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Chapter 13: Q17P (page 675)

If the imaginary replacement of either of two protons forms enantiomers, then those protons are said to be enantiotopic.The NMR is not a chiral probe, and it cannot distinguish between enantiotopic protons. They are seen to be 鈥渆quivalent by NMR鈥�.

  1. Use the imaginary replacement technique to show that the two allylic protons (those on) of allyl bromide are enantiotopic.
  2. Similarly, show that the two HCprotons in cyclobutanol are enantiotopic.
  3. What other protons in cyclobutanol are enantiotopic?

Short Answer

Expert verified

a.

b.

The products formed are enantiomers and hence, HCprotons are enantiotopic.

c.The Hd protons are enantiotopic.

Step by step solution

01

Step-1. Explanation of part (a):

a.

Allylic protons of allyl bromide are enantiotopic in nature. By replacing the protons HA and HBwith some atom 鈥淶鈥�, we get two products and these products are enantiomers of each other. Enantiomers are non-superimposable mirror images of each other and enantiotopic protons are not distinguishable by NMR.

02

Step-2. Explanation of part (b)

b.

Cyclobutanol has protons HC and Hdand both are enantiotopic protons. On replacing the protons Hc with some atom 鈥淶鈥�, we get the two products which are enantiomers of each other as they are non-superimposable mirror images of each other.

03

Step-3: Explanation of part  (c):

c.

The Hd protons are also enantiotopic in cyclobutanol.

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Most popular questions from this chapter

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?

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 usingNMR.

  1. Describe how carbon NMR distinguishes these three isomers.
  2. Explain why they are difficult to distinguish using proton NMR.

Determine the ratios of the peak areas in the following spectra. Then use this information, together with the chemical shifts, to pair up the compounds with their spectra. Assign the peaks in each spectrum to the protons they represent in the molecular structure.

Possible structures:

A laboratory student was converting cyclohexanol to cyclohexyl bromide by using one equivalent of sodium bromide in a large excess of concentrated sulfuric acid. The major product she recovered was not cyclohexyl bromide, but a compound of formula C6H10that gave the following 13CNMR spectrum:

  1. Propose a structure for this product.
  2. Assign the peaks in the 13CNMR spectrum to the carbon atoms in the structure.
  3. Suggest modifications in the reaction to obtain a better yield of cyclohexyl bromide.

A new chemist moved into an industrial lab where work was being done on oxygenated gasoline additives. Among the additives that had been tested, she found an old bottle containing a clear, pleasant-smelling liquid that was missing its label. She took the quick NMR spectrum shown and was able to determine the identity of the compound without any additional information. Propose a structure and assign the peaks. (Hint: This is a very pure sample.)
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