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Chapter 13: Q1P (page 650)

Question:In a 300-MHz spectrometer, the protons in bromomethane absorb at a position 660 Hz downfield from TMS.

(a) What is the chemical shift of these protons?

(b) What is the chemical shift of the bromomethane protons in a 60-MHz spectrometer?

(c) How many hertz downfield from TMS would they absorb at 60 MHz?

Short Answer

Expert verified

Answer

  1. The chemical shift value is 2.20 ppm.
  2. The chemical shift value is 2.20 ppm.
  3. The answer is 132 Hz.

Step by step solution

01

Chemical shift

The chemical shift is the fraction and its formula is as shown:

Chemical shift=shift downfieldHzspectrometer frequencyMHz

02

Subpart (a)

The spectrometer frequency given is 300 MHz.

Downfield shift in bromomethane is 660 Hz.

The value of chemical shift can be calculated as shown below:

Chemical shift=shift downfieldHzspectrometer frequencyMHz=660 Hz300 MHz=2.20 ppm

03

Subpart (b)

The chemical shift value of bromomethane remains the same and it is 2.20 ppm.

04

Subpart (c)

The frequency shift is calculated as shown below:

frequency shift=chemical shift×spectrometer frequency=2.20 ppm×60 MHz=132  Hz

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Most popular questions from this chapter

The three isomers of dichlorobenzene are commonly named ortho-chlorobenzene, meta-chlorobenzene, and para-chlorobenzene. These three isomers are difficult to distinguish using proton NMR, but they are instantly identifiable using 13C NMR

(a)Describe how carbon NMR distinguishes these three isomers.
(b)Explain why they are difficult to distinguish using proton NMR.

Draw the expected broadband-decoupled 13C NMR spectra of the following compounds. Use Figure 13-41 (page 686) to estimate the chemical shift.

Predict the theoretical number of different NMR signals produced by each compound and give approximate chemical shifts. Point out any diastereotopic relationships.

  1. 2-bromobutane
  2. Cyclopentanol
  3. Ph-CHBr-CH2Br
  4. Vinyl chloride

Each of these four structures has molecular formula . Match the structure with its characteristic proton NMR signals. (Not all of the signals are listed in each case.)

(a) Sharpsinglet atδ8.0 andtriplet atδ4.0

(b) Sharpsinglet atδ2.0 andquartet atδ4.1

(c) Sharpsinglet atδ3.7 andquartet atδ2.3

(d) Broadsinglet atδ11.5 andtriplet atδ2.3

Phenyl Grignard reagent adds to 2-methylpropanal to give the secondary alcohol shown. The proton NMR of 2-methylpropanal shows the two methyl groups as equivalent (one doublet at1.1), yet the product alcohol, a racemic mixture, shows two differentdoublets, one at0.75 and one around1.0.

  1. Draw a Newmann projection of the product along the C1-C2 axis.
  2. Explain why the two methyl groups have different NMR chemical shifts. What is the term applied to protons such as these?
See all solutions

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