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Chapter 10: Problem 26

How might you use IR spectroscopy to distinguish among the three isomers but-1-yne, buta-1,3-diene, and but-2-yne?

Short Answer

Expert verified
IR spectra show distinct C鈮 or C=C peaks; use these to distinguish the isomers.

Step by step solution

01

Understanding Isomers

Begin by identifying the three isomers: but-1-yne, buta-1,3-diene, and but-2-yne. But-1-yne is an alkyne with a triple bond at the first carbon. Buta-1,3-diene is a conjugated diene with double bonds at the first and third carbons. But-2-yne is an alkyne with a triple bond at the second carbon.
02

Identifying Unique IR Features

Different functional groups absorb IR light at different characteristic frequencies. Alkynes and dienes exhibit unique features in the IR spectrum. Alkynes have a strong absorption around 2100-2260 cm鈦宦� due to the C鈮 bond, while dienes show peaks due to C=C stretching between 1620-1680 cm鈦宦�.
03

IR Spectrum of But-1-yne

But-1-yne has a terminal alkyne group. Its IR spectrum would show a sharp peak near 2100-2260 cm鈦宦� for the C鈮 stretch. Additionally, a terminal alkyne would have a hydrogen bonded to the sp-carbon, resulting in a C-H stretch around 3300 cm鈦宦�.
04

IR Spectrum of Buta-1,3-diene

Buta-1,3-diene, being a conjugated diene, will show peaks around 1620-1680 cm鈦宦� for the C=C stretching vibrations. This is distinct from the alkyne C鈮 stretch seen in the other isomers.
05

IR Spectrum of But-2-yne

But-2-yne is an internal alkyne, so it shows a peak for the C鈮 stretch around 2100-2260 cm鈦宦�. Unlike but-1-yne, it lacks the C-H stretch near 3300 cm鈦宦� because it does not have a terminal hydrogen on the alkyne.
06

Comparison of IR Spectra

By comparing the IR spectra of these compounds, one can distinguish them based on the presence and position of the C鈮 and C=C peaks and the presence or absence of the terminal alkyne hydrogen peak.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Alkyne Functional Groups
Alkynes are hydrocarbons that contain carbon-carbon triple bonds, denoted as C鈮. These functional groups are significant in organic chemistry due to their unique chemical properties and reactivity. In IR spectroscopy, alkynes exhibit distinct absorptions associated with their triple bonds. The C鈮 stretching vibration typically appears as a strong and sharp peak between 2100 cm鈦宦� and 2260 cm鈦宦�.

Alkynes can be categorized into terminal and internal alkynes based on the positioning of the triple bond:
  • Terminal Alkynes: Have the C鈮 bond at the end of the molecule. These show an additional IR absorption around 3300 cm鈦宦� due to the hydrogen atom attached to the sp-hybridized carbon.
  • Internal Alkynes: Feature the C鈮 bond within the carbon chain, typically display the C鈮 stretch but lack the terminal hydrogen C-H stretching peaks.
Understanding these distinctions is crucial for interpreting IR spectra and identifying alkyne-containing compounds.
Conjugated Dienes
Conjugated dienes consist of two double bonds separated by a single bond, leveraging a system of conjugation that enables electron delocalization. This structure influences the energy levels within the molecule, thus altering its absorption characteristics in the IR spectrum.

In the case of conjugated dienes like buta-1,3-diene, the IR spectrum reveals peaks for C=C stretching vibrations around 1620 cm鈦宦� to 1680 cm鈦宦�. These absorptions are characteristic of the influence of conjugation, which lowers the typical C=C stretching frequency found in simple alkenes.
  • Conjugation stabilizes the molecule, leading to unique reactivity and spectral properties.
  • Electrons can move more freely across the conjugated system, affecting the IR absorption features.
Recognizing the IR signature of conjugated dienes aids in identifying and studying these compounds.
C鈮 Bond Stretching
The C鈮 bond stretching is a feature in IR spectroscopy that directly relates to alkynes. It is marked by an absorption band often found between 2100 cm鈦宦� and 2260 cm鈦宦�. This range allows us to identify the presence of a triple bond in an organic molecule.

Key points about C鈮 bond stretching include:
  • The triple bond involves sp-hybridized carbon atoms, contributing to the unique position of this absorption in the IR spectrum.
  • It is usually more intense and narrower compared to other types of carbon-carbon bonds, making it a distinctive marker.
By examining the C鈮 stretching vibration in the IR spectrum, chemists can confirm the presence of alkynes, as seen when analyzing compounds like but-1-yne and but-2-yne.
C=C Bond Stretching
C=C bond stretching is significant for identifying alkenes and conjugated dienes in IR spectroscopy. It manifests in a different region compared to single or triple bonds. The typical stretch for an isolated C=C bond is between 1640 cm鈦宦� and 1680 cm鈦宦�.

For conjugated dienes, like buta-1,3-diene, the C=C bond stretching appears slightly lower due to the effect of conjugation, ranging from 1620 cm鈦宦� to 1680 cm鈦宦�. Key aspects include:
  • Conjugation typically lowers the stretching frequency of the C=C bond.
  • The IR absorption band is often medium to strong in intensity.
Recognizing C=C bond stretching patterns helps distinguish compounds within the alkene family, enabling precise analysis of different organic molecules.
Terminal Alkyne Hydrogen
Terminal alkynes, in addition to the signature C鈮 stretch, have a characteristic IR absorption due to the C-H bond at about 3300 cm鈦宦�. This peak is associated with the hydrogen bonded directly to the sp-hybridized carbon at the end of the alkyne.

This absorption is distinct and noticeable because:
  • The C-H stretch for terminal alkynes is sharp and prominent.
  • It confirms the presence of a hydrogen atom on the terminal carbon of the C鈮 group.
Identifying this feature is crucial for differentiating between terminal and internal alkynes, as seen in but-1-yne versus but-2-yne, which lacks this frequency due to its internal C鈮 bond placement.

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Most popular questions from this chapter

4-Methylpentan-2-one and 3-methylpentanal are isomers. Explain how you could tell them apart, both by mass spectrometry and by infrared spectroscopy.

Benzene has an ultraviolet absorption at \(\lambda_{\max }=204 \mathrm{~nm}\), and \(p\) -toluidine has \(\lambda_{\max }=235 \mathrm{~nm}\). How do you account for this difference?

Halogenated compounds are particularly easy to identify by their mass spectra because both chlorine and bromine occur naturally as mixtures of two abundant isotopes. Chlorine occurs as \({ }^{35} \mathrm{Cl}(75.8 \%)\) and \({ }^{37} \mathrm{Cl}(24.2 \%)\); bromine occurs as \({ }^{79} \mathrm{Br}(50.7 \%)\) and \({ }^{81} \mathrm{Br}(49.3 \%)\). At what masses do the molecular ions occur for the following formulas? What are the relative percentages of each molecular ion? (a) Bromomethane, \(\mathrm{CH}_{3} \mathrm{Br}\) (b) 1-Chlorohexane, \(\mathrm{C}_{6} \mathrm{H}_{13} \mathrm{Cl}\)

Propose structures for compounds that meet the following descriptions: (a) \(\mathrm{C}_{5} \mathrm{H}_{8}\), with IR absorptions at 3300 and \(2150 \mathrm{~cm}^{-1}\) (b) \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}\), with a strong IR absorption at \(3400 \mathrm{~cm}^{-1}\) (c) \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}\), with a strong IR absorption at \(1715 \mathrm{~cm}^{-1}\) (d) \(\mathrm{C}_{8} \mathrm{H}_{10}\), with IR absorptions at 1600 and \(1500 \mathrm{~cm}^{-1}\)

Propose structures for compounds that meet the following descriptions: (a) An optically active compound \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}\) with an IR absorption at \(1730 \mathrm{~cm}^{-1}\) (b) An optically inactive compound \(\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{~N}\) with an IR absorption at \(2215 \mathrm{~cm}^{-1}\)
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