Organic Chemistry Learning Module

Introduction: Nomenclature of Alkanes  Go to Alkenes and Alkynes

You may test your knowledge of knowledge of alkane nomenclature prior to studying this module, by using the Interactive Quiz on Alkanes

Carbon, because of its valence electrons, can form four bonds and hydrogen can form only one bond. The names of methane, ethane, propane, butane, and pentane are used for alkanes containing one, two, three, four, and five carbon atoms, respectively.

methane	ethane
propane	butane

Except for the first four members of the family, the name is simply derived from the Greek (or Latin) prefix for the particular number of carbon atoms in the alkane; thus PENTane for five, HEXane for six, HEPtane for seven, OCTane for eight, NONane for nine, DECane for ten and so on. You should certainly know the first ten. The structures drawn above are called "normal" alkanes because they form in a straight line without side chains. These, as well as others, form the base of a multitude of organic compounds. From these normal alkanes, we derive the names of certain groups that constantly appear as structural units of organic molecules. For instance, chloromethane, CH₃Cl, is also known as methyl chloride. The CH₃- group is called "methyl" wherever it appears. CH₃Br is thus called methyl bromide, CH₃I is called methyl iodide, and CH₃OH is called methyl alcohol. In the same way, the C₂H₅ group is "ethyl"; C₃H₇ - is propyl; C₄H₉ is butyl, and so on. The . . . means that something is attached at that point. Remember that carbon forms 4 bonds and hydrogen forms 1 bond, so when you see, for instance, C₄H₉ - , it must for instance, it must have a structure of:

With these things in mind, we will begin to name and draw structures for compounds using the system devised by IUPAC (International Union of Pure and Applied Chemistry). The rules devised by IUPAC for the alkanes are as follows:

1. Select as the parent structure the longest continuous chain, and then consider the compound to have been derived from this structure by the replacement of hydrogen by various alkyl groups.
2. Where necessary, as in the isomeric methyl pentanes, indicate by a number the carbon to which the branching alkyl group is attached. In numbering the parent carbon chain, start at whichever end results in the use of the lowest numbers; thus the naming of the following isomers:

2-methylpentane

3-methylpentane

Now, you might be wondering why there is no 1-methylpentane...

If you analyze the structure closely, you see that the added methyl group actually creates a longest chain that is now 6 carbon atoms in length. Thus, the structure above is correctly named hexane.

3. If the alkyl group appears more than once as a side chain, indicate this by the prefix di-, tri-, tetra-, etc. These prefixes are used to show how many of these alkyl groups there are, and indicate by various numbers the positions of each group as shown below.

2,2,4-trimethylpentane

4. If there are several different alkyl groups attached to the parent chain, name them in alphabetical order. The ethyl group is named before the methyl group because "e" (as in ethyl) comes alphabetically before "m" (as in methyl). The prefix does not contribute to the alphabetical order of the functional groups.

4-ethyl-1,4-dimethylheptane

5. The prefix "iso-" is used to designate any alkyl group (of six carbons or less) that has a single one-carbon branch on the next-to-last carbon of a chain and has the point of attachment at the opposite end of the chain.

isopropyl group

isopropyl alcohol

Isopropyl alcohol could also be (correctly) named 2-propanol, denoting the presence of the alcohol functional group (OH-) attached to the second carbon in the 3-carbon propane chain.

Isomers

Different compounds that have the same molecular formula are called isomers. They contain the same numbers of the same kinds of atoms, but the atoms are attached to one another in different ways. Isomers are different compounds because they have different molecular structures. For instance, the two isomers of C₄H₁₀ are:

butane

2-methyl propane

As you can see, structural formulas can take up a lot of space when drawn out. In order to save space, structural formulas will often be written out in one line. In fact, I have already taken the liberty of using some shorthand in writing several of the alkyl groups as CH₃ . and C₂H₅- . This is commonly done even when writing complete structural formulas, so to the difficulty of confining the bond lines to two dimensions. When you want to indicate that a chain is meant to be a side chain, it is written in parentheses. For instance, the drawn out structure for 2 - methylpropane is:

Want to test your newfound knowledge of alkane nomenclature? Use the Interactive Quiz on Alkanes

Introduction to Alkenes and Alkynes

There are two other types of basic hydrocarbons: alkenes and alkynes. Alkenes contain less hydrogen, carbon for carbon, than the alkanes. To account for this difference, the carbon atoms are joined by a double bond. The carbon-carbon double bond is the distinguishing feature of the alkene structure.

ethene

Alkynes contain an even smaller proportion of hydrogen than the alkenes. To account for this difference, the carbon atoms share three pairs of electrons . that is, they are joined by a triple bond. The carbon-carbon triple bond is the distinguishing feature of the alkyne structure.

propyne

Rules for Naming Alkenes

1. Select as the parent structure the longest continuous chain that contains the carbon-carbon double bond; then consider the compound to have been derived from this structure by replacement of hydrogen by various alkyl groups. The parent structure is known as ethane, propene, butene, pentene, and so on, depending upon the number of carbon atoms . each name is derived by changing the ending "-ane" of the corresponding alkane name to "-ene".

propene

2. If the parent chain is longer than three carbons, indicate by a number the position of the double bond in the parent chain. Although the double bond involves two carbon atoms designate its position by the number of the first doubly-bonded carbon encountered when numbering from the end of the chain nearest the double bond.

1-butene

2-butene

3. Indicate by numbers the positions of the alkyl groups attached to the parent chain.

3,3-dimethyl-1-butene

4-methyl-2-pentene

Rules for Naming Alkynes

The alkynes can also be named by the IUPAC system. The rules are exactly the same as for the naming of alkenes except that the ending "-yne" replaces "-ene". The parent structure is the longest continuous chain that contains the triple bond, and the positions both of the substituents and of the triple bond are indicated by numbers. The triple bond is given the number of the first triply-bonded carbon encountered, starting from the end of the chain nearest the triple bond.

1-butyne

2-butyne

4-methyl-2-pentyne

Substituted Hydrocarbons

Hydrocarbons that are substituted with groups other than the alkyl groups are named in the same way. Other groups that are often substituted are the halogens (bromo-, chloro-, iodo-) as well as nitro (-NO₂) and amine (-NH₂).

2-bromopropane	1-chloro-2-butene
2,4-dinitro-2-pentene	5-bromo-2-methyl-2-propene

The following table contains a summary of functional groups that substitute in hydrocarbons:

An alcohol
An aldehyde
An amine
A carboxylic acid
An ester
An ether
A halide
A ketone

Test your recall of these hydrocarbon substitutions by taking the Interactive Quiz on Functional Groups