Identify (B) and (D) in the following sequence of reactions:
`H_(2)C = CH_(2) overset("conc". H_(2)SO_(4))to(A) overset(H_(2)O)underset(Delta)to (B) overset(PB_(3))to ( C)overset((D))to (H_(2)C=CH_(2))`

Options

A. `CH_(3)OH` and `C_(2)H_(5)Br`
B. `C_(2)H_(5)HSO_(4)` and alc. KOH
C. `C_(2)H_(5)OH` and alc. KOH
D. `C_(2)H_(5)HSO_(4)` and aq. KOH

Answer

To solve the problem, we will analyze the sequence of reactions step by step to identify compounds (B) and (D).

### Step-by-Step Solution:

1. **Starting Compound Identification**:
- The starting compound is **ethene (H₂C=CH₂)**.

2. **Reaction with Concentrated H₂SO₄**:
- When ethene reacts with concentrated sulfuric acid (H₂SO₄), it undergoes an electrophilic addition reaction. The double bond attacks the proton (H⁺) from H₂SO₄, leading to the formation of a carbocation.
- The bisulfate ion (HSO₄⁻) then adds to the carbocation, resulting in the formation of **ethyl hydrogen sulfate (A)**:
\[ \text{H₂C=CH₂} + \text{conc. H₂SO₄} \rightarrow \text{C₂H₅OSO₃H (A)} \]

3. **Hydrolysis of Compound A**:
- When ethyl hydrogen sulfate (A) is treated with water (H₂O) under heating conditions, it undergoes hydrolysis. The sulfate group is replaced by a hydroxyl group (OH), yielding **ethanol (B)**:
\[ \text{C₂H₅OSO₃H} + \text{H₂O} \xrightarrow{\Delta} \text{C₂H₅OH (B)} + \text{H₂SO₄} \]

4. **Reaction with PBr₃**:
- Ethanol (B) reacts with phosphorus tribromide (PBr₃) in a nucleophilic substitution reaction, where the hydroxyl group (OH) is replaced by a bromine atom (Br), forming **ethyl bromide (C)**:
\[ \text{C₂H₅OH (B)} + \text{PBr₃} \rightarrow \text{C₂H₅Br (C)} + \text{H₃PO₃} \]

5. **Dehydrohalogenation with Alcoholic KOH**:
- Finally, ethyl bromide (C) undergoes a dehydrohalogenation reaction when treated with alcoholic KOH (D). This reaction leads to the elimination of HBr, resulting in the regeneration of **ethene (H₂C=CH₂)**:
\[ \text{C₂H₅Br (C)} + \text{KOH (D)} \rightarrow \text{H₂C=CH₂} + \text{KBr} + \text{H₂O} \]

### Final Identification:
- **Compound (B)**: Ethanol (C₂H₅OH)
- **Reagent (D)**: Alcoholic KOH

### Summary:
- (B) is ethanol (C₂H₅OH).
- (D) is alcoholic KOH.