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ChemWiki: The Dynamic Chemistry E-textbook > Organic Chemistry > Hydrocarbons > Alkanes > Properties of Alkanes > Cycloalkanes > Nomenclature of Cycloalkanes

Nomenclature of Cycloalkanes

Cycloalkanes are cyclic hydrocarbons, meaning that the carbons of the molecule are arranged in the form of a ring, and are also saturated, meaning that all the carbons atoms that makes up the ring are single bonded to other atoms (no double or triple bonds). There are also polycyclic alkanes which are molecules that contain two or more cycloalkanes which are joined together, forming multiple rings.

Introduction

Many organic compounds found in nature or created in a laboratory contain rings of carbon atoms with distinguishing chemical properties, also known as cycloalkanes. Cycloalkanes again only contain carbon-hydrogen bonds and carbon-carbon single bonds, but this time the carbon atoms are joined up in a ring. The smallest cycloalkane is cyclopropane.

 

If you count the carbons and hydrogens, you will see that they no longer fit the general formula CnH2n+2. By joining the carbon atoms in a ring, you have had to lose two hydrogen atoms. You are unlikely to ever need it, but the general formula for a cycloalkane is CnH2n.

Don't imagine that these are all flat molecules. All the cycloalkanes from cyclopentane upwards exist as "puckered rings". Cyclohexane, for example, has a ring structure which looks like this:

Figure 2: This is known as the "chair" form of cyclohexane - from its shape which vaguely resembles a chair. Note:  The cyclohexane molecule is constantly changing, with the atom on the left which is currently pointing down flipping up, and the one on the right flipping down. During the process, another (slightly less stable) form of cyclohexane is formed known as the "boat" form. In this arrangement, both of these atoms are either pointing up or down at the same time

In addition to being saturated cyclic hydrocarbons, cycloalkanes may have multiple substituents or functional groups which further determines their unique chemical properties. The most common and useful cycloalkanes in organic chemistry are cyclopentane and cyclohexane, though other cycloalkanes varying in the number of carbons can be synthesized. Understanding cycloalkanes and their properties are crucial in that many of the biological processes that occur in most living things have cycloalkane-like structures.

drawing012.gifGlucose (6 carbon sugar)            drawing013.gifRibose (5 carbon sugar)

 

cholesterol.gif

Cholesterol (polycyclic)

Though polycyclic compounds are important, they highly complex and typically have common names accepted by IUPAC. However, the common names do not generally follow the basic IUPAC nomenclature rules. The general formula of the cycloalkanes is CnH(2n) where n=the number of carbons. The naming of cycloalkanes follow a simple set of rules which are built upon the same basic steps in naming alkanes. Cyclic hydrocarbons has the prefix "cyclo-".

Contents

For simplicity purposes, cycloalkane molecules can be drawn in the form of skeletal structures where each intersection between two lines are assumed to have a carbon atom with its corresponding number of hydrogens.

2 (1).gif    same as    4.gif    same as    cyclohexane.gif

 

Cycloalkane Molecular Formula Basic Structure
Cyclopropane C3H6 cyclopropane.gif
Cyclobutane C4H8 cyclobutane.gif
Cyclopentane C5H10  cyclopentane.gif
Cyclohexane C6H12  cyclohexane.gif
Cycloheptane C7H14  cycloheptane.gif
Cyclooctane C8H16  cyclooctane.gif
Cyclononane C9H18  cyclononane.gif
Cyclodecane C10H20  cyclodecane.gif

IUPAC Rules for Nomenclature

  1. Determine the cycloalkane to use as parent chain. The parent chain is one with the most amount of carbon atoms. If there are two cycloalkanes, use the cycloalkane with more carbons as the parent chain.
  2. If there is an alkyl straight chain that is greater in the number of carbons than the cycloalkane, the alkyl chain must be used as the primary parent chain. Cycloalkane acting as a substituent to an alkyl chain has an ending "-yl" and therefore must be named as a cycloalkyl.
Cycloalkane Cycloalkyl
cyclopropane cyclopropyl
cyclobutane cyclobutyl
cyclopentane cyclopentyl
cyclohexane cyclohexyl
cycloheptane cycloheptyl
cyclooctane cyclooctyl
cyclononane cyclononanyl
cyclodecane cyclodecanyl

Example  

drawing020.gif

The longest straight chain contains 10 carbons, compared to the cyclopropane which only contains 3 carbons. Since cyclopropane is a substituent, it would be named a cyclopropyl-substituted alkane.

Example  

3) Determine any functional groups or other alkyl groups.

4) Number the carbons of the cycloalkane so that the carbons with functional groups or alkyl groups have the lowest possible number. A carbon with multiple substituents should have a lower number than a carbon with only one substituent or functional group. One way to make assure the lowest number possible is to number the carbons in such a way that when the numbers corresponding to the substituents are added, its sum is the lowest possible.

drawing01 (2).gif(1+3=4)    NOT    drawing02 (4).gif(1+5=6)

5) When naming the cycloalkane, substituents and functional groups must be placed by alphabetical order.

noname20.gif    drawing03.gif 

(ex: 2-bromo-1-chloro-3-methylcyclopentane)

6) Indicate the carbon number with the functional group with the highest priority according to alphabetical order. A dash"-" must be placed between numbers and name of the substituent.  After the carbon number and the dash, the name of substituent can follow.  When there is only one substituent on the parent chain, indicating the number of the carbon atoms with the substituent is not necessary.

(ex: 1-chlorocyclohexane or cholorocyclohexane is acceptable) drawing3.gif

7) If there is more than one of the same functional group on one carbon, write the number of the carbon two, three, or four times, depending on how many of the same functional group is present on that carbon.  The numbers must be seperated by commas and the name of the functional group that follows must be seperated by a dash.  When there are two of the functional group, it must have the prefix "di". When there are three of the functional group, it must have the prefix "tri". When there are four of the functional group, it must have the prefix "tetra".  However, these prefixes cannot be used when determining alphabetical priorities.

There must always be commas between numbers and dashes between numbers and names.

Example  

noname21.gif      drawing.gif    (2-bromo-1,1-dimethylcyclohexane)

Example  

Notice that "f" of fluoro is higher in alphabetical than "m" of methyl. Even though "di" is higher in alphabet than "f", it is not used in determining alphabetical order.

drawing07.gif   (2-fluoro-1,1,-dimethylcyclohexane    NOT    1,1-dimethyl-2-fluorocyclohexane)


8) If the substituents of the cycloalkane have a relation of by way of cis or trans, indicate so by placing "cis-" or "trans-" in front of the name of the structure.

3D2.gif

Blue=Carbon    Yellow=Hydrogen   Green=Chlorine

Notice that chlorine and methyl group are both pointed in the same direction on the axis of the molecule and therefore are cis.

drawing13.gif     cis-1-chloro-2-methylcyclopentane

9) After all the functional groups and substituents have been mentioned with their corresponding numbers, the name of the cycloalkane can follow.

Reactivity

Cycloalkanes are very similar to the alkanes in reactivity, except for the very small ones - especially cyclopropane. Cyclopropane is much more reactive than you would expect. The reason has to do with the bond angles in the ring. Normally, when carbon forms four single bonds, the bond angles are about 109.5°. In cyclopropane, they are 60°.

With the electron pairs this close together, there is a lot of repulsion between the bonding pairs joining the carbon atoms. That makes the bonds easier to break.

Alcohol Substituents on Cycloalkanes

Alcohol (-OH) substituents take the highest priority for carbon atom numbering in IUPAC nomenclature. The carbon atom with the alcohol substituent must be labeled as 1. Molecules containing an alcohol group has an ending "-ol", indicating the presense of an alcohol group. If there are two alcohol groups, the molecule will have a "di-" prefix before "-ol" (diol). If there are three alcohol groups, the molecule will have a "tri-" prefix before "-ol" (triol) and etc.

Example  

(Alcohol substituent is given the lowest number even though the two methyl groups are on the same carbon atom and labeling 1 on that carbon atom would give the lowest possible numbers. Numbering the location of the alcohol substituent is unnecessary since the ending "-ol" suggests one alcohol group present on carbon atom number 1, but it is acceptable.)

drawing04.gif    2,2-dimethylcyclohexanol    NOT    1,1-dimethyl-cyclohexane-2-ol

Example  

drawing05.gif    3-bromo-2-methylcyclopentanol    NOT    1-bromo-2-methyl-cyclopentane-2-ol

Example  

noname30 (1).gif

Blue=Carbon    Yellow=Hydrogen    Red=Oxygen

 

      drawing06.giftrans-cyclohexane-1,2-diol


Other Substituents on Cycloalkanes

There are many other functional groups like alcohol which are later covered in an organic chemistry course and they also determine the ending name of a molecule. The naming of these functional groups will be explained in depth later as their chemical properties are explained.

Name Name ending
alkene -ene
alkyne -yne
alcohol -ol
ether -ether
nitrile -nitrile
amine -amine
aldehyde -al
ketone -one
carboxylic acid -oic acid
ester -oate
amide -amide

Though alkynes determine the name ending of a molecule, alkyne as a substituent on a cycloalkane is not possible since alkynes are planar, and would require that the carbon that is part of the ring to make 5 bonds, giving the carbon atom a negative charge.

pz1.gif

However, a cycloalkane with a triple bond-containing substituent is possible so long as the triple bond is not directly attatched to the ring.

Example  

pz2.gifethynylcyclooctane

Example  

drawing6.gif1-propylcyclohexane


Summary

  1. Determine the parent chain: the parent chain contains the most carbon atoms.
  2. Number the substituents of the chain so that the sum of the numbers are the lowest possible.
  3. Name the substituents and place them in alphabetical order.
  4. If stereochemistry of compound is shown, indicate as part of the nomenclature.
  5. Cyclic hydrocarbons has the prefix "cyclo-" and has an "-alkane" ending unless there is an alcohol substituent present. When an alcohol substituent is present, the molecule has an ending "-ol".

Glossary

  • alcohol: An oxygen and hydrgen atom containing -OH hydroxyl group that is bonded a substituted alkyl group.
  • alkyl: A structure that is formed when a hydrogen atom is removed from an alkane.
  • cyclic: Chemical compounds arranged in the form of a ring or an closed chain form.
  • cycloalkanes: Cyclic saturated hydrocarbons with a general formula of CnH(2n). Cycloalkanes are alkanes with carbon atoms attached in the form of a closed ring.
  • functional groups: An atom or groups of atoms that substitutes in the place of an hydrogen atom on an organic compound, giving the compound unique chemical properties and determining its reactivity.
  • hydrocarbon: A chemical compound containing only carbon and hydrogen atoms.
  • saturated: All atoms that makes up a compound are single bonded to other atoms, with no double or triple bonds.
  • skeletal structure: A simplified structure where each intersection between two lines are assumed to have a carbon atom with its corresponding number of hydrogens.

Problems

Name the following structures. (Note: The structures are complex for practice purposes and may not be found in nature.)

1) cyclodecane.gif   2)  drawing08.gif      3) drawing09.gif    4)drawing2.gif    5)drawing4.gif      6)drawing7.gif   

 

7)drawing010.gif   

Draw the following structures.

8) 1,1-dibromo-5-fluoro-3-butyl-7-methylcyclooctane    9) trans-1-bromo-2-chlorocyclopentane   

10) 1,1-dibromo-2,3-dichloro-4-propylcyclobutane   11) 2-methyl-1-ethyl-1,3-dipropylcyclopentane    12) cycloheptane-1,3,5-triol

Name the following structures. 

Blue=Carbon    Yellow=Hydrogen    Red=Oxygen    Green=Chlorine

13)noname05 (1).gif    14)noname06.gif    15)noname07.gif    16)noname08.gif    17)noname09.gif            

18)noname10.gif    19)noname11.gif

Answers to Practice Problems

1) cyclodecane    2) chlorocyclopentane or 1-chlorocyclopentane    3) trans-1-chloro-2-methylcycloheptane   

4) 6-methyl-3-cyclopropyldecane    5) cyclopentylcyclodecane or 1-cyclopentylcyclodecane    6) 1,3-dibromo-1-chloro-2-fluorocycloheptane   

7) 1-cyclobutyl-4-isopropylcyclohexane

8)drawing10.gif        9)drawing11.gif        10)drawing12.gif        11)drawing9.gif    12)drawing011.gif    

13) cyclohexane    14) cyclohexanol    15) chlorocyclohexane    16) cyclopentylcyclohexane    17) 1-chloro-3-methylcyclobutane   

18) 2,3-dimethylcyclohexanol    19) cis-1-propyl-2-methylcyclopentane

References

  1. ACD/ChemSketch Freeware, version 11.0, Advanced Chemistry Development, Inc., Toronto, ON, Canada, www.acdlabs.com, 2008.
  2. Bruice, Paula Yurkanis. Oragnic Chemistry. 5th. CA. Prentice Hall, 2006.
  3. Fryhle, C.B. and G. Solomons. Organic Chemistry. 9th ed. Danvers, MA: Wiley, 2008.
  4. McMurry, John. Organic Chemistry. 7th ed. Belmont, California: Thomson Higher Education, 2008.
  5. Sadava, Heller, Orians, Purves, Hillis. Life The Science of Biology. 8th ed. Sunderland, MA: W.H. Freeman, 2008.
  6. Vollhardt, K. Peter C., and Neil E. Schore. Organic Chemistry. 5th ed. New York: W.H. Freeman, 2007.

Contributors

  • Pwint Zin
  • Jim Clark (ChemGuide)

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