You are asked to design a laboratory procedure to determine the bond stretching…
CHEMISTRY · Multi-part · A Level · Chemistry · Organic analysis and spectroscopy · Organic analysis and spectroscopy
Problem
You are asked to design a laboratory procedure to determine the bond stretching frequency of the carbon–oxygen double bond in gaseous acetaldehyde, CH3CHO. The laboratory owns a tunable light source that can generate continuous radiation from 0.01 cm−1 (longest wavelength) up to 35 000 cm−1 (shortest wavelength).
(a) Select the portion of the electromagnetic spectrum you would tune the light source to in order to excite the C=O stretching mode. Provide a numerical range of wavenumbers (cm−1) typical for this mode and justify your selection.
(b) Describe the key components (besides the light source) required in the experimental setup to detect and record the absorption due to the C=O stretch, explaining the role each component plays.
(c) The laboratory acquires isotopically labeled CH3C18HO. Predict qualitatively how the stretching frequency you measure for the labeled molecule will differ from that of unlabeled acetaldehyde, and explain the physical basis for this shift.