Anti—Markownikoff’s addition of HBr is not observed in -

Options

A. Propene
B. But-2-ene
C. Butene
D. 1-Methylcyclohexene

Answer

To determine where the Anti-Markownikoff's addition of HBr is not observed, we need to analyze the given options based on the Markownikoff's rule and the conditions under which Anti-Markownikoff's addition occurs.

### Step-by-Step Solution:

1. **Understand Markownikoff's Rule**:
- According to Markownikoff's rule, when HX (like HBr) is added to an alkene, the hydrogen (H) will attach to the carbon atom that has the greater number of hydrogen atoms already attached. The halide (Br in this case) will attach to the carbon with fewer hydrogen atoms.

2. **Identify the Options**:
- We have four options to analyze:
1. Propene
2. But-2-ene
3. But-1-ene
4. Cyclohexane with a methyl group

3. **Analyze Each Option**:
- **Option 1: Propene (C3H6)**
- Structure: CH3-CH=CH2
- According to Markownikoff's rule, H will go to the CH2 (higher H count), and Br will go to the CH (lower H count). Thus, Markownikoff's addition is valid.

- **Option 2: But-2-ene (C4H8)**
- Structure: CH3-CH=CH-CH3
- Here, both carbons in the double bond have the same number of hydrogen atoms (1 each). Therefore, neither Markownikoff nor Anti-Markownikoff's addition can be applied effectively. This means that Anti-Markownikoff's addition is not observed here.

- **Option 3: But-1-ene (C4H8)**
- Structure: CH2=CH-CH2-CH3
- According to Markownikoff's rule, H will go to the CH2 (higher H count), and Br will go to the CH (lower H count). Anti-Markownikoff's addition can also be applied here, where H would go to the less substituted carbon.

- **Option 4: Cyclohexane with a methyl group (C7H12)**
- Structure: A cyclohexane ring with a methyl group.
- In this case, when HBr is added, H will go to the carbon with fewer hydrogens (the one attached to the methyl group), and Br will go to the carbon with more hydrogens. Thus, Anti-Markownikoff's addition can also be applied.

4. **Conclusion**:
- The only case where Anti-Markownikoff's addition is not observed is in **Option 2: But-2-ene**, where both carbons in the double bond have equal hydrogen counts.

### Final Answer:
**Anti-Markownikoff’s addition of HBr is not observed in But-2-ene.**