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Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoBruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblema 64
Capitolo 13, Problema 64

How can IR spectroscopy distinguish between 1-hexyne, 2-hexyne, and 3-hexyne?

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Identify the functional group of interest in the molecules: All three compounds (1-hexyne, 2-hexyne, and 3-hexyne) are alkynes, which contain a carbon-carbon triple bond (C≡C). IR spectroscopy can be used to detect the presence and location of this bond.
Understand the key IR absorption regions for alkynes: The C≡C triple bond typically absorbs in the range of 2100–2260 cm⁻¹. Additionally, terminal alkynes (like 1-hexyne) have a characteristic ≡C-H stretch around 3300 cm⁻¹, which is absent in internal alkynes (like 2-hexyne and 3-hexyne).
Analyze 1-hexyne: Since 1-hexyne is a terminal alkyne, it will show both the C≡C stretch in the 2100–2260 cm⁻¹ region and the ≡C-H stretch around 3300 cm⁻¹. This distinguishes it from the other two compounds.
Analyze 2-hexyne and 3-hexyne: Both are internal alkynes, so they will show the C≡C stretch in the 2100–2260 cm⁻¹ region but will lack the ≡C-H stretch around 3300 cm⁻¹. To differentiate between these two, additional structural analysis (e.g., NMR spectroscopy) may be required, as IR alone cannot distinguish between different internal alkyne positions.
Summarize the distinguishing features: IR spectroscopy can differentiate 1-hexyne from 2-hexyne and 3-hexyne based on the presence of the ≡C-H stretch at 3300 cm⁻¹. However, it cannot distinguish between 2-hexyne and 3-hexyne, as both are internal alkynes with similar IR spectra in the relevant regions.

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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 absorb characteristic frequencies of IR radiation, leading to distinct peaks in the spectrum. This allows chemists to determine the presence of specific functional groups and structural features in molecules.
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General Features of IR Spect

Alkyne Structure and Isomerism

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. The position of this triple bond can vary, leading to structural isomers such as 1-hexyne, 2-hexyne, and 3-hexyne. Each isomer has a unique arrangement of atoms, which affects its IR absorption characteristics, making it possible to distinguish between them using IR spectroscopy.
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Alkyne Hydration

Characteristic IR Absorption Peaks

Each type of bond in a molecule has a specific range of IR absorption frequencies. For alkynes, the C≡C triple bond typically shows a strong absorption around 2100-2260 cm⁻¹. Additionally, the position of the triple bond in 1-hexyne, 2-hexyne, and 3-hexyne results in different peak patterns and intensities, allowing for their differentiation in an IR spectrum.
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Major IR absorptions
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The IR spectrum of compound A with a molecular formula of C5H12O is shown below. Compound A is oxidized to give compound B, a ketone with a molecular formula of C5H10O. When compound A is heated with H2SO4, compounds C and D are obtained. Considerably more D is obtained than C. Reaction of compound C with O3, followed by treatment with dimethyl sulfide, gives two products: formaldehyde and compound E, with a molecular formula of C4H8O. Reaction of compound D with O3, followed by treatment with dimethyl sulfide, gives two products: compound F, with a molecular formula of C3H6O, and compound G, with a molecular formula of C2H4O. What are the structures of compounds A through G?

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