Identify compounds P, Q and R in the following reaction?
`CH_3 CH_3 Br overset("alc. KOH")to P overset(+ HBr)to Q overset(+ AgNO_2) to R `

Options

A. `CH_2 = CH_2 , CH_3 CH_2 Br . CH_3 CH_2 ON =O`
B. `CH_3 CH_2 OH , CH_3 CH_2 Br . CH_3 CH_2 ON =O`
C. `CH_2 = CH_2 , CH_3 CH_2 Br , CH_3 CH_2 NO_2 `
D. `CH_3 CH_2 OH , CH_3 CH_2 Br , CH_3 CH_2 NO_2`

Explanation

To identify the compounds P, Q, and R in the given reaction sequence, we will analyze each step of the reaction carefully.

### Step 1: Reaction of CH3CH2Br with Alcoholic KOH
The first compound in the reaction is CH3CH2Br (bromoethane). When bromoethane reacts with alcoholic KOH, it undergoes an elimination reaction (dehydrohalogenation) to form an alkene.

- **Reaction**:
\[ CH_3CH_2Br \overset{\text{alc. KOH}}{\rightarrow} CH_2=CH_2 + HBr \]
- **Compound P**: The product formed is ethene (C2H4), so **P = C2H4**.

### Step 2: Reaction of Ethene with HBr
Next, ethene (C2H4) reacts with HBr. Since ethene is a symmetrical alkene, the addition of HBr will yield bromoethane again.

- **Reaction**:
\[ CH_2=CH_2 + HBr \rightarrow CH_3CH_2Br \]
- **Compound Q**: The product formed is bromoethane (C2H5Br), so **Q = C2H5Br**.

### Step 3: Reaction of Bromoethane with AgNO2
Finally, bromoethane (C2H5Br) reacts with silver nitrite (AgNO2). In this reaction, the bromine atom is substituted by a nitro group (NO2) through an SN2 mechanism.

- **Reaction**:
\[ CH_3CH_2Br + AgNO_2 \rightarrow CH_3CH_2NO_2 + AgBr \]
- **Compound R**: The product formed is ethyl nitrite (C2H5NO2), so **R = C2H5NO2**.

### Summary of Compounds
- **P = C2H4 (Ethene)**
- **Q = C2H5Br (Bromoethane)**
- **R = C2H5NO2 (Ethyl nitrite)**