Question

Compound A, \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}\), is one of the basic building blocks of nature. All steroids and many other naturally occurring compounds are built from compound A. Spectroscopic analysis of A yields the following information: IR: \(\quad 3400 \mathrm{~cm}^{-1} ; 1640 \mathrm{~cm}^{-1}\) \({ }^{1} \mathrm{H}\) NMR: \(\quad 1.63 \delta(3 \mathrm{H},\) singlet \() ; 1.70 \delta(3 \mathrm{H},\) singlet \()\) \(3.83 \delta(1 \mathrm{H},\) broad singlet \() ; 4.15 \delta(2 \mathrm{H},\) doublet, \(J=7 \mathrm{~Hz})\) \(5.70 \delta(1 \mathrm{H},\) triplet, \(J=7 \mathrm{~Hz})\) (a) From the IR spectrum, what is the nature of the oxygen-containing functional group? (b) What kinds of protons are responsible for the NMR absorptions listed? (c) Propose a structure for A.

Short answer

Compound A is isoprenol (3-methyl-2-buten-1-ol).

Step-by-step solution

Analyze the IR Spectrum

The IR spectrum shows a peak at 3400 cm^{-1} indicating the presence of an -OH (alcohol) group, which typically shows a broad absorption around this region. There is also a peak at 1640 cm^{-1}, which is characteristic of a C=C (alkene) bond, suggesting the compound contains both an alcohol and an alkene functional group.

Analyze the Proton NMR Spectrum for Singlet Peaks

The NMR spectrum has two singlet peaks at 1.63 δ (3H) and 1.70 δ (3H), each likely indicating the presence of methyl groups attached to non-proton-bearing carbons or possibly at two ends of a double bond (vinyl groups).

Analyze the Proton NMR Spectrum for Broad Singlet and Doublet

There is a broad singlet at 3.83 δ (1H), which suggests the presence of an OH proton that often appears as a singlet due to exchange effects. The doublet at 4.15 δ (2H, J=7 Hz) indicates two protons on a carbon adjacent to a double bond (likely -CH2 connected to the vinyl group). The coupling constant (J=7 Hz) suggests vicinal coupling, possibly with the vinyl hydrogen.

Analyze the Proton NMR Spectrum for Triplet Peak

The triplet at 5.70 δ (1H, J=7 Hz) must be from a vinyl hydrogen (involved in the alkene bond detected in IR), which is splitting with no more than two hydrogens (consistent with the previously analyzed doublet).

Propose the Structure for Compound A

Considering all spectral data, IR indicating an alcohol and alkene, NMR indicating two singlet methyl groups, an OH group, -CH2 adjacent to a double bond, and a vinyl hydrogen: A plausible structure is isoprenol (3-methyl-2-buten-1-ol). The structure contains an allylic alcohol with structural formula C5H10O.

Key concepts

IR Spectroscopy

IR spectroscopy, or infrared spectroscopy, is a powerful technique used in organic chemistry to identify functional groups within a compound. This method measures the absorption of infrared radiation by the compound, resulting in a spectrum that displays distinct peaks. Each peak corresponds to a particular bond or functional group within the molecule. In the case of Compound A, an IR spectrum with a peak at 3400 cm⁻¹ suggests the presence of an -OH group, which is typical for alcohols, as they exhibit broad absorptions in this region.
Another significant peak in the IR spectrum of Compound A appears at 1640 cm⁻¹. This is characteristic of a carbon-carbon double bond (C=C), commonly found in alkenes. By analyzing these peaks, we can deduce that Compound A likely possesses both an alcohol group and an alkene bond. This dual presence helps narrow down the potential structure of the compound significantly.

Proton NMR

Proton Nuclear Magnetic Resonance (NMR) spectroscopy is another crucial tool for structural analysis in organic chemistry. It provides insights into the types of hydrogen environments present in a molecule. When analyzing the NMR data for Compound A, several chemical shifts and splitting patterns give us clues.
The spectrum reveals three distinct types of peaks: singlets, a broad singlet, and a doublet. Singlet peaks at 1.63 δ and 1.70 δ, each integrating to 3 hydrogens, suggest the presence of methyl groups. These groups likely attach to carbons without nearby protons, possibly creating vinyl groups.
The broad singlet at 3.83 δ (1H) raises the likelihood of it being an -OH proton, which often appears as a singlet in NMR due to hydrogen bonding effects causing chemical shift exchange. Further, the doublet at 4.15 δ (2H, J=7 Hz) suggests a -CH₂ group adjacent to a functional group, typically an alkene. This is supported by the coupling pattern, reinforcing our understanding of the molecule's structure.

Functional Groups Identification

Identifying functional groups is a core aspect of organic chemistry analysis. The IR and NMR spectroscopy methods provide complementary information for this purpose. In IR spectroscopy, specific frequency absorptions indicate particular groups, such as -OH and C=C, which we identified in Compound A based on peaks at 3400 cm⁻¹ and 1640 cm⁻¹, respectively.
Meanwhile, proton NMR helps further confirm these groups and their contexts within the molecule. Methyl protons exhibit singlet peaks if attached to non-hydrogen-bearing carbons as seen at chemical shifts of 1.63 δ and 1.70 δ in our analysis. Additionally, the broad singlet at 3.83 δ, aligning with OH protons, strengthens the alcohol identification from IR analysis.

• Methyl group - confirmed by singlet in NMR
• OH group - indicated by broad singlet in NMR, corroborated by IR
• Alkene bond - reinforced by IR and supported by NMR peaks

These insights piece together the puzzle of determining the precise structure of Compound A.

Chemical Structure Analysis

The ultimate goal of using spectroscopic methods like IR and NMR is to determine the chemical structure of a compound. By integrating knowledge gained from both approaches, we can finalize the structure of Compound A.
The IR spectrum's indication of both alcohol and alkene groups, coupled with the NMR data's singlets and splitting patterns, suggests that Compound A could be isoprenol, also known as 3-methyl-2-buten-1-ol. This proposed structure aligns with the empirical formula, C₅H₁₀O, and matches all the spectroscopic markers:

• Presence of a functional -OH group
• A carbon-carbon double bond indicating a vinyl group
• Methyl groups validated through NMR singlets
• A CH₂ group confirmed by the NMR doublet

Thus, by linking the information from spectroscopy, the chemical structure analysis of Compound A becomes clear. The collective data decisively points to its identification as isoprenol.