Characteristic Absorptions of Carbonyl Compounds

The IR spectrum of dipropylamine shows a broad N — H stretching absorption centered around 3300 cm . Notice the spike in this broad absorption. 

The C=0 stretching vibrations of simple ketones, aldehydes, and carboxylic acids occur at frequencies around 1710 cm . These frequencies are higher than tho.se for C=C double bonds because the C = 0 double bond is stronger and stiffen 

Infrared spectra of (a) 2-heptanone and (b) butyraldehyde. Both the ketone and the aldehyde show intense carbonyl absorptions near 1710 cm . In the aldehyde spectrum, there are two peaks (2720 and 2820 cm ) characteristic of the aldehyde C — H stretch. 

PROBLEM-SOLVING /—f/yft/7 Real spectra are rarely perfect. Samples often contain traces of water, giving weak absorptions in the O — H region. 

Many compounds oxidize in air. For example, alcohols often give weak C=0 absorptions from oxidized impurities. 

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The Infrared Region 515 12-4 Molecular Vibrations 516 12-5 IR-Active and IR-lnactive Vibrations 518 12-6 Measurement of the IR Spectrum 519 12-7 Infrared Spectroscopy of Hydrocarbons 522 12-8 Characteristic Absorptions of Alcohols and Amines 527 12-9 Characteristic Absorptions of Carbonyl Compounds 528 12-10 Characteristic Absorptions of C—N Bonds 533 12-11 Simplified Summary of IR Stretching Frequencies 535 12-12 Reading and Interpreting IR Spectra (Solved Problems) 537 12-13 Introduction to Mass Spectrometry 541 12-14 Determination of the Molecular Formula by Mass Spectrometry 545... 

The (n, it ) state plays a very important role in the photochemistry of carbonyl compounds and many heterocyclic systems. In conjugated hydrocarbons (it -> it ) transitions are most important and give intense characteristic absorption bands. Because of some overlap forbidden character, the (n - it ) transitions have low probability and hence weak absorption bands. 

As we shall see, n —> tt singlet and triplet states of carbonyl compounds play an important role in photochemistry. Aldehydes and ketones display all the characteristics of absorption, fluorescence, phosphorescence, and intersystem crossing (5, —> T,) illustrated in Figure 28-1. Generally, they are more efficient at intersystem crossing than are unsaturated hydrocarbons, perhaps because the energies of the S and T states involved are not widely different. 

The investigations performed using IR-spectroscopy showed that the spectrum of the hydrocarbon fraction contains 2920, 2960, 2850, 1380, 1460 cm absorption bands, typical for C-H oscillations. The IR spectrum of carbonyl compounds reveals 1730, 1250 cm" absorption bands characteristic for C=0 groups. The spectrum of alcohols has 1730, 1250,... 

Solution The spectrum shows an intense absorption at 1725 cm- due to a carbonyl group (perhaps an aldehyde, -CHO), a series of weak absorptions from 1800 to 2000 cm-1, characteristic of aromatic compounds, and a C—H absorption near 3030 cm-1, also characteristic of aromatic compounds. In fact, the compound is phenylacetaldehyde. 

Both ionic forms have the same set of characteristic bands in their IR spectra and thus cannot be identified in solution. The 3IP NMR spectrum contains only one averaged signal. The IR study revealed tautomeric transformations in solution. The characteristic absorption band of the P—H bond and carbonyl group appeared in solution spectra that is possible only on dissociation of the a-hydroxyalkyl fragment, present only in the second tautomeric form. An X-ray single-crystal study showed the compound (128) to be in a cyclic form. 

Solvent polarity may affect the absorption characteristics, in particular since the polarity of a molecule usually changes when an electron is moved from one orbital to another. Solvent effects of up to 20 nm may be observed with carbonyl compounds. Thus the absorption of acetone occurs at 279 nm in n-hexane, 270 nm in... 

Recently from a comparison of the infra-red absorption spectrum of this compound with that of o-benzoquinone Glowiak [10] came to the conclusion that dinitro-benzenediazo-oxide has a quinonoid structure. Both substances show the presence of the strong absorption band of the carbonyl group 1666 cm-1 for dinitrobenzene-diazo-oxide and 1680 cm-1 for o-benzoquinone. In addition dinitrobenzenediazo-oxide gives a band with a frequency of 2190 cm-1, characteristic of a double bond between nitrogen atoms. (Some derivatives of this compound may also have the diazo structure (Ha), which is discussed later on.)... 

All of these compounds have characteristic carbonyl absorptions that help identify them these are listed in Table 21.3. 

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