Question

How could you distinguish between diphenyl ketone (also called benzophenone) and benzaldehyde using ir only?

Answer 1

To differentiate between diphenyl ketone and benzaldehyde using IR spectroscopy, look for the aldehyde C-H stretch in the 2700-2850 cm-1 range unique to benzaldehyde and the sharper ketone C=O stretch around 1700-1725 cm-1.

Step-by-step explanation:

To distinguish between diphenyl ketone (benzophenone) and benzaldehyde using infrared (IR) spectroscopy, one can focus on the characteristic absorption frequencies of their functional groups. Benzaldehyde features an aldehyde functional group, whereas diphenyl ketone features a ketone functional group. In IR spectroscopy, aldehydes typically show a distinct absorption around 1700-1725 cm-1 due to the C=O stretching vibration as well as a peak in the 2700-2850 cm-1 range due to the C-H stretch of the aldehyde hydrogen. Diphenyl ketone, though also displaying the C=O stretch near 1700-1725 cm-1, lacks the aldehyde C-H stretch, and hence will not show the latter peak in its IR spectrum.

Additionally, the absorption peak for the carbonyl group in ketones is typically sharper and more intense compared to the broader absorption seen in aldehydes. Thus, by carefully analyzing the IR spectrum and observing the presence or absence of characteristic peaks, one can distinguish between these two compounds.

Answer 2

Answer is: benzaldehyde will have one or two bands of moderate intensity in the region 2695 - 2830 cm⁻¹ because of C-H stretch. Presence of a moderate band near 2720 cm⁻¹ is helpful in determining whether or not a compound is an aldehyde. Aldehyde, always have peak around 2720 cm⁻¹.