It is not always easy to predict the position of attack on multiply substituted benzene. If the benzene ring bears different ortho//para directing group at the `1` and `4` positions, the position of further substitution is not immediately clear. Sometimes steric effects determine the outcome. In other cases, electronic factors determine the outcome, and further reaction will be at the position activated by the more strongly activating group.
Some substituens are so strongly activating that no catalyst is needed, and it is often diffcult to stop substitution after mono substituion. Mild condition are needed to restrict the reaction to mono-substitution.
It is possible to reduce the activity such groups by side chain reaction so that the reaction can be stopped use can sometimes be made of removable blocking groups on the ring.
Which of the following side reaction//s can be used to reduce the activity of strongly activating groups like `-OH` ?

Options

A. benzoylation
B. acetylation
C. both of the above
D. none of the above

Answer

To solve the question regarding which side reactions can be used to reduce the activity of strongly activating groups like -OH on a benzene ring, we can follow these steps:

### Step 1: Understand the Activating Effect of the -OH Group
The -OH group (hydroxyl group) is a strong activating group for electrophilic aromatic substitution reactions. It increases the electron density on the benzene ring, particularly at the ortho and para positions, making these positions more reactive.

**Hint:** Recall that activating groups increase the electron density on the benzene ring, making it more reactive towards electrophiles.

### Step 2: Identify Methods to Reduce the Activity of the -OH Group
To reduce the activity of the -OH group, we can use side reactions such as benzoylation and acetylation. These reactions involve replacing the hydrogen atom of the -OH group with another group, thereby reducing its activating effect.

**Hint:** Think about how replacing the hydrogen atom in the -OH group could affect the reactivity of the benzene ring.

### Step 3: Benzoylation Reaction
In the benzoylation reaction, phenol is reacted with benzoyl chloride in the presence of a base (like NaOH). The hydroxyl hydrogen is replaced by a benzoyl group (-C(=O)C6H5), which reduces the activating effect of the -OH group.

**Hint:** Remember that benzoylation introduces a bulky group that can hinder further electrophilic substitution.

### Step 4: Acetylation Reaction
In the acetylation reaction, phenol is reacted with acetyl chloride. The hydroxyl hydrogen is replaced by an acetyl group (-C(=O)CH3), which also reduces the activating effect of the -OH group.

**Hint:** Consider how the introduction of an acetyl group affects the electron density on the benzene ring.

### Step 5: Conclusion
Both benzoylation and acetylation effectively reduce the activity of the -OH group on the benzene ring. Therefore, the correct answer to the question is that both reactions can be used to reduce the activity of strongly activating groups like -OH.

**Final Answer:** Both benzoylation and acetylation can reduce the activity of the -OH group, so the answer is option C: both of the above.