Question

The \(^{1} \mathrm{H}\) NMR spectrum of acetaldehyde (14.24) consists of signals at \(\delta 2.21\) and 9.79 ppm. (a) Assign the spectrum. (b) The inequivalent protons couple \((J 2.9 \mathrm{Hz}) .\) What will be the multiplicities of the signals?

Short answer

The signal at 2.21 ppm is a doublet (CH₃), and the signal at 9.79 ppm is a quartet (CHO).

Step-by-step solution

Identify the chemical shifts

In the given NMR spectrum of acetaldehyde, signals are observed at \(\delta 2.21\) ppm and \(\delta 9.79\) ppm. We need to assign these shifts to the specific hydrogen environments in acetaldehyde, which is \(CH_3CHO\). The hydrogens in the \(CH_3\) group (methyl group) typically show up at lower ppm values, while the hydrogen at the aldehyde \(CHO\) group shows up at higher ppm values.

Assign chemical shifts to hydrogens

The signal at \(\delta 2.21\) ppm can be assigned to the methyl group protons \((CH_3)\) because it is at a lower ppm and indicates a more shielded environment. The signal at \(\delta 9.79\) ppm is characteristic of aldehyde protons \(\text{-CHO}\), known to appear downfield due to deshielding, matching the usual chemical shift range for such protons.

Determine coupling constant

The coupling constant \(J\) provided is 2.9 Hz, indicating the interaction between the methyl protons \(CH_3\) and the aldehyde proton \(CHO\). This suggests that each set of inequivalent protons is splitting the signal of the other set.

Determine multiplicities of signals

For the \(CH_3\) group (three equivalent protons), according to the \(n+1\) rule (where \(n\) is the number of neighboring protons), the splitting by one neighboring aldehyde proton \((n = 1)\) results in a doublet peak (\(n+1 = 2\)). Similarly, for the aldehyde \(\text{-CHO}\) group, the splitting by the three \(CH_3\) protons \((n = 3)\) results in a quartet peak (\(n+1 = 4\)).

Key concepts

Chemical Shifts in NMR Spectroscopy

In NMR spectroscopy, chemical shifts reveal the environment of hydrogen atoms in a molecule. The chemical shift is measured in parts per million (ppm) and reflects how shielded or deshielded a proton is by its surrounding electronic environment. Chemical shifts help us to determine the type of hydrogen we are observing and its environment within a molecule.

For instance, in the NMR spectrum of acetaldehyde, you observe signals at

• a relatively lower ppm (b4 2.21 ppm), which typically indicates hydrogens in a more shielded environment, such as those in a methyl group (bdash;CH₃).
• The signal at b4 9.79 ppm is much higher and characteristic of hydrogens attached to the electronegative aldehyde group (bdash;CHO).

These values give us clues about the positions of the hydrogens within the molecule, facilitating their identification in the spectrum.

Understanding Coupling Constants

The coupling constant, denoted as \(J\), is a measure of the interaction between neighboring, non-equivalent protons. It is expressed in Hertz (Hz) and provides insight into the connectivity between protons.

In the case of acetaldehyde, a coupling constant of 2.9 Hz indicates a moderate interaction between the methyl (bdash;CH₃) and aldehyde (bdash;CHO) protons. This

• value reflects the level of interaction that influences how the signals for overlapping hydrogens spread out or split in relation to one another.
• The size of the coupling constant can give information on the distance and position, even the angle, between interacting protons.

Understanding these interactions is crucial for interpreting complex NMR spectra.

Signal Multiplicity: How Peaks Split

Signal multiplicity, often described by terms like singlet, doublet, triplet, etc., reveals the number of neighboring protons influencing a particular hydrogen atom's signal. The rule often used is the \(n+1\) rule, where \(n\) is the number of neighboring nonequivalent protons.

For the methyl group (\(-CH_3\)) in acetaldehyde, there is one proton neighbor in the aldehyde group (bdash;CHO). By applying the \(n+1\) rule:

• \(n = 1\), the methyl group's protons' signal splits into a doublet (\(n+1 = 2\)).
• Conversely, the aldehyde group, surrounded by the three protons of the methyl group, will display a quartet (\(n+1 = 4\)).

These multiplicities provide richer information about the molecular structure, aiding chemists in mapping out how atoms connect in a molecule.