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    I'm studying AS chemistry and was revising the organic topic, answering end of topic questions. I came across this answer as being the displayed formulae for "methylbutanone". I'm just a bit confused as I thought there should be a locant there, making it "2-methylbutanone"?

    or is it simply methylbutanone because you only count the chain I've circled in the image (which would mean it would be 1-methylbutanone on either position of the two carbon chain)? If this is the case, why is the prefix still "but", meaning 4 carbons long?
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    (Original post by piano8myhomework)
    I'm studying AS chemistry and was revising the organic topic, answering end of topic questions. I came across this answer as being the displayed formulae for "methylbutanone". I'm just a bit confused as I thought there should be a locant there, making it "2-methylbutanone"?

    or is it simply methylbutanone because you only count the chain I've circled in the image (which would mean it would be 1-methylbutanone on either position of the two carbon chain)? If this is the case, why is the prefix still "but", meaning 4 carbons long?
    First, if you think about butanone - you don't need to specify where the ketone is because it can't be on the end carbons (that'd be an aldehyde), and either of the other two would put the ketone on carbon 2.

    Then, adding the methyl in, it doesn't go on 1 or 4 as then you'd have a pentanone. So it has to go on carbon 3. Since there's only one way of arranging the functional groups to get methylbutanone there's no need to put the numbers in.

    I don't think you'd be penalised for adding numbers in, but it'd have to be 3-methylbutanone. The ketone has the highest priority so it sits on carbon 2. The methyl group can't then go on carbon 2 as you'd have 5 bonds coming from that carbon.
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    Welcome Squad
    This link explaining Nomenclature might help you
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    (Original post by piano8myhomework)
    I'm studying AS chemistry and was revising the organic topic, answering end of topic questions. I came across this answer as being the displayed formulae for "methylbutanone". I'm just a bit confused as I thought there should be a locant there, making it "2-methylbutanone"?

    or is it simply methylbutanone because you only count the chain I've circled in the image (which would mean it would be 1-methylbutanone on either position of the two carbon chain)? If this is the case, why is the prefix still "but", meaning 4 carbons long?
    Simply, the methyl group can only go on that carbon to so using a number isn't necessary x


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