JEE Challenger
More from Aldehydes, Ketones and Carboxylic Acids
ChemHardAdv 2014 • Paper 1

Ketones of MW 100 Yielding Racemic Product on Reduction

Consider every possible isomeric ketone, including all stereoisomers, possessing a molecular weight MW=100\text{MW} = 100. Each of these isomers is independently treated with NaBH4\text{NaBH}_4 (NOTE: stereoisomers are also treated separately). Determine the total number of ketones that yield a racemic mixture as product(s).

Official Numerical Answer5

Topics & Concepts

Aldehydes, Ketones and Carboxylic AcidsAldehydes and KetonesIsomerismOxidation and Reduction (Organic)

Step-by-Step Solution

To determine the total number of ketones of molecular weight MW=100\text{MW} = 100 that yield a racemic mixture upon reduction with NaBH4\text{NaBH}_4, we first find the molecular formula.

1. Molecular Formula Determination

For an acyclic aliphatic ketone of formula CnH2nO\text{C}_n\text{H}_{2n}\text{O}: MW=12n+2n+16=14n+16=100\text{MW} = 12n + 2n + 16 = 14n + 16 = 100 14n=84  ⟹  n=614n = 84 \implies n = 6

Thus, the molecular formula is C6H12O\text{C}_6\text{H}_{12}\text{O}.

2. Identifying All Isomeric Ketones

The possible structural isomers of acyclic ketones with the formula C6H12O\text{C}_6\text{H}_{12}\text{O} and their reduction behavior with NaBH4\text{NaBH}_4 are:

  1. Hexan-2-one: CH3−CO−CH2−CH2−CH2−CH3→NaBH4CH3−C∗H(OH)−CH2−CH2−CH2−CH3\text{CH}_3-\text{CO}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3 \xrightarrow{\text{NaBH}_4} \text{CH}_3-\text{C}^*\text{H(OH)}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3

    • Starting ketone is achiral.
    • Reduction generates a single chiral center at C-2\text{C-2}. Hydride attack occurs with equal probability from the re and si faces, yielding a racemic mixture of (R)(R)- and (S)(S)-hexan-2-ol.

  2. Hexan-3-one: CH3−CH2−CO−CH2−CH2−CH3→NaBH4CH3−CH2−C∗H(OH)−CH2−CH2−CH3\text{CH}_3-\text{CH}_2-\text{CO}-\text{CH}_2-\text{CH}_2-\text{CH}_3 \xrightarrow{\text{NaBH}_4} \text{CH}_3-\text{CH}_2-\text{C}^*\text{H(OH)}-\text{CH}_2-\text{CH}_2-\text{CH}_3

    • Starting ketone is achiral.
    • Generates a single chiral center at C-3\text{C-3}, yielding a racemic mixture of (R)(R)- and (S)(S)-hexan-3-ol.

  3. 4-Methylpentan-2-one: CH3−CO−CH2−CH(CH3)2→NaBH4CH3−C∗H(OH)−CH2−CH(CH3)2\text{CH}_3-\text{CO}-\text{CH}_2-\text{CH}(\text{CH}_3)_2 \xrightarrow{\text{NaBH}_4} \text{CH}_3-\text{C}^*\text{H(OH)}-\text{CH}_2-\text{CH}(\text{CH}_3)_2

    • Starting ketone is achiral.
    • Generates a single chiral center at C-2\text{C-2}, yielding a racemic mixture of (R)(R)- and (S)(S)-4-methylpentan-2-ol.

  4. 2-Methylpentan-3-one: (CH3)2CH−CO−CH2−CH3→NaBH4(CH3)2CH−C∗H(OH)−CH2−CH3(\text{CH}_3)_2\text{CH}-\text{CO}-\text{CH}_2-\text{CH}_3 \xrightarrow{\text{NaBH}_4} (\text{CH}_3)_2\text{CH}-\text{C}^*\text{H(OH)}-\text{CH}_2-\text{CH}_3

    • Starting ketone is achiral.
    • The product contains four different groups at C-3\text{C-3} (−H-\text{H}, −OH-\text{OH}, −CH(CH3)2-\text{CH(CH}_3)_2, −CH2CH3-\text{CH}_2\text{CH}_3), generating a chiral center and resulting in a racemic mixture.

  5. 3,3-Dimethylbutan-2-one: CH3−CO−C(CH3)3→NaBH4CH3−C∗H(OH)−C(CH3)3\text{CH}_3-\text{CO}-\text{C}(\text{CH}_3)_3 \xrightarrow{\text{NaBH}_4} \text{CH}_3-\text{C}^*\text{H(OH)}-\text{C}(\text{CH}_3)_3

    • Starting ketone is achiral.
    • Generates a single chiral center at C-2\text{C-2}, yielding a racemic mixture of (R)(R)- and (S)(S)-3,3-dimethylbutan-2-ol.

  6. 3-Methylpentan-2-one: CH3−CO−C∗H(CH3)CH2CH3\text{CH}_3-\text{CO}-\text{C}^*\text{H}(\text{CH}_3)\text{CH}_2\text{CH}_3

    • Contains a pre-existing chiral center at C-3\text{C-3} and exists as two stereoisomers: (R)(R)-3-methylpentan-2-one and (S)(S)-3-methylpentan-2-one.
    • When each enantiomer is reduced independently, it yields a mixture of diastereomers in unequal amounts (diastereoselective reduction). The product is optically active and not racemic.

Conclusion

Only the 5 achiral ketones yield a racemic mixture upon reduction.

Correct Answer: 5

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