Hell-Volhard-Zelinsky reaction involves the replacement of an .............. Atom from the alkyl group of a monocarboxylic acid by a ........ Atom.

Answer

**Step-by-Step Solution:**

1. **Understanding the Reaction**: The Hell-Volhard-Zelinsky (HVZ) reaction is a chemical reaction involving carboxylic acids, where a hydrogen atom from the alkyl group is replaced by a halogen atom.

2. **Identifying the Components**: In the context of the HVZ reaction, we focus on monocarboxylic acids (which have one carboxylic acid group) and recognize that the reaction does not apply to formic acid (HCOOH) since it lacks an alkyl group.

3. **Reaction Mechanism**: The reaction typically involves the following steps:
- The carboxylic acid reacts with halogens (like Cl2 or Br2) in the presence of red phosphorus.
- This leads to the substitution of an alpha hydrogen atom (the hydrogen atom adjacent to the carboxylic acid group) with a halogen atom.

4. **Final Statement**: Therefore, in the HVZ reaction, an **alpha hydrogen atom** from the alkyl group of a monocarboxylic acid is replaced by a **halogen atom** (such as chlorine or bromine).

**Answer**: Hell-Volhard-Zelinsky reaction involves the replacement of an **alpha hydrogen atom** from the alkyl group of a monocarboxylic acid by a **halogen atom**.

---