Question

Compound \(A, M W=86,\) shows an IR absorption at \(1730 \mathrm{~cm}^{-1}\) and a very simple \({ }^{1} \mathrm{H}\) NMR spectrum with peaks at \(9.78(1 \mathrm{H},\) singlet \()\) and 1.2 \(\delta(9 \mathrm{H},\) singlet \() .\) Propose a structure for \(\mathrm{A}\)

Short answer

The structure of compound A is tert-butyraldehyde (2,2-dimethylpropanal).

Step-by-step solution

Understand the Given Data

We have a compound with a molecular weight (MW) of 86. The IR absorption at 1730 cm⁻¹ suggests the presence of a carbonyl group (C=O). The NMR spectrum shows two signals: one at 9.78 ppm (a singlet for 1H) and another at 1.2 ppm (a singlet for 9H). These NMR signals will help identify the structure of compound A.

Analyze the NMR Spectrum

The signal at 9.78 ppm indicates a deshielded proton, often associated with an aldehyde proton since they appear in the regions between 9-10 ppm in a H NMR spectrum. The 1.2 ppm singlet for 9H suggests three equivalent methyl groups (tert-butyl group) because each methyl group in tert-butyl has equivalent environments and will integrate to 3 H each.

Deduce the Structure

Combine the information from both spectroscopic methods: 1. The IR data indicates the presence of a carbonyl group (C=O). 2. The 9.78 ppm suggests an aldehyde group (H-C=O), a component of an acyl group. 3. The 1.2 ppm singlet accounts for 9H total, suggesting three methyl groups connected to the same carbon, likely forming a tert-butyl group. Assembling these clues, the structure that fits all the data is tert-butyraldehyde, also known as 2,2-dimethylpropanal, with the formula (CH₃)₃CCHO.

Key concepts

IR spectroscopy

Infrared (IR) spectroscopy is a technique used in organic chemistry to help identify functional groups in compounds. The basic principle is that molecules absorb specific frequencies of IR radiation, which causes vibrations such as stretching and bending of chemical bonds. Each type of chemical bond absorbs IR light at a specific wavenumber. An IR spectrum is a plot of absorption versus wavenumber and can provide insight into the functional groups present in a molecule.
The given IR spectrum of compound A shows an absorption at 1730 cm⁻¹. This wavenumber is characteristic of the carbonyl group (C=O) stretching. Carbonyl groups are typically found in compounds like ketones, aldehydes, and carboxylic acids. The precise absorption can vary slightly depending on the specific group, but aldehydes often fall close to this value.

• This evidence strongly suggests that compound A contains a carbonyl group.

Identifying this functional group is crucial as it narrows down the potential structures for the compound. It's the IR absorption property that gives us the initial clue about the functional groups present, directing us to consider specific possibilities when combined with other analytical data.

NMR spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool in organic chemistry for elucidating the structure of organic compounds. It provides detailed information about the electronic environment surrounding specific atoms, usually protons (H) or carbons (C).
The NMR spectrum of compound A provides two distinct peaks: one at 9.78 ppm and another at 1.2 ppm. These peaks are both singlets, meaning there's no splitting and each signal is due to non-equivalent protons.

• The peak at 9.78 ppm corresponds to a highly deshielded proton. This is typically indicative of a proton attached to a carbonyl carbon, such as in an aldehyde, where the proton is significantly influenced by the electronegative oxygen atom in the carbonyl group.
• The signal at 1.2 ppm is a singlet associated with 9 equivalent protons, suggesting the presence of three methyl groups that are chemically equivalent, as seen in a tert-butyl group.

From this analysis, the NMR spectrum helps us further confirm that compound A not only includes a carbonyl, aligning with aldehyde characteristics, but also features a tert-butyl group. This detailed NMR interpretation is key to piecing together the overall structure.

aldehyde structure determination

Determining the structure of an aldehyde involves using spectroscopic clues to piece together how atoms in a molecule are connected. In the case of compound A, IR and NMR spectroscopy provide complementary information.
The IR spectrum highlights the carbonyl presence with its peak at 1730 cm⁻¹. This is highly suggestive of an aldehyde due to the specific range of absorption.

The NMR signals provide further specificity:

• The 9.78 ppm signal is the aldehyde proton, a vital clue as it places a proton directly connected to a carbonyl group, characteristic of the H-C=O structure.
• The 1.2 ppm singlet is attributable to a tert-butyl group containing three methyl groups, contributing a significant 9 hydrogen atoms, all in equivalent environments.

Combining these observations leads to a plausible structure: tert-butyraldehyde, known chemically as 2,2-dimethylpropanal. This structure fits the molecular weight and spectral data, accurately reflecting the presence of both an aldehyde group and a tert-butyl group. Hence, aldehyde structure determination incorporates both the identification of specific functional groups and alignment with expected instrumental data, ensuring a correct molecular identification.