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JEE Chemistry 2023 Question with Solution

The product formed in the following multistep reaction is:

CH3−CH=CH2  →(ii)H2O2, NaOH(i)B2H6  →(iii)PCC  →(iv)CH3MgBr\mathrm{CH_3-CH=CH_2} \;\xrightarrow[(ii) \mathrm{H_2O_2,\,NaOH}]{(i) \mathrm{B_2H_6}} \;\xrightarrow{(iii) PCC} \;\xrightarrow{(iv) \mathrm{CH_3MgBr}}

  • A

    (CH3)3COH\mathrm{(CH_3)_3COH}

  • B

    CH3−CH2−CH(OH)−CH3\mathrm{CH_3-CH_2-CH(OH)-CH_3}

  • C

    CH3−CH2−COOCH3\mathrm{CH_3-CH_2-COOCH_3}

  • D

    CH3−CH2−CH2−CH2−OH\mathrm{CH_3-CH_2-CH_2-CH_2-OH}

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: CH3−CH=CH2\mathrm{CH_3-CH=CH_2} is treated successively with B2H6\mathrm{B_2H_6}, then H2O2/NaOH\mathrm{H_2O_2/NaOH}, then PCC, and finally CH3MgBr\mathrm{CH_3MgBr}.

Find: The final product formed after the complete reaction sequence.

Hydroboration-oxidation of propene gives anti-Markovnikov addition of water, so the alkene first forms the primary alcohol:

CH3−CH=CH2⟶CH3−CH2−CH2OH\mathrm{CH_3-CH=CH_2 \longrightarrow CH_3-CH_2-CH_2OH}

PCC oxidizes a primary alcohol to an aldehyde without further oxidation, so:

CH3−CH2−CH2OH⟶CH3−CH2−CHO\mathrm{CH_3-CH_2-CH_2OH \longrightarrow CH_3-CH_2-CHO}

Now propanal reacts with methyl magnesium bromide. The methyl group adds to the carbonyl carbon, and after hydrolysis a secondary alcohol is obtained:

CH3−CH2−CHO→CH3MgBrCH3−CH2−CH(OH)−CH3\mathrm{CH_3-CH_2-CHO \xrightarrow{CH_3MgBr} CH_3-CH_2-CH(OH)-CH_3}

Therefore, the final product is 2-butanol, and the correct option is B.

Reaction Sequence Shortcut

Given: Propene undergoes hydroboration-oxidation, then PCC oxidation, then reaction with CH3MgBr\mathrm{CH_3MgBr}.

Find: The final product quickly.

Use the pattern directly:

  • Hydroboration-oxidation of a terminal alkene gives a primary alcohol at the terminal carbon.
  • PCC converts that primary alcohol into the corresponding aldehyde.
  • A Grignard reagent adds one alkyl group to the aldehyde carbonyl carbon to give a secondary alcohol after hydrolysis.

So the sequence is:

CH3−CH=CH2⟶CH3−CH2−CH2OH⟶CH3−CH2−CHO⟶CH3−CH2−CH(OH)−CH3\mathrm{CH_3-CH=CH_2 \longrightarrow CH_3-CH_2-CH_2OH \longrightarrow CH_3-CH_2-CHO \longrightarrow CH_3-CH_2-CH(OH)-CH_3}

Hence, the correct option is B.

Common mistakes

  • Assuming hydroboration-oxidation follows Markovnikov addition. That is incorrect because B2H6/H2O2,NaOH\mathrm{B_2H_6/H_2O_2,NaOH} gives anti-Markovnikov alcohol formation. First convert propene to 1-propanol, not 2-propanol.

  • Treating PCC as if it oxidizes the primary alcohol all the way to a carboxylic acid. That is wrong because PCC stops at the aldehyde stage under these conditions. Use propanal as the intermediate.

  • Forgetting that CH3MgBr\mathrm{CH_3MgBr} adds a methyl group to the carbonyl carbon. This changes the carbon chain length by one carbon. After Grignard addition to propanal, the product is a four-carbon secondary alcohol.

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