Question

Propose structures for compounds with the following formulas that show only one peak in their \({ }^{1} \mathrm{H}\) NMR spectra: (a) \(\mathrm{C}_{5} \mathrm{H}_{12}\) (b) \(\mathrm{C}_{5} \mathrm{H}_{10}\) (c) \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\)

Short answer

(a) Neopentane, (b) Cyclopentane, (c) Symmetrical ester or acid derivative.

Step-by-step solution

Understanding Proton NMR

Proton (1H) NMR spectroscopy identifies different types of hydrogen atoms in a compound's structure. A molecule that shows only one peak in its 1H NMR spectrum has all its hydrogen atoms in equivalent environments.

Analyzing Compound (a) C₅H₁₂

For a compound with the formula C₅H₁₂ to show only one peak, all hydrogen atoms must be equivalent. This suggests the structure is a symmetrical molecule. The only alkane isomer with such symmetry is neopentane, or 2,2-dimethylpropane.

Analyzing Compound (b) C₅H₁₀

Compound C₅H₁₀ has two fewer hydrogens than C₅H₁₂, indicating it might be a cycloalkane or an alkene. All hydrogens being equivalent suggests a symmetrical cycloalkane is present. The molecule that fits this requirement is cyclopentane.

Analyzing Compound (c) C₄H₈O₂

Compound C₄H₈O₂ could be an ester or a symmetrical compound with an internal symmetry that results in one unique hydrogen environment. Considering symmetrical esters, ethyl acetate is an option, but you would need a symmetrical structure like ethyl 1,1-dichloroacetate, or possibly cyclobutanecarboxylic acid.

Key concepts

1H NMR Peaks

In the world of organic chemistry, uclear magnetic resonance (NMR) spectroscopy is a tool used to determine the structure of organic compounds. The "1H" refers to hydrogen atoms, or protons, whose magnetic properties are analyzed in this type of NMR. The detectable peaks in a 1H NMR spectrum represent different types of hydrogen environments within a molecule. A single peak indicates that all hydrogen atoms in the molecule are equivalently experiencing the same magnetic environment. Let's use compound a), C₅H₁₂, as an example. Because C₅H₁₂ shows only one peak in its NMR spectrum, all the hydrogen protons must be equivalent from a magnetic point of view. This highly symmetrical distribution of hydrogens results in a single peak, simplifying the analysis of complex structures. When every hydrogen atom in a molecule is magnetically identical, you'll see such simple, singular peaks.

Molecular Symmetry

Molecular symmetry is a fundamental aspect of chemistry that significantly influences the NMR spectrum of a compound. When a molecule has a high degree of symmetry, its NMR spectrum shows fewer peaks, oftentimes just one. For instance, in the case of C₅H₁₂ (neopentane), its symmetrical structure means every hydrogen atom is in the same electronic environment. This alignment of environments corresponds to just one peak in the NMR spectrum. Symmetrical molecules are usually easier to analyze because their environments are consistent throughout their structure. Symmetry is not just a theoretical exercise but a practical tool that helps chemists quickly predict the behavior of molecules in various conditions.

Structural Isomers

Structural isomers are compounds with the same molecular formula but different structural arrangements. This results in distinct physical and chemical properties. In the case of C₅H₁₂, multiple structural isomers are possible, including straight chains and branched chains. However, for that single NMR peak observed, 2,2-dimethylpropane stands out. It has a completely symmetrical structure where all the protons are equivalent. This is not the case for other isomers, which typically feature a variety of peaks, reflecting the different environments the hydrogens occupy. Structural isomers play a vital role in chemistry, offering diverse properties within the same overall composition.

Equivalent Hydrogen Environments

The concept of equivalent hydrogen environments is critical for interpreting 1H NMR spectra. Hydrogens are considered equivalent if they are in indistinguishable magnetic environments, often due to symmetry. The single peak in the NMR spectrum means all hydrogen atoms sit equally in magnetic and electronic landscapes. For compound C₅H₁₂, this equivalence is achieved in the structure of 2,2-dimethylpropane (neopentane). Each hydrogen in this molecule experiences identical surrounding atoms, leading to equivalence. A similar scenario occurs with cyclopentane (C₅H₁₀), where rotational symmetry creates an equivalence among all hydrogens. Understanding these equivalent sites helps simplify predictions and supports chemical understanding when designing or analyzing molecular structures.