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Chapter 12: Q.27P (page 641)

Question: (Another true story.) A student who was checking into her lab desk found an unlabeled sample from a previous student. She was asked to identify the sample. She did an IR spectrum and declared, “It looks like an alkane.” But it seemed too reactive to be an alkane, so she did a GC–MS. The mass spectrum is shown next. Identify the compound as far as you can, and state what part of your identification is uncertain. Propose fragments corresponding to the numbered peaks.

Short Answer

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Answer

The unknown compound is 1-iodo-3-methylbutyne

Step by step solution

01

Introduction

The graph between the intensity (abundance) and mz(mass to charge ratio) values of the ions is known as the mass spectrum. It is highly useful in predicting the structure of the given unknown compounds.

02

Explanation

From the mass spectrum, it can be said that no chlorine (Cl) or bromine (Br) are present in the unknown compound since isotopic peaks are not seen. No peak at value ofmz indicates the absence of benzene ring in the molecule. Again, the peak at value of mzindicates the presence of iodine in the molecule. Lastly, the peak at value of mzshows the presence of isopropyl group. Therefore, the unknown compound will be

The structure of can be drawn as:

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Most popular questions from this chapter

The mass spectrum of n-octane shows a prominent molecular ion peak (m/z 114). There is also a large peak at m/z 57, but it is not the base peak. The mass spectrum of 3,4-dimethylhexane shows a smaller molecular ion, and the peak at mass 57 is the base peak. Explain these trends in abundance of the molecular ions and the ions at mass 57 and predict the intensities of the peaks at masses 57 and 114 in the spectrum of 2,2,3,3-tetramethylbutane.

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.

Question: A C-D (carbon–deuterium) bond is electronically much like a C-H bond, and it has a similar stiffness, measured by the spring constant, k. The deuterium atom has twice the mass (m) of a hydrogen atom, however.

(a) The infrared absorption frequency is approximately proportional tokm , when one of the bonded atoms is much heavier than the other, and m is the lighter of the two atoms (H or D in this case). Use this relationship to calculate the IR absorption frequency of a typical C-D bond. Use as a typical C-H absorption frequency.

(b) A chemist dissolves a sample in deuterochloroform (CDCl3) and then decides to take the IR spectrum and simply evaporates most of theCDCl3 . What functional group will appear to be present in this IR spectrum as a result of theCDCl3 impurity?

Convert the following infrared wavenumbers into wavelengths.

(a) 1600cm-1, typical for an aromatic

(b) 3000cm-1, typical for a saturated bond

(c) 1715cm-1, typical for a ketone carbonyl

(d) 1750cm-1, typical for an ester carbonyl

(e) 2220cm-1, typical for a nitrile

(f) 3300cm-1, typical for an alcohol

Question: Which of the bonds shown in red are expected to have IR-active stretching frequencies?
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