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Chapter: Organic Chemistry: Alkanes and cycloalkanes

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Alkanes and cycloalkanes: Nomenclature

Alkanes and cycloalkanes: Nomenclature
The names of the first 10 simple alkanes are methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, and decane.

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Key Notes

Simple alkanes

The names of the first 10 simple alkanes are methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, and decane.

Branched alkanes

Branched alkanes have alkyl substituents branching off from the main chain. When naming a branched alkane, identify the longest chain and number it from the end nearest the branch point. Identify the substituent and its position on the longest chain. The name is n-alkylalkane where n is the position of the substituent, alkyl is the substituent and alkane is the longest chain.

Multi-branched alkanes

If there is more than one substituent present, the substituents are named in alphabetical order. Identical substituents are identified by prefixing them with di-, tri-, tetra-, etc., but the order of naming still depends on the alpha-betical order of the substituents themselves. If there are two different sub-stituents at equal distances from either end of the chain, the substituent with alphabetical priority has the lowest numbering. This rule may be sup-planted if there are several substituents so placed.

Cycloalkanes

Cycloalkanes are named according to the number of carbon atoms making up the ring, that is, cyclopropane (C3H6), cyclobutane (C4H8), cyclopentane (C5H10), cyclohexane (C6H12), etc.

Branched cycloalkanes

Cycloalkanes linked to an alkane are usually named such that the cycloalkane is considered the parent system and the alkane group is an alkyl substituent (i.e. alkylcycloalkane). However, the opposite holds true if the alkane portion has more carbon atoms than the cycloalkane in which case the cycloalkane is considered a substituent of the alkane portion (i.e. n-cycloalkylalkane).

Multi-branched cycloalkanes

Cycloalkanes having several substituents are numbered such that the sub-stituent with alphabetical priority is at position 1. Numbering is then car-ried out such that the total obtained from the substituent positions is a minimum.

 

Simple alkanes

The names of the simplest straight chain alkanes are shown in Fig. 1.


Branched alkanes

Branched alkanes are alkanes with alkyl substituents branching off from the main chain. They are named by the following procedure:


·           identify the longest chain of carbon atoms. In the example shown (Fig. 2a), the longest chain consists of five carbon atoms and a pentane chain;

·           number the longest chain of carbons, starting from the end nearest the branch point (Fig. 2b);

·           identify the carbon with the branching group (number 2 in Fig. 2b);

·           identify and name the branching group. (In this example it is CH3. Branching groups (or substituents) are referred to as alkyl groups (CnH2n1) rather than alkanes (CnH2n2). Therefore, CH3 is called methyl and not methane.)

·           name the structure by first identifying the substituent and its position in the chain, then naming the longest chain. The structure in Fig. 1 is called 2-methylpentane. Notice that the substituent and the main chain is one complete word, that is, 2-methylpentane rather than 2-methyl pentane.

Multi-branched alkanes

If  there  is  more  than  one  alkyl  substituent  present  in  the  structure  then  the substituents are named in alphabetical order, numbering again from the end of the chain nearest the substituents. The structure in Fig. 3 is 4-ethyl-3-methyloctane and not 3-methyl-4-ethyloctane.


If a structure has identical substituents, then the prefixes di-, tri-, tetra-, et ceteraare used to represent the number of substituents. For example, the structure in Fig. 4 is called 2,2-dimethylpentane and not 2-methyl-2-methylpentane.


The prefixes di-, tri-, tetra- etc. are used for identical substituents, but the order in which they are written is still dependent on the alphabetical order of the substituents themselves (i.e. ignore the di-, tri-, tetra-, et cetera). For example, the structure in Fig. 5 is called 5-ethyl-2,2-dimethyldecane and not 2,2-dimethyl-5-ethyldecane.


Identical substituents can be in different positions on the chain, but the same rules apply. For example, the structure in Fig. 6 is called 5-ethyl-2,2,6-trimethyldecane.


In some structures, it is difficult to decide which end of the chain to number from. For example, two different substituents might be placed at equal distances from either end of the chain. If that is the case, the group with alphabetical prior-ity should be given the lowest numbering. For example, the structure in Fig. 7a is 3-ethyl-5-methylheptane and not 5-ethyl-3-methylheptane.


However, there is another rule which might take precedence over the above rule. The structure (Fig. 7c) has ethyl and methyl groups equally placed from each end of the chain, but there are two methyl groups to one ethyl group. Num-bering should be chosen such that the smallest total is obtained. In this example, the structure is called 5-ethyl-3,3-dimethylheptane (Fig. 7c) rather than 3-ethyl-5,5-dimethylheptane (Fig. 7b) since 5 3 3 = 11 is less than 3 5 5 = 13.

Cycloalkanes

Cycloalkanes are simply named by identifying the number of carbons in the ring and prefixing the alkane name with cyclo (Fig. 8).


Branched cyclohexanes

Cycloalkanes consisting of a cycloalkane moiety linked to an alkane moiety are usually named such that the cycloalkane is the parent system and the alkane moiety is considered to be an alkyl substituent. Therefore, the structure in Fig. 9a is methylcyclohexane and not cyclohexylmethane. Note that there is no need to number the cycloalkane ring when only one substituent is present.


If the alkane moiety contains more carbon atoms than the ring, the alkane moiety  becomes  the  parent  system  and  the  cycloalkane  group  becomes  the substituent. For example, the structure in Fig. 9b is called 1-cyclohexyloctane and not octylcyclohexane. In this case, numbering is necessary to identify the position of the cycloalkane on the alkane chain.

Multi-branched cycloalkanes

Branched cycloalkanes having different substituents are numbered such that the alkyl substituent having alphabetical priority is at position 1. The numbering of the rest of the ring is then carried out such that the substituent positions add up to a minimum. For example, the structure in Fig. 9c is called 1-ethyl-3-methyl-cyclohexane rather than 1-methyl-3-ethylcyclohexane or 1-ethyl-5-methylcyclo-hexane. The last name is incorrect since the total obtained by adding the substituent positions together is 5 1 6 which is higher than the total obtained from the correct name (i.e. 1 3 4)

 

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