Final answer:
Benzoic acid is the compound used as a food preservative, and benzoyl chloride is the compound where a chlorine atom replaces a hydrogen atom.
Step-by-step explanation:
The compound commonly used as a food preservative among benzoic acid, benzamide, methylbenzoate, and benzoyl chloride is benzoic acid. Benzoic acid is widely used in the food industry for its ability to kill microorganisms that can lead to spoilage, especially when added to acidic foods such as fruit juices and soft drinks. It can be used in the form of the carboxylic acid itself or as a more soluble salt like sodium benzoate.
In the benzene derivatives provided, the compound where a chlorine atom replaces a hydrogen atom is benzoyl chloride. This compound is named using the substituent group 'chloro', indicating the presence of a chlorine atom within its structure, which is not the case for benzoic acid, benzamide, or methylbenzoate.