Consider all the isomers of pentane (C5H12).
(a) Write the molecular formula for pentane. (b) Name each isomer by the IUPAC system. (c) Write a condensed structural formula and draw a ‘stick’ representation of each isomer. (d) What type of isomerism is illustrated by these structures?
The correct answer and explanation is:
(a) The molecular formula for pentane is C5H12.
(b) The isomers of pentane are:
- n-Pentane (normal pentane): This is the straight-chain isomer where all five carbon atoms are connected in a single chain.
- Isopentane (methylbutane): This is a branched-chain isomer with a methyl group (–CH3) attached to the second carbon of a four-carbon chain.
- Neopentane (dimethylpropane): This is another branched-chain isomer with two methyl groups (–CH3) attached to a central carbon in a “T” shape.
(c) The condensed structural formulas and stick representations for each isomer:
- n-Pentane:
- Condensed Formula: CH3(CH2)3CH3
- Stick Representation:
CH3-CH2-CH2-CH2-CH3
- Isopentane:
- Condensed Formula: CH3CH(CH3)CH2CH3
- Stick Representation:
CH3-CH-CH2-CH3 | CH3
- Neopentane:
- Condensed Formula: (CH3)2C(CH3)2CH3
- Stick Representation:
CH3 | CH3-C-CH3 | CH3
(d) The type of isomerism illustrated by these structures is structural isomerism, which occurs when molecules have the same molecular formula but different structural arrangements of atoms. Specifically, these are chain isomers, meaning they differ in the branching of the carbon chain. Structural isomerism arises because the carbon atoms in pentane can be arranged in different ways to form distinct compounds with the same number of carbon and hydrogen atoms. These different structural forms exhibit different physical and chemical properties, even though they share the same molecular formula.