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I'm having trouble with this question since I'm getting (A), when the answer says its (B).

My logic was that there is a methyl group on the first carbon, and a hydroxide functional group on the second carbon, hence the name 1-methyl-2-pentanol.

Could someone please explain why I am wrong?

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Don't let the graphical layout of the molecule deceive you! There are no branches in the carbon chain. (That is, no carbon atom is bound to more than two carbon atoms.) The molecule ethane is not called "methylmethane", so calling your molecule a methylpentane derivative would similarly be an error.

With no branches (and no rings and no double bonds etc.), and with six carbon atoms, we must be dealing with a hexane derivative. Once you know that, you just need to apply IUPAC rules that minimize the numbering of atoms to which substituents are bound. So answer B makes more sense than C, for that reason.

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    $\begingroup$ Alright, I understand now! Thankyou! So, just to be 100% sure, under what circumstances would the molecule be a pentane derivative with a methyl chain branch? $\endgroup$
    – chemistryyo
    Mar 8, 2015 at 4:56
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    $\begingroup$ For simple alkanes with methyl "branches", the numbering gives it away. You can't have a "1-methyl" substituent on an otherwise unsubstituted alkane, because it would make the chain longer, thus making it a hexane. So "1-methylpentane" doesn't exist, but 2-methylpentane and 3-methylpentane do. (But not "4-methylpentane" or "5-methylpentane" because the former is the same as the 2-methyl compound, and the latter thas the same "not really a branch" problem as the "1-methyl".) $\endgroup$
    – Curt F.
    Mar 8, 2015 at 5:00
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Could someone please explain why I am wrong?

  1. Find the longest chain! This determines the base name: hexane.
  2. Find the position of the $\ce{OH}$ substituent. Lower number wins.
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