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Cycloalkanes

Chapter 13

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Cycloalkanes are named much like acyclic alkanes. Substituted

cycloalkanes use the cycloalkane for the base name, with the alkyl

groups named as substituents. If there is just one substituent, no

numbering is needed.

Cycloalkanes:

Homocyclic

Heterocyclic

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Compounds that contain two fused rings are called bicyclic compounds, and they can be drawn in different ways:

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The C-C bonds of cyclopropane bend outward (on the dotted red lines) to alleviate some of the angle strain.

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Cyclohexane can adopt many conformations, as we will soon see.

For now, we will explore two conformations: the chair

conformation and the boat conformation

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An energy diagram showing the conformational analysis of cyclohexane.

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to occupy an axial position, which is parallel to a vertical axis

passing through the center of the ring.

The other group is said to occupy an equatorial position, which is

positioned approximately along the equator of the ring

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The term “ring flip” is used to describe the conversion of one chair

conformation into the other.

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At room temperature, 95% of the molecules will be in the chair

conformation that has the methyl group in an equatorial position.

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When the substituent is in an axial position, there are steric interactions with

the other axial H’s on the same side of the ring.

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These interactions are called 1,3-diaxial interactions, where the numbers “1,3”

describe the distance between the substituent and each of the H’s.

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cis-1,2-Dimethylcyclohexane and trans-1,2-dimethylcyclohexane are stereoisomers

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Each stereoisomer of 1,2-dimethylcyclohexane has two chair conformations.

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Another common polycyclic system is norbornane. Norbornane is

the common name for bicyclo[2.2.1]heptane.

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Epoxides

Epoxides are named in three different ways—epoxyalkanes, oxiranes, or

alkene oxides.

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Epoxides don’t have a good leaving group either, but they have one

characteristic that neither alcohols nor ethers have: the “leaving group” is

contained in a strained three-membered ring.

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Reactions of Epoxides

Nucleophilic attack opens the strained three-membered ring, making it a favorable

process even with the poor leaving group.

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Crown Ethers