15. Analytical Techniques:IR, NMR, Mass Spect

How many signals would be expected in the ¹³C NMR spectrum for the following molecule?

An alkene/aromatic signal will often appear ~ 160–170 ppm in a ¹³C NMR spectrum. Why?

How many signals would you expect to see in the 13C NMR of the following compounds? In each case, show which carbon atoms are equivalent in the 13C NMR.

(a) Show which carbon atoms correspond with which peaks in the 13C NMR spectrum of butan-2-one (Figure 13-45)

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Hexamethylbenzene undergoes free-radical chlorination to give one monochlorinated product (C12H17Cl) and four dichlorinated products (C12H16Cl2). These products are easily separated by GC-MS, but the dichlorinated products are difficult to distinguish by their mass spectra. Draw the monochlorinated product and the four dichlorinated products, and explain how 13C NMR would easily distinguish among these compounds.

(a) Draw all six isomers of formula C4H8 (including stereoisomers). (b) For each structure, show how many types of H would appear in the proton NMR spectrum. (c) For each structure, show how many types of C would appear in the 13C NMR spectrum. (d) If an unknown compound of formula C4H8 shows two types of H and three types of C, can you determine its structure from this information?

The three isomers of dimethylbenzene are commonly named ortho-xylene, meta-xylene, and para-xylene. 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.

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 C6H10 that gave the following 13C NMR spectrum:

(a) Propose a structure for this product. (b) Assign the peaks in the 13C NMR spectrum to the carbon atoms in the structure. (c) Suggest modifications in the reaction to obtain a better yield of cyclohexyl bromide.

An inexperienced graduate student was making some 4-hydroxybutanoic acid. He obtained an excellent yield of a different compound, whose 13C NMR spectrum is shown here.

(a) Propose a structure for this product. (b) Assign the peaks in the 13C NMR spectrum to the carbon atoms in the structure.

A bottle of allyl bromide was found to contain a large amount of an impurity. A careful distillation separated the impurity, which has the molecular formula C3H6O. The following 13C NMR spectrum of the impurity was obtained:

(a) Propose a structure for this impurity. (b) Assign the peaks in the 13C NMR spectrum to the carbon atoms in the structure. (c) Suggest how this impurity arose in the allyl bromide sample.

How many signals are produced by each of the following compounds in its b. 13C NMR spectrum? 6.

How many signals are produced by each of the following compounds in its b. 13C NMR spectrum? 5.

How many signals are produced by each of the following compounds in its b. 13C NMR spectrum? 4.

Would it be better to use 1H NMR or 13C NMR spectroscopy to distinguish 1-butene, cis-2-butene, and 2-methylpropene? Explain your answer.

How many signals are produced by each of the following compounds in its b. 13C NMR spectrum? 2.

How many signals are produced by each of the following compounds in its b. 13C NMR spectrum? 1.

How can 1,2-, 1,3-, and 1,4-dinitrobenzene be distinguished by b. 13C NMR spectroscopy?

Sketch the following spectra that would be obtained for 2-chloroethanol: d. The proton-coupled 13C NMR spectrum.

Sketch the following spectra that would be obtained for 2-chloroethanol: c. The 13C NMR spectrum.

Compound A, with molecular formula C4H9Cl, shows two signals in its 13C NMR spectrum. Compound B, an isomer of compound A, shows four signals, and in the proton-coupled mode, the signal farthest downfield is a doublet. Identify compounds A and B.

Describe the proton-coupled 13C NMR spectra for compounds 5 in Problem 41, indicating the relative positions of the signals.

Answer the following questions for each compound: a. How many signals are in its 13C NMR spectrum? b. Which signal is at the lowest frequency? 9.

Answer the following questions for each compound: a. How many signals are in its 13C NMR spectrum? b. Which signal is at the lowest frequency? 7.

Answer the following questions for each compound: a. How many signals are in its 13C NMR spectrum? b. Which signal is at the lowest frequency? 8.

Describe the proton-coupled 13C NMR spectra for compounds 1 in Problem 41, indicating the relative positions of the signals.

Describe the proton-coupled 13C NMR spectra for compounds 3 in Problem 41, indicating the relative positions of the signals.

Identify each compound below from its molecular formula and its 13C NMR spectrum. a. C11H22O

Identify the compound with molecular formula C7H14O that gives the following proton-coupled 13C NMR spectrum:

(•) For the molecules in Assessment 15.58, give an approximate chemical shift for each indicated carbon. [The range of correct answers is large here.]. a)

(•) For the molecules in Assessment 15.58, give an approximate chemical shift for each indicated carbon. [The range of correct answers is large here.]. c)

Draw the ¹³C NMR spectrum of each molecule in Assessment 15.48.

Draw the ¹³C NMR spectrum of each molecule in Assessment 15.48.

Draw the ¹³C NMR spectrum of each molecule in Assessment 15.48.

How many signals would you expect in the ¹³C NMR spectrum of each molecule shown? (b)

How many signals would you expect in the ¹³C NMR spectrum of each molecule shown? (c)

Assign each signal in the ¹³C NMR spectra to the molecule shown. (a)

Assign each signal in the ¹³C NMR spectra to the molecule shown. (b)

(•••) Draw the ¹³C NMR spectrum you would expect to see for each of the molecules shown. (a)

(•••) Draw the ¹³C NMR spectrum you would expect to see for each of the molecules shown. (b)

(•••) Draw the ¹³C NMR spectrum you would expect to see for each of the molecules shown. (c)

(••) SPECTROSCOPY INSIGHTS How might you use ¹³C NMR spectroscopy to differentiate between the possible ortho, meta, and para products of the electrophilic aromatic substitution reaction shown?

Sketch the following spectra that would be obtained for 2-chloroethanol: e. The four parts of a DEPT 13C NMR spectrum.

Identify the compound with molecular formula C6H10O that gives the following DEPT 13C NMR spectrum:

The standard 13C NMR spectrum of phenyl propanoate is shown here. Predict the appearance of the DEPT-90 and DEPT-135 spectra.

Different types of protons and carbons in alkanes tend to absorb at similar chemical shifts, making structure determination difficult. Explain how the 13C NMR spectrum, including the DEPT technique, would allow you to distinguish among the following four isomers. (a)

(b)

(c)

(d)