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Ch. 12 - Infrared Spectroscopy and Mass Spectrometry
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 12 - Infrared Spectroscopy and Mass SpectrometryProblem 13a,b,c
Chapter 12, Problem 13a,b,c

Convert the following infrared wavelengths to cm-1. (a) 6.24 πœ‡m, typical for an aromatic C=C (b) 3.38 πœ‡m, typical for a saturated C-H bond (c) 5.85 πœ‡m, typical for a ketone carbonyl

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1
Step 1: Understand the relationship between wavelength (πœ†) and wavenumber (πœˆΜ…). The wavenumber is the reciprocal of the wavelength and is expressed in cm⁻¹. The formula is: Ξ½Μ…=1Ξ», where πœ† is the wavelength in cm.
Step 2: Convert the given wavelengths from micrometers (πœ‡m) to centimeters (cm). Recall that 1 πœ‡m = 10⁻⁴ cm. Multiply each wavelength by 10⁻⁴ to convert to cm.
Step 3: Apply the formula for wavenumber to each converted wavelength. For example, for part (a), substitute the converted wavelength value into the formula Ξ½Μ…=1Ξ».
Step 4: Repeat the calculation for parts (b) and (c) using their respective converted wavelengths. Ensure that each calculation uses the correct wavelength value in cm.
Step 5: Verify the units of the final wavenumber values to ensure they are expressed in cm⁻¹, as this is the standard unit for infrared spectroscopy.

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Key Concepts

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

Infrared Spectroscopy

Infrared spectroscopy is a technique used to identify molecular structures by measuring the absorption of infrared light by a sample. Different functional groups absorb infrared radiation at characteristic wavelengths, allowing chemists to deduce the presence of specific bonds or groups in a molecule.
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Wavelength and Wavenumber Conversion

Wavelength is the distance between successive peaks of a wave, typically measured in micrometers (πœ‡m) in infrared spectroscopy. Wavenumber, expressed in cm-1, is the reciprocal of the wavelength and is commonly used in spectroscopy because it directly relates to energy levels. The conversion from πœ‡m to cm-1 involves taking the reciprocal of the wavelength in centimeters.
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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 infrared spectroscopy, different functional groups absorb infrared light at specific wavelengths, allowing for their identification. For example, C=C bonds, C-H bonds, and carbonyl groups each have distinct absorption features that can be correlated with their respective infrared wavelengths.
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Related Practice
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Predict the characteristic infrared absorptions of the functional groups in the following molecules.

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Four infrared spectra are shown, corresponding to four of the following compounds. For each spectrum, determine the structure and explain how the peaks in the spectrum correspond to the structure you have chosen.

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