Q. 14 Two equivalents of 𝐏 react with one equivalent of 𝐐 to produce a major product 𝐑.

The number of double bonds present in the major product 𝐑 is ____ .

Question Overview

Two equivalents of compound P react with one equivalent of compound Q to produce a major product R.

The question asks:

What is the number of double bonds present in the major product R?

This problem is based on Wittig reaction chemistry and tests your understanding of:

• Phosphonium ylides

• Carbonyl reactions

• Double bond formation

• Conjugated polyene systems

Identification of Reactants

Compound P

From the structure shown:

• P contains a terminal aldehyde group (–CHO)

• It already has conjugated C=C double bonds

• Acts as a carbonyl compound

Compound Q

From the given structure:

• Q is a bis-phosphonium ylide

• Contains two Wittig-active phosphorus ylides

• Each ylide can react with one carbonyl group

Reaction Concept (Core Chemistry)

🔬 Key Reaction: Wittig Reaction

• Each phosphonium ylide reacts with one aldehyde

• Forms one new C=C double bond

• Produces triphenylphosphine oxide (by-product)

Important Observation

• Q has two ylide centers

• Two equivalents of P are required

• Hence, two new C=C double bonds are formed

Step-by-Step Double Bond Counting

Step 1: Existing Double Bonds in P

From the given structure of P:

• Three C=C double bonds

• One C=O double bond (aldehyde)

Step 2: Effect of Wittig Reaction

• The C=O double bond is replaced by C=C

• Each reaction adds one new alkene

Since two equivalents of P react:

• Two C=O bonds are converted

• Two new C=C bonds are formed

Total Double Bonds in Product R

✅ Correct Answer

✔️ Number of double bonds in the major product R = 8

Why Other Answers Are Incorrect

❌ 6 Double Bonds

• Ignores formation of new C=C bonds from Wittig reaction

❌ 7 Double Bonds

• Counts only one Wittig reaction instead of two

❌ 9 or More Double Bonds

• Overcounts by assuming additional unsaturation not formed in reaction

Final Conclusion

✔️ Since:

• Q contains two phosphonium ylides

• Each reacts with one aldehyde group

• Two new C=C bonds are formed

• And existing conjugation remains intact

✅ The major product R contains exactly 8 double bonds

Exam Tip 🚀

In Wittig reaction problems:

• Always count existing unsaturation

• Then add one C=C for each ylide used

• Carbonyl C=O becomes C=C, not extra