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I am currently studying organic chemistry at school and I came across a question in my textbook which asked me to find the full displayed formula of methylpropene. I knew that there would be three carbons because of the 'prop' and that there would be a one-carbon side chain on the first carbon. However, in my chemistry textbook the one-carbon side chain is on the second carbon (the middle one). I don't know whether this is correct or not. Could someone please explain to me what the correct answer is?

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    $\begingroup$ There's only one place the methyl group can go for the compound to be a propene derivative i.e. the middle one. If you put it on the first carbon then it is a butene because you have a C4 chain; 1-Methyl-propene IS butene. $\endgroup$ – Waylander Jan 3 '19 at 14:28
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    $\begingroup$ Related: Naming of 2-methylpropane. Nevertheless, the name "methylpropene" is not in accordance with IUPAC nomenclature; see also IUPAC rules on omission of locants in carboxylic acid $\endgroup$ – Faded Giant Jan 3 '19 at 15:52
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"Propene" is what defines the main chain of three atoms so that the maximum number of carbon atoms in any other chain cannot exceed this number. Hence your proposition that

there would be a one-carbon side chain on the first carbon

is wrong as in this case there would be a 4-membered chain of either 1-butene or 2‐butene:

1-butene and 2‐butene

In order to comply with the methylpropene name, one can only attach methyl group to the second carbon, and not to the terminal one:

methylpropene

To sum it up, your textbook is correct.

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