Spectroscopic Analysis of Aldehydes and Ketones

Aldehydes and ketones exhibit several characteristic signals in their infrared (IR) and nuclear magnetic resonance (NMR) spectra. We will now summarize these characteristic signals. 

The carbonyl group produces a strong signal in an IR spectrum, generally around 1715 or 1720 cm However, a conjugated carbonyl will produce a signal at a lower wavenumber as a result of electron delocalization via resonance effects  

Ring strain has the opposite effect on a carbonyl group. That is, increasing ring strain tends to increase the wavenumber of absorption  

In a NMR spectrum, the carbonyl group itself does not produce a signal. However, it has a pronounced eflfect on the chemical shift of neighboring protons. We saw in Section 16.5 that a carbonyl group typically adds approximately +1 ppm to the chemical shift of its neighbors  

43 Compound A has molecular formula CiqHioO and exhibits a strong signal at 1720 cm i in its IR spectrum. Treatment with 1,2-eth-anedithiol followed by Raney nickel affords the product shown below. Identify the structure of compound A. 

The 300-MHz H NMR spectrum of 2-methylpropanal, showing the aldehyde proton as a doublet at low field (8 9.6). 

Chapter 17 Aldehydes and Ketones Nucleophilic Addition to the Carbonyl Group 

The NMR spectrum of 3-heptanone. Each signal corresponds to a single carbon. The carbonyl carbon is the least shielded and appears at 8 210. 

Mass Spectrometry Aldehydes and ketones typically give a prominent molecular ion peak in their mass spectra. Aldehydes also exhibit an M-1 peak. A major fragmentation pathway for both aldehydes and ketones leads to formation of acyl cations (acyhum ions) by cleavage of an alkyl group from the carbonyl. The most intense peak in the mass spectrum of diethyl ketone, for example, is tn/z 57, corresponding to loss of ethyl radical from the molecular ion. 

The chemistry of the carbonyl group is probably the single most important aspect of organic chemical reactivity. Classes of compounds that contain the carbonyl group include many derived from carboxylic acids (acyl chlorides, acid anhydrides, esters, and amides) as well as the two related classes discussed in this chapter aldehydes and ketones. 

UV-VIS Aldehydes and ketones have two absorption bands in the ultraviolet region. Both involve excitation of an electron to an antibonding ir. In one, called a 

FIGURE 17.15 The 200-MHz H NMR spectrum of 2-butanone. The triplet-quartet pattern of the ethyl group is more clearly seen in the scale-expanded insets. 

In addition to a peak for C=0 stretching, the CH=0 group of an aldehyde exhibits two weak bands for C—H stretching near 2720 and 2820 cm  

Mechanistically, these reactions probably proceed through the hydrate of the aldehyde and follow a course similar to that of alcohol oxidation. 

Aldehydes are more easily oxidized than alcohols, which is why special reagents such as PCC and PDC (Section 15.9) have been developed for oxidizing primary alcohols to aldehydes and no further. PCC and PDC are effective not only because they are sources of Cr(VI), but also because they are used in nonaqueous media (dichloromethane). By keeping water out of the reaction mixture, the aldehyde is not converted to its hydrate, which is the necessary intermediate that leads to the carboxylic acid. 

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