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Branched Alkanes

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Branched Alkanes

Example of a family tree

Credit: CK-12 Foundation - Joy Sheng
License: CC BY-NC 3.0

Who is my great-aunt?

Tracing your family tree can be both fun and exciting. But, it helps to know the exact names of your family members in order to do this correctly. Just a first name or middle name or last name is not enought. A traceable family tree is one in which all relatives are carefully and precisely identified. After all, you would prefer that great-great-great-uncle to be royalty and not a horse thief.

Branched Alkanes

Beginning with butane, there is an alternate structure possible that is not a straight chain. The structural formulas below show a structure with a three-carbon chain that has a –CH 3 group attached to the middle carbon.

  Example of a branched alkane

Credit: CK-12 Foundation - Christopher Auyeung and Joy Sheng
License: CC BY-NC 3.0

The name of this molecule is 2-methylpropane. The molecular formula is still C 4 H 10 , which is the same formula as butane. A structural isomer is one of multiple molecules that have the same molecular formula, but different structural formulas. Butane and 2-methylpropane are structural isomers.

2-methylpropane is an example of a type of alkane called a branched alkane. The IUPAC system of nomenclature for branched alkanes follows a set of steps which will be applied to the example molecule below.

  Example nomenclature of a complicated branched alkene

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

  1. Find the longest continuous chain of carbon atoms in the molecule. This is called the parent chain. In the example, the longest chain is eight carbon atoms and so the parent hydrocarbon is octane.
  2. Number the carbon atoms in the parent chain. To do this, start at the end that will give the smallest numbers possible to the carbon atoms where the branches originate. In the example above, the branches are on carbons 3 and 5 when the chain is numbered left-to-right. If it were to be numbered right-to-left, the branches would be on carbons 4 and 6, so the left-to-right order is preferable.
  3. The atoms attached to the parent chain are called substituents. A substituent that is a hydrocarbon is called an alkyl group . The names of alkyl groups use the same prefixes as the alkanes, but with a –yl suffix. So a 1-carbon alkyl group is a methyl group, a 2-carbon alkyl group is an ethyl group, and so on. The substituents are named by placing the number from the parent carbon chain in front of the name of the substituent. In the current example, we have 3-methyl and 4-ethyl substituents.
  4. Use a prefix to indicate the appearance of more than one of the same substituent in the structural formula. Two of the same group is tri-, three is tri-, four is tetra-, etc. For example, if methyl groups were attached to both carbons 2 and 3, that part of the name would be 2, 3-dimethyl-. This rule does not apply to the current structure above.
  5. Multiple different substituents are listed in alphabetical order. Ignore any of the prefixes from rule 4. In the current example, the 5-ethyl- comes before the 3-methyl.
  6. Commas are used to separate multiple numbers. Hyphens come between the number and the name of a substituent. The parent name comes immediately after the last substituent. There are no blank spaces in the name.

The correct name for the above structure according to the IUPAC system is 5-ethyl-3-methyloctane.

Summary

Practice

Work the problems at the link below:

http://chemware.co.nz/orgquiz.htm

Review

Questions

  1. What is an alkyl group?
  2. How are alkyl substituents listed?
  3. If there are three methyl groups on the same carbon, what is the prefix used?

Image Attributions

  1. [1]^ Credit: CK-12 Foundation - Joy Sheng; License: CC BY-NC 3.0
  2. [2]^ Credit: CK-12 Foundation - Christopher Auyeung and Joy Sheng; License: CC BY-NC 3.0
  3. [3]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0

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