How many of the following alkenes on addition of HBr would give the sae product in the presence or absence of peroxide propene, 1-butene, 2-butene, 2-methylpropene, 3-methyl-1-butene, 2,3-dimethyl-1-butene, 2-pentene, 1-pentene, 4-methyl-2-pentene

Explanation

To determine how many of the given alkenes will produce the same product upon the addition of HBr in the presence or absence of peroxide, we need to analyze each alkene based on its structure and symmetry.

### Step-by-Step Solution:

1. **Identify the Alkenes**: The alkenes given are:
- Propene
- 1-butene
- 2-butene
- 2-methylpropene
- 3-methyl-1-butene
- 2,3-dimethyl-1-butene
- 2-pentene
- 1-pentene
- 4-methyl-2-pentene

2. **Understand the Reaction Mechanism**:
- When HBr is added to alkenes, it can follow two pathways:
- **Markovnikov's Rule**: The hydrogen atom from HBr adds to the carbon with more hydrogen atoms (more substituted carbon), leading to the more stable carbocation.
- **Anti-Markovnikov Addition**: In the presence of peroxides, the addition occurs in such a way that the bromine atom adds to the less substituted carbon.

3. **Classify the Alkenes**:
- **Symmetrical Alkenes**: These will give the same product regardless of whether HBr is added in the presence or absence of peroxide.
- Example: 2-butene (CH3-CH=CH-CH3) is symmetrical.
- **Unsymmetrical Alkenes**: These will give different products based on the method of addition.
- Example: Propene (CH2=CH-CH3) is unsymmetrical.

4. **Analyze Each Alkene**:
- **Propene**: Unsymmetrical, different products.
- **1-butene**: Unsymmetrical, different products.
- **2-butene**: Symmetrical, same product.
- **2-methylpropene**: Unsymmetrical, different products.
- **3-methyl-1-butene**: Unsymmetrical, different products.
- **2,3-dimethyl-1-butene**: Unsymmetrical, different products.
- **2-pentene**: Symmetrical, same product.
- **1-pentene**: Unsymmetrical, different products.
- **4-methyl-2-pentene**: Unsymmetrical, different products.

5. **Count the Symmetrical Alkenes**:
- From the analysis, the symmetrical alkenes that give the same product are:
- 2-butene
- 2-pentene
- Thus, there are **2 alkenes** that will give the same product upon the addition of HBr, regardless of the presence of peroxide.

### Final Answer:
**2 alkenes (2-butene and 2-pentene) will give the same product upon addition of HBr in the presence or absence of peroxide.**