True about alkyl halides is/are -

Options

A. Tertiary alkyl halides undergo `S_(N)^(2)` substitutions
B. Alkyl iodides on exposure to sunlight gradually darken
C. Alkyl chlorides do not give beilstein test
D. A nucleophilic substitution is most difficult in alkyl iodides

Answer

To determine the true statements about alkyl halides, we will analyze each statement provided in the question step by step.

### Step 1: Analyze the first statement
**Statement:** Tertiary Alkyl Halides undergo SN2 Substitution.

**Analysis:**
- Tertiary alkyl halides have three alkyl groups attached to the carbon atom bonded to the halogen.
- The SN2 mechanism involves a backside attack by the nucleophile, which is hindered by steric bulk.
- Therefore, tertiary alkyl halides do not undergo SN2 reactions; instead, they typically undergo SN1 reactions due to the stability of the tertiary carbocation formed during the reaction.

**Conclusion:** This statement is **false**.

### Step 2: Analyze the second statement
**Statement:** If I expose Alkyl Iodide in sunlight, it gradually turns dark.

**Analysis:**
- Alkyl iodides can decompose upon exposure to sunlight, leading to the formation of iodine, which is responsible for the dark coloration.
- The decomposition reaction of alkyl iodides produces iodine, which can appear as a brown or violet color.

**Conclusion:** This statement is **true**.

### Step 3: Analyze the third statement
**Statement:** Nucleophilic substitution is most difficult in Alkyl Iodide.

**Analysis:**
- Alkyl iodides generally undergo nucleophilic substitution more easily than alkyl chlorides or bromides because the carbon-iodine bond is weaker.
- The weaker bond allows the leaving group (iodine) to depart more readily, facilitating the nucleophilic substitution process.

**Conclusion:** This statement is **false**.

### Step 4: Analyze the fourth statement
**Statement:** Alkyl Chlorides do not show Bilstein test.

**Analysis:**
- The Bilstein test is a qualitative test for organic halides, where a clean copper wire is heated, and a green flame indicates the presence of copper halide.
- Alkyl chlorides do indeed give a positive Bilstein test, indicating that this statement is incorrect.

**Conclusion:** This statement is **false**.

### Final Conclusion:
The only true statement about alkyl halides is that if alkyl iodide is exposed to sunlight, it gradually turns dark.

### Summary of True Statements:
- The true statement is: **If I expose Alkyl Iodide in sunlight, it gradually turns dark.**