http://www.ck12.org Chapter 3. Chemical Interactions
3.34 Isomers
- Describe isomers of hydrocarbons.
- Contrast properties of isomers of the same hydrocarbon.
- Relate the number of isomers to the number of carbon atoms in a hydrocarbon.
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
(CH 4 ), contains just one carbon atom and four hydrogen atoms. Larger hydrocarbons contain many more. Hydro-
carbons 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, calledisomers. 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 theFigure3.60 you
can see structural formulas for normal butane (orn-butane) and its only isomer, namediso-butane. Both molecules
have four carbon atoms as well as ten hydrogen atoms (C 4 H 10 ), but the atoms are arranged differently in the two
compounds. Inn-butane, all four carbon atoms are lined up in a straight chain. Iniso-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.
http://www.worldofmolecules.com/3D/butane_3d.htm
http://www.chemspider.com/Chemical-Structure.6120.html
The next smallest hydrocarbon is pentane, which has five carbon atoms and twelve hydrogen atoms (C 5 H 12 ). Pentane
has three isomers:n-pentane,iso-pentane, andneo-pentane. Their structural formulas are shown in the images below.
Look at the carbon atoms in each isomer. Inn-pentane (seeFigure3.61), the carbon atoms form a straight chain. In
iso-pentane (seeFigure3.62), one carbon atom branches off from the main chain. Inneo-pentane (seeFigure3.63),
two carbon atoms branch off from the main chain.