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Introduction
The frequency of the stretching vibration of the carbonyl group is studied using the infrared spectrum. The force constant of the bond is affected by the electron distribution in the bond and on adjacent atoms. Substituents that withdraw electrons from the carbonyl group will increase the force constant, thereby increasing the observed stretching frequency.
There are two effects that are responsible for the shift in the C=O frequency: inductive (polarization of σ-bonds) and mesomeric (overlap of lone pairs with the carbonyl π-orbitals). Reduced mass changes due to substituents should be small, and hydrogen bonding will be minimized by using a nonpolar solvent.
Procedure
The instructor will provide you with a selection of reagents, all of which have the same general formula : CH3-C(O)Y , where Y is the substituent. Prepare a 5% solution of each reagent, using carbon tetrachloride as a solvent, and acquire the infrared spectrum of each, using a 0.1 mm fixed path liquid cell (the reference should be neat carbon tetrachloride in a matched cell). Determine the location of the maximum absorption of the carbonyl band (~1700 cm-1) as accurately as possible.
Calculations
Determine the shift (relative to acetaldehyde: Y= H) in the carbonyl stretching frequency for each substituent. Plot the shift vs. σ and σ* : determine the degree of correlation for each.