Arrange the alkenes 2-methyl but-l-ene (I), 2-methyl but-2-ene (II) and 3-methylbut-1-one (III) in order of decreasing reactivity towards bromine.

Answer

To determine the order of decreasing reactivity of the alkenes 2-methyl but-1-ene (I), 2-methyl but-2-ene (II), and 3-methylbut-1-ene (III) towards bromine, we need to consider the structure and stability of each alkene. The reactivity of alkenes towards electrophilic addition reactions, such as bromination, generally increases with the degree of substitution of the double bond.

### Step-by-Step Solution:

1. **Identify the Structures of the Alkenes:**
- **2-methyl but-1-ene (I)**: This alkene has a double bond between the first and second carbon atoms, with a methyl group on the second carbon.
- **2-methyl but-2-ene (II)**: This alkene has a double bond between the second and third carbon atoms, with two methyl groups attached to the second carbon.
- **3-methylbut-1-ene (III)**: This alkene has a double bond between the first and second carbon atoms, with a methyl group on the third carbon.

2. **Analyze the Degree of Substitution:**
- **2-methyl but-1-ene (I)**: This is a monosubstituted alkene (one alkyl group attached to the double bond).
- **2-methyl but-2-ene (II)**: This is a tetrasubstituted alkene (two alkyl groups attached to the double bond).
- **3-methylbut-1-ene (III)**: This is also a monosubstituted alkene (one alkyl group attached to the double bond).

3. **Determine Reactivity Based on Substitution:**
- Tetrasubstituted alkenes are generally more reactive than monosubstituted alkenes due to the stability of the double bond. Therefore, 2-methyl but-2-ene (II) will be the most reactive.
- Between the two monosubstituted alkenes (2-methyl but-1-ene (I) and 3-methylbut-1-ene (III)), the reactivity can be influenced by steric hindrance and the position of substituents, but generally, they will have similar reactivity.

4. **Order of Reactivity:**
- The order of reactivity towards bromine is as follows:
1. **2-methyl but-2-ene (II)** (most reactive)
2. **2-methyl but-1-ene (I)** and **3-methylbut-1-ene (III)** (less reactive, similar reactivity)

### Final Order:
- **II > I = III**