Reactivity of alkenes towards `HX` decreases in the order

Options

A. Butene `gt` Propene `gt` Ethene
B. Butene `gt` Ethene `gt` Propene
C. Ethene `gt` Propene `gt` Butene
D. None of these

Explanation

To determine the reactivity of alkenes towards HX, we need to analyze the stability of the carbocation intermediates formed during the reaction. The order of reactivity is influenced by the stability of these intermediates.

### Step-by-Step Solution:

1. **Identify the Alkenes**: We have three alkenes to compare: butene, propene, and ethene.

2. **Draw the Structures**:
- **Butene**: C=C with two CH3 groups on one side (but-1-ene or but-2-ene).
- **Propene**: C=C with one CH3 and one H on each side.
- **Ethene**: C=C with two H atoms on each carbon.

3. **Add HX to Each Alkene**: When HX is added to an alkene, it results in the formation of a carbocation intermediate.
- **Butene**: The addition of HX leads to a secondary carbocation (more stable).
- **Propene**: The addition of HX leads to a secondary carbocation (also stable).
- **Ethene**: The addition of HX leads to a primary carbocation (least stable).

4. **Stability of Carbocations**: The stability of carbocations increases with the number of alkyl groups attached to the positively charged carbon.
- **Butene**: Forms a secondary carbocation (more stable).
- **Propene**: Also forms a secondary carbocation (stable but less than butene).
- **Ethene**: Forms a primary carbocation (least stable).

5. **Order of Reactivity**: Since the stability of the carbocations formed determines the reactivity of the alkenes towards HX, we can conclude:
- Butene (most stable carbocation) > Propene (stable carbocation) > Ethene (least stable carbocation).

6. **Final Answer**: Therefore, the reactivity of alkenes towards HX decreases in the order:
- **Butene > Propene > Ethene**.