How many structural isomers does pentane have?

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Multiple Choice

How many structural isomers does pentane have?

Counting constitutional isomers means counting distinct carbon skeletons, i.e., different connectivities of the five carbons that still give the same molecular formula C5H12. For pentane there are three unique skeletons. First is a straight chain of five carbons, n-pentane. Second is a four-carbon chain with a methyl group attached to the second carbon, giving 2-methylbutane (isopentane). Third is a three-carbon chain with two methyl groups on the central carbon, giving 2,2-dimethylpropane (neopentane). In each case the remaining valences are filled with hydrogens to yield C5H12. These three skeletons are distinct, and no other arrangements produce a new connectivity. Hence pentane has three structural isomers.

Counting constitutional isomers means counting distinct carbon skeletons, i.e., different connectivities of the five carbons that still give the same molecular formula C5H12.

For pentane there are three unique skeletons. First is a straight chain of five carbons, n-pentane. Second is a four-carbon chain with a methyl group attached to the second carbon, giving 2-methylbutane (isopentane). Third is a three-carbon chain with two methyl groups on the central carbon, giving 2,2-dimethylpropane (neopentane). In each case the remaining valences are filled with hydrogens to yield C5H12.

These three skeletons are distinct, and no other arrangements produce a new connectivity. Hence pentane has three structural isomers.

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