Arrange the following compound in order decreasing reactivity in the E2 reaction:
(i) `H_(3)C-underset(CH_(3))underset(|)overset(CH_(3))overset(|)(C)-Br`
(ii) `D_(3)C-underset(CD_(3))underset(|)overset(CD_(3))overset(|)(C)-Cl`
(iii) `H_(3)C-underset(CH_(3))underset(|)overset(CH_(3))overset(|)(C)-Cl`
(iv) `H_(3)C-underset(CH_(3))underset(|)overset(CH_(3))overset(|)(C)-l`

Options

A. `igtiigtiiigtiv`
B. `ivgtigtiiigtii`
C. `ivgtigtiigtiii`
D. `iigtiiigtigtiv`

Answer

To determine the order of decreasing reactivity in the E2 reaction for the given compounds, we need to analyze the leaving groups and the nature of the beta-hydrogens in each compound. The E2 reaction involves the elimination of a hydrogen atom and a leaving group (like Br or Cl) to form a double bond.

### Step-by-Step Solution:

1. **Identify the Compounds and Their Structures:**
- (i) `H₃C-CH(CH₃)-C(Br)(CH₃)`
- (ii) `D₃C-CH(CD₃)-C(Cl)(CD₃)`
- (iii) `H₃C-CH(CH₃)-C(Cl)(CH₃)`
- (iv) `H₃C-CH(CH₃)-C(I)(CH₃)`

2. **Determine the Leaving Groups:**
- The leaving groups in the compounds are Br, Cl, and I.
- The order of leaving group ability is: I > Br > Cl. This means that compounds with I will be more reactive than those with Br, which in turn are more reactive than those with Cl.

3. **Analyze the Beta-Hydrogens:**
- In E2 reactions, the reactivity also depends on the ease of removal of the beta-hydrogen.
- The presence of deuterium (D) instead of hydrogen (H) can affect the reactivity due to the kinetic isotope effect. The C-H bond is weaker than the C-D bond, making it easier to remove H than D.

4. **Rank the Compounds:**
- **Compound (iv)** has I as the leaving group, making it the most reactive.
- **Compound (i)** has Br as the leaving group and has three beta-hydrogens available for elimination, making it the second most reactive.
- **Compound (iii)** has Cl as the leaving group and has beta-hydrogens available, but since it has only H, it will be more reactive than (ii).
- **Compound (ii)** has Cl as the leaving group and deuterium instead of hydrogen, making it the least reactive due to the stronger C-D bond.

5. **Final Order of Reactivity:**
- (iv) > (i) > (iii) > (ii)

### Final Answer:
The order of decreasing reactivity in the E2 reaction is:
**(iv) > (i) > (iii) > (ii)**