Logo
Jeeify
MCQMediumJEE 2026Aldehydes & Ketones

JEE Chemistry 2026 Question with Solution

'x' is the product from propenenitrile + SnCl2/HClSnCl_2/HCl followed by hydrolysis. 'y' is the product from but-2-ene by ozonolysis. Which product is not obtained when 'x' and 'y' react in alkali with heating?

  • A

    Pent-2-enal

  • B

    2-Methylpent-2-enal

  • C

    3-Methylbut-2-enal

  • D

    2-Methylbut-2-enal

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: 'x' is formed from propenenitrile by SnCl2/HClSnCl_2/HCl followed by hydrolysis, and 'y' is formed from but-2-ene by ozonolysis.

Find: Which product is not obtained when 'x' and 'y' react in alkali with heating.

First identify the two carbonyl compounds.

CH2=CH−CN→Stephen’s ReductionCH2=CH−CHOCH_2=CH-CN \xrightarrow{\text{Stephen's Reduction}} CH_2=CH-CHO

So, x is acrolein

CH2=CH−CHOCH_2=CH-CHO

For ozonolysis of but-2-ene:

CH3−CH=CH−CH3→O32 CH3CHOCH_3-CH=CH-CH_3 \xrightarrow{O_3} 2 \, CH_3CHO

So, y is acetaldehyde.

Now consider all possible aldol condensation reactions in alkali with heating:

  1. Self-aldol of y:
CH3CHO+CH3CHO→But-2-enalCH_3CHO + CH_3CHO \rightarrow \text{But-2-enal}
  1. Cross-aldol with enolate of y and carbonyl of x:
CH3CHO+CH2=CH−CHO→Pent-2-enalCH_3CHO + CH_2=CH-CHO \rightarrow \text{Pent-2-enal}
  1. Cross-aldol with enolate of x and carbonyl of y:
CH2=CH−CHO+CH3CHO→2-Methylbut-2-enalCH_2=CH-CHO + CH_3CHO \rightarrow \text{2-Methylbut-2-enal}
  1. Self-aldol of x:
x+x→2-Methylpent-2-enalx + x \rightarrow \text{2-Methylpent-2-enal}

Thus, Pent-2-enal, 2-Methylpent-2-enal, and 2-Methylbut-2-enal are obtained. 3-Methylbut-2-enal is not formed because it would require a different branching pattern not available from these reactants.

Therefore, the correct option is C.

Carbon-count and branching check

Given: The products are formed through self-aldol and cross-aldol condensations of acrolein and acetaldehyde.

Find: Which listed aldehyde cannot arise.

Use a quick structure check:

  • Acrolein provides an unsaturated aldehyde framework.
  • Acetaldehyde provides a two-carbon aldehyde unit.
  • Possible products must match combinations obtainable from these two aldehydes through aldol condensation followed by dehydration.

The listed products Pent-2-enal, 2-Methylbut-2-enal, and 2-Methylpent-2-enal fit the possible self or cross combinations. However, 3-Methylbut-2-enal has a branching pattern that does not match the enolate-carbonyl combinations available here.

Therefore, the product not obtained is 3-Methylbut-2-enal, so the correct option is C.

Common mistakes

  • Identifying x incorrectly. Stephen reduction of a nitrile followed by hydrolysis gives an aldehyde, not an amine or alcohol. Here propenenitrile gives acrolein, so first convert CH2=CH−CNCH_2=CH-CN to CH2=CH−CHOCH_2=CH-CHO.

  • Forgetting that ozonolysis of but-2-ene gives two molecules of acetaldehyde. Missing this leads to the wrong identity of y. Write the cleavage explicitly before attempting aldol products.

  • Considering only one cross-aldol product. Both carbonyl compounds have α\alpha-hydrogen, so each can form the enolate. Therefore, both cross combinations must be checked separately.

  • Choosing a product only by carbon count. 3-Methylbut-2-enal has the right number of carbons, but its branching pattern is not obtainable from the available enolates. Always check connectivity, not just total carbons.

Practice more Aldehydes & Ketones questions

Get unlimited AI-adaptive practice, mastery tracking, and an AI tutor that explains every step — free to start.

Start free trialMore on Aldehydes & Ketones

Related questions