Question

\- Propose a structure consistent with the following spectral data for a compound \(\mathrm{C}_{8} \mathrm{H}_{18} \mathrm{O}_{2}\) : \(\mathrm{IR}: \quad 3350 \mathrm{~cm}^{-1}\) 1H NMR: \(1.24 \delta(12 \mathrm{H}\), singlet \() ; 1.56 \delta(4 \mathrm{H}\), singlet \() ; 1.95 \delta\) (2 \(\mathrm{H}\), singlet )

Short answer

The compound is likely 2,2,4,4-tetramethylpentane-1,5-diol.

Step-by-step solution

Consider the molecular formula

The compound has the molecular formula \(\mathrm{C}_{8} \mathrm{H}_{18} \mathrm{O}_{2}\). This suggests a potential presence of alcohol or ether groups due to the two oxygen atoms. The high number of hydrogens (18) indicates a saturated or highly substituted system.

Interpret IR data

The IR peak at 3350 \(\text{cm}^{-1}\) typically indicates an \(\text{-OH}\) (hydroxyl) group, which confirms the presence of alcohol groups in the molecule. Since there are two oxygen atoms, they could be part of two separate \(\text{-OH}\) groups.

Analyze 1H NMR Data

The NMR shows three signals: a 12H singlet at 1.24 \(\delta\), a 4H singlet at 1.56 \(\delta\), and a 2H singlet at 1.95 \(\delta\). The absence of splitting suggests that these protons are in symmetrical environments, leading us to consider symmetrical alkyl groups.

Construct a Possible Structure

One possible structure that fits the data is 2,2,4,4-tetramethylpentane-1,5-diol. This structure accounts for the 12 hydrogens in the four \(\text{-CH}_3\) groups (singlet at 1.24 \(\delta\)), the 4 hydrogens in the two \(\text{-CH}_2\) groups (singlet at 1.56 \(\delta\)), and the 2 hydrogens in the two \(\text{-OH}\) groups (singlet at 1.95 \(\delta\)). The \(\text{-OH}\) groups create the IR peak at 3350 \(\text{cm}^{-1}\).

Key concepts

Molecular Formula Interpretation

When interpreting a molecular formula such as \( \mathrm{C}_8 \mathrm{H}_{18} \mathrm{O}_2 \), several key aspects need to be considered. Firstly, the number and types of atoms provide clues about the possible functional groups. The presence of two oxygen atoms suggests functionalities like hydroxyl (\( \text{-OH} \)) groups or ether linkages, as both of these contain oxygen. Furthermore, with eighteen hydrogen atoms, it's indicative of either a saturated compound or a highly branched structure. Recognizing these features early helps to streamline the process of structural determination.

IR Spectroscopy

Infrared (IR) spectroscopy is a powerful tool for identifying functional groups within a molecule. In our compound, the IR spectrum shows a significant peak at \( 3350 \, \text{cm}^{-1} \). This peak is characteristic of an \( \text{-OH} \) stretch, which is indicative of an alcohol group presence. This observation is consistent with the molecular formula, which has adequate oxygen atoms to support the idea of two separate \( \text{-OH} \) groups. The frequency of this peak aligns well with typical alcohol vibrations, reinforcing this assumption.

NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy provides detailed insights into the environment of hydrogen atoms within a molecule. The NMR data show three distinct signals: a 12 hydrogen singlet at \( 1.24 \, \delta \), a 4 hydrogen singlet at \( 1.56 \, \delta \), and a 2 hydrogen singlet at \( 1.95 \, \delta \). The singlet nature of these peaks indicates an absence of neighboring protons, suggesting highly symmetric environments.

• The 12H singlet likely points to a set of four \( \text{-CH}_3 \) groups in a symmetric setting, accounting for the large integral value.
• The 4H singlet can be attributed to two \( \text{-CH}_2 \) groups, again indicating a symmetric configuration.
• The 2H singlet is consistent with two hydroxyl groups, further supporting the IR findings.

This pattern of signals aligns with a molecule that is significantly branched and has symmetrical repetitive units.

Structural Proposal

Using the spectral data, a plausible structure can be proposed for the compound \( \mathrm{C}_8 \mathrm{H}_{18} \mathrm{O}_2 \). A structure that fits well with all the data points is 2,2,4,4-tetramethylpentane-1,5-diol. In this structure:

• The 12 hydrogens are from four \( \text{-CH}_3 \) groups, explaining the singlet at \( 1.24 \, \delta \).
• The 4 hydrogens correspond to two \( \text{-CH}_2 \) groups, explaining the singlet at \( 1.56 \, \delta \).
• The 2 hydrogens from two separate \( \text{-OH} \) groups account for the singlet at \( 1.95 \, \delta \).
• The presence of the \( \text{-OH} \) groups explains the IR peak at \( 3350 \, \text{cm}^{-1} \).

This structural proposal integrates all aspects of the spectral data into a cohesive model that is in agreement with the given molecular formula.