Which of the following reactions follows Markovnikov's rule?

Options

A. `C_2H_4+HBr`
B. `C_3H_6+Cl_2`
C. `C_3H_6+HBr`
D. `C_3H_6+Br_2`

Explanation

To determine which of the given reactions follows Markovnikov's rule, we need to understand the conditions under which this rule applies. Markovnikov's rule is relevant when an asymmetric alkene reacts with a hydrogen halide (like HBr, HCl, etc.). According to this rule, when HX is added to an asymmetric alkene, the hydrogen atom (H) will attach to the carbon with the greater number of hydrogen atoms, while the halide (X) will attach to the carbon with fewer hydrogen atoms.

### Step-by-Step Solution:

1. **Identify the Reactions**: We need to analyze each of the given reactions to see if they involve an asymmetric alkene reacting with a hydrogen halide.

2. **Reaction 1: C2H4 + HBr**
- C2H4 (Ethylene) is a symmetrical alkene (CH2=CH2).
- When HBr is added, the product is CH3-CH2-Br.
- Since it is symmetrical, Markovnikov's rule does not apply.
**Conclusion**: This reaction does not follow Markovnikov's rule.

3. **Reaction 2: C3H6 + Cl2**
- C3H6 can be represented as CH3-CH=CH2 (an asymmetric alkene).
- However, Cl2 does not involve a hydrogen halide; it is a halogen addition reaction.
- The product will be CH3-CHCl-CH2Cl, which does not follow Markovnikov's rule.
**Conclusion**: This reaction does not follow Markovnikov's rule.

4. **Reaction 3: C3H6 + HBr**
- C3H6 can be represented as CH3-CH=CH2 (an asymmetric alkene).
- When HBr is added, according to Markovnikov's rule, H will attach to the carbon with more hydrogen atoms (the CH3 group), and Br will attach to the carbon with fewer hydrogen atoms (the CH2 group).
- The product will be CH3-CHBr-CH3 (2-bromopropane).
**Conclusion**: This reaction follows Markovnikov's rule.

5. **Reaction 4: C3H6 + Br2**
- C3H6 is again CH3-CH=CH2 (an asymmetric alkene).
- The addition of Br2 does not follow Markovnikov's rule as it is a halogen addition reaction, leading to the formation of a vicinal dibromide (CH3-CHBr-CH2Br).
**Conclusion**: This reaction does not follow Markovnikov's rule.

### Final Answer:
The reaction that follows Markovnikov's rule is **Option 3: C3H6 + HBr**.