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Term 'Iso' in naming (common name)

  1. Feb 27, 2015 #1
    Why 2-Bromopropane is called isopropyl bromide?I am concern about term iso.I have been told if there is one CH3 group as a substituent on the c atom next to end c atom then we use term iso but here in 2-Bromopropane
    there is no ch3 group on the c atom next to end c atom one ch3 group does exist but it is not substituent.Infact I have been thinking this iso configuration as it is only applied to branched compounds otherwise it is we use n-....
    and 2-Bromopropane is not branched at all ,although one halo group is present as substituent but carbon chain is not branched.
  2. jcsd
  3. Feb 27, 2015 #2


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    Some guidance is found here - http://www.chem.ucla.edu/harding/IGOC/C/common_name.html

    and here - http://colapret.cm.utexas.edu/courses/organicnomenclature.pdf - "Note that the prefix “iso” means “same”, so isobutane received its name because it has the same formula as butane."

    In the isopropyl bromide, no matter where the lone Br is, it will have the same formula. Conventionally, the Br is on the second carbon, as opposed to the end carbon on the end position or side of the end carbon, so in the middle, it is isopropyl bromide.

    Isopropyl alcohol has the OH on the middle C.
  4. Feb 27, 2015 #3


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    The term you are looking for here is 'isomer':


    If you study organic chem in any detail, you should be familiar with this concept. :wink:
  5. Feb 28, 2015 #4
    But there are many types of isomerism,how can I differentiate between neo and iso?
  6. Feb 28, 2015 #5


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    That's where naming conventions come into play.

    Because there are millions of possible organic compounds, a naming system, or nomenclature, was developed to organize these various compounds by their basic structure.

    The following is just one article which discusses the basics:

    http://chemwiki.ucdavis.edu/Organic_Chemistry/Hydrocarbons/Alkanes/Nomenclature_of_Alkanes [Broken]

    IUPAC has established rules for naming various chemical compounds, which may have superseded older conventions:


    Last edited by a moderator: May 7, 2017
  7. Feb 28, 2015 #6


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    n- and iso- are just shortcuts, used for a limited number of cases - there is no stiff rule that make them applicable to all compounds.

    n- means carbon chain is not branched, so you can have n-propyl, n-butyl and so on.

    iso- means something is different. The only two carbon chains I can think of where this is commonly used are isopropyl and isobutyl. Note that the isobutyl is one of four possible arrangements of atoms for butyl (n-buty, isobutyl, sec-butyl, tert-butyl). These are not systematic names, and while they are often convenient to use, it is not a shame to not know them as long as you can name the chain in a systematic way.
  8. Jun 11, 2015 #7
    The term iso is used to specify that the compound has a single branch. That branch is a methyl group (CH3) and it appears at the 2nd carbon in the chain. The longest chain of an iso compound is the root name minus one.

    So isobutane has a 3 carbon chain with a methyl group at carbon 2 (2-methyl propane). And isopentane is a 4 carbon chain also known as 2-methyl butane. Isopropane of course is a misnomer since 2-methyl ethane is simply just propane.

    The rules are the same even if you have a functional group like an alcohol or a halide (like in your example). In an iso compound, the functional group will appear at the omega carbon (the last one in the chain). When I say the last carbon in the chain, I mean with respect to labeling the methyl group as carbon #2. Please realize that the addition of a functional group will almost always change the way you would have to number the carbon chain for IUPAC naming. The only time the functional group does not appear on the omega carbon is when the compound is isopropyl. Because if it did, it would simply be an n-propyl compound instead.

    So take isopentanol for example. The isopentan- portion of the name means a 4 carbon chain with a methyl on carbon #2. Then on the omega carbon (#4) put the alcohol (-ol). Like I mentioned before, the IUPAC name would not be 2-methylbutan-4-ol. It would be 3-methylbutan-1-ol to give the alcohol higher priority.

    And finally, take a look at your example. Isopropyl bromide means a 2 carbon chain with a methyl group on carbon #2. In other words, just propane. Then, the bromide substituent would go on the omega carbon, UNLESS that would result in n-propyl which it clearly would. So in that case the substituent goes on the 2nd-to-last carbon.

    Common names for organic compounds are interesting, but ultimately not very straightforward sometimes. IUPAC nomenclature is preferred, but when you go into the organic chemistry community, everyone uses common names so you have to get used to them. No one says ethanoic acid, trichloromethane, or 1-methylethyl benzene. We say acetic acid, chloroform, and cumene.
  9. Jun 12, 2015 #8


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    Do you have any source for that? That's the first time I see this rule.
  10. Jun 12, 2015 #9
    Common names in organic nomenclature are just that; common names given by people before the standardized rules existed. By nature, you won't find any source other than something straight from the horse's mouth (any organic chemist). There are no set rules akin to IUPAC nomenclature. But there were some commonly accepted ways of doing things.

    form = 1 carbon chain
    acet = 2 carbon chain

    acetic acid, a 2 carbon carboxylic acid
    acetone, a 2 carbon methyl ketone (this one annoys students to no end since the chain is 3)
    formaldehyde, a 1 carbon aldehyde

    Iso was/is commonly used to refer to a specific arrangement of 2 methyl groups attached to a carbon. Like:


    Anything with this group and no other branched hydrocarbons is assigned the appropriate iso name depending on what R is. I simply generalized the nomenclature in my last post.
  11. Jun 12, 2015 #10


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    Funny thing, I just checked my edition of Morrison and Boyd and they list the rule, so it seems I lied when I said I have never seen it, I must have read it in 1985 when I bought the book. I was always under impression iso- is much less precise (or rather - it has a well defined meaning only for two or three light alkanes).
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