds using IR spectroscopy, one must compare the IR spectra of the substances in question. For example, cyclohexene will show a distinct C=C stretching vibration absent in cyclohexane, while primary and tertiary amines can be differentiated by the presence of N-H stretching vibrations, which appear differently based on the number of hydrogen atoms attached to the nitrogen.
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