Question

The NMR spectrum of a compound can reveal whether the atoms within the molecule are equivalent or different. For example, the hydrogen atoms in \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) are equivalent: they all exist in an identical environment. However, in \(\mathrm{CH}_{2}=\mathrm{CHCl}\), the two hydrogen atoms attached to the same carbon atom are equivalent, but different from the hydrogen atom on the other carbon atom. Predict how many different types of hydrogen atoms can be identified in the \({ }^{1} \mathrm{H}\) NMR spectrum of each of the following molecules: (a) \(\mathrm{C}_{2} \mathrm{H}_{2}\); (b) cis- \(\mathrm{C}_{2} \mathrm{H}_{2} \mathrm{Cl}_{2}\); (c) trans- \(\mathrm{C}_{2} \mathrm{H}_{2} \mathrm{Cl}_{2}\); (d) \(\mathrm{CH}_{3} \mathrm{OH}\). (c) In the molecule \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\), the hydrogen atoms would all be different if the molecule existed in only one conformation. However, only two types of hydrogen atoms are detected. Propose an explanation.

Short answer

The predicted number of different types of hydrogen atoms in the 1H NMR spectrum: (a) C2H2 is 1, (b) cis-C2H2Cl2 is 2, (c) trans-C2H2Cl2 is 2, (d) CH3OH is 2. In C2H5Cl, two types of hydrogen atoms are detected due to rapid interchanging environments.

Step-by-step solution

Identifying Hydrogen Environments in Ethyne (C2H2)

Ethyne (C2H2) has a triple bond between the two carbon atoms. Each carbon atom is bonded to only one hydrogen. Therefore, these hydrogens are in the same chemical environment. The predicted number of different types of hydrogen atoms in the 1H NMR spectrum of ethyne is one.

Determining Hydrogen Types in cis-C2H2Cl2

In cis-C2H2Cl2, each carbon atom is double-bonded to the other and has one hydrogen and one chlorine atom attached. The hydrogens on each carbon atom are equivalent to each other but in a different environment due to the chlorine atom on each carbon. This gives us two types of hydrogen environments.

Evaluating Hydrogen Types in trans-C2H2Cl2

The molecule trans-C2H2Cl2 also has two carbon atoms double-bonded to each other. The hydrogens are on opposite sides with respect to the chlorine atoms. This spatial arrangement still leaves us with two types of hydrogen environments, similar to the cis isomer.

Counting Hydrogen Types in CH3OH

CH3OH (methanol) has three hydrogen atoms attached to the carbon and one hydrogen to the oxygen (as part of the hydroxyl group, OH). The three hydrogen atoms on the carbon are equivalent, and the hydroxyl hydrogen is in a different environment. Thus, two different hydrogen environments can be predicted.

Explaining Hydrogen Equivalence in C2H5Cl

In C2H5Cl (ethyl chloride), the hydrogens would all be in different environments if the molecule were rigid. However, due to free rotation around the single C-C bond in ethyl chloride, the hydrogens are exposed to rapidly interchanging environments which average to produce spectra typical of only two distinct environments: one for the three hydrogens on the methyl group (CH3) and another for the two hydrogens on the methylene (CH2) group.

Key concepts

1H NMR Spectrum

Understanding the basics of 1H NMR spectrum is crucial for any student diving into the realm of NMR spectroscopy. Simply put, a 1H NMR spectrum displays the resonance frequencies of hydrogen atoms within a molecule and provides insights into the molecular structure. Each peak corresponds to a distinct set of equivalent hydrogen atoms, that is, hydrogens in the same chemical environment.

In NMR terminology, the position of a peak along the horizontal axis is called its chemical shift and is unique for each type of hydrogen environment. Through careful examination of a molecule's 1H NMR spectrum, one can glean information about the number of hydrogen environments, their relative positions, and even the possible molecular conformation. This powerful analytical technique is indispensable for organic chemists and other scientists in identifying and characterizing organic compounds.

Chemical Environments in Molecules

Atoms within a molecule can experience different chemical environments based on their location and the surrounding atoms or functional groups. This concept is integral to analyzing NMR spectrums. In NMR spectroscopy, chemically equivalent hydrogens—those in identical environments—will give rise to the same signal, whereas hydrogens in different environments will result in distinct signals.

For instance, in methyl alcohol (CH3OH), the three hydrogen atoms connected to the carbon are in a similar chemical environment and thus contribute to a single peak, while the hydrogen in the hydroxyl group (OH) has a different environment and produces a separate peak. When it comes to chemical environments, factors such as electronegativity of adjacent atoms and the molecular geometry play pivotal roles in influencing the chemical shifts observed in the 1H NMR spectrum.

Hydrogen Equivalence

The notion of hydrogen equivalence is another fundamental aspect when interpreting 1H NMR spectra. Hydrogen atoms are considered equivalent when they are indistinguishable from each other by the NMR instrument due to their identical spatial and electronic environments. In other words, they are mirror images or symmetrically equivalent.

For example, in ethane (C2H6), all hydrogen atoms are equivalent, leading to a single peak in the NMR spectrum. However, the presence of different functional groups or varying spatial arrangements—such as in ethyl chloride (C2H5Cl)—can create an environment where different types of hydrogen become non-equivalent. Their rapid movements or rotations can sometimes result in an averaged environment, thus appearing equivalent in the NMR spectrum. This is exactly what happens with the hydrogens in C2H5Cl, causing only two distinct hydrogen signals instead of three.

Molecular Conformation

Lastly, molecular conformation significantly affects the 1H NMR spectral data. Molecular conformation refers to the spatial arrangement of a molecule's atoms and the rotation around its single bonds. This rotation can cause atoms to experience various chemical environments over time. In rigid structures, atoms may be locked into distinct environments, resulting in multiple peaks in an NMR spectrum.

In contrast, for molecules with free rotation around bonds, such as C2H5Cl, hydrogen atoms can interchange between different environments rapidly. This situation leads to an averaged chemical environment, seen in NMR as fewer distinct peaks than there are types of hydrogen atoms. Therefore, the understanding of molecular conformation is vital for correctly interpreting the NMR data and determining the structure and dynamics of organic molecules.