JEE Challenger
More from Aldehydes, Ketones and Carboxylic Acids
ChemHardAdv 2010 • Paper 2

Identification of Aliphatic Aldehydes P and Q

Comprehension Passage

Paragraph for Questions 12 to 14.
Two aliphatic aldehydes P and Q react in the presence of aqueous K2CO3\text{K}_2\text{CO}_3 to give compound R, which upon treatment with HCN provides compound S. On acidification and heating, S gives the product shown below :

Passage Diagram 0

The compounds P and Q respectively are

Options

A
Option A Diagram 1
B
Option B Diagram 1
Correct
C
Option C Diagram 1
D
Option D Diagram 1

Topics & Concepts

Aldehydes, Ketones and Carboxylic AcidsAldehydes and Ketones

Step-by-Step Solution

To determine the structures of the aliphatic aldehydes PP and QQ, we can perform a retrosynthetic analysis starting from the final product.

Step 1: Analysis of the Final Product

The final product shown is a 5-membered γ\gamma-lactone with a hydroxyl group and two methyl substituents:
4,4-dimethyl-3-hydroxydihydrofuran-2(3H)-one\text{4,4-dimethyl-3-hydroxydihydrofuran-2(3H)-one}

The formation of a lactone occurs when a molecule containing both a carboxylic acid group (−COOH-\text{COOH}) and a hydroxyl group (−OH-\text{OH}) undergoes intramolecular esterification upon heating in acidic medium.

Opening the lactone ring gives the precursor α,γ\alpha,\gamma-dihydroxy carboxylic acid:
HO−CH2−C(CH3)2−CH(OH)−COOH\text{HO}-\text{CH}_2-\text{C}(\text{CH}_3)_2-\text{CH}(\text{OH})-\text{COOH}

Step 2: Identification of Compound SS

Compound SS undergoes acidification (H+\text{H}^+) and heating (Δ\Delta) to yield the lactone. Since acidification of a cyanohydrin hydrolyzes the nitrile group (−CN-\text{CN}) into a carboxylic acid group (−COOH-\text{COOH}), compound SS must be the cyanohydrin derivative:
S=HO−CH2−C(CH3)2−CH(OH)−CNS = \text{HO}-\text{CH}_2-\text{C}(\text{CH}_3)_2-\text{CH}(\text{OH})-\text{CN}

Step 3: Identification of Compound RR

Cyanohydrins are formed by the nucleophilic addition of HCN\text{HCN} to an aldehyde or ketone. Since SS has a −CH(OH)CN-\text{CH}(\text{OH})\text{CN} group, compound RR must contain an aldehyde group (−CHO-\text{CHO}) at that carbon:
R=HO−CH2−C(CH3)2−CHOR = \text{HO}-\text{CH}_2-\text{C}(\text{CH}_3)_2-\text{CHO}

Step 4: Identification of Aldehydes PP and QQ

Compound RR is a β\beta-hydroxy aldehyde formed via a crossed-aldol reaction between two aldehydes PP and QQ in the presence of aqueous K2CO3\text{K}_2\text{CO}_3:

  1. Enolate donor (Nucleophile): 2-Methylpropanal (isobutyraldehyde), (CH3)2CH−CHO(\text{CH}_3)_2\text{CH}-\text{CHO}, has one α\alpha-hydrogen. Deprotonation by K2CO3\text{K}_2\text{CO}_3 forms the carbanion:
    (CH3)2Cˉ−CHO(\text{CH}_3)_2\bar{\text{C}}-\text{CHO}

  2. Electrophile: Formaldehyde, HCHO\text{HCHO}, has no α\alpha-hydrogens and is extremely reactive towards nucleophilic attack.

  3. Aldol Addition:
    (CH3)2Cˉ−CHO+HCHO→H2OHO−CH2−C(CH3)2−CHO(R)(\text{CH}_3)_2\bar{\text{C}}-\text{CHO} + \text{HCHO} \xrightarrow{\text{H}_2\text{O}} \text{HO}-\text{CH}_2-\text{C}(\text{CH}_3)_2-\text{CHO} \quad (R)

Thus, the two aliphatic aldehydes PP and QQ are:
P=(CH3)2CH−CHOandQ=HCHOP = (\text{CH}_3)_2\text{CH}-\text{CHO} \quad \text{and} \quad Q = \text{HCHO}

This corresponds to Option B.

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