THEORETICAL PROBLEM 29 Infrared Spectroscopy (IR) — Analytical Chemistry — Spectroscopy Chemistry Question
Infrared Spectroscopy (IR)
THEORETICAL PROBLEM 29
Infrared Spectroscopy (IR)
Substitution of an amino group on the para position of acetophenone shifts the C=O frequency from about 1685 to 1652 cm−1, whereas a nitro group attached to the para position yields a C=O frequency of 1693 cm−1. Explain the shift for each substituent from the 1685 cm−1 base value for acetophenone.
Model Answer
Resonance (conjugation) effect: the amino group pushes electron density into the ring and into the carbonyl group resulting in a lower frequency carbonyl group (more single bond character). A nitro group withdraws electrons resulting in higher frequency carbonyl absorption (more double bond character).
Conjugation of a C=C double bond with either a carbonyl group or another double bond provides the multiple bond with more single-bond character (through resonance, as the following example shows [VISUAL]), a lower force constant K, and thus a lower frequency of vibration. For example, the vinyl double bond in styrene gives an absorption band at 1630 cm−1. Esters show a very strong band for the C=O group that appears in the range of 1750–1735 cm−1 for simple aliphatic esters. The C=O band is shifted to lower frequencies when it is conjugated to a C=C or phenyl group.
(Hint: [VISUAL] μ, reduced mass; c, speed of light).
Assign a structure to each of the spectra shown. Choose from the following 5-carbon esters:
[VISUAL]
Spectrum A:
[VISUAL]
Spectrum B:
[VISUAL]
Model Answer
Spectrum A : Methyl acrylate. The absorption band appears at 1726 cm−1 belong to the C=O group that conjugates to double carbon-carbon double bond. Similarly, the C=C bond in this molecule has the absorption bond at 1639 cm−1 due to the stretching vibration.
Spectrum B : Allyl acetate. The stretching vibrations of C=O and C=C double bonds appear at the normal positions for these vabrations, at 1743 and 1650 cm−1, respectively.
There are only the separated C=C and C=O double bonds in vinyl propionate and allyl acetate, so the stretching bands appear at the normal positions.