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Help! Nomenclature of this molecule:

Discussion in 'MCAT Study Question Q&A' started by confusedgradu8, Jun 13, 2008.

  1. confusedgradu8

    confusedgradu8 2+ Year Member

    May 13, 2008
    file:///C:/DOCUME%7E1/LOCALS%7E1/Temp/moz-screenshot.jpg[​IMG] View attachment mol.JPG

    What would be the name of this molecule?

    I thought it would be 2-methyl-trans-3-pentene.
    But the books solution says its 4-methyl-trans-2-pentene.

    Im not sure why...but I thought you have to pick the numbering with the lowest sum...
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  3. koopa_troopa

    koopa_troopa Junior Member 7+ Year Member

    Mar 28, 2005
    The alkene is the main group of this molecule and should have the lowest number (2), which is why the book chose 2-pentene.
  4. DoctorRx1986

    DoctorRx1986 10+ Year Member

    Feb 11, 2008

    In the case of alkenes and alkynes, you start numbering at the carbon nearest the first carbon atom of the double bond. You assign the lowest numbers to double or triple bonds. Take this molecule, CH3CH=CHCH2CH2CH3. You start numbering it from the first carbon of the methyl group on the left. Upon doing so, you will realize the first carbon of the double bond will be a 2. The second carbon of that bond will be a 3. Once you realize this, you will see that the correct name of this molecule, stereochemistry assigned as well, is Trans-2-hexene.

    Numbering the molecule beginning at the first carbon of the methyl group on the right would have resulted in assigning a 4 to the first carbon of the double bond and a 5 to the second carbon. In this case, you would incorrectly be naming the molecule as Trans-4-hexene. So, always remember to assign priority in numbering to a double bond and give it the lowest number.
  5. supertrooper66

    supertrooper66 Banned

    Apr 28, 2008
    i don't think i've ever even seen nomenclature on a single practice test. definitely wasn't on my real mcat either!

    but yea, like they said, you # the carbons based on what takes highest priority. methyl is like the LAST priority of everything haha.
  6. gogopogo

    gogopogo Canadian stereotype, eh? 2+ Year Member

    Apr 27, 2008
    Triangle Island
    I agree. Much easier to be, like "certain classes of sulfonamides are blah blah blah..." and just expect it to be understood.

    P.S. - Lactone: Cyclic ester (Looks like a guitar sorta)
  7. Quantum Chem

    Quantum Chem 2+ Year Member

    Jun 10, 2008
    College Park, Md
    Although this really doesn't apply to this molecule, I want to remind everyone that there are E and Z identifiers for some alkenes. Remember to rate each side of the alkene with the highest molecular weight substituent and if the two are facing the same side its Z and if they are opposite it would be E. So if done on this molecule it would be (E) 4-methyl-2-pentene.

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