Arrange the following hydrogen halides in order of their decreasing reactivity with propene.

Options

A. `HCl gt HBr gt HI`
B. `HBr gt HI gt NCl`
C. `HI gt NBr gt HCl`
D. `HCl gt HI gt HBr`

Explanation

To arrange the hydrogen halides (HCl, HBr, and HI) in order of their decreasing reactivity with propene, we need to consider the mechanism of their reaction and the stability of the carbocations formed during the reaction.

### Step-by-Step Solution:

1. **Understanding the Reaction Mechanism**:
- Propene (CH3-CH=CH2) reacts with hydrogen halides (HX) through an electrophilic addition mechanism. The double bond in propene acts as a nucleophile and attacks the hydrogen atom of HX.

2. **Formation of Carbocations**:
- When the double bond attacks the hydrogen, it forms a carbocation. The stability of the carbocation formed is crucial in determining the reactivity of the hydrogen halide.
- In this case, two possible carbocations can be formed:
- A secondary carbocation (more stable) when the hydrogen adds to the terminal carbon.
- A primary carbocation (less stable) when the hydrogen adds to the middle carbon.

3. **Stability of Carbocations**:
- The stability of carbocations follows the order: tertiary > secondary > primary > methyl.
- Since the secondary carbocation formed in this reaction is more stable due to hyperconjugation, this will be the major product.

4. **Bond Strength of HX**:
- The reactivity of the hydrogen halides is also influenced by the strength of the H-X bond. The weaker the bond, the more reactive the hydrogen halide will be.
- The bond strengths of the hydrogen halides decrease in the order: HCl > HBr > HI.
- This means HI has the weakest bond and will break more easily, leading to higher reactivity.

5. **Arranging the Hydrogen Halides**:
- Based on the bond strengths and the stability of the carbocations, we can arrange the hydrogen halides in order of their decreasing reactivity with propene:
- HI > HBr > HCl

### Final Answer:
The order of decreasing reactivity of hydrogen halides with propene is:
**HI > HBr > HCl**