Organic Chemistry/Alkanes/Cycloalkanes

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Cycloalkanes are hydrocarbons containing one or more rings. (Alkanes without rings are referred to as aliphatic.)

→ + H₂

Under certain reaction conditions, propane can be transformed into cyclopropane. (H₂ comes off as a sideproduct.)

Cyclopropane (unstable, lots of ring strain)

Cyclobutane (ring strain)

Cyclopentane (little ring strain)

Cyclohexane (Next to no ring strain)

Cyclodecane

Rings with thirteen or more carbons have virtually no ring strain.

Naming cycloalkanes

Cycloalkanes are named similarly to their straight-chain counterparts. Simply add the root "cyclo-" before the alkane part of the name.

Example: Propane >> Cyclopropane

When naming cycloalkanes, the cyclo prefix is used for alphabetization.

Substituents

If a cycloalkane has only one substituent, it is not necessary to assign that substituent a number. If there is more than one substituent, then it is necessary to number the carbons and specify which substituent is on which carbon.

Methylcyclopentane

1,1-dimethylcyclopentane

1,2-dimethylcyclopentane

1,3-dimethylcyclopentane

The organic compound

could be named and numbered

1-cyclopropyl-5-ethyl-2-methylcyclohexane

but instead should be named

2-cyclopropyl-4-ethyl-1-methylcyclohexane

because it produces a lower numbered name (1+5+2=8 vs. 2+4+1=7).

In the following examples, notice that the longer chain is the parent and the cycloalkane is the substituent.

2-Cyclopropylbutane

1,3-dicyclopropylpropane

Multicyclic alkanes

Multicyclic alkanes are hydrocarbons that have more than one bonded cyclic ring. These abound in biology as all kinds of hormones, steroids, cholesterol,carbohydrates, etc.

They are named as bicycloalkanes, tricycloalkanes, etc.

They are named slightly differently than singularly cyclic alkanes.

Bicyclo[2.1.0]pentane

Multicyclic alkanes are found frequently in living beings:

Part of Cholesterol

We will get to some of the most interesting multicyclic rings later on when we study benzene and aromaticity.

Stereochemistry

Because the C-C bonds in cycles cannot rotate through 360 degrees, substituted cycloalkanes and similar compounds can exhibit diastereomerism. This is comparable to alkenes which show cis/trans (or E/Z) isomerism. The isomers can be named using cis/trans notation, or more rigorously using R-S notation.

cis-1,2-dichlorocyclohexane	trans-1,2-dichlorocyclohexane
1(R),2(S)-dichlorocyclohexane	1(S),2(S)-dichlorocyclohexane

Conformers, or conformational isomers, are different arrangements of the same molecule in space. Do not confuse them with any kind of true isomer as they are in every way the same molecule. The difference is in how the molecule is bent or twisted is space in any one instant of time.

Cyclohexane

The first molecule that is generally presented in a discussion of cycloalkane conformers is cyclohexane. It comes in several flavors; the main ones are the chair conformation and the boat conformation.

Boat conformation	Chair conformation

Note: In the above models, the straight lines represents single bonds, the lumps represent carbon atoms, and the open ends represent hydrogen atoms.

Consider getting a good molecular model set if you do not yet have one. They are not as inexpensive as you would hope but they help most people immensely to understand the way molecules look in three dimensions. Follow this link to places you can buy a molecular model kit.

The chair conformation (can you see how it looks like a chair?) is lower in energy than the boat conformation. This is because the two ends of the molecule are farther apart and avoid steric hindrance.

Hydrogen atoms in a cyclohexane can be divided into two types:

1. Axial, that point towards the top and bottom, and
2. Equitorial, that point out away from the edge of the molecule

When hydrogens are replaced with other, bulkier groups, it becomes apparent that the axial positions are less energetically favored than the equatorial positions. That means that, if given a choice, bulkier groups will tend to bond to cyclohexane in equatorial positions, as this reduces their steric hinderance and potential energy.

Other cycloalkanes

Cyclopentane flips between slightly different conformers as well.

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