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Nomenclature of alkanes

Naming straight-chain alkanes

Naming of straight chain alkanes (alkanes that do not branch) is a straightforward process. To give an alkane a name, a prefix indicating the number of carbons in the molecule is added to the suffix ane, identifying both the kind of molecule (an alkane) and how many carbons the molecule has (the prefix). The name pentane, for example, tells you that the molecule is an alkane (thus the ane ending) and that it has five carbons (pent indicates five). Prefixes for alkanes that have 1-4 carbons are rooted historically. These are methane, ethane, propane, and butane, respectively. On the other hand, for 5 carbons and up a prefix derived from greek is given. (An easy way to remember the first four names is the anagram Mary eats peanut butter, standing for methane, ethane, propane, butane). Learning the prefixes for up to twelve carbons is a good idea, and they are listed in the figure below.

Number of Carbons Prefix Structure
1 Methane CH4
2 Ethane CH3CH3
3 Propane CH3CH2CH3
4 Butane CH3(CH2)2CH3
5 Pentane CH3(CH2)3CH3
6 Hexane CH3(CH2)4CH3
7 Heptane CH3(CH2)5CH3
8 Octane CH3(CH2)6CH3
9 Nonane CH3(CH2)7CH3
10 Decane CH3(CH2)8CH3
11 Undecane CH3(CH2)9CH3
12 Dodecane CH3(CH2)10CH3

Naming branched alkanes

The nomenclature becomes more complex if the alkane branches. In such a case, there are several rules that you must follow to give the alkane the correct name.

  1. Find the longest chain of carbons in the molecule. The number of carbons in the longest chain becomes the parent name (refer to the above table)
  2. After finding the parent chain, you number the parent chain starting with the end nearest the first substituent (a substituent is any fragment that juts off the main chain).
  3. Next, determine the names of all substituents. Substituents are named as if the piece were a separate molecule, except that the suffix of yl is used rather than ane. Thus, a two-carbon substituent would be an ethyl substituent (not an ethane substituent).
  4. Put the substituents in alphabetical order (ie. ethyl before methyl) in front of the parent name.
  5. Next, identify the positions of all substituents in the name by placing the carbon number where the substituent attaches to the parent chain in front of it. For example, 2-methylheptane indicates that a methyl substituent is attached to the number 2 carbon.

 Continue tutorial :: applying these rules to an example

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