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Chapter 14: Problem 9

How many signals would you expect to find in the \({ }^{1} \mathrm{H}\) NMR spectrum of each of the following compounds? (a) 1 -Bromobutane (b) 1-Butanol (c) Butane (d) 1,4 -Dibromobutane (e) 2, 2-Dibromobutane (f) \(2,2,3,3\) -Tetrabromobutane (g) \(1,1,4\) -Tribromobutane (h) \(1,1,1\) -Tribromobutane

Short Answer

Expert verified
(a) 4 signals, (b) 4 signals, (c) 2 signals, (d) 2 signals, (e) 3 signals, (f) 0 signals, (g) 3 signals, (h) 2 signals.

Step by step solution

01

Identify unique hydrogen environments for 1-Bromobutane

1-Bromobutane is a straight-chain alkane with a bromine atom attached to the first carbon. We identify different hydrogen (proton) environments based on bond connectivity and symmetry. In 1-bromobutane, there are four distinct sets of protons: - One set from the CH3 group at the end of the chain. - One set from the methylene group (CH2) next to the CH3 group. - Another set from the methylene group (CH2) next to the bromine. - One set from the CH2Br group directly bonded to bromine. Thus, the NMR spectrum would show 4 signals.
02

Identify unique hydrogen environments for 1-Butanol

1-Butanol has an -OH group at the end of the straight-chain alkane. We must identify unique hydrogen environments. There are four different proton environments in 1-butanol: - One set from the terminal CH3 group. - One set from the CH2 group next to the CH3. - One set from the CH2 group next to the OH group. - One set from the CH2OH methylene group. Therefore, the NMR spectrum will show 4 signals.
03

Identify unique hydrogen environments for Butane

Butane is a simple straight-chain alkane. Due to the symmetry across the molecule, we identify proton environments. Butane has two unique sets of protons: - Terminal CH3 groups (equivalent by symmetry). - Central CH2 groups (equivalent by symmetry). Thus, the NMR spectrum would show 2 signals.
04

Identify unique hydrogen environments for 1,4-Dibromobutane

1,4-Dibromobutane has bromine atoms at both ends of the four-carbon chain. The symmetry alters the proton environments. There are two distinct sets of protons: - Central CH2 groups (equivalent by symmetry). - Terminal CH2Br groups (equivalent by symmetry). Thus, the NMR spectrum would show 2 signals.
05

Identify unique hydrogen environments for 2,2-Dibromobutane

2,2-Dibromobutane has two bromine atoms on the same central carbon of the four-carbon chain. We find three distinct proton environments: - The methyl group at the terminal end. - The CH2 group near the methyl end. - The methine (CH) group bonded to the two bromine atoms. Thus, the NMR spectrum would show 3 signals.
06

Identify unique hydrogen environments for 2,2,3,3-Tetrabromobutane

2,2,3,3-Tetrabromobutane has all protons replaced by bromines around the central carbons, resulting in no protons to detect. Thus, there are 0 signals in the NMR spectrum.
07

Identify unique hydrogen environments for 1,1,4-Tribromobutane

1,1,4-Tribromobutane has bromines at one end and the fourth carbon. This structure supports three distinct proton environments: - Terminal CH2Br group. - Intermediate CH2 group. - The CH group with two bromines attached. Thus, the NMR spectrum would show 3 signals.
08

Identify unique hydrogen environments for 1,1,1-Tribromobutane

1,1,1-Tribromobutane has three bromine atoms on the first carbon. The molecule contains two distinct proton environments: - Terminal CH3 group. - Two CH2 groups equivalent by symmetry. Thus, the NMR spectrum would show 2 signals.

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Key Concepts

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

Proton Environments
In NMR spectroscopy, a proton environment refers to a specific hydrogen atom (or group of atoms) that exists in a unique magnetic setting within a molecule. Each unique proton environment can lead to a distinct signal in the NMR spectrum. Understanding proton environments helps chemists determine how hydrogen atoms are distributed in a molecule.
  • **Bond Connectivity:** Hydrogen atoms bonded to different atoms or functional groups experience different magnetic environments, leading to distinct signals. For example, the protons in a methyl group \(\text{-CH}_3\) differ from those in a methylene group \(\text{-CH}_2\).
  • **Symmetry:** Molecules with symmetrical structures can reduce the number of unique proton environments. Symmetric parts in a molecule cause protons to be equivalent, hence sharing the same environment.
Identifying proton environments involves looking closely at the molecular structure to see how protons are positioned relative to atoms like carbon, nitrogen, oxygen, and others.
Signal Prediction
Predicting the number of signals in an NMR spectrum involves counting unique proton environments within a molecule. This requires understanding of both the molecule's structural layout and any symmetry present.
  • **Equivalent Protons:** If two or more hydrogen atoms are in the same environment, they produce a single signal. For example, in butane, the protons in terminal \(\text{-CH}_3\) groups are all in the same environment, leading to one signal.
  • **Different Environments:** Each different type of hydrogen atom environment gives rise to a separate signal. For instance, in 1-bromobutane, hydrogen atoms in \(\text{-CH}_3\), \(\text{-CH}_2\), and \(\text{-CH}_2Br\) environments produce four different signals.
By mapping out these environments, chemists can predict the number of signals and better understand the chemical structure.
Chemical Structure Analysis
Chemical structure analysis using NMR spectroscopy involves interpreting the NMR signals to deduce the structure of a molecule. This technique is paramount in organic chemistry to identify and confirm molecular composition.
  • **Number of Signals:** The total number of signals reflects the number of distinct proton environments. Analyzing these can provide insights into symmetrical aspects and the overall architecture of a molecule.
  • **Signal Splitting:** Sometimes, a signal might be split into multiple peaks. This splitting reveals information about neighboring hydrogens, known as "spin-spin coupling." This can further help pinpoint structural details.
By examining the NMR spectrum, one can infer not only the number of types of hydrogen but also gather clues about the connections between these atoms.
Symmetry in Molecules
Symmetry plays a significant role in determining the number of unique proton environments in a molecule. A symmetric molecule often has fewer distinct environments as equivalent protons are indistinguishable under NMR.
  • **Simple Alkanes** like butane display symmetry, with protons in equivalent groups generating fewer signals. Both the symmetrically arranged \(\text{-CH}_3\) and \(\text{-CH}_2\) groups contribute to simplified spectra.
  • **Complex Molecules:** In compounds like 1,4-dibromobutane, symmetry around the central carbons results in two sets of equivalent protons, reducing the expected signals.
A detailed examination of symmetric features helps predict the NMR spectrum's outcome, making symmetry a crucial factor in molecular analysis. Understanding symmetry aids in interpreting experimental data and verifying the correctness of proposed molecular structures.

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Most popular questions from this chapter

The \({ }^{1} \mathrm{H}\) NMR signal for bromoform \(\left(\mathrm{CHBr}_{3}\right)\) appears at \(2065 \mathrm{~Hz}\) when recorded on a \(300-\mathrm{MHz}\) NMR spectrometer. (a) What is the chemical shift of this proton? (b) If the spectrum was recorded on a 400-MHz instrument, what would be the chemical shift of the \(\mathrm{CHBr}_{3}\) proton? (c) How many hertz downfield from TMS is the signal when recorded on a \(400-\mathrm{MHz}\) instrument?

Consider carbons \(x, y\), and \(z\) in p-methylanisole. One has a chemical shift of \(\delta 20\), another has \(\delta 55\), and the third \(\delta 157\). Match the chemical shifts with the appropriate carbons.

Hydrogen bonding between the oxygen of dimethyl sulfoxide (DMSO) and the proton of an OH group is relatively strong. How could recording an NMR spectrum in DMSO instead of \(\mathrm{CDCl}_{3}\) be helpful in distinguishing between primary, secondary, and tertiary alcohols?

Assign the chemical shifts \(\delta 1.6, \delta 4.0, \delta 7.5, \delta 8.2\), and \(\delta 12.0\) to the appropriate protons of 2 -(p-nitrophenyl)propanoic acid .

How many signals would you expect to see in the \({ }^{13} \mathrm{C}\) NMR spectrum of each of the following compounds? (a) Propylbenzene (b) Isopropylbenzene (c) \(1,2,3\) -Trimethylbenzene (d) \(1,2,4\) -Trimethylbenzene (e) \(1,3,5\) -Trimethylbenzene
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