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Naming Organic Compounds

Organic Nomenclature - Alkanes, Alkenes, Alkynes
Dr. MJ Patterson

Naming organic compounds can be a challenge to any chemist at any level.  Historically, chemists developed names for new compounds without any systematic guidelines.  In this century, the need for standardization was recognized.  For simple molecules, the nomenclature system worked out by the International Union of Pure and Applied Chemists (IUPAC) works well.  For complex molecules, the IUPAC names are so long that no one in their right mind would use them.  The net result is that a hodgepodge of IUPAC names and historic or common names is used.  Any one compound may have five or six different names.

Practically speaking, as you work with different organic chemicals in the lab you will learn the various names for them.  So, what we want to accomplish in this module is simply to establish the fundamentals of the IUPAC system and apply them to naming alkanes, alkenes and alkynes.

Numerical Prefixes = Number of Backbone Carbon Atoms
 

The prefix in the name of an organic molecule indicates the number of carbon atoms found in the longest continuous chain of carbon atoms in the molecule.  You need to memorize the following prefixes:
 

Prefix 

# C atoms

meth-

1

eth-

2

prop-

3

but-

4

pent-

5

hex-

6

hept-

7

oct-

8

non-

9

dec-

10

Alkanes = -ane ending

The alkane family uses the -ane ending.

Example 1:
Name the following compounds:
a.    CH4
b.    C2H6 or CH3CH3
c.
    C3H8 or CH3CH2CH3

Solution 1:
All of the bonds in these compounds are single bonds, so they are alkanes.  Use the numerical prefix for the number of carbon atoms with the -ane ending.
a.    one C atom = methane
b.    two C atoms = ethane
c.    three C atoms = propane

Alkenes = -ene ending

The alkene family uses the -ene ending.  However, since the double bond could appear at various sites in a typical molecule, we have to specify where it is.  To do so, number the carbon backbone so that the lowest possible number is used to describe the double bond position.  The lowest number of the two C atoms involved in the double bond is used in front of the name to indicate the C=C position.  The number is place at the beginning of the name and is separated with a dash.

In the expanded structure formulas shown below, it is understood that since H only forms one bond, any double bonds are between carbon atoms.  The expanded structures give a bit more information about how many H atoms are attached to each C atom.

Example 2:
Name the following compounds.
a.    C2H4 or H2C=CH2
b.    C3H6 or CH3CH=CH2
c.    C4H8 or H2C=CHCH2CH3
d.    C4H8 or CH3CH2=CH2CH3
e.    C5H10 or CH3CH2CH2CH=CH2

Solution 2:
a.    2 C atoms = ethene (since there are no options for the position of the C=C, we do not need to specify the position, as in 1-ethene)

  1. b.    3 C atoms = propene (again, since there are no options for the position of the C=C, we do not need to specify 1-propene.  Convince yourself that 1-propene and 2-propene are really the same molecule.)
  2. 4 C atoms with the C=C after the #1 C atom = 1-butene
  3. 4 C atoms with the C=C after the #2 C atom = 2-butene
  4. 5 C atoms with the C=C after the #1 C atom = 1-pentene (Did you say 4-pentene?  Remember that we want to number the backbone of C atoms so that the lowest numbers are used in the name.  In this case, you want to number the C backbone from right to left.  This same molecule could also be written H2C=CHCH2CH2CH3).

Alkynes = -yne ending

The alkyne family contains a triple bond between two C atoms.  Just as in the alkene family, the position of the triple bond is specified with a number at the beginning of the name.

(Since I cannot indicate a triple bond with 3 stacked dashes in this medium, I will use a double bond followed by a single bond or =- to indicate a triple bond.)

Example 3:
Name the following compounds.
a.    CH=-CH
b.    CH=-CCH2CH2CH2CH3
c.    CH3C=-CCH2CH2CH3
d.    CH3CH2C=-CCH2CH3
e.    CH3CH2CH2C=-CCH3
f.    CH3CH2CH2CH2C=-CH

Solution 3:
a.    2 C atoms = ethyne (this compound is commonly known as acetylene)

  1. b.    6 C atoms, triple bond after the #1 C atom = 1-hexyne
  2. 6 C atoms, triple bond after the #2 C atom = 2-hexyne
  3. 6 C atoms, triple bond after the #3 C atom = 3-hexyne
  4. 6 C atoms, triple bond after the #2 C atom = 2-hexyne (number the backbone from right to left)
  5. f.    6 C atoms, triple bond after the #1 C atom = 1-hexyne (number the backbone from right to left)