11th Class Chemistry Hydrocarbons

  • question_answer 75)   An alkane \[{{C}_{8}}{{H}_{18}}\] is obtained as the only product on subjecting a primary alkyl halide to Wurtz reaction. On monobromination this alkane yields a single isomer of a tertiary bromide. Write the structure of alkane and the tertiary bromide.

    Answer:

      Two possible primary alkyl, halides having four carbonatoms each are \[I\] and \[II\]. \[C{{H}_{3}}-C{{H}_{2}}-\underset{(I)}{\mathop{C{{H}_{2}}}}\,-C{{H}_{2}}-X\]                 \[C{{H}_{3}}-\underset{\underset{\begin{smallmatrix}  \text{C}{{\text{H}}_{\text{3}}} \\  (II) \end{smallmatrix}}{\mathop{|}}\,}{\mathop{CH}}\,-C{{H}_{2}}-X\] Since, the alkane \[{{C}_{8}}{{H}_{18}}\] on monobromination yields a single isomer of tertiary alkyl halide, therefore the alkane, \[{{C}_{8}}{{H}_{18}}\]must contain a tertiary hydrogen. This is possible only if the starting primary alkyl halide has a tertiary hydrogen(structure\[II\]). \[C{{H}_{3}}-\underset{\underset{\underset{\begin{smallmatrix}  1-Halo-2-methylpropane \\  (Primary\,alkayl\,halide) \end{smallmatrix}}{\mathop{C{{H}_{3}}}}\,}{\mathop{|}}\,}{\mathop{CH}}\,-C{{H}_{2}}-X\xrightarrow[(Wutrz\,reaction)]{Na/Dry\,ether}\]                 \[\underset{2,5Dimethylhexane\,({{C}_{8}}{{H}_{18}})}{\mathop{C{{H}_{3}}-\underset{\underset{\text{C}{{\text{H}}_{\text{3}}}}{\mathop{|}}\,}{\mathop{CH}}\,-C{{H}_{2}}-C{{H}_{2}}-\underset{\underset{\text{C}{{\text{H}}_{\text{3}}}}{\mathop{|}}\,}{\mathop{CH}}\,-C{{H}_{3}}\xrightarrow[hv]{B{{r}_{2}}}}}\,\]                 \[\underset{2-Bromo-2,5-\dimethyl\,hexane\,(tertiary\,bromide)}{\mathop{C{{H}_{3}}-\underset{\underset{C{{H}_{3}}}{\mathop{|}}\,}{\overset{\overset{\text{Br}}{\mathop{|}}\,}{\mathop{C}}}\,-C{{H}_{2}}-C{{H}_{2}}-\underset{\underset{\text{C}{{\text{H}}_{\text{3}}}}{\mathop{|}}\,}{\mathop{CH}}\,-C{{H}_{3}}}}\,\]


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