Q.15.

Question

Question: Draw the product formed when cyclohepta-1,3,5-triene (pKa = 39) is treated with a strong base. Why is its pKa so much higher than the pKa of cyclopentadiene?

Step-by-Step Solution

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Answer

Answer

Product on reaction with a strong base:

Cyclohepta-1,3,5-triene has higher $p K a$ because its conjugate base is unstable

1Step 1: Stability of conjugate base and p k a value

When an acid loses a proton, it forms a conjugate base.

Smaller value of $p k_{a}$ signifies stronger acid and weak conjugate base.

And higher value of $p k_{a}$ suggests weak acid and more stable conjugate base.

2Step 2: Aromatic molecules

Aromatic molecules have closed electronic configuration and filled orbitals due to which it is less reactive and more stable than non-aromatic molecules.

Aromatic molecules follow the (4n+2) pi electrons rule (n is a natural number).

3Step 3: p k a of cyclohepta-1,3,5-triene and cyclopentadiene

When a molecule is treated with a strong base, a proton is abstracted and a conjugate base is formed.

The conjugate base of cyclohepta-1,3,5-triene has 8 pi electrons.

It is an antiaromatic base and hence, unstable.

Therefore, the acid has a high $p k_{a}$ value.

Reaction of cyclohepta-1,3,5-triene with a strong base

The conjugate base of cyclopentadiene has 6 pi electrons.

It is aromatic conjugate base and hence more stable.

Therefore, the acid has a low $p k_{a}$ value.

Reaction of cyclopentadiene with a strong base

Q.14.