ALKANES 1
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Naming Cycloalkanes

Cycloalkanes are hydrocarbons that have the general molecular formula CnH2n and in which some, or all, of the carbon atoms form a ring. The condensed structural and bond-line formulas for cyclopentane are as follows:



To name cycloalkanes, use the following steps.

Step 1 Determine the parent name of the compound by counting the number of carbons in the ring. Use the same parent namefor the ring that you would use for the normal alkane containing that number of carbons.

Step 2 Add the prefix cyclo– to the parent name.



Step 3 Name any alkyl group substituents the same way that you name any other alkyl group.

Step 4 Determine the position of the alkyl group or groups on the ring.

a. For a ring with only one alkyl group attached, you do not need a number to designate the group's position. The carbon bearing a single group is always carbon number 1. For example, the following compound is methylcyclohexane:




b. When a ring has more than one alkyl group attached, number the ring to give the lowest sum of numbers. If there are two groups, assign the number one to the first alkyl group alphabetically. Then count the shortest distance to the second substituent. With three or more substituents, determine a set of numbers to give the lowest sum of numbers. For example, the following compound is 1-ethyl-2-methylcyclohexane, not 1-ethyl-6-methylcyclohexane:


c. If the sum of numbers is identical either direction around the ring, then count towards the second group alphabetically on the ring. The following compound is 1-ethyl-3-methyl-5-propylcyclohexane:


The more complex ring-containing alkanes need the following additional steps to name the compound.

Step 5 When the alkane chain is complex or has more carbons than the ring, name the ring as a substituent on the alkane chain. Follow the above steps for naming a normal alkane and for naming a cycloalkane. Call the ring a cycloalkyl group.

Step 6 When one ring is attached to another ring, call the larger ring the parent compound. Use the above steps for naming the cycloalkane and cycloalkyl portions of the compound.


When naming molecules with a cycloalkyl substituent, consider the prefix cyclo– when alphabetizing.





Naming Complex Alkyl Groups


When using this system to name one of the complex alkyl groups, follow these steps.

Step 1 Use the IUPAC system to name the rest of the compound and to determine the position of the alkyl group.

Step 2 Count the number of carbons in the complex alkyl group to determine its name and use the appropriate prefix to show its arrangement.


a. The prefix n–

An alkyl group with one or two carbons forms only one possible isomer, so call it a methyl or ethyl group. An alkyl group with three or more carbons can form two or more possible isomers; thus, you need a way to show that the group is the straight chain isomer. The following structure illustrates the generalized structure of a straight chain alkyl group isomer. The values of n are 2 or more.

CH3(CH2)n

The prefix n– is derived from the word normal, meaning an unbranched chain.

Use the prefix n– to name a straight chain isomer that contains three or more carbons. When naming molecules with the prefix n–, ignore the prefix when alphabetizing the name of the group. Two specific examples are the n-propyl and n-butyl groups.




b. The iso prefix

The prefix iso indicates a structure in which two methyl groups are bonded to the end of an alkyl side chain farthest from the point where the side chain bonds to the parent chain. The following structure illustrates a generalized structure for an isoalkyl group. The value of n is 0, 1, 2, or occasionally 3.

The iso prefix is not italicized because it is not separated from the name by a dash. The other prefixes are separated from the name by a dash and are italicized.



Two specific examples of the iso structure are the isopropyl and the isobutyl groups. When naming molecules with an iso alkyl group, consider the prefix when alphabetizing the name of the group.




c. The prefix sec–

A secondary carbon bears two carbon—carbon bonds and is the carbon at the point where the group attaches to the main chain.
The prefix sec– indicates a structure in which the carbon that bonds the alkyl side chain to the parent chain bears a methyl group branch, making that carbon a secondary carbon. Following is the generalized structure of a sec-alkyl group. The value of n is 1, 2, or occasionally 3.


A secondary carbon bears two carbon—carbon bonds and is the carbon at the point where the group attaches to the main chain.


A specific example is the sec-butyl group.


To name a molecule with a sec-alkyl group, ignore the prefix when alphabetizing the name of the group.
An exception to the use of the prefix sec– is when n=0. According to this system the name of the group should be sec-propyl. However, sec-propyl is not the accepted name for this group. Call the group isopropyl instead.
d. The prefix tert–

The prefix tert– describes a structure in which the carbon that bonds the alkyl side chain to the parent chain bears two methyl group branches, making it a tertiary carbon. Below is the generalized structure for a tert-alkyl group. The value of n is 0, 1, or 2.




A tertiary carbon is the carbon at the point where the group attached to the main chain bears three carbon—carbon bonds.



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