Name a Hydrocarbon Chain Using the IUPAC Method

Hydrocarbons, or compounds based on a chain of hydrogen and carbon, are the basis of organic chemistry. You need to learn to name them according to the IUPAC, or International Union of Pure and Applied Chemistry, which is the currently accepted method for naming hydrocarbon chains.

Steps

  1. Know why the rules exist. The IUPAC rules were created to phase out old names (such as "toluene") and replace them with a system that would be consistent, as well as providing information about the placement of substituents (the atoms or molecules attached to a hydrocarbon chain).
  2. Keep a list of prefixes on-hand. These prefixes will help you name your hydrocarbons. They're based on the number of carbons in the parent chain (not altogether). For instance, CH3-CH3 would be ethane. Your professor probably won't expect you to know prefixes above 10; take note if he or she does expect it.
    • 1: meth-
    • 2: eth-
    • 3: prop-
    • 4: but-
    • 5: pent-
    • 6: hex-
    • 7: hept-
    • 8: oct-
    • 9: non-
    • 10: dec-
  3. Practice. Learning the IUPAC system takes practice. Read through the following methods to see some examples, then find links to practice problems under Sources and Citations at the bottom of the page.

Alkanes

  1. Understand what an alkane is. An alkane is a hydrocarbon chain that contains no double or triple bonds between carbon molecules. The suffix at the end of an alkane should always be -ane.
  2. Draw out your molecule. You can draw out all the symbols, or you can use a skeleton structure. Find out which one your instructor wants you to use, and stick with it.
  3. Number the carbons on the parent chain. The parent chain is the longest continuous carbon chain in the molecule. Number it starting by the nearest substituent group. Each substituent will be noted by its numerical location on the chain.
  4. Assemble the name in alphabetical order. Substituents should be named alphabetically (excluding prefixes such as di, tri or tetra), not in numerical order.
    • If you have two of the same substituents on the hydrocarbon chain, put "di" before the substituent. Even if they're attached to the same numbered carbon, list the number twice.

Alkenes

  1. Know what an alkene is. An alkene is a hydrocarbon chain that contains one or more double bonds between carbons, but no triple bonds. The suffix at the end of an alkene should always be -ene.
  2. Draw out the molecule.
  3. Find the parent chain. The parent chain of an alkene must contain any double bonds between carbons. Additionally, it should be numbered from the end that is nearest to a carbon-carbon double bond.
  4. Note where the double bond is. In addition to noting where substituents are on an alkene chain, you must also note where the double bond is. Do this such that the lowest number on the double bond is used.
  5. Modify the suffix based on the number of double bonds in the parent chain. If your parent chain has two double bonds, your name will end in "-diene." Three is "-triene," and so on.
  6. Name substituents alphabetically. As with alkanes, you must list substituents alphabetically in the final name. Exclude prefixes such as di-, tri- and tetra-.

Alkynes

  1. Learn what an alkyne is. An alkyne is a hydrocarbon chain that contains one or more triple bonds between carbons. The suffix at the end of an alkyne should always be -yne.
  2. Draw out the molecule.
  3. Locate the parent chain. The parent chain of an alkyne must contain any carbons that are triple-bonded. Number it from the end nearest to a triple-bonded carbon.
    • If you're dealing with a molecule that has both double and triple bonds, start the numbering from the end located closest to any multiple bond.
  4. Note where the triple bond is located. In addition to noting where substituents are on an alkyne chain, you must also note where the triple bond is. Do this such that the lowest number on the triple bond is used.
    • If your molecule contains double bonds as well as triple bonds, you must locate these as well.
  5. Modify the suffix based on the number of triple bonds in the parent chain. If your parent chain has two triple bonds, the name will end in "-diyne." Three is "-triyne," and so on.
  6. Name the substituents alphabetically. As with alkanes and alkenes, you must list substituents alphabetically in the final name. Exclude prefixes such as di-, tri- and delta-.
    • If your molecule contains double bonds as well as triple bonds, the double bonds must be named first.

Cyclic Hydrocarbons

  1. Figure out what sort of cyclic hydrocarbon you have. Cyclic hydrocarbons work like non-cyclic hydrocarbons as far as naming - those containing no multiple bonds are cycloalkanes, those with double bonds are cycloalkenes, and those with triple bonds are cycloalkynes. For instance, a 6-carbon ring with no multiple bonding is cyclohexane.
  2. Know the difference in naming a cyclic hydrocarbon. There are a few notable differences between naming cyclic and non-cyclic hydrocarbons:
    • Because all the carbons in a cyclic hydrocarbon ring are equal, you don't need to use a number if your cyclic hydrocarbon only has one constituent.
    • If the alkyl group attached to the cyclic hydrocarbon is larger or more complex than the ring, then the cyclic hydrocarbon may become a substituent instead of the parent chain.
    • If two substituents are on the ring, they're numbered in alphabetical order. The first (alphabetically) substituent is 1; the next is numbered going counterclockwise or clockwise - whichever results in a lower number for the second substituent.
    • If more than two substituents are on the ring, the first one alphabetically is said to be attached to the first carbon. The others are numbered counterclockwise or clockwise - whichever results in the lowest numbers.
    • Like non-cyclic hydrocarbons, the final molecule is named in alphabetical order, excluding prefixes like di-, tri- and tetra-.

Benzene Derivatives

  1. Understand what a benzene derivative is. A benzene derivative is based on a benzene molecule, C6H6, which three evenly-spaced double bonds.
  2. Don't use numbering if there's only one substituent. As with other cyclic hydrocarbons, there's no need to use a number if the ring only has one substituent.
  3. Learn benzene naming conventions. It's possible to name your benzene molecule as you would any other cyclic hydrocarbon, starting alphabetically with the first substituent and assigning numbers going around. However, there are some special designations for substituent positions on benzene molecules:
    • Ortho, or o-: The two substituents are located at 1 and 2.
    • Meta, or m-: The two substituents are located at 1 and 3.
    • Para, or p-: The two substituents are located at 1 and 4.
  4. If your benzene molecule has three substituents, name it as you would a normal cyclic hydrocarbon.

Tips

  • If there are two possibilities for the longest chain, choose the chain with the most branches. If the two chains have the same number of branches, choose the one which branches first. If the two chains are identical with regards to branching, then just pick one.
  • If a hydrocarbon has an OH (hydroxyl group) anywhere in the compound, it becomes an alcohol and is named with the suffix -ol instead of -ane.
  • Keep practicing! When you run into these problems on a test, the professor has likely designed them so that there is only one right answer. Remember the rules, and go through them step by step.

Warnings

  • Many compounds are called by their common names instead of using the new IUPAC system. For example, a side chain that would be named under the IUPAC system as (1-methylethyl), is also known as an isopropyl group. Be careful not to mix your naming systems.

Related Articles

Sources and Citations

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