Spectroscopy of Ketones and Aldehydes

Aldehydes and ketones show a strong C=0 bond absorption in the IR region from 1660 to 1770 as the spectid of benzaldehyde aiid cyclohexanone 

Tlie values given in Table 19.2 are remarkably constant from one aldehyde or ketone to another. As a result, IR spectroscopy is a powerful tool for identifying the kind of a carbonyl group in a molecule of unknown structure. An unknown that shows an IR absorption at 1730 cm is almost certainly an aldehyde rather than a ketone an unknown that shows an IR absorption at 1750 cm is almost certainly a cyxlopentanone, and so on. 

Problem 19.23 l-low might you use IR spectroscopy to determine whether reaction between 2-cyclo- 

Where would you expect each of the following compounds to absorb in the IK spectrum  

Aldehyde protons (RCHO) absorb near 10 5 in the Ml N MR spectrum and are very distinctive because no other absorptions occur in this region. The aldehyde proton shows spin-spin coupling with protons on the neighboring carbon, with coupling constant / 3 Hz, Acetaldehyde, tor example, shows a quartet at 9.8 5 

The C=0 stretching absorption is one of the most important and characteristic absorptions because it varies predictably as a result of differences in structure. The absorption, which is extremely intense, occurs in the vicinity of 1700 cm . Because the C=0 bond is stronger than the C=C bond, the carbonyl absorption occurs at higher wavenumber. 

Simple ketones absorb at 1710-1715 cm simple aldehydes absorb at 1720-1725 cm. Aldehydes also have a characteristic absorption near 2710 cm for the aldehyde C—H bond. Cyclohexanones have carbonyl absorptions at the same position as simple acyclic ketones. However, decreased ring size results in shifts to higher wavenumber. Cyclopentanone, cyclobutanone, and cyclopropanone absorb at 1745, 1780, and 1850 cm , respectively. 

CHAPTER 14 ALDEHYDES AND KETONES NUCLEOPHILIC ADDITION REACTIONS 

The carbonyl-group carbon atoms of aldehydes and ketones have characteristic NMR resonances in the range 190 to 215 8. Since no other kinds of carbons absorb in this range, the presence of an NMR absorption near 200 8 is clear evidence for a carbonyl group. Saturated aldehyde or ketone carbons usually absorb in the region from 200 to 215 8, while aromatic and a, 8-unsaturated carbonyl carbons absorb in the 190 to 200 8 region. 

FIGURE 14.17 Mass spectrum of 5-methylhexan-2-one. The peak at mjz = 58 is due to McLafferty rearrangement. The abundant peak at m/z = 43 is due to a cleavage at the more highly substituted side of the carbonyl group. Note that the peak due to the molecular ion is very small. 

The exact position of the C=0 absorption is diagnostic of the nature of the carbonyl group. As the data in Table 19.2 indicate, saturated aldehydes usually 

Where would you expect each of the following compounds to absorb in the IR spectrum (a) 4-Penten-2-one (b) 3-Penten-2-one 

Another nucleophilic addition reaction—this time in reverse—is involved in the chemical defense mechanism by which the millipede Aphe-loria corrugata protects itself from predators. When attacked by ants, it secretes the cyanohydrin mandelonitrile and an enzyme that catalyzes the decomposition of mandelonitrile into benzaldehyde and HCN. The millipede actually protects itself by discharging poisonous HCN at its attackers. 

TABLS 19.2 Infrared Absorptions of Some Aldehydes and Ketones  

CHAPTER 19 Aldehydes and Ketones NudeopMIU Addition Reactions 

Fred Warren McLafferty (1923-) was born in Evanston, Illinois, and received his Ph.D. in 1950 at Cornell University. He was a scientist at the Dow Chemical Company from 1950 to 1964 before becoming professor of chemistry at Purdue University. In 1968, he returned to Cornell University as professor. 

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Spectroscopy of Aldehydes and Ketones A Deeper Look— Enantioselective Synthesis... 

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