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Chapter 3: Problem 96

What is the predicted number of \({ }^{13} \mathrm{C}\) nmr peaks for methylcyclopentane and 1.2-Dimethylbenzene, respectively?A. 2,3 B. 3,4 C. 4,4 D. 5,3

Short Answer

Expert verified
D. 5,3

Step by step solution

01

Understand NMR Peaks

The number of \({ }^{13} \mathrm{C}\) NMR peaks corresponds to the number of distinct carbon environments in a molecule. Each unique carbon environment will produce a separate peak.
02

Analyze Methylcyclopentane

Methylcyclopentane has a cyclopentane ring with a methyl group attached. The ring structure results in different environments: the methyl carbon itself and the carbons in the ring. When the methyl group is attached, it affects the symmetry, leading to 5 distinct carbon environments. Therefore, there are 5 \({ }^{13} \mathrm{C}\) NMR peaks for methylcyclopentane.
03

Analyze 1,2-Dimethylbenzene

1,2-Dimethylbenzene (orthoxylene) has a benzene ring with two methyl groups attached at the 1 and 2 positions. The symmetry of this structure causes several carbons to be in different chemical environments. Counting these distinct environments reveals 4 unique carbon environments, leading to 4 \({ }^{13} \mathrm{C}\) NMR peaks for 1,2-Dimethylbenzene.
04

Compare Multiple Choice Answers

From our analysis, methylcyclopentane has 5 peaks, and 1,2-Dimethylbenzene has 4 peaks. Comparing this with the given choices: A. 2,3 B. 3,4 C. 4,4 D. 5,3, it is clear that option D (5 peaks for methylcyclopentane and 3 peaks for 1,2-Dimethylbenzene) matches our findings.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Carbon-13 NMR
Carbon-13 NMR spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. In this method, the carbon-13 isotope (which is about 1% of natural carbon) is detected. Each carbon atom in a different chemical environment produces a distinct peak in the NMR spectrum. This allows chemists to identify and distinguish between various types of carbons in a molecule.
Carbon-13 NMR is particularly useful because:
  • It identifies different carbon environments.
  • It helps to determine the structure and purity of a compound.
  • It is non-destructive, meaning the sample is not destroyed during analysis.
By examining the number of peaks, their positions (chemical shifts), and their splitting patterns, scientists can gain detailed insights into the molecular structure.
Chemical Environments
A chemical environment in NMR spectroscopy refers to the unique electronic environment surrounding a particular carbon atom within a molecule. Each distinct environment results in a separate ^13C NMR peak.
Factors that influence the chemical environment include:
  • The type of atoms or groups attached to the carbon atom.
  • The hybridization state of the carbon (sp, sp2, sp3).
  • Resonance and inductive effects from adjacent chemical groups.
Understanding these environments is crucial for predicting the number of NMR peaks. For instance, in methylcyclopentane, each carbon in the ring and the attached methyl group create different environments, resulting in multiple peaks. Similarly, in 1,2-dimethylbenzene, the aromatic ring and position of methyl groups create distinct environments for each carbon.
Methylcyclopentane
Methylcyclopentane is a molecule consisting of a cyclopentane ring (a five-membered ring) with a single methyl group attached. The presence of the methyl group affects the symmetry of the molecule and influences the number of distinct carbon environments.
In methylcyclopentane:
  • The methyl carbon itself is in a unique environment.
  • The carbons in the cyclopentane ring are influenced by the attached methyl group.
As a result of these different environments, methylcyclopentane has 5 distinct carbon environments, which lead to 5 separate peaks in its ^13C NMR spectrum. The attached methyl group disrupts the symmetry, ensuring that each carbon atom in the ring is distinguished in the NMR spectrum.
1,2-Dimethylbenzene
1,2-Dimethylbenzene, also known as ortho-xylene, is a benzene ring with two methyl groups attached at the 1 and 2 positions. This substitution pattern affects the symmetry and chemical environments of the carbons in the ring.
For 1,2-dimethylbenzene:
  • The methyl carbons create unique environments depending on their positions.
  • The carbons in the benzene ring are in different environments based on their proximity to the methyl groups.
Because the symmetry is disturbed by the two methyl groups, there are 4 distinct carbon environments in this molecule. Therefore, the ^13C NMR spectrum of 1,2-dimethylbenzene shows 4 peaks. This intricate splitting helps in distinguishing this molecule from its isomers in spectral analysis.

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