Anti-Markownikoff's addition of HBr is not observed in

Options

A. propene
B. butene
C. but-2-ene
D. pent-2-ene

Explanation

To determine in which compound Anti-Markovnikov's addition of HBr is not observed, we need to analyze the structures of the given compounds and understand the principles of Markovnikov's and Anti-Markovnikov's addition.

### Step-by-Step Solution:

1. **Identify the Compounds**:
We are given several compounds, including propene, butene, and pentene. We need to draw their structures to analyze them.

2. **Draw Structures**:
- **Propene (C3H6)**: CH2=CH-CH3
- **Butene (C4H8)**: There are two isomers:
- 1-Butene: CH2=CH-CH2-CH3
- 2-Butene: CH3-CH=CH-CH3
- **Pentene (C5H10)**: There are two isomers:
- 1-Pentene: CH2=CH-CH2-CH2-CH3
- 2-Pentene: CH3-CH=CH-CH2-CH3

3. **Understanding Markovnikov's and Anti-Markovnikov's Addition**:
- **Markovnikov's Rule**: When HX (like HBr) adds to an alkene, the hydrogen atom (H) attaches to the carbon with the greater number of hydrogen atoms (the less substituted carbon), while the halide (Br) attaches to the more substituted carbon.
- **Anti-Markovnikov's Addition**: In this case, the bromine (Br) will attach to the less substituted carbon (the one with fewer hydrogen atoms).

4. **Analyze Each Compound**:
- **Propene**: Has one degree 1 carbon and one degree 2 carbon. Anti-Markovnikov's addition can occur here.
- **1-Butene**: Similar to propene, it has a degree 1 carbon and a degree 2 carbon. Anti-Markovnikov's addition can occur.
- **2-Butene**: Both carbons involved in the double bond are degree 2 carbons. Anti-Markovnikov's addition cannot occur here since there is no degree 1 carbon.
- **1-Pentene**: Similar to propene and 1-butene, it has a degree 1 carbon and a degree 2 carbon. Anti-Markovnikov's addition can occur.
- **2-Pentene**: Similar to 2-butene, both carbons involved in the double bond are degree 2 carbons. Anti-Markovnikov's addition cannot occur here.

5. **Conclusion**:
Based on the analysis, Anti-Markovnikov's addition of HBr is not observed in **2-butene** and **2-pentene** because both have symmetrical double bonds with only degree 2 carbons. However, the question specifically asks for one compound, which is **butane** (2-butene), where Anti-Markovnikov's addition does not occur.

### Final Answer:
Anti-Markovnikov's addition of HBr is not observed in **butane (2-butene)**.