Question

Propose structures for compounds that fit the following descriptions: (a) A hydrocarbon with seven lines in its \({ }^{13} \mathrm{C}\) NMR spectrum (b) A six-carbon compound with only five lines in its \({ }^{13} \mathrm{C}\) NMR spectrum (c) A four-carbon compound with three lines in its \({ }^{13} \mathrm{C}\) NMR spectrum

Short answer

(a) Heptane; (b) 2,3-dimethylbutane; (c) 2-methylpropane

Step-by-step solution

Analyze hydrocarbon with seven lines

The compound must be a hydrocarbon with 7 distinct carbon environments in its \( ^{13} \mathrm{C} \) NMR spectrum. One possibility is heptane. Heptane is a straight chain alkane with 7 carbon atoms, each occupying a unique chemical environment due to its linear and non-symmetrical structure.

Consider six-carbon compound with five lines

For a compound with 6 carbons and 5 unique carbons in an NMR spectrum, the compound has symmetry. One possible structure is 2,3-dimethylbutane. In this isomer, there are two identical methyl groups methyl (creating identical environments) and a total of 5 different carbon environments.

Determine four-carbon compound with three lines

This suggests symmetrical structure and/or identical groups. One candidate is 2-methylpropane. This compound, commonly known as isobutane, has three different types of carbon environments: one primary, one tertiary, and two secondary carbons (which are identical).

Key concepts

Carbon Environments

The concept of carbon environments is crucial in understanding NMR spectroscopy. In an NMR spectrum, each peak corresponds to a unique carbon environment. A unique carbon environment is determined by the distinct chemical surroundings of each carbon atom.

These environments are altered by factors such as the types of neighboring atoms (e.g., hydrogen or other carbons) and the overall molecular structure. For example:

• If a carbon atom is bonded to three hydrogen atoms and one other carbon atom, it is in a different environment compared to a carbon connected to two hydrogen atoms and two other carbon atoms.
• The presence of electronegative atoms or groups nearby can also shift carbon environments, affecting how each carbon is represented in the NMR spectrum.
• Understanding carbon environments allows chemists to deduce structural information about the compound.

Given these nuances, analyzing NMR spectra involves identifying the number and types of carbon environments present to determine the potential molecular structure.

Hydrocarbon Structures

Hydrocarbon structures play a key role in determining how many unique carbon environments are present, which is directly reflected in ewline the NMR spectrum. Hydrocarbons are organic compounds composed solely of hydrogen and carbon atoms. They can be either aliphatic, or cyclic, each affecting carbon environments differently.

For instance:

• A straight-chain hydrocarbon like heptane has no branching and thus has a fixed number of carbon environments, each is accompanied by a separate peak on the NMR spectrum.
• Branched hydrocarbons, like 2,3-dimethylbutane, introduce symmetry which can lead to fewer distinct environments since some carbons might be in identical environments.
• Cyclic hydrocarbons can further change the pattern due to ring strain and angle, influencing the chemical shifts in the NMR spectrum.

Analyzing the structure of hydrocarbons is essential to deciphering the corresponding NMR spectrum and understanding how structural variations can affect the magnetic resonance signals.

Symmetrical Compounds

Symmetry in chemical compounds is a fascinating aspect that plays a significant role in NMR spectroscopy. When there's symmetry in a molecule, several carbon atoms can experience identical chemical environments. This results in fewer distinct peaks in an NMR spectrum than the actual number of carbons present in the compound.

Take, for instance, compounds such as 2,3-dimethylbutane or 2-methylpropane. These molecules exhibit a degree of symmetry that causes some carbon atoms to appear identical in the NMR spectrum.

• In 2,3-dimethylbutane, the presence of identical methyl groups on the same carbon skeleton introduces symmetry, leading to five unique peaks despite having six carbon atoms.
• Similarly, 2-methylpropane, or isobutane, displays only three peaks instead of four due to its symmetrical arrangement.

Understanding symmetry helps chemists predict and interpret the number of signals in an NMR spectrum accurately, which is critical in identifying and verifying molecular structures.