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Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 62c
Chapter 14, Problem 62c

Five compounds are shown for each of the IR spectra below. Indicate which of the five compounds is responsible for each spectrum.
c.
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Step 1: Analyze the IR spectrum provided. The spectrum shows a strong absorption around 1700 cm⁻¹, which is characteristic of a carbonyl (C=O) functional group. This indicates the presence of a compound with a ketone, aldehyde, or similar carbonyl-containing functional group.
Step 2: Look for additional peaks in the spectrum. There is a broad absorption around 3000 cm⁻¹, which corresponds to C-H stretching vibrations. This suggests the compound contains alkyl or aromatic groups.
Step 3: Compare the IR spectrum with the provided compounds. Compound (i) contains a ketone group (C=O) and alkyl groups, which matches the strong absorption at 1700 cm⁻¹ and the C-H stretching around 3000 cm⁻¹. Compound (ii) also contains a carbonyl group, but it is part of an aldehyde, which would typically show a distinct peak around 2700 cm⁻¹ for the aldehyde C-H stretch, not observed here.
Step 4: Eliminate compounds that do not match the spectrum. Compound (iii) contains a nitrile group (C≡N), which would show a sharp peak around 2200 cm⁻¹, not present in the spectrum. Compound (iv) contains an alcohol group (-OH), which would show a broad peak around 3200-3600 cm⁻¹, also not observed.
Step 5: Based on the analysis, compound (i) is the most likely match for the IR spectrum, as its functional groups and structure align with the observed peaks.

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Key Concepts

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

Infrared Spectroscopy (IR)

Infrared spectroscopy is an analytical technique used to identify molecular structures based on the absorption of infrared light by a sample. Different functional groups absorb characteristic wavelengths of IR radiation, leading to distinct peaks in the spectrum. By analyzing these peaks, chemists can deduce the presence of specific bonds and functional groups in a compound.
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Wavenumbers and Transmittance

In IR spectroscopy, wavenumbers (measured in cm⁻¹) represent the frequency of the absorbed light, while transmittance indicates the amount of light that passes through the sample. Peaks in the spectrum correspond to specific wavenumbers where absorption occurs, indicating the presence of certain functional groups. A lower transmittance value at a given wavenumber signifies stronger absorption.
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Functional Group Identification

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In IR spectroscopy, each functional group has a unique absorption pattern, allowing chemists to identify them based on the spectrum. For example, O-H bonds typically show strong absorption around 3200-3600 cm⁻¹, while C=O bonds appear around 1700 cm⁻¹.
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Related Practice
Textbook Question

The IR spectrum of compound A with a molecular formula of C5H12O is shown below. Compound A is oxidized to give compound B, a ketone with a molecular formula of C5H10O. When compound A is heated with H2SO4, compounds C and D are obtained. Considerably more D is obtained than C. Reaction of compound C with O3, followed by treatment with dimethyl sulfide, gives two products: formaldehyde and compound E, with a molecular formula of C4H8O. Reaction of compound D with O3, followed by treatment with dimethyl sulfide, gives two products: compound F, with a molecular formula of C3H6O, and compound G, with a molecular formula of C2H4O. What are the structures of compounds A through G?

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Textbook Question

Which one of the following five compounds produced the IR spectrum shown below?

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Textbook Question

Give approximate wavenumbers for the major characteristic IR absorption bands that would be given by each of the following compounds:

e.

f.

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Textbook Question

How can IR spectroscopy distinguish between 1-hexyne, 2-hexyne, and 3-hexyne?

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Textbook Question

Give approximate wavenumbers for the major characteristic IR absorption bands that would be given by each of the following compounds:

a.

b.

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Textbook Question

Each of the IR spectra shown below is accompanied by a set of four compounds. In each case, indicate which of the four compounds is responsible for the spectrum.

a.

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