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Part A shows butane and isobutane are structural isomers. Both molecules have four carbons and ten hydrogens, but in butane the carbons form a single chain, while in isobutane the carbons form a branched chain. Part B shows cis dash 2 butene and trans dash 2 butene each consist of a four-carbon chain. The two central carbons are connected by a double bond resulting in a planar, or flat shape. In the cis isomer, both terminal upper case C upper case H subscript 3 baseline groups are on the same side of the plane, and two hydrogen atoms are on the opposite side. Imagine a person with arms stretched out and upwards and legs spread apart, with a glove on the left hand and a sock on the left foot: this represents a cis configuration. In cis-butene the terminal upper C upper H subscript 3 baseline groups are on opposite sides of the plane. Now, imagine a person with outstretched arms and legs, but this time with a glove on the left hand and a sock on the right foot: this is what a trans configuration looks like. Part C shows two enantiomers, each with different arrangement of hydrogen, bromine, chlorine and fluorine around a central carbon. The molecules are mirror images of one another.
We call molecules that have the same number and type of atoms arranged differently isomers. (a) Structural isomers have a different covalent arrangement of atoms. (b) Geometric isomers have a different arrangement of atoms around a double bond. (c) Enantiomers are mirror images of each other.

Source: OpenStax Biology 2e

OpenStax Biology 2e

The three-dimensional placement of atoms and chemical bonds within organic molecules is central to understanding their chemistry. We call molecules that share the same chemical formula but differ in the placement (structure) of their atoms and/or chemical bonds isomers. Structural isomers differ in the placement of their covalent bonds: both molecules have four carbons and ten hydrogens (C4H10), but the different atom arrangement within the molecules leads to differences in their chemical properties. For example, butane is suited for use as a fuel for cigarette lighters and torches; whereas, isobutane is suited for use as a refrigerant and a propellant in spray cans.

Butane was discovered by the chemist Edward Frankland in 1849.

Geometric isomers, alternatively have similar placements of their covalent bonds but differ in how these bonds are made to the surrounding atoms, especially in carbon-to-carbon double bonds. In the simple molecule butene (C4H8), the two methyl groups (CH3) can be on either side of the double covalent bond central to the molecule. When the carbons are bound on the same side of the double bond, this is the cis configuration. If they are on opposite sides of the double bond, it is a trans configuration. In the trans configuration, the carbons form a more or less linear structure; whereas, the carbons in the cis configuration make a bend (change in direction) of the carbon backbone.

– What indicates that the functional groups are on the same side of the carbon chain?

In triglycerides (fats and oils), long carbon chains known as fatty acids may contain double bonds, which can be in either the cis or trans configuration. Fats with at least one double bond between carbon atoms are unsaturated fats. When some of these bonds are in the cis configuration, the resulting bend in the chain’s carbon backbone means that triglyceride molecules cannot pack tightly, so they remain liquid (oil) at room temperature. Alternatively, triglycerides with trans double bonds (popularly called trans fats), have relatively linear fatty acids that are able to pack tightly together at room temperature and form solid fats. In the human diet, trans fats are linked to an increased risk of cardiovascular disease, so many food manufacturers have reduced or eliminated their use in recent years. In contrast to unsaturated fats, we call triglycerides without double bonds between carbon atoms saturated fats, meaning that they contain all the hydrogen atoms available. Saturated fats are a solid at room temperature and usually of animal origin.

Ground beef contains one percent of trans fat while shortenings can contain up to 33 percent.

Oleic acid and eliadic acid both consist of a long carbon chain. In oleic acid the chain is kinked due to the presence of a double bond about half way down, while in eliadic acid the chain is straight.
These space-filling models show a cis (oleic acid) and a trans (eliadic acid) fatty acid. Notice the bend in the molecule caused by the cis configuration.

Source: OpenStax Biology 2e
– What disease is associated with high trans fat consumption?


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: