11th Class Chemistry Hydrocarbons

  • question_answer 5) Write structures for each of the following compounds. Why are the given names incorrect? Write correct IUPAC names. (i) 2-Ethylpentane (ii) 5-Ethyl-3-methylheptane  

    Answer:

    (i) \[C{{H}_{3}}-\underset{\underset{{{\,}^{2}}C{{H}_{2}}\overset{1}{\mathop{C}}\,{{H}_{3}}}{\mathop{|}}\,}{\mathop{\overset{3}{\mathop{C}}\,}}\,H-\overset{4}{\mathop{C}}\,{{H}_{2}}-\overset{5}{\mathop{C}}\,{{H}_{2}}-\overset{6}{\mathop{C}}\,{{H}_{3}}\] The longest chain is of six carbon atoms and not five. Therefore, the correct name of the compound is 3-methylhexane. (ii) \[\overset{1}{\mathop{C{{H}_{3}}}}\,-\overset{2}{\mathop{C}}\,{{H}_{2}}-\underset{\underset{{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}}{\mathop{\text{ }\!\!|\!\!\text{ }}}\,}{\mathop{\overset{3}{\mathop{C}}\,}}\,H-\overset{4}{\mathop{C{{H}_{2}}}}\,-\underset{\underset{\text{C}{{\text{H}}_{\text{3}}}}{\mathop{\text{ }\!\!|\!\!\text{ }}}\,}{\overset{5}{\mathop{CH}}}\,-\overset{6}{\mathop{C{{H}_{2}}}}\,-\overset{7}{\mathop{C}}\,{{H}_{3}}\] In case there are different (ethyl and methyl) alkyl substituents at equivalent positions then numbering is to be started in alphabetical order. Since 'e' comes before m, hence, the correct name is 3-ethyl-5-methylheptane.  


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