Class 11 NEET Chemistry Organic Chemistry Some Basic Principles And Techniques Part 2

Concept:IUPAC naming of branched alkanes requires selecting the longest continuous carbon chain as the parent, numbering it to give the lowest set of locants, and arranging substituents alphabetically.Explanation:First, identify the longest carbon chain in the given structure.The longest chain contains seven carbon atoms, so the parent name is heptane.Number the chain from the end that gives the lowest possible locants to the substituents.Here, numbering from left to right gives substituents at positions 3 and 5; numbering from right to left would give positions 3 and 5 as well (same set).Thus, the locants are 3 and 5.The substituents are: an ethyl group ($\mathrm{-C_2H_5}$) at carbon 3 and a methyl group ($\mathrm{-CH_3}$) at carbon 5.When writing the name, substituents are listed in alphabetical order: ethyl comes before methyl.Therefore, the correct IUPAC name is 3-ethyl-5-methylheptane.Shortcut:Always check the longest chain (not necessarily the one drawn horizontally). Use the “first point of difference” rule: start numbering from the end that gives a lower locant at the first branching.Answer:Option A: 3-ethyl-5-methylheptane