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Thread: Naming hydrocarbons

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    Naming hydrocarbons


    Can someone tell me why the ans is c and not a?
    Thank you

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    Re: Naming hydrocarbons

    I reckon its A.

    There was a thread on this and no one really gave a clear answer. AFAIK the rule is to put the lowest number at the front.

    If you look at Hurlstone's 2013 Trial you'll see the MC was corrected to be A.

    In the thread some people have suggested in order of electronegativity. So since Fluorine has higher electronegativity it's named less.

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    Re: Naming hydrocarbons

    ah ok, thanks for that! my teacher told us it was in order of weight, and as chlorine is heavier it is first , not sure tho

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    Re: Naming hydrocarbons

    Quote Originally Posted by horndog huang 2 View Post

    Can someone tell me why the ans is c and not a?
    Thank you

    Sent from my LG-D855 using Tapatalk
    Using current naming techniques, answer should be (a).

    Both (a) and (c) could apply to the molecule and both produce the same value for the set of locants ( 3 + 5 = 5 + 3 ) and when this happens, RACI guidelines state that the lowest number is given to the halogen which is named first. Chloro is named first as naming is now done alphabetically, rather than by electronegativity.

    https://www.raci.org.au/document/item/1012

    Bottom of page 4, points 5 & 6.
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    Exalted Member BlueGas's Avatar
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    Re: Naming hydrocarbons

    These type of questions scream out "free marks". The way you go about answering these questions is by following two simple rules: Naming is done alphabetically and the numbering/positioning is done from the side closest to the first alphabetical atom. So in this case A B Chloro D E Floro, so you start with 3-chloro, and then you stay consistent and continue reading along to reach 5-fluoro.

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    Re: Naming hydrocarbons

    Fluorine is more electronegative than Cl therefore gets higher priority, in IUPAC at least. Then we write in alphabetical order, therefore the answer is C.

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    Re: Naming hydrocarbons

    Quote Originally Posted by Queenroot View Post
    Fluorine is more electronegative than Cl therefore gets higher priority, in IUPAC at least. Then we write in alphabetical order, therefore the answer is C.
    IUPAC does not use electronegativity for haloalkanes. Functional groups different story, but not haloalkanes.

    IUPAC uses alphanumeric.

    Please see the RACI guidelines I linked in a previous post.

    In red writing at the very beginning

    "IUPAC uses the alphanumeric nomenclature system. Older methods using electronegativity are NO LONGER VALID."

    I think it's been this way for the last 5 years or so?
    Last edited by Fizzy_Cyst; 20 Jul 2016 at 1:19 PM.
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    Re: Naming hydrocarbons

    Quote Originally Posted by Fizzy_Cyst View Post
    IUPAC does not use electronegativity for haloalkanes. Functional groups different story, but not haloalkanes.

    IUPAC uses alphanumeric.

    Please see the RACI guidelines I linked in a previous post.

    In red writing at the very beginning

    "IUPAC uses the alphanumeric nomenclature system. Older methods using electronegativity are NO LONGER VALID."

    I think it's been this way for the last 5 years or so?
    maybe the question paper is old then?

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    Re: Naming hydrocarbons

    Yeah, I'd say so! The 2012 HSC has a MC question based on the old naming system and hence there was no correct answer for it! That would have been written towards the end of 2011, so the change probably occurred around 2011/12. Papers before then regarding naming of haloalkanes would have invalid answers according to current naming system
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