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.
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.
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-".
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.
same as same as
|Cycloalkane||Molecular Formula||Basic Structure|
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.
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.
(1+3=4) NOT (1+5=6)
5) When naming the cycloalkane, substituents and functional groups must be placed by alphabetical order.
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)
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.
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.
(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.
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.
9) After all the functional groups and substituents have been mentioned with their corresponding numbers, the name of the cycloalkane can follow.
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 (-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.
(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.)
2,2-dimethylcyclohexanol NOT 1,1-dimethyl-cyclohexane-2-ol
3-bromo-2-methylcyclopentanol NOT 1-bromo-2-methyl-cyclopentane-2-ol
Blue=Carbon Yellow=Hydrogen Red=Oxygen
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.
|carboxylic acid||-oic acid|
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.
However, a cycloalkane with a triple bond-containing substituent is possible so long as the triple bond is not directly attatched to the ring.
Name the following structures. (Note: The structures are complex for practice purposes and may not be found in nature.)
1) 2) 3) 4) 5) 6)
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) 14) 15) 16) 17)
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
8) 9) 10) 11) 12)
13) cyclohexane 14) cyclohexanol 15) chlorocyclohexane 16) cyclopentylcyclohexane 17) 1-chloro-3-methylcyclobutane
18) 2,3-dimethylcyclohexanol 19) cis-1-propyl-2-methylcyclopentane
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