Question

The number of isomeric tetraenes (NOT containing $s p$-hybridized carbon atoms) that can be formed from the following reaction sequence is ______.

Answer 1

Correct Answer: 2

Correct answer is 2

Answer 2

To solve this problem, we need to analyze the given reaction sequence step by step and determine how many isomeric tetraenes (not containing sp-hybridized carbon atoms) can be formed.

1. First Step: Reaction with Sodium in Liquid Ammonia (Na, liquid NH₃)
This step represents the Birch reduction, where the benzene ring undergoes partial hydrogenation. This reaction adds hydrogen atoms to the carbon-carbon double bonds in the benzene ring, converting it into a cyclohexadiene intermediate. The product is a 1,4-cyclohexadiene, where two hydrogen atoms are added to the ring in a specific manner.

2. Second Step: Addition of Bromine (Br₂, excess)
In the presence of excess bromine (Br₂), the cyclohexadiene intermediate undergoes electrophilic addition to the double bonds. Since there are two double bonds, the excess bromine adds across both of them, leading to the formation of a dibromo intermediate. The product of this step is a dibromocyclohexane.

3. Third Step: Reaction with Alcoholic Potassium Hydroxide (alc. KOH)
Alcoholic KOH induces elimination reactions. In this case, the reaction leads to the elimination of HBr from the dibromocyclohexane, resulting in the formation of a conjugated diene. This step can form two isomers due to the positions of the double bonds in the final product, leading to the formation of two isomeric tetraenes (the conjugated dienes).

Conclusion:
From the reaction sequence, two isomeric tetraenes are formed, and neither of them contains sp-hybridized carbon atoms (as no triple bonds are involved).

Final Answer:
The number of isomeric tetraenes that can be formed from the given reaction sequence is 2.