Look up the IR stretching frequency for an acyclic ketone (like acetone) and compare that...

Question:

Look up the IR stretching frequency for an acyclic ketone (like acetone) and compare that frequency to the IR stretching frequency for an alpha,beta-unsaturated ketone (like methyl vinyl ketone or but-3-en-2-one). Which has a stronger C=O bond based on IR stretching frequency? Use resonance structures to explain this observation?

Infrared Spectroscopy:

Every bond vibrates at a specific frequency. A molecule can absorb radiation when the energy of the radiation matches the energy of the vibration as long as the bond has a dipole moment that can change over time. Infrared spectroscopy thus uses light to determine the types of bonds present in a molecule and the strength of those bonds. Peaks for a certain type of bond appear in a general region of an IR spectra, however, the IR spectra of two molecules will never be identical since two molecules which contain the same type of bond have slightly different environments around that bond. The positions, shapes, and intensity of peaks can be used as diagnostic information for the presence of certain types of bonds.

Answer and Explanation:

For acyclic ketones the stretch appears in the 1720-1780 cm{eq}^{-1} {/eq} range. For an alpha-beta-unsaturated ketone the stretch appears at 1700-1675 cm{eq}^{-1} {/eq}

The wavelength of the photon that induces these C=O stretches is larger for the alpha-beta-unsaturated ketone than for the acyclic ketone. Since the wavelength is larger that means that the energy of the stretch is smaller and so the C=O bond in the alpha-beta-unsaturated ketone is weaker. This can be explained through the structures of these ketones. An aliphatic ketone has a C=O double bond, however an alpha-beta-unsaturated ketone has a resonance structure where there is a C-O single bond. The structures for both types of ketones is below

Double are stronger than single bonds and since the C=O double bond has some single bond character in the alpha-beta-unsaturated ketone the bond is weaker in this ketone.


Learn more about this topic:

Infrared Spectroscopy & Molecule Identification
Infrared Spectroscopy & Molecule Identification

from AQA A-Level Chemistry: Practice and Study Guide

Chapter 2 / Lesson 13
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