Methylbutane is called methylbutane because its structure consists of a butane backbone with a methyl group attached to it. The name reflects the molecule's composition based on IUPAC nomenclature rules.
Here's a breakdown of why it's named Methylbutane:
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Butane Backbone: The longest continuous chain of carbon atoms in the molecule contains four carbons. As stated in the reference about isopentane (which is also called methylbutane in systematic nomenclature), a four-carbon chain is named "butane."
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Methyl Group: A methyl group (CH3-) is attached as a branch to the butane chain. The presence of this CH3- group as a "branch" is reflected in the "methyl" portion of the name.
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Location of the Methyl Group: While the question doesn't specify which methylbutane isomer is being referred to, the most common one is 2-methylbutane, also known as isopentane. The reference states that, "A methyl group (CH3-) branches off from the second carbon along the chain, so isopentane is called 2-methylbutane." In the case of 2-methylbutane, the methyl group is attached to the second carbon atom of the butane chain. (Note that this location information is implied, not stated, in the question.)
Therefore, the name "Methylbutane" (and specifically "2-methylbutane") describes a butane molecule with a methyl group attached to it.
