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Alcohols Phenols and Ethers question

2010 · Shift 0 · Q14
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Alcohols Phenols and Ethers question

2010 · Shift 0 · Q14

JEE MainChemistryAlcohols Phenols and EthersMCQ+4 / −1
From amongst the following alcohols the one that would react fastest with conc. HClHClHCl and anhydrous ZnCl2ZnCl_2ZnCl2​, is
  1. A
    2–Butanol
  2. B
    2–Methylpropan–2–ol
  3. C
    2–Methylpropanol
  4. D
    1–Butanol
View written solutionFree

Correct answer: B

  1. This reaction uses conc. HClHClHCl and anhydrous ZnCl2ZnCl_2ZnCl2​, which is Lucas reagent.

  2. Lucas reagent converts alcohols to alkyl chlorides. The rate depends on how easily the alcohol forms a carbocation:

    • Tertiary alcohols react fastest
    • Secondary alcohols react slower
    • Primary alcohols react very slowly at room temperature

  3. Now classify each option:

    A: 2–Butanol
    Structure: CH3−CH(OH)−CH2−CH3CH_3-CH(OH)-CH_2-CH_3CH3​−CH(OH)−CH2​−CH3​
    This is a secondary alcohol.

    B: 2–Methylpropan–2–ol
    Structure: (CH3)3C−OH(CH_3)_3C-OH(CH3​)3​C−OH
    This is a tertiary alcohol.

    C: 2–Methylpropanol
    This refers to a primary alcohol such as 2-methylpropan-1-ol, (CH3)2CH−CH2OH (CH_3)_2CH-CH_2OH(CH3​)2​CH−CH2​OH, which is primary.

    D: 1–Butanol
    Structure: CH3−CH2−CH2−CH2OHCH_3-CH_2-CH_2-CH_2OHCH3​−CH2​−CH2​−CH2​OH
    This is a primary alcohol.

  4. Since tertiary alcohols react fastest with Lucas reagent due to easiest carbocation formation, option B will react the fastest.

  5. Order of reactivity: tertiary>secondary>primary\text{tertiary} > \text{secondary} > \text{primary}tertiary>secondary>primary

    Hence, B>A>C, D\text{B} > \text{A} > \text{C, D}B>A>C, D

Therefore, the alcohol that reacts fastest is 2–Methylpropan–2–ol.

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