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Naming of Haloalkane (Disputed)

  1. Jun 5, 2014 #1

    Qube

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    Gold Member

    My teacher insists the name of this compound is (2S)-chloro-3-methyl-3-propylhexane

    Shouldn't it be 4-(1-chloroethyl)-4-methylheptane? (Ignore R,S configuration for now).

    Chemoffice also tells me 4-(1-chloroethyl)-4-methylheptane.

    Do52y.png

    I think it's the latter because I've consulted three textbooks; all say pick out the longest chain of carbon atoms.

    I've also consulted Wikipedia; it says pick the parent chain with the following rules in mind (in descending order of importance):

    1) Pick the most substituted chain - the chain with the most substituents of the parent suffix ("ane") - so that would mean either the hexane or the heptane chain, right? The hexane chain has a methyl and an propyl substituent, and a chloro substituent (it's a halogen, however, so it doesn't count). The heptane chain has a complex alkyl substituent and a methyl substituent too.

    2) Pick the longest chain. So the heptane chain should win here.
     
    Last edited: Jun 5, 2014
  2. jcsd
  3. Jun 6, 2014 #2

    DrClaude

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    Staff: Mentor

    This is a tough one. I've been going over the IUPAC rules, and I would lean towards your teacher being right.

    From the 1979 rules, we have
    and also
    In the 1993 rules, we find
    From this, I would conclude that the chain containing the chlorine is to take precedence (as this makes it a "substitutive nomenclature"), even if the chain length is shorter.

    The one thing that makes me uncertain is the fact that a halogen can only be named as a prefix. If the functional group was an alcohol (OH), then it would clealy take precedence: 3-methyl-3-propylhexan-2-ol. My guess is that the same approach should apply here.
     
  4. Jun 10, 2014 #3

    Qube

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    Gold Member

    Thank you for the thorough response. It seems that our textbooks greatly simplify naming considerations to picking out the longest chain. My teacher also changed the answer to 4-(1-chloroethyl)-4-methylheptane. Oh well.
     
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