Question
Asked Mar 4, 2020
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The 1H-NMR spectrum of 1-bromopropane shows signals at 1.03, 1.88, and 3.39 ppm. The 1H-NMR spectrum of 2-bromopropane shows signals at 1.73 and 4.21 ppm. Draw these two molecules and assign the correct chemical shift to each proton environment.

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Expert Answer

Step 1

NMR or nuclear magnetic resonance spectroscopy is a technique used to determine a compound’s unique structure. It identifies the carbon-hydrogen framework of an organic compound.

 

Each group of chemically equivalent protons gives rise to a signal.

Chemically Equivalent Protons are protons that are in the same environment, and they must be identical in every way. They experience is the same magnetic force, and therefore, will create overlapping signals on the spectrum.

Therefore, we can determine how many sets of equivalent protons there are in a molecule by looking at the number of signals in its H-NMR spectrum.

 

The position of the signals depends on the chemical shift.

Chemical Shift is a measure of how far the signal produced from the proton is from the reference compound signal, and it usually measured using the ? or ppm scale.

Chemical shift is affected by Electronegativity :

The electron cloud shields the nucleus from the applied magnetic field, and electronegativity is defined as the tendency of an atom to pull electrons toward itself. Therefore, electronegative atoms remove electron density from the proton. This causes the proton to have less electron density, and this leads to less shielding. If the proton has less shielding, it will feel the applied magnetic field more, and this leads to a higher chemical shift.

Protons that are closer to the electronegative atom are in a less electron dense environment, which means that they’re chemical shifts will be larger.

Step 2

Given,

The 1H-NMR spectrum of 1-bromopropane shows signals at 1.03, 1.88, and 3.39 ppm.

The 1H-NMR spectrum of 2-bromopropane shows signals at 1.73 and 4.21 ppm.

 

The structure of these two molecules are :

Chemistry homework question answer, step 2, image 1

Step 3

There are three different proton environments in 1-broopropane and two different proton environments in 2-broopropane.

The protons same environment is grouped together as a, b, c in 1-bromopropnae and d, e in 2-bromopropane as :

Chemistry homework question answer, step 3, image 1

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