For the reaction performed in this experiment, the IR spectra for the reactants and product are shown below. For the IR spectra of acetone and the product, identify the important IR signals for each spectrum. Specifically: for the IR spectrum of acetone, identify the signal which indicates the presence of the C=O bond and the signal which represents the CH bond (indicate the hybridization of the carbon). For the IR spectrum of the product, indicate which signal represents the CH bond (hybridization?) and which signal represents the O-H bond.
The Correct Answer and Explanation is:
Key IR Absorptions
Acetone IR Spectrum:
- C=O (Carbonyl Stretch):
- Wavenumber: ~1715 cm⁻¹
- Explanation: This strong, sharp signal corresponds to the stretching vibration of the carbonyl (C=O) bond, a key functional group in ketones like acetone.
- C–H Stretch (sp³ Hybridized):
- Wavenumber: ~2850–2960 cm⁻¹
- Explanation: These medium-intensity signals represent the stretching vibrations of C–H bonds where the carbon is sp³ hybridized (from the methyl groups in acetone).
Product IR Spectrum:
Assuming the product is an alcohol formed by reduction of acetone:
- O–H Stretch (Hydroxyl group):
- Wavenumber: ~3200–3600 cm⁻¹
- Explanation: This broad, strong absorption is indicative of hydrogen-bonded O–H stretching from alcohols. Its breadth distinguishes it from other X–H stretches.
- C–H Stretch (sp³ Hybridized):
- Wavenumber: ~2850–2960 cm⁻¹
- Explanation: Similar to acetone, the product contains sp³ C–H bonds, typically from CH₃ or CH₂ groups, showing in this same region.
Explanation
Infrared (IR) spectroscopy is a vital analytical technique used to identify functional groups in organic molecules by measuring their vibrational transitions. Each functional group absorbs IR radiation at characteristic frequencies, allowing us to interpret molecular structures based on their IR spectra.
In acetone, a simple ketone, the most prominent feature is the carbonyl (C=O) stretch around 1715 cm⁻¹. This is a strong, sharp absorption due to the polar double bond of the carbonyl group. The high polarity of the C=O bond results in a strong dipole moment change during vibration, making it one of the most diagnostic signals in IR spectroscopy. In addition, acetone’s methyl C–H stretches appear in the region of 2850–2960 cm⁻¹, characteristic of sp³ hybridized carbon atoms bonded to hydrogen.
If acetone is reduced to an alcohol (e.g., 2-propanol), its IR spectrum changes significantly. Most notably, the carbonyl peak at ~1715 cm⁻¹ disappears, and a new O–H stretch appears between 3200–3600 cm⁻¹. This broad, strong signal is due to hydrogen bonding among hydroxyl groups, which causes the band to broaden significantly compared to non-hydrogen-bonded X–H stretches. The C–H stretches from sp³ carbons remain visible at 2850–2960 cm⁻¹, indicating the retention of alkyl groups.
By comparing these IR spectra, one can confirm the transformation of acetone (a ketone) into an alcohol: the loss of the C=O signal and the appearance of the O–H stretch in the product provide clear evidence of chemical change.
