Question

How many carbon resonance lines would you expect in the \({ }^{13} \mathrm{C}\) NMR spectra of the following compounds? (a) Methylcyclopentane (b) 1-Methylcyclohexene (c) 1,2 -Dimethylbenzene (d) 2-Methylbut-2-ene (e) CCC(=O)C(C)C (f) CCC(C)=C(C)C

Short answer

Methylcyclopentane: 6 lines; 1-Methylcyclohexene: 7 lines; 1,2-Dimethylbenzene: 4 lines; 2-Methylbut-2-ene: 5 lines; SMILES 3-pentanone: 5 lines; SMILES 2,3-dimethylbut-2-ene: 3 lines.

Step-by-step solution

Understand the Theory

In \(^{13}C\) NMR spectroscopy, each unique carbon environment will generally give rise to a separate resonance line. Symmetrical structures or equivalent carbon environments may have fewer lines due to atom equivalency.

Analyze Methylcyclopentane

Methylcyclopentane has a cyclopentane ring with a methyl group attached. Since the ring carbons and methyl carbon are unique, there are 6 unique carbon environments, resulting in 6 resonance lines.

Analyze 1-Methylcyclohexene

This compound features a methyl group and a double bond on a cyclohexene ring. Each carbon is unique due to the presence of the double bond and substituents, resulting in 7 unique carbon environments and thus 7 resonance lines.

Analyze 1,2-Dimethylbenzene

In 1,2-dimethylbenzene (o-xylene), the aromatic ring symmetry leads to the equivalency of opposite carbon positions, resulting in only 4 unique carbon environments and thus 4 resonance lines.

Analyze 2-Methylbut-2-ene

The double bond and methyl groups in 2-methylbut-2-ene create unique carbon environments, leading to 5 unique resonance lines in the NMR spectrum.

Analyze SMILES: CCC(=O)C(C)C

This SMILES represents 3-pentanone with a methyl and an ethyl group on the ketone, each creating a unique carbon environment. Hence, it results in 5 resonance lines.

Analyze SMILES: CCC(C)=C(C)C

This SMILES represents 2,3-dimethylbut-2-ene, a compound symmetrical around the double bond. The carbon environments lead to 3 unique resonance lines due to symmetry.

Key concepts

carbon resonance lines

In mr theory@^{13}C NMR spectroscopy@, each unique carbon environment in a molecule contributes to a separate resonance line.
Think of resonance lines as unique fingerprints for carbon atoms in a compound.
To determine the number of resonance lines, it's essential to analyze the number of different carbon environments present. Here are some key points:

• Each distinct carbon atom environment in a molecule will usually give rise to its own resonance line.
  
• Equivalent carbon environments—those that are structurally and electronically the same—will produce the same resonance line.
  
• The presence of molecular symmetry can reduce the number of unique environments by making certain carbons equivalent.
  

Understanding these principles is crucial for predicting the number of carbon resonance lines in the NMR spectra of organic compounds.

organic compounds

Organic compounds contain primarily carbon and hydrogen atoms, and often other elements like oxygen, nitrogen, sulfur, and halogens.
Among these, carbon is pivotal due to its bonding versatility, forming long chains, rings, and complex structures that are fundamental to biological and synthetic chemistry.
In mr theory@^{13}C NMR spectroscopy@, compounds are analyzed based on the chemical shifts of the carbon nuclei, which gives insight into their structure. Key characteristics:

• The structure determines the number and types of carbon environments.
  
• Functional groups influence the electronic environment of carbon atoms, affecting resonance lines.
  
• Analyzing organic compounds with mr theory@^{13}C NMR@ helps in identifying the number of different carbon environments and hence the total number of resonance lines.
  

Simple examples like alkanes, alkenes, and aromatics highlight how structural nuances in organic compounds reflect in mr theory@^{13}C NMR@ analysis.

molecular symmetry

Molecular symmetry significantly influences the number of unique carbon resonance lines observed in mr theory@^{13}C NMR spectroscopy@.
When a molecule has symmetrical features, certain carbon atoms become equivalent, reducing the number of distinct resonance lines. Helpful insights include:

• Symmetry can occur in various forms, such as rotational, mirror, or inversion, affecting how carbon atoms appear in mr theory@^{13}C NMR spectra@.
  
• In symmetrical compounds, like o-xylene, equivalent carbons across the symmetric axis contribute to a single resonance line.
  
• Understanding symmetry helps identify redundant carbon environments, simplifying the interpretation of NMR spectra.
  

Students should develop a keen eye for symmetry to accurately predict resonance lines in compound analysis.

NMR theory

NMR theory represents the foundation for understanding mr theory@NMR spectroscopy@, particularly mr theory@^{13}C NMR@.
The technique involves applying a magnetic field to a sample and measuring the response of nuclear spins, usually of carbon nuclei in mr theory@^{13}C NMR@, which provide insight into molecular structure. Core aspects of mr theory@NMR theory@:

• Nuclei in an external magnetic field absorb radiofrequency radiation at a resonance frequency specific to their environment.
  
• The chemical shift, measured in parts per million (ppm), offers detailed information about the electronic environment surrounding a nucleus.
  
• NMR spectra directly reflect the number of different nuclei environments, which in mr theory@^{13}C NMR@ correlates with distinct carbon environments.
  

Understanding NMR theory helps students gain deeper insights into analyzing and predicting the outcomes of NMR experiments, crucial for interpreting carbon resonance lines.