The IR spectrum of acetic acid exhibits several characteristic absorption ranges corresponding to its distinct functional groups. These ranges are crucial for identifying the presence of acetic acid through infrared spectroscopy.
Acetic acid (CH₃COOH) is a carboxylic acid, and its IR spectrum is dominated by absorptions from the O-H (hydroxyl), C=O (carbonyl), and C-O (ester-like, but part of carboxylic acid) bonds within its carboxyl group, along with C-H bonds from the methyl group.
Characteristic IR Absorption Ranges for Acetic Acid
The key absorption ranges observed in the infrared (IR) spectrum of acetic acid are as follows:
- O-H Bond (Hydroxyl Group): A very broad and intense peak due to the hydrogen-bonded hydroxyl group of the carboxylic acid, typically observed in the region of 2500 – 3300 cm⁻¹. This broadness is a hallmark of carboxylic acid O-H stretching, often overlapping with C-H stretching vibrations.
- C=O Bond (Carbonyl Group): A strong and sharp absorption peak resulting from the carbonyl stretching vibration, appearing in the region of 1680 – 1750 cm⁻¹. This is a highly characteristic absorption for carboxylic acids.
- C-O Bond (Single Bond within Carboxyl Group): An absorption related to the C-O single bond stretching, generally found in the region of 1075 – 1350 cm⁻¹. This peak helps distinguish carboxylic acids from other carbonyl-containing compounds.
These specific absorption ranges provide a unique fingerprint that allows for the identification of acetic acid and its functional groups.
Summary of IR Absorption Ranges for Acetic Acid
To summarize, here are the primary IR absorption ranges for acetic acid:
| Functional Group/Bond | Type of Vibration | Wavenumber Range (cm⁻¹) | Characteristics |
|---|---|---|---|
| O-H (Carboxylic Acid) | Stretching | 2500 – 3300 | Broad, intense peak, characteristic of hydrogen-bonded O-H in carboxylic acids. |
| C=O (Carboxylic Acid) | Stretching | 1680 – 1750 | Strong, sharp peak, indicative of the carbonyl group. |
| C-O (Carboxylic Acid) | Stretching | 1075 – 1350 | Medium intensity, contributes to the overall fingerprint of the carboxylic acid. |
In addition to these, the C-H stretching vibrations from the methyl (CH₃) group would typically be observed around 2850-3000 cm⁻¹, often somewhat obscured by the broad O-H absorption.
