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Difficulty: Very hardAlkanals and Alkanones: Oxidation, Reduction, and Distinction Tests

An organic compound ZZ with the molecular formula C5H10OC_5H_{10}O gives a positive orange precipitate when treated with 2,4-dinitrophenylhydrazine. When warmed with acidified potassium heptaoxodichromate(VI) (K2Cr2O7K_2Cr_2O_7), compound ZZ is readily oxidized to a carboxylic acid containing five carbon atoms. Upon reduction with lithium tetrahydridoaluminate(III) (LiAlH4LiAlH_4), compound ZZ yields a primary alcohol that exhibits optical activity (chirality). What is the IUPAC name of compound ZZ?

  1. 2-methylbutanalAnswer
  2. B
    pentan-2-one
  3. C
    pentanal
  4. D
    3-methylbutanal

Answer

2-methylbutanal
2-methylbutanal has the molecular formula C5H10OC_5H_{10}O and contains an alkanal functional group. It reacts with 2,4-dinitrophenylhydrazine to form a precipitate. Because it is an alkanal, it undergoes oxidation with acidified potassium heptaoxodichromate(VI) to produce 2-methylbutanoic acid (a 5-carbon carboxylic acid). Upon reduction with lithium tetrahydridoaluminate(III), it forms 2-methylbutan-1-ol, which has an asymmetric carbon atom at position 2 bonded to four different substituent groups (−H-H, −CH3-CH_3, −CH2CH3-CH_2CH_3, −CH2OH-CH_2OH), rendering the molecule chiral and optically active.

Step-by-Step Solution

1
Identify the functional group class from the 2,4-DNPH test.
A positive test with 2,4-dinitrophenylhydrazine confirms that compound ZZ is a carbonyl compound (either an alkanal or an alkanone).
Both alkanals and alkanones form colored hydrazone precipitates with 2,4-DNPH.
2
Distinguish between an alkanal and an alkanone using oxidation behavior.
Compound ZZ readily oxidizes with acidified K2Cr2O7K_2Cr_2O_7 to a carboxylic acid with 5 carbons, proving ZZ is an alkanal (pentanal isomer).
Alkanals are easily oxidized to carboxylic acids with the same number of carbon atoms, whereas alkanones resist oxidation under mild conditions.
3
Analyze the reduction product for chirality.
Reduction of an alkanal with LiAlH4LiAlH_4 yields a primary alcohol. Among 5-carbon alkanals (pentanal, 2-methylbutanal, 3-methylbutanal, 2,2-dimethylpropanal), only 2-methylbutanal reduces to 2-methylbutan-1-ol, CH3CH2CH(CH3)CH2OHCH_3CH_2CH(CH_3)CH_2OH.
In 2-methylbutan-1-ol, C-2 is bonded to four distinct groups: −H-H, −CH3-CH_3, −CH2CH3-CH_2CH_3, and −CH2OH-CH_2OH, making it a chiral molecule capable of optical isomerism.

Key Concept

Distinction between alkanals and alkanones via oxidation, reduction of carbonyls to alcohols, and optical isomerism in branched primary alcohols.
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Topics

Alkanals and Alkanones: Oxidation, Reduction, and Distinction Tests

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