Which of the following syntheses could not be doen without involving blocking positions on the ring?

Options

A.
B.
C.
D.

Explanation

To determine which synthesis could not be done without involving blocking positions on the ring, we need to analyze the options provided and understand the directing effects of the substituents on the aromatic ring.

### Step-by-Step Solution:

1. **Identify the Directing Groups**:
- The groups attached to the aromatic ring in the options are either ortho/para directing or meta directing. In this case, OH (hydroxyl) and NH2 (amino) are ortho/para directing groups.

2. **Analyze Each Option**:
- **Option A: Para Bromo Phenol**:
- The para product can be easily obtained from phenol and bromine (Br2) without any blocking. Thus, this synthesis does not require blocking.

- **Option B: 2,4,6-Tribromo Phenol**:
- This compound can also be synthesized by reacting phenol with Br2 in a polar solvent, leading to substitution at all ortho and para positions. No blocking is needed here either.

- **Option C: Ortho Bromo Phenol**:
- To obtain the ortho product, we need to block the para position. If we directly react phenol with Br2, the major product will be the para bromo phenol. Therefore, we must block the para position first to direct the bromination to the ortho position. This requires the use of a blocking group.

- **Option D: Para Bromo Aniline**:
- Similar to option A, the para product can be synthesized easily from aniline without blocking, as the amino group directs substitution to the ortho and para positions.

3. **Conclusion**:
- The only synthesis that cannot be achieved without blocking is **Option C: Ortho Bromo Phenol**. This is because the para position must be blocked to ensure that bromination occurs at the ortho position.

### Final Answer:
- **Option C** is the synthesis that could not be done without involving blocking positions on the ring.