Question

What functional groups might the following molecules contain? (a) A compound with a strong absorption at \(1710 \mathrm{~cm}^{-1}\) (b) A compound with a strong absorption at \(1540 \mathrm{~cm}^{-1}\) (c) A compound with strong absorptions at \(1720 \mathrm{~cm}^{-1}\) and at 2500 to \(3100 \mathrm{~cm}^{-1}\)

Short answer

(a) Ketone or aldehyde; (b) Nitro group; (c) Carboxylic acid.

Step-by-step solution

Understanding IR Absorption at 1710 cm^-1

Absorption peaks around 1710 cm^{-1} typically indicate the presence of a carbonyl group, often found in ketones, aldehydes, or carboxylic acids. In IR spectroscopy, the presence of a C=O stretch is a strong indicator of this group.

Analyzing IR Absorption at 1540 cm^-1

An absorption around 1540 cm^{-1} is often associated with the N-O stretching vibration in nitro compounds. This range is specific for nitro groups due to their asymmetric stretching vibrations.

Understanding Combined Absorptions at 1720 cm^-1 and 2500-3100 cm^-1

A peak around 1720 cm^{-1} typically suggests a carbonyl group similar to 1710 cm^{-1}. The additional broad absorption between 2500 and 3100 cm^{-1} is characteristic of -OH stretches in carboxylic acids. Hence, the compound most likely contains a carboxylic acid functional group.

Key concepts

Functional Groups Identification

Infrared (IR) spectroscopy is a powerful tool for identifying functional groups within molecules based on their vibrational transitions. By analyzing the absorption peaks in an IR spectrum, chemists can determine which functional groups are present. Each functional group absorbs IR radiation at specific frequencies, appearing as peaks. For instance:

• Carbonyl groups (C=O) absorb sharply around 1710-1720 cm^-1.
• Nitro groups (N-O) usually show absorption near 1540 cm^-1.
• Hydroxyl groups (-OH), particularly in carboxylic acids, produce broad absorptions between 2500 to 3100 cm^-1.

Understanding these key absorption ranges allows for the effective identification of functional groups within a compound. This method simplifies the characterization of unknown molecules and helps predict their structural features.

Carbonyl Group Analysis

The carbonyl group is a common functional group found in various organic compounds, including ketones, aldehydes, and carboxylic acids. It is defined by a carbon double-bonded to an oxygen atom (C=O). In IR spectroscopy, the carbonyl group is easily identifiable by its strong absorption peak, usually located between 1710 and 1720 cm^-1. - The exact position of the carbonyl absorption can provide insight into the specific type of carbonyl-containing compound: - **Aldehydes**: often show an additional C-H stretch near 2720-2820 cm^-1. - **Carboxylic Acids**: present a broad O-H absorption alongside the carbonyl peak. - **Ketones**: typically have a sharp carbonyl peak without the broad O-H stretch. Detecting these peaks is vital for differentiating between various carbonyl-containing compounds, as their chemical environment influences the exact frequency of absorption.

Nitro Compounds Detection

Nitro compounds are characterized by the presence of one or more nitro groups (NO₂), which are known for their distinctive IR absorption patterns. In the infrared spectrum, nitro groups typically exhibit two key stretching vibrations: - The strong asymmetric N-O stretch appears around 1540 cm^-1. - A symmetric N-O stretch may also be observed between 1350-1370 cm^-1, though it is less intense. These unique spectral features allow for the straightforward identification of nitro compounds. It's also important to recognize that nitro compounds can exhibit additional peaks depending on their specific structure and other present functional groups, but the N-O stretching frequencies remain primary markers for their detection using IR spectroscopy.