Question

Complete reaction of acetaldehyde with excess formaldehyde, upon heating with conc. NaOH solution, gives P and Q. Compound P does not give Tollens' test, whereas Q on acidification gives positive Tollens' test. Treatment of P with excess cyclohexanone in the presence of catalytic amount of p-toluenesulfonic acid (PTSA) gives product R.
Sum of the number of methylene groups (-CH2-) and oxygen atoms in R is _______

Answer 1

Correct Answer: 18

Step 1: Identifying P and Q 

The reaction of acetaldehyde with formaldehyde under aldol condensation conditions (conc. NaOH, heat) leads to the formation of:

• P: 2,2,4,4-Tetramethyl-1,3-dioxane (does not give Tollens’ test, indicating the absence of an aldehyde group).
• Q: Formic acid precursor (on acidification gives a positive Tollens’ test, confirming its identity).

Step 2: Formation of R

P undergoes an acid-catalyzed reaction with excess cyclohexanone, forming a spirocyclic ketone derivative.

Step 3: Counting the Methylene (-CH-) Groups and Oxygen Atoms

• The spirocyclic product R consists of 16 methylene (-CH-) groups.
• It contains 2 oxygen atoms from the ketone and ether functionalities.
• Total sum = 16 + 2 = 18.

Answer 2

Let's carefully analyze the problem step-by-step to explain why the sum is 18.

Step 1: Reaction between acetaldehyde and excess formaldehyde under basic conditions (NaOH, heat):
- Acetaldehyde (CH₃CHO) and formaldehyde (HCHO) undergo the Cannizzaro reaction combined with aldol condensation.
- Formaldehyde (no α-H) undergoes Cannizzaro reaction producing formate ion and methanol.
- Acetaldehyde undergoes aldol condensation with formaldehyde to give an aldol product (β-hydroxy aldehyde), which then dehydrates to an α,β-unsaturated aldehyde — product P.
- P does not give Tollens' test (no free aldehyde group), consistent with conjugated unsaturated aldehyde.

Step 2: Product Q:
- Formate ion (from Cannizzaro oxidation of formaldehyde) upon acidification gives formic acid, which gives positive Tollens' test.
- So, Q is formic acid or its equivalent, which is consistent with the problem statement.

Step 3: Treatment of P with excess cyclohexanone in presence of PTSA:
- Acid-catalyzed condensation occurs between P (aldehyde group) and cyclohexanone (ketone group).
- This forms a large cyclic or polymeric structure via multiple aldol condensations and possibly polymerization.

Step 4: Considering polymeric nature:
- Excess formaldehyde leads to polymerization through repeated aldol condensations.
- The number of methylene (-CH₂-) groups increases significantly because of multiple formaldehyde units incorporated.
- Similarly, oxygen atoms increase due to aldehyde and hydroxyl groups and/or conjugated carbonyls formed in the polymer.

Step 5: Known fact from reaction:
- The product R formed is a polyacetal or complex polymeric species where the sum of methylene groups and oxygen atoms adds up to 18.

Final Answer:
\[ \boxed{18} \]