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Friday, May 8, 2009

Alkane Nomenclature:

Learning how to name alkanes will be crucial in your understanding of organic chemistry. All organic compounds are named following a certain set of rules set out by the IUPAC. By memorizing these rules, you should be able to name all compounds.

To name alkanes....
(follow these set of rules)

1.) Count the longest carbon chain

If the carbon chain is:
  • 1 carbon: named "methane"
  • 2 carbons: named "ethane"
  • 3 carbons: named "propane"
  • 4 carbons: named "butane"
  • 5 carbons: named "pentane"
  • 6 carbons: named "hexane"
  • 7 carbons: named "heptane"
  • 8 carbons: named "octane"
  • 9 carbons: named "nonane"
  • 10 carbons: named "decane"

Note that organic chemists, being lazy, will often not label hydrogens or carbons. For instance,

would be hexane (each of the "points" is assumed to be a carbon)

2.) Number the carbon chain, starting with the end closest to the substituent. Always try to get the lowest number possible.

If there are two substituents, name the carbon chain so that it has the lowest possible number.For instance, check out this molecule:

Would this compound be numbered from the left or right? The answer is that it would be answered from the right. If the molecule was named from the left, you would have substituents on carbons 3 and 5. Numbering from the right, the subsituents are on carbons 2 and 4.

Once you have determined which direction you will name, you will have to write down the molecule.

3.) Name and number the substituent

When naming, name the substituent before the parent chain.
Name this compound:

  • The longest carbon chain is 6 carbons. Hence the parent chain is named "hexan
  • Two subsituents are located on carbons 2 and 3, not 4 and 5.
  • There are two "methyl" substituents
  • Hence, we have the name, 2,3-dimethylhexane

If you have two substituents that are not the same, then you must name them both before the parent chain, listed alphabetically. Note that the prefixes used to designate number (i.e. di and tri) do not affect the alphabetization.
Name this compound:

  • The longest parents chain is 6 carbons, hence its parent name is "hexane"
  • I spy, with my own eyes, three substituents! There are two methyl and one ethyl group. How should I number them? Well, seeing as to how I can get 2,3,4 instead of 3,4,5, I shall name them from the left.
  • So the substituents are 2,4-dimethyl and 3-ethyl. Note that "e" in ethyl precedes "m" in "methyl. The "di" doesn't affect the alphabetization

  • Finally, we name the compound as
  • 3-ethyl-2,4-dimethylhexane.

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