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Basic Organic
Nomenclature
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Mono-Cyclic Alkanes
The bonding of carbon atoms is not limited to simple chains. Carbon atoms
can form cyclic structures as well. The general formula for a linear or
branched chain is CnH2n+2 where n is the number of
carbon atoms. In a mono cyclic ring the general formula is CnH2n
. The loss of the two hydrogen atoms represents the additional carbon-carbon
bond in the ring.
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Naming cyclic rings follows the same rules as for linear chains, with the
addition of the prefix cyclo to the name of the compound.
Names of the simple cyclic hydrocarbons
Substitution on the ring is also possible. When the ring is larger
than the longest continuous chain of carbon atoms in a substituent, the
ring will be the parent compound.
Example 1.)
The ring is larger than the ethyl group so the name is:
ethylcyclopentane
NOTE: no number is needed as there is only a single substituent.
- Multiple substituents are located on the ring by numbering clockwise or
counter-clockwise from one substituent to the other(s), thus there will always
be a substituent at carbon number 1. The substituent, and direction of numbering,
are chosen to give the lowest total for all substituents,
i.e. starting at one substituent what is the shortest distance around the
ring, in either direction, to the next substituent? If the numbers for the
substituents will be the same in either direction, then the substituent that
comes first alphabetically should have the lower number.
Example 2.)
The ring is larger than both groups so the name is:
1-ethyl-2-methylcyclobutane
NOTE: the ethyl group is first alphabetically so it gets the lower number.
Example 3.)
The name is: 4-isopropyl-1,2-dimethylcyclohexane
NOTE:
- this gives the lowest total number for the three substituents, i.e. the
shortest distance from the two methyl groups to the isopropyl group.
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the isopropyl comes before the dimethyl
When the chain is longer than the ring, the ring becomes a substituent
on the chain. As a substituent, the ring is named following the rules for
alkanes, drop the -ane and replace it with -yl. For example a cyclopropane
ring as a substituent becomes a cyclopropyl group.
Example 4.)
The chain is larger so the name is: 4-cyclobutyl-5-cyclopropyl-2-methylheptane
Next Page: Cycloalkanes - Stereochemistry
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