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Hydrocarbons question

2005 · Shift 0 · Q14
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Hydrocarbons question

2005 · Shift 0 · Q14

JEE MainChemistryHydrocarbonsMCQ+4 / −1
Of the five isomeric hexanes, the isomer which can give two monochlorinated compounds is
  1. A
    n-hexane
  2. B
    2, 3-dimethylbutane
  3. C
    2,2-dimethylbutane
  4. D
    2-methylpentane
View written solutionFree

Correct answer: B

  1. Idea: The number of monochlorinated products depends on the number of distinct types of hydrogen atoms in the molecule.

    If all hydrogens of a certain kind are equivalent by symmetry, replacing any one of them by Cl gives the same product.

  2. Check each isomer:

Option A: nnn-hexane

Structure: CH3−CH2−CH2−CH2−CH2−CH3\mathrm{CH_3-CH_2-CH_2-CH_2-CH_2-CH_3}CH3​−CH2​−CH2​−CH2​−CH2​−CH3​

By symmetry:

  • C1 and C6 are equivalent
  • C2 and C5 are equivalent
  • C3 and C4 are equivalent

So there are 3 distinct positions for substitution.

Thus, nnn-hexane gives 3 monochlorinated products.

Option B: 2,32,32,3-dimethylbutane

Structure: CH3−CH(CH3)−CH(CH3)−CH3\mathrm{CH_3-CH(CH_3)-CH(CH_3)-CH_3}CH3​−CH(CH3​)−CH(CH3​)−CH3​

This molecule is highly symmetrical.

Types of hydrogens:

  • The four CH3\mathrm{CH_3}CH3​ groups are equivalent in pairs and actually all methyl hydrogens lead to the same product by symmetry.
  • The two hydrogens on C2 and C3 are equivalent.

So there are only 2 distinct kinds of hydrogens:

  1. methyl hydrogens
  2. methine hydrogens

Hence, it gives 2 monochlorinated products.

Option C: 2,22,22,2-dimethylbutane

Structure: CH3−C(CH3)2−CH2−CH3\mathrm{CH_3-C(CH_3)_2-CH_2-CH_3}CH3​−C(CH3​)2​−CH2​−CH3​

Distinct hydrogen types:

  • The three methyl groups attached to C2 are equivalent? Not all. The two methyl substituents on C2 and the left terminal methyl are equivalent around the quaternary carbon.
  • The CH2\mathrm{CH_2}CH2​ hydrogens are different from methyl hydrogens.
  • The terminal CH3\mathrm{CH_3}CH3​ on the ethyl side is another type.

This gives more than 2 monochlorinated products (actually 3).

Option D: 222-methylpentane

Structure: CH3−CH(CH3)−CH2−CH2−CH3\mathrm{CH_3-CH(CH_3)-CH_2-CH_2-CH_3}CH3​−CH(CH3​)−CH2​−CH2​−CH3​

Distinct positions:

  • Two methyl groups attached to C2 are equivalent
  • C3 is different
  • C4 is different
  • terminal C5 methyl is different
  • C2 methine hydrogen is different

So this gives more than 2 monochlorinated products.

  1. Conclusion: Only 2,32,32,3-dimethylbutane gives exactly two monochlorinated compounds.

Therefore, the correct option is: B\boxed{\text{B}}B​

  1. Comparison with stored answer:
  • Derived answer: B\boxed{\text{B}}B​
  • Stored correct answer: B\boxed{\text{B}}B​

They match.

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