Look at the desks in the photos above. Both pictures show the same classroom with the same number of desks, but the desks are arranged differently in each picture. The different arrangements work better for different classroom activities. Desks in rows facing the front of the classroom are better for watching the teacher do demonstrations. Desks facing each other in small groups are better for working on group projects. What does the arrangement of desks in a classroom have to do with chemistry? Like desks in a classroom, atoms in a hydrocarbon molecule can be arranged in different ways, and the different arrangements give them different properties.
Same Atoms, Different Shapes
Hydrocarbons are compounds that contain only carbon and hydrogen atoms. The smallest hydrocarbon, methane (CH4), contains just one carbon atom and four hydrogen atoms. Larger hydrocarbons contain many more. Hydrocarbons with four or more carbon atoms can have different shapes. Although they have the same chemical formula, with the same numbers of carbon and hydrogen atoms, they form different compounds, called isomers. Isomers are compounds whose properties are different because their atoms are bonded together in different arrangements.
Examples of Isomers
The smallest hydrocarbon that has isomers is butane, which has just four carbon atoms. In the Figure below you can see structural formulas for normal butane (or n-butane) and its only isomer, named iso-butane. Both molecules have four carbon atoms as well as ten hydrogen atoms (C4H10), but the atoms are arranged differently in the two compounds. In n-butane, all four carbon atoms are lined up in a straight chain. In iso-butane, one of the carbon atoms branches off from the main chain. You can see three-dimensional models of these two isomers at the URLs below. Rotate the molecule models to get a better idea of their shapes.
The next smallest hydrocarbon is pentane, which has five carbon atoms and twelve hydrogen atoms (C5H12). Pentane has three isomers: n-pentane, iso-pentane, and neo-pentane. Their structural formulas are shown in the images below. Look at the carbon atoms in each isomer. In n-pentane (see Figure below), the carbon atoms form a straight chain. In iso-pentane (see Figure below), one carbon atom branches off from the main chain. In neo-pentane (see Figure below), two carbon atoms branch off from the main chain.
How Many Isomers?
Butane has only two isomers and pentane has just three, but some hydrocarbons have many more isomers than these. As you increase the number of carbon atoms in a hydrocarbon, the number of isomers quickly increases. For example, heptane, with seven carbon atoms, has nine isomers; and dodecane, with twelve carbon atoms, has 355 isomers. Some hydrocarbons with many more carbon atoms have billions of isomers!
Q: Why does the number of carbon atoms in a hydrocarbon determine how many isomers it has?
A: The more carbon atoms there are, the greater the number of possible arrangements of carbon atoms.
Properties of Isomers
Because isomers are different compounds, they have different properties. Generally, branched-chain isomers have lower boiling and melting points than straight-chain isomers. For example, the boiling and melting points of iso-butane are -12 °C and -160 °C, respectively, compared with 0 °C and -138 °C for n-butane. The more branching there is, the lower the boiling and melting points are.
Q: The boiling point of n-pentane is 36 °C. Predict the boiling points of iso-pentane and neo-pentane.
A: The boiling point of iso-pentane is 28 °C, and the boiling point of neo-pentane is 10 °C.
- Hydrocarbons with the same numbers of atoms but different shapes form different compounds called isomers.
- Hydrocarbons with four or more carbon atoms have isomers. The more carbon atoms a hydrocarbon has, the greater the number of isomers.
- Isomers are different compounds with different properties, such as different boiling and melting points.
At the following URL, watch the video about isomers of hexane. Then answer the questions below.
- What is the chemical formula for hexane?
- How many isomers does hexane form?
- How do boiling points of hexane isomers vary?
- What are isomers?
- Name and describe the isomers of butane.
- Identify the hydrocarbon that has nine isomers. What is its chemical formula?