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: \(3400 \mathrm{~cm}^{-1} ; 1640 \mathrm{~cm}^{-1}\) \({ }^{1} \mathrm{H}\) NMR: \(1.63 \delta\) ( \(3 \mathrm{H}\), 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

(a) Alcohol (O-H group). (b) Methyls, methylene, alcohol proton, alkene proton. (c) 3-Methyl-3-buten-1-ol.

Step-by-step solution

Analyze the IR Spectrum

The IR spectrum shows peaks at 3400 cm\(^{-1}\) and 1640 cm\(^{-1}\). A peak at 3400 cm\(^{-1}\) usually indicates the presence of an O-H group, suggesting an alcohol. The peak at 1640 cm\(^{-1}\) often indicates a C=C double bond.

Analyze the NMR Spectrum

The \(^1\text{H}\) NMR spectrum shows different types of proton signals:- A singlet at 1.63 ppm and a singlet at 1.70 ppm, each integrating to 3 protons, suggest the presence of two methyl groups (likely near a carbon with no protons that affect splitting).- A broad singlet at 3.83 ppm, 1 proton, supports the presence of an alcohol (-OH group).- A doublet at 4.15 ppm (2 protons, \(J=7\) Hz) indicates a methylene group adjacent to a carbon with a single hydrogen (CH).- A triplet at 5.70 ppm is a single proton, suggesting an alkene proton (\(=CH-\)) adjacent to the methylene group.

Deduce the Structure

Combine the information:- The IR indicates an alcohol group and possibly a C=C bond.- The NMR data suggests two methyl groups, an alcohol group, a methylene group adjacent to a CH, and an alkene.Thus, the structure must accommodate an alcohol and an alkene, likely being 3-methyl-3-buten-1-ol. This fits all spectral data with a structure of:``` H H \ / C=C / \ H CH3HO-CH2-CH-C-CH3 \ CH3```This structure matches the molecular formula C\(_5\)H\(_{10}\)O and the spectroscopic data.

Key concepts

The Basics of IR Spectroscopy

Infrared (IR) spectroscopy is a powerful tool used in organic chemistry to determine the presence of different functional groups in a compound. It works by measuring the vibration frequencies of bonds within molecules. These vibrations appear as peaks on the IR spectrum.

In the IR spectrum of Compound A, we observe two significant peaks:

• A peak at 3400 cm^-1, indicative of an O-H stretching vibration, commonly associated with alcohol functional groups.
• A peak at 1640 cm^-1, which typically signifies a C=C double bond, often found in alkenes.

By identifying these key peaks, IR spectroscopy allows us to infer the presence of specific bonds and, hence, suggest possible functional groups in the compound.

Understanding NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy provides detailed information about the molecular structure of a compound, specifically the environments of hydrogen atoms. In this technique, signals are observed at chemical shifts (measured in ppm), which reveal the electronic environment around different types of protons.

For Compound A, the ^1H NMR spectrum shows:

• Singlets at 1.63 ppm and 1.70 ppm, each indicating the presence of 3 protons, typical of isolated methyl groups.
• A broad singlet at 3.83 ppm, representing a single proton, often pointing to a hydrogen atom in an alcohol group (O-H).
• A doublet at 4.15 ppm, consisting of 2 protons with a coupling constant of 7 Hz, suggesting a methylene group adjacent to a single hydrogen atom.
• A triplet at 5.70 ppm, indicating a single proton likely to be a part of an alkene moiety.

This spectral data helps us decipher how the hydrogen atoms are connected within the molecular framework of the compound.

Decoding Functional Groups

Functional groups are specific groups of atoms within molecules that determine the characteristic chemical reactions of those molecules. Identifying them is crucial for deducing the structure and properties of a compound.

From the IR and NMR data of Compound A:

• We identify an alcohol functional group from the O-H peak in the IR and the broad singlet in NMR.
• The C=C peak in the IR and the NMR triplet suggest an alkene functional group.
• The presence of methyl and methylene signals in the NMR spectrum further supports the overall structural arrangement.

The combination of these functional groups gives insight into not only the compound's structural makeup, but also its potential reactivity and behavior in chemical reactions.

Linking Molecular Structure

Molecular structure describes the arrangement of atoms within a molecule, determining its shape and physical properties. By integrating data from IR and NMR spectroscopy, we can construct a plausible structure of the compound.

For Compound A, molecular formula \(C_5H_{10}O\):

• The alcohol group information aligns with the inferred presence of an O-H group, suggesting part of the structure includes 3-methyl-3-buten-1-ol.
• Alkene characteristics deduced from both IR and NMR help propose the presence of a double bond within the structure.
• The observed methyl and methylene groups reflect in the proposed architecture.

The outcome is a predicted and reasonable molecular structure that matches both the spectroscopic signatures and the molecular formula.