Three common lab experiments are shown. In each case, describe how the IR spectrum of the product would differ from that of the reactant. Give approximate frequencies for distinctive peaks in the IR spectrum of the reactant and also that of the product.

Verified step by step guidance

1. Step 1: Analyze reaction A (Pinacol rearrangement). The reactant contains two hydroxyl (-OH) groups, which will show broad IR peaks around 3200-3600 cm⁻¹. The product is a ketone, which will exhibit a strong IR peak for the carbonyl (C=O) group around 1700 cm⁻¹. The disappearance of the -OH peak and the appearance of the C=O peak are key differences in the IR spectra.
2. Step 2: Analyze reaction B (Esterification of salicylic acid). The reactant contains a carboxylic acid group (-COOH), which will show a broad peak for the O-H stretch around 2500-3000 cm⁻¹ and a sharp peak for the C=O stretch around 1700 cm⁻¹. The product is an ester, which will show a C=O stretch around 1735 cm⁻¹ and a C-O stretch around 1200-1300 cm⁻¹. The disappearance of the broad O-H peak and the shift in the C=O peak are key differences.
3. Step 3: Analyze reaction C (Reduction of cinnamaldehyde). The reactant contains an aldehyde group (-CHO), which will show a C=O stretch around 1720 cm⁻¹ and a C-H stretch around 2700-2900 cm⁻¹. The product is an alcohol, which will show a broad O-H stretch around 3200-3600 cm⁻¹. The disappearance of the aldehyde peaks and the appearance of the O-H peak are key differences.
4. Step 4: For each reaction, identify the functional group transformation and correlate it with the IR spectral changes. Functional group transformations (e.g., hydroxyl to ketone, carboxylic acid to ester, aldehyde to alcohol) directly impact the IR spectrum by introducing or removing characteristic peaks.
5. Step 5: Approximate frequencies for distinctive peaks: Hydroxyl (-OH) stretch: 3200-3600 cm⁻¹, Carboxylic acid O-H stretch: 2500-3000 cm⁻¹, Carbonyl (C=O) stretch: 1700-1735 cm⁻¹, Ester C-O stretch: 1200-1300 cm⁻¹, Aldehyde C-H stretch: 2700-2900 cm⁻¹. Use these values to interpret the IR spectra of reactants and products in each case.

Key Concepts

Infrared (IR) Spectroscopy

Infrared spectroscopy is a technique used to identify functional groups in organic compounds by measuring the absorption of infrared light at specific wavelengths. Different functional groups absorb IR radiation at characteristic frequencies, allowing chemists to deduce the molecular structure of a compound based on its IR spectrum.

Functional Group Transformations

Functional group transformations refer to the chemical reactions that convert one functional group into another. In the provided reactions, the conversion of pinacol to pinacolone and the reduction of cinnamaldehyde to cinnamyl alcohol illustrate how the presence or absence of certain functional groups affects the IR spectrum, particularly in terms of peak positions and intensities.

Characteristic IR Peaks

Characteristic IR peaks correspond to specific bond vibrations within a molecule. For example, alcohols typically show broad O-H stretching around 3200-3600 cm⁻¹, while carbonyl groups (C=O) exhibit strong peaks near 1700 cm⁻¹. Understanding these peaks helps in predicting how the IR spectrum of a product will differ from that of its reactants after a reaction.