How to distinguish between phenol and benzoic acid?

Phenol and benzoic acid are two compounds with different chemical structures, which are widely used in chemical, pharmaceutical, pesticide and other industries. Although their names and some properties can be confusing, we can easily distinguish them with some simple chemical tests and methods. This article will explain in detail how to distinguish between phenol and benzoic acid and give you some common identification techniques.

Phenol and Benzoic Acid Basic Chemical Structures

We need to understand the basic chemical structure differences between phenol and benzoic acid:

• Phenol: Phenol (C≡H≡OH) is a compound consisting of a benzene ring (C≡H≡I) and a hydroxyl group (OH). The hydroxyl group in the phenol molecule makes phenol acidic, but its acidity is much weaker than benzoic acid.

• Benzoic acid: Benzoic acid (C-HYCOOH) is an organic acid consisting of a benzene ring and a carboxyl group (COOH). The presence of carboxyl groups makes benzoic acid more acidic and can release hydrogen ions more obviously in water.

Although both molecules contain benzene rings, their functional groups make them significantly different in chemical properties.

Acid-base difference: how to judge from acid?

The distinction between phenol and benzoic acid can first be judged by their acidity and basicity. Although phenol can also react with water, it is far less acidic than benzoic acid. Benzoic acid can dissociate in water and release hydrogen ions, thus showing strong acidity.

Experimental method: The aqueous solutions of both were tested with pH paper. Benzoic acid solution showed obvious acidity (low pH value), while the pH value of phenol solution was higher, showing neutral or weak acidity.

Oxidation Reaction: Phenol and Benzoic Acid Differences

The oxidation properties of phenol and benzoic acid are also different. Phenol is easily oxidized in air to form quinones, especially at high temperatures or in the presence of oxygen. The oxidation of benzoic acid is weak, and a significant oxidation reaction usually occurs under the action of a strong oxidant.

Experimental method: The phenol and benzoic acid solutions are treated with an oxidizing agent (such as potassium permanganate or chromate), respectively. Phenol will quickly change color, showing a dark quinone product, and benzoic acid is slow reaction, not easy to appear obvious color change.

Water solubility difference: phenol and benzoic acid in water solubility

There are also significant differences in the water solubility of phenol and benzoic acid. Phenol has a low solubility in water, but in the presence of a small amount of alkaline material (such as sodium hydroxide solution), phenol can react with alkali to form a water-soluble phenol sodium salt. In contrast, benzoic acid has lower solubility, but it can react with strong alkali in water to form sodium benzoate salt, and the salt solution formed is more stable.

Experimental method: the same amount of phenol and benzoic acid were added to the water, and the solubility difference was observed. Phenol is less soluble and benzoic acid is less soluble. If sodium hydroxide solution is added to the two, phenol can form water-soluble sodium phenolate relatively quickly, while benzoic acid produces sodium benzoate.

Infrared Spectroscopy (IR) Analysis: How to Use Spectroscopy to Distinguish?

Infrared spectroscopy is a commonly used analytical method that can accurately identify the functional groups of phenol and benzoic acid. In the infrared spectrum of phenol, a strong-OH stretching vibrational absorption peak usually appears in the 3200-3600 cm¹ region, while benzoic acid has a distinct carbonyl (C = O) stretching vibrational peak near 1700 cm¹.

Experimental methods: Phenol and benzoic acid were analyzed by infrared spectroscopy, and their unique absorption peaks were observed. Phenol has a hydroxyl-OH absorption peak, while benzoic acid has a clear carboxyl-C = O absorption peak.

Summary: How to effectively distinguish between phenol and benzoic acid?

Through the above methods, we can summarize some practical techniques to distinguish between phenol and benzoic acid:

1. Acidic: benzoic acid than phenol more acid, pH paper test can be used as a preliminary judgment tool.
2. Oxidation: Phenol is more easily oxidized, showing a dark product, while benzoic acid oxidation is relatively slow.
3. Solubility: phenol in water solubility is low, but in alkaline solution solubility is better; benzoic acid in water solubility is also low, but the solubility of phenol is slightly weaker.
4. Infrared spectra: Phenol and benzoic acid IR spectral characteristics are different, phenol has-OH peak, benzoic acid has-C = O peak.

With the above method, you can easily distinguish between phenol and benzoic acid in a laboratory environment, avoiding confusion and misuse.