Aldehydes C-H stretch

Infrared spectra of (a) heptan-2-one and (b) butyraldehyde. Both the ketone and the aldehyde show intense carbonyl absorptions near 1720 cm-1. In the aldehyde spectrum, there are two peaks (2720 and 2820 cm-1) characteristic of the aldehyde C — H stretch. 

Compound 1 This spectrum is most useful for what it does not show. There is a carbonyl absorption at 1714 cm-1 and little else. There is no aldehyde C—H, no hydroxyl O—H, and no N—H. The weak absorption at 3400 cm-1 is probably an overtone of the strong C=0 absorption. The carbonyl absorption could indicate an aldehyde, ketone, or acid, except that the lack of aldehyde C—H stretch eliminates an aldehyde, and the lack of O — H stretch eliminates an acid. There is no visible C=C stretch and no unsaturated C—H absorption above 3000 cm-1, so the compound appears to be otherwise saturated. The compound is probably a simple ketone. 

Gilman and Marion (12) examined a sample of -pelletierine sulfate prepared by C. Tanret in 1880. The IR-spectrum of the base showed carbonyl absorption at 1710 cm i but no absorption due to an aldehyde C—H stretching vibration. The NMR-spectrum showed a three-proton... 

C-H Stretch, aldehyde hydrogen (—CHO), consists of a pair of weak bands, one at 2860-2800 cm and the other at 2760-2700 cm It is easier to see the band at the lower frequency because it is not obscured by the usual C—H bands from the alkyl chain. The higher-frequency aldehyde C—H stretch is often buried in the aliphatic C—H bands. 

The doublet that is observed in the range 2860-2700 cm for an aldehyde is a result of Fermi resonance (p. 17). The second band appears when the aldehyde C—H stretching vibration is coupled with the first overtone of the medium intensity aldehyde C—H bending vibration appearing in the range 1400-1350 cm . ... 

Aldehyde (—CHO) " Aldehydic C—H stretching, m 2690-2830 (two peaks often seen)... 

Aldehydes and ketones Aldehyde C-H stretching Aliphatic aldehyde C=0 stretching Aliphatic ketone C=0 stretching Aromatic aldehyde C=0 stretching Aromatic ketone C=0 stretching... 

Examine the spectrum of 2-methylbenzaldehyde shown below in Figure 4.6 and identify the aldehyde C-H stretching band. Describe the effect responsible for the doublet formation. 

Ashdown and Kletz [60] have reported a number of such cases, and have shown that a corresponding reduction in the intensities of the aldehydic C—H stretching and deformation modes also occurs, whilst a new band appears near 1020—1110 cm which may be associated with the C—O— linkage. 

Next consider the spectra of an aliphatic aldehyde, hexanal (Figure 12.22), in comparison to the aromatic aldehyde benzaldehyde. In the case of the aliphatic aldehyde note that in the IR spectrum, while the carbonyl intensity remains more intense than the aldehyde C—H stretch (one leg of the doublet is resolved), the aliphatic C—H stretch (11 C—H groups) is more intense than the single carbonyl oscillator. In the Raman spectrum, however, the unconjugated carbonyl intensity has clearly collapsed relative to the aldehyde C—H (2740 cm ) stretch which it is... 

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