Ch. 12 - Infrared Spectroscopy and Mass Spectrometry

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 12 - Infrared Spectroscopy and Mass Spectrometry

Problem 26b

Chapter 12, Problem 26b

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 143 °C. GC–MS analysis of the distillate showed one major product with a few minor impurities. The mass spectrum of the major product is shown here.
(b) Explain why the molecular ion is or 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.
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Verified step by step guidance

Step 1: Analyze the reaction sequence. The student added 1-bromobutane to magnesium in dry ether, initiating the formation of a Grignard reagent (butylmagnesium bromide). This reagent reacts with acetone to form a tertiary alcohol after hydrolysis with dilute acid.

Step 2: Identify the major product. The boiling point of 143 °C and the reaction with acetone suggest the major product is 2-methyl-2-butanol, a tertiary alcohol formed from the Grignard reaction.

Step 3: Examine the mass spectrum. The molecular ion (M⁺) peak corresponds to the molecular weight of the major product. For 2-methyl-2-butanol, the molecular weight is 88 g/mol, so the peak at m/z = 88 is likely the molecular ion.

Step 4: Explain the visibility of the molecular ion. The molecular ion is visible because tertiary alcohols are relatively stable under electron ionization conditions, allowing the molecular ion to persist without extensive fragmentation.

Step 5: Identify likely fragment ions. Common fragments include m/z = 73 (loss of a methyl group, CH₃), m/z = 55 (loss of an ethyl group, C₂H₅), and m/z = 43 (loss of isopropyl group, C₃H₇). These peaks are consistent with the fragmentation pattern of 2-methyl-2-butanol.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Grignard Reagents

Grignard reagents are organomagnesium compounds formed by the reaction of alkyl or aryl halides with magnesium in dry ether. They are highly reactive and can act as nucleophiles, attacking electrophiles such as carbonyl compounds. In this experiment, 1-bromobutane reacts with magnesium to form a Grignard reagent, which subsequently reacts with acetone to produce a tertiary alcohol.

Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. It provides information about the molecular weight and structure of compounds. In the context of the question, the mass spectrum shows peaks corresponding to different ions, allowing for the identification of the major product and its fragmentation pattern, which is crucial for understanding why the molecular ion may or may not be visible.

Fragmentation Patterns

Fragmentation patterns refer to the specific ways in which a molecule breaks apart during mass spectrometry. These patterns are influenced by the structure of the molecule and can lead to the formation of characteristic ions. In the provided mass spectrum, the presence of peaks at m/z 43 and 73 suggests specific fragments that can help deduce the structure of the major product, while the absence of a molecular ion peak may indicate that the compound is unstable or has undergone significant fragmentation.

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Common Splitting Patterns

Related Practice

Textbook Question

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

(a) sharp, 2254 cm^–1

(b) very broad, centered about 3330 cm^–1

(d) broad with spikes at 3367 and 3292 cm^–1

(e) strong, sharp 1717 cm^–1

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Textbook Question

A common lab experiment is the dehydration of cyclohexanol to cyclohexene.

(a) Explain how you could tell from the IR spectrum whether your product was pure cyclohexene, pure cyclohexanol, or a mixture of cyclohexene and cyclohexanol. Give approximate frequencies for distinctive peaks.

(b) Explain why mass spectrometry might not be a good way to distinguish cyclohexene from cyclohexanol.

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Textbook Question

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.

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Textbook Question

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 143 °C. GC–MS analysis 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.

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Textbook Question

Three common lab experiments are shown. In each case, describe how the IR spectrum of the product would differ from that of the reactant. Give approximate frequencies for distinctive peaks in the IR spectrum of the reactant and also that of the product.

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Textbook 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 to $\sqrt{\frac{k}{m}}$ , 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 3000 cm^–1 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 the CDCl₃. What functional group will appear to be present in this IR spectrum as a result of the CDCl₃ impurity?

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