Which of the following can't be prepared by the typical Williamson reaction ?

Options

A. `R_3COCR_3`
B. `ArOAr`
C. `RCH=CHOCH=CHR`
D. `C_6H_5CH_2OC_2H_5`

Explanation

To determine which of the given compounds cannot be prepared by the typical Williamson reaction, we first need to understand the Williamson Ether Synthesis. This reaction involves the nucleophilic substitution of an alkyl halide (R-X) with a sodium alkoxide (R-O-Na), leading to the formation of an ether (R-O-R').

### Step-by-Step Solution:

1. **Understanding Williamson Ether Synthesis**:
- The Williamson reaction is a method to synthesize ethers by reacting an alkyl halide with a sodium alkoxide.
- The general reaction can be represented as:
\[ R-X + R'-O^{-} \rightarrow R-O-R' + NaX \]

2. **Analyzing the Options**:
- **Option A: R3CO-CR3** (Tertiary ether)
- To prepare this compound, a tertiary alkyl halide is required. However, tertiary alkyl halides tend to undergo elimination reactions rather than substitution due to steric hindrance. Therefore, this compound cannot be prepared by the Williamson reaction.

- **Option B: C6H5-OC6H5** (Diphenyl ether)
- This compound requires the use of halobenzene. The carbon-halogen bond in halobenzene has partial double bond character due to resonance, making it difficult to break. Thus, this compound cannot be prepared by the Williamson reaction.

- **Option C: RCH=CH-CHO-CHR** (Allylic ether)
- Similar to option B, the carbon-halogen bond in this compound also exhibits resonance, which strengthens the bond and makes it resistant to nucleophilic attack. Therefore, this compound cannot be prepared by the Williamson reaction.

- **Option D: C6H5-CH2-O-C2H5** (Benzyl ether)
- This compound can be prepared using benzyl chloride (C6H5-CH2Cl) and sodium ethoxide (C2H5O-Na). The reaction proceeds smoothly, resulting in the formation of the ether.

3. **Conclusion**:
- Based on the analysis, options A, B, and C cannot be prepared by the typical Williamson reaction, while option D can be prepared.

### Final Answer:
**Options A, B, and C cannot be prepared by the typical Williamson reaction.**

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