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Give the name and draw the graphical formula of an alkene that is an isomer of but-1-ene and that has a different carbon skeleton.

I think it's 2-methylpropene - but I'm not sure whether the correct name is 2-methylprop-1-ene or 2-methylpropene?

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When considering whether numbering is necessary you must see whether there are alternative structures which have the same basic name but different numbering.

In this example putting the methyl group on the 1 or the 3 position of the longest carbon chain changes its name from prop- to but-. Since the only other position that the methyl group can go is the 2 position then both numbers are unnecessary and so the shortest unambiguous name is methylpropene.

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