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Chapter 12: Q30P (page 642)

These five structures all have distinguishing absorption in the IR. Match each structure with its characteristic absorption.

Structure 1

Structure 2

Structure 3

Structure 4

Structure 5

(a)sharp, 2254 cm-1 (b) very broad, centered about3330cm-1 (c) strong, slightly broadened, 1645cm-1 (d) broad with spikes at 3367cm-1and 3392cm-1(e) strong, sharp 1717cm-1

Short Answer

Expert verified

The characteristic absorption of given structures are as follows:

Structure 1:

very broad, centered about 3330 cm-1.

Structure 2:

strong, sharp 1717 cm-1 .

Structure 3:

broad with spikes at 3367cm-1 and 3392cm-1.

Structure 4:

sharp, 2254cm-1.

Structure 5:

strong, slightly broadened, 1645cm-1.

Step by step solution

01

Frequencies of functional groups

Different functional groups will give different IR frequencies value.The frequency values experimentally determined in cm-1.

02

Characteristic absorption of structure 1

The given Structure is as follows:

Structure 1

The functional group in the given structure is 2o-alcohol. The frequency in cm-1 for 2o-alcohol is 3330cm-1 .

Therefore, it gives very broad, centered IR absorption about3330cm-1 .

03

Characteristic absorption of structure 2

The given Structure is as follows:

Structure 2

The functional group in the given structure is ketone. The frequency in cm-1 for ketone is 1717cm-1.

Therefore, it gives strong, sharp IR absorption at 1717cm-1 .

04

Characteristic absorption of structure 3

The given Structure is as follows:

Structure 3

The functional group in the given structure is 1o-amine. The frequency in cm-1 for 1o-amine is 3367cm-1 and 3292 cm-1 .

Therefore, it gives broad IR absorption with spikes at 3367cm-1 and 3292 cm-1 .

05

Characteristic absorption of structure 4

The given Structure is as follows:

Structure 4

The functional group in the given structure is cyanide. The frequency in cm-1 for cyanide is 2254cm-1

Therefore, it gives sharp IR absorption at 2254cm-1 .

06

Characteristic absorption of structure 5

The given Structure is as follows:

Structure 5

The functional group in the given structure is N-substituted amide. The frequency in cm-1 for N-substituted amide is 1645cm-1 .

Therefore, it gives strong, slightly broadened IR absorption at 1645cm-1.

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Most popular questions from this chapter

Question: (A true story.) While organizing the undergraduate stockroom, a new chemistry professor found a half-gallon jug containing a cloudy liquid (bp 100–105 °C), marked only “STUDENT PREP.” She ran a quick mass spectrum, which is printed below. As soon as she saw the spectrum (without even checking the actual mass numbers), she said, “I know what it is.”

(a) What compound is the “student prep”? Any uncertainty in the structure?

(b) Suggest structures for the fragments at 136, 107, and 93. Why is the base peak (at m/z 57) so strong?

Question: Consider the following four structures, followed by mass spectral data. Match each structure with its characteristic molecular ion or fragment. In each case, give a likely structure of the ion responsible for the base peak

(a) base peak at 105

(b) base peak at 72

(c) doublet at 198 and 200, base peak at 91

(d) base peak at 91, large peak at 43

The ultimate test of fluency in MS and IR is whether you can determine a moderately complex structure from just the MS and the IR, with no additional information. The IR and MS of a compound are shown below. Use everything you know about IR and MS, plus reasoning and intuition, to determine a likely structure. Then show how your proposed structure is consistent with these spectra.

A laboratory student added 1-bromobutane to a flask containing dry ether and magnesium turnings. An exothermic reaction resulted, and the ether boiled vigorously for several minutes. Then she added acetone to the reaction mixture and the ether boiled even more vigorously. She added dilute acid to the mixture and separated the layers. She evaporated the ether layer, and distilled a liquid that boiled at 143C∘. role="math" localid="1649504068494" GC-MSanalysis of the distillate showed one major product with a few minor impurities. The mass spectrum of the major product is shown here.

(a) Draw out the reactions that took place and show the product that was formed.

(b) Explain why the molecular ion is not visible in the mass spectrum, and show what ions are likely to be responsible for the strong peaks at m/z, 59 and 101.

Question: (a) Use the mass spectrum to propose a molecular formula. How many elements of unsaturation are there? (b) Use the IR spectrum to determine the functional group(s), if any. (c) Propose one or more structures for this compound. What parts of the structure are uncertain? If you knew that hydrogenation of the compound gives n-octane, would the structure still be uncertain? (d) Propose structures for the major fragments at 39, 67, 81, and 95 in the mass spectrum.
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