14.13: Cycloalkanes
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Learning Objectives
- To name cycloalkanes given their formulas and write formulas for these compounds given their names.
The hydrocarbons we have encountered so far have been composed of molecules with open-ended chains of carbon atoms. When a chain contains three or more carbon atoms, the atoms can join to form ring or cyclic structures. The simplest of these cyclic hydrocarbons has the formula C 3 H 6 . Each carbon atom has two hydrogen atoms attached (Figure \(\PageIndex{1}\)) and is called cyclopropane.
To Your Health: Cyclopropane as an Anesthetic
With its boiling point of −33°C, cyclopropane is a gas at room temperature. It is also a potent, quick-acting anesthetic with few undesirable side effects in the body. It is no longer used in surgery, however, because it forms explosive mixtures with air at nearly all concentrations.
The cycloalkanes—cyclic hydrocarbons with only single bonds—are named by adding the prefix cyclo- to the name of the open-chain compound having the same number of carbon atoms as there are in the ring. Thus the name for the cyclic compound C 4 H 8 is cyclobutane. The carbon atoms in cyclic compounds can be represented by line-angle formulas that result in regular geometric figures. Keep in mind, however, that each corner of the geometric figure represents a carbon atom plus as many hydrogen atoms as needed to give each carbon atom four bonds.
Some cyclic compounds have substituent groups attached. Example \(\PageIndex{1}\) interprets the name of a cycloalkane with a single substituent group.
Example \(\PageIndex{1}\)
Draw the structure for each compound.
- cyclopentane
- methylcyclobutane
Solution
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The name
cyclopentane
indicates a cyclic (cyclo) alkane with five (pent-) carbon atoms. It can be represented as a pentagon.
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The name
methylcyclobutane
indicates a cyclic alkane with four (but-) carbon atoms in the cyclic part. It can be represented as a square with a CH
3
group attached.
Exercise \(\PageIndex{1}\)
Draw the structure for each compound.
- cycloheptane
- ethylcyclohexane
The properties of cyclic hydrocarbons are generally quite similar to those of the corresponding open-chain compounds. So cycloalkanes (with the exception of cyclopropane, which has a highly strained ring) act very much like noncyclic alkanes. Cyclic structures containing five or six carbon atoms, such as cyclopentane and cyclohexane, are particularly stable. We will see later that some carbohydrates (sugars) form five- or six-membered rings in solution.
The cyclopropane ring is strained because the C–C–C angles are 60°, and the preferred (tetrahedral) bond angle is 109.5°. (This strain is readily evident when you try to build a ball-and-stick model of cyclopropane; see Figure \(\PageIndex{1}\).) Cyclopentane and cyclohexane rings have little strain because the C–C–C angles are near the preferred angles.
Key Takeaway
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Many organic compounds have cyclic structures.