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Chapter 7: Problem 164

Indicate the features you would expect for the infrared and n.m.r. spectra of the following substances. (a) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}\) (expect a long-range n.m.r. coupling of \(3 \mathrm{cps}\) ) (C) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CCH}_{2} \mathrm{CH}_{3}\)

Short Answer

Expert verified
For all three substances, we expect an IR absorption around 2100-2260 cm^{-1} due to the carbon-carbon triple bond stretching in the alkyne functional group. In the NMR spectra, compound (a) will have two singlet peaks for methyl and terminal methyl protons; compound (b) will have a singlet peak for methyl protons and a doublet for the terminal alkyne proton due to long-range coupling; compound (c) will have separate peaks for terminal methylene, adjacent methylene, and interior protons, with specific splitting patterns depending on proton coupling constants.

Step by step solution

01

Identify the functional groups

In the compound \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{3}\), we have an alkyne functional group formed by the carbon-carbon triple bond.
02

Predict the infrared features

For the alkyne functional group, we should expect an IR absorption at around 2100-2260 \(cm^{-1}\) for the carbon-carbon triple bond stretching. #Step 2: Compound (a) NMR features#
03

Identify the distinct proton environments

There are two distinct proton environments in this compound: the four equivalent methyl protons and two terminal methyl protons.
04

Predict the NMR features

The methyl protons should appear as two singlet peaks in the proton NMR spectrum. #Step 3: Compound (b) Infrared features#
05

Identify the functional groups

In the compound \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}\), we also have the alkyne functional group with a carbon-carbon triple bond.
06

Predict the infrared features

For the alkyne functional group, we will also expect an IR absorption at around 2100-2260 \(cm^{-1}\) for the carbon-carbon triple bond stretching. #Step 4: Compound (b) NMR features#
07

Identify the distinct proton environments

There are two distinct proton environments: the three equivalent methyl protons and the terminal alkyne proton.
08

Predict the NMR features

Due to the long-range coupling mentioned in the problem, the terminal alkyne proton should have a splitting pattern of a doublet due to coupling with the methyl protons. The methyl protons should appear as a singlet peak. #Step 5: Compound (c) Infrared features#
09

Identify the functional groups

In the compound \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CCH}_{2} \mathrm{CH}_{3}\), we have the alkyne functional group, formed by the carbon-carbon triple bond.
10

Predict the infrared features

The alkyne functional group will also produce an IR absorption at around 2100-2260 \(cm^{-1}\) for the carbon-carbon triple bond stretching. #Step 6: Compound (c) NMR features#
11

Identify the distinct proton environments

There are four distinct proton environments: the two terminal methylene protons, the methylene protons adjacent to carbon-carbon triple bond, and the two interior protons.
12

Predict the NMR features

As the carbon-carbon triple bond affects the adjacent proton environments, we should expect each proton environment to produce a separate peak in the NMR spectrum. The specific splitting patterns will depend on the relative coupling constants between the different protons.

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Most popular questions from this chapter

Suppose you were given four unlabeled bottles, each of which is known to contain one of the following compounds: n-pen-tane, 1-pentene, 2-pentyne, and 1-pentyne. Explain how you could use- simple chemical tests (preferably test-tube reactions) to identify the contents of each bottle. (Note that all four compounds are low-boiling liquids.)

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