In the presence of a small amount of phosphorous, aliphatic carboxylic acids react with chlorine or bromine to yield a compound in which `alpha`- hydrogen has been replaced by halogen. This reaction is known as :

Explanation

To solve the question, we need to identify the reaction that occurs when aliphatic carboxylic acids react with halogens (chlorine or bromine) in the presence of phosphorus. The key points to consider are the reactants, the conditions, and the product formed.

### Step-by-Step Solution:

1. **Identify the Reactants**:
- We have aliphatic carboxylic acids, which can be represented as RCH2COOH, where R is an alkyl group.
- The halogens involved in the reaction are chlorine (Cl2) or bromine (Br2).
- A small amount of phosphorus (P) is also present in the reaction.

2. **Understand the Reaction Mechanism**:
- In the presence of phosphorus, the carboxylic acid undergoes halogenation at the alpha position (the carbon adjacent to the carboxyl group).
- This reaction replaces one of the alpha hydrogens (the hydrogen on the carbon next to the carboxylic acid) with a halogen atom (X).

3. **Write the Reaction**:
- The general reaction can be represented as:
\[ \text{RCH}_2\text{COOH} + \text{X}_2 \xrightarrow{\text{P}} \text{RCHXCOOH} + \text{HX} \]
- Here, X represents the halogen (Cl or Br), and HX is the hydrogen halide (HCl or HBr) formed as a byproduct.

4. **Identify the Product**:
- The product formed is an alpha halo acid, represented as RCHXCOOH, where one alpha hydrogen has been replaced by a halogen atom.

5. **Name the Reaction**:
- This specific reaction is known as the **Helmholtz-Hartz-Zelenskyi reaction**, commonly abbreviated as the **HVZ reaction**. It is characterized by the halogenation of the alpha carbon of carboxylic acids.

### Final Answer:
The reaction described is known as the **Helmholtz-Hartz-Zelenskyi reaction (HVZ reaction)**.

---