Naming Organic Compounds

Here is a web site that contains some additional organic stuff. (NOTE: to see the models displayed on many of these pages you will need the MDL Chemscape chime plugin for your browser. When you click on the link you will need to scroll the displayed page to find the link to the plugin download page.)

http://www.ouc.bc.ca/chem/nomenclature/index-2.htm

Carbon is the third most abundant element in the human body. The other elements that combine with carbon range from hydrogen to oxygen, nitrogen, and sulfur in those compounds in the body. The range of elements that combine with carbon outside the body include all the halogens, and phosphorus.

We'll begin with the compounds of carbon and hydrogen. Compounds containing only carbon and hydrogen are called hydrocarbons. The simplest hydrocarbon contains one carbon atom and four hydrogen atoms. The formula is CH4, and the name is methane. If we continue adding carbon atoms and hydrogen atoms the formulas and names for the first ten hydrocarbons are;

Formula

Name

CH4

Methane

C2H6

Ethane

C3H8

Propane

C4H10

Butane

C5H12

Pentane

C6H14

Hexane

C7H16

Heptane

C8H18

Octane

C9H20

Nonane

C10H22

Decane

We can also write the formula as a condensed formula which shows us a little more information for these compounds.

Formula

Condensed Formula

Name

CH4

CH4

Methane

C2H6

CH3CH3

Ethane

C3H8

CH3CH2CH3

Propane

C4H10

CH3CH2CH2CH3 or CH3(CH2)2CH3

Butane

C5H12

CH3CH2CH2CH2CH3 or CH3(CH2)3CH3

Pentane

C6H14

CH3CH2CH2CH2CH2CH3 or CH3(CH2)4CH3

Hexane

C7H16

CH3CH2CH2CH2CH2CH2CH3 or CH3(CH2)5CH3

Heptane

C8H18

CH3(CH2)6CH3

Octane

C9H20

CH3(CH2)7CH3

Nonane

C10H22

CH3(CH2)8CH3

Decane

What do these ten compounds 'look' like?

Formula

Name

Structure

CH4

Methane

C2H6

Ethane

C3H8

Propane

C4H10

Butane

C5H12

Pentane

C6H14

Hexane

C7H16

Heptane

C8H18

Octane

C9H20

Nonane

C10H22

Decane

What do these structures look like on paper? Note: the drawings you see below depict the C-C or C-H bonds in a different form than we have seen before. To get a better perspective of the models review the key in the table below. There are three different C-H bonds shown in methane.

The solid 'thin' line between the carbon and hydrogen atom shows a C-H bond in the plane defined by the page.

The dashed lines depict a C-H bond with the bond going behind (away from the viewer) the plane of the page.

The solid wedge shaped lines depict a C-H bond projected in front (towards the viewer) of the plane of the page.

Models of the first ten alkanes.

Formula

Name

Structure

CH4

Methane

C2H6

Ethane

C3H8

Propane

C4H10

Butane

C5H12

Pentane

C6H14

Hexane

C7H16

Heptane

C8H18

Octane

C9H20

Nonane

C10H22

Decane

So we now know all the formulas, condensed formulas and how to draw a structure of the first ten members of this group of hydrocarbons. This particular series of hydrocarbons are called the alkanes. Notice all of the names of these compounds have an 'ane' ending.

So if I were to draw the structure;

 

You would call it octane because it has eight carbon atoms and 18 hydrogen atoms and all of the bonds are single bonds.

In laboratory you worked with models of these compounds and discovered that for some formulas there were different ways to arrange the carbon atoms! That's is alkanes with the same formula had different structures. Such occurrences are called structural isomers.

Structural isomers first occur in butane. Two compounds have the same formula but different structures;

 and

Both structures have the same formula, C4H10, but different arrangements of carbon atoms. With different arrangements they will have a different name.

So the question comes up, how do we name these hydrocarbons? There are some rules which we must follow to correctly name these compounds. Let's look at the rules.

  1. Find the longest continueous chain of carbon atoms. Called the parent chain this chain establishes the base alkane name for the compound.

  2. Number the carbon atoms in the parent chain beginning with the end closest to any substituents.

  3. Using the correct substituent name (methyl, ethyl, chloro,etc.), and the carbon atom number on the parent chain.

  4. If a particular substituent occurs more than once, use a prefix (di-, tri-, tetra-) to indicate the number of those substituents.

  5. List the alkyl substituents in alphabetical order.

Following these rules the first structure which has four carbon atoms in the longest parent chain with no substituents is called butane, or more properly, n-butane. The second isomer has a parent chain (longest chain) of only three carbon atoms. So the parent base chain is propane. There is a CH3 group attached to the 2nd carbon atom. So the name is 2-methylpropane.

Lets look at the structural isomers for pentane, C5H12. There are three.

 

n-pentane

 2-methylbutane

2,2-dimethylpropane

So it is probably time for some practice problems!!

Name the following compound or draw its structure:

a)

answer

b) 2-methyl-4-ethylheptane

answer