Benzaldehyde and benzoic acid are two chemicals that are similar to one another in terms of their molecular structures. Despite this, there are many differences in their physical properties and applications. Understand the reasons for these differences by considering the variation in their chemical composition.
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Both benzaldehyde and benzoic acid are organic chemicals based on the so-called "aromatic" compound benzene, a circular ring made of six carbons. There are differences in their chemical structures and composition, however. Benzaldehyde, which is known by several other names, including benzoic aldehyde, has the chemical formula C7H6O. Benzoic acid, also known as benzenecarboxylic acid, has the formula C7H6O2. As evidenced by these formulas, the molecular compositions of these two substances differ by only one oxygen atom.
Attached Atom Differences
A molecule of benzaldehyde is composed of the benzene ring structure with a carbon attached to it. This carbon in turn has a hydrogen atom attached to it and is also doubly bonded to an oxygen. The C=O group is known as a "carbonyl," and a chemical that contains a carbonyl attached to a hydrogen is called an "aldehyde," hence the name benzaldehyde. Benzoic acid also has a carbon attached to the ring, and this carbon is also doubly bonded to oxygen. However, the carbon is also attached to an -OH group. A compound that contains this -(C=O)OH portion is a "carboxylic acid." Therefore, these chemicals are in two different categories of organic chemicals.
Physical Property Differences
Despite the small difference in their chemical structures, benzaldehyde and benzoic acid have quite different properties. Benzaldehyde melts at --9.44 degrees C and boils at 178 degrees C, so it is a liquid at room temperature. It has a density of 1.05 grams per millilitre. Benzoic acid melts at 122 degrees C and boils at 248 degrees C, so it is a solid at room temperature. It has a greater density of 1.32 grams per millilitre. Benzoic acid is also quite acidic; a saturated water solution will have a pH of 2.8.
The toxicity of benzoic acid is fairly low. When tested on rats, its LD50 (the dose that kills half of the test animals) is 1.7 grams per kilogram body weight. When ingested or inhaled, the main effects are irritation, sore throat and nausea. Benzaldehyde is slightly more toxic with an LD50 of 1.3 grams per kilogram. It also produces symptoms of irritation and discomfort, but ingestion of large amounts can have a narcotic effect.
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- National Institute of Standards and Technology: Benzaldehyde
- National Institute of Standards and Technology: Benzoic Acid
- Michigan State University: Aldehydes and Ketones
- Michigan State University: Carboxylic Acids
- J.T. Baker: Material Safety Data Sheet: Benzaldehyde
- J.T. Baker: Material Safety Data Sheet: Benzoic Acid