Section Two

Chapter Eleven, Section Two

OBJECTIVES

Learn the IUPAC nomenclature of alkanes and alkyl halides
Learn some nomenclature for cycloalkanes.

I. ALKANES AND ALKYL HALIDES

The International Union of Pure and Applied Chemistry (IUPAC) has established guidelines for naming compounds so that people from all over the world can communicate with each other regarding chemical compounds. Certainly we would have chaos if everyone could call any chemical compound whatever seemed good to them. There are still some compounds that are called by their "common" names (such as aspirin) and we just have to live with that.

Here are some rules to guide you in the naming process:

Pick the longest carbon chain and that is the name of the "parent" compound. There are certain prefixes that are used to communicate the number of carbons in the longest chain. These prefixes are in your book and should be memorized. Alkanes always end in -ane such as pentane.
Always number the group attached to the parent compound using the smallest possible numbers.
When there are more than one of the same type of added goup, use the prefixes di, tri, tetra, etc.
When there are more than one type of added groups to the parent compound name the groups in alphabetical order.
If there are halogen atoms added to the alkane parent, the compounds are alkyl halides, but are named the same as the alkanes, but with the prefixes fluoro, chloro, bromo, and iodo.
Some common names are used because they are simpler names. These occur when branched groups are involved such as the isopropyl group. Your text illustrates some of these.
The IUPAC names use dashes between the numbers and letters and commas between the numbers.

The following compound illustrates all of the above (except the common name)

alkylhalide1

We note the parent compound is pentane and then we use the other rules to arrive at the name

1-bromo-4,4-difluoro-2-methylpentane.

Easy!! Again, a little practice should have you naming such compounds easily. Do note the examples in your book illustrating that the main chain does not have to be in a strict left to right straight line. The main chain can go up and down, zig or zag, or whatever. The following compound is the same as the one above. Study the two until you are convinced that they are the same.

alkylhalide2

II. CYCLOALKANES

Carbon atoms can also be connected to each other in a ring structure. The naming of the ring structures is rather straightforward. We just put the prefix "cyclo" in front of the appropriate name for the number of carbon atoms involved in the ring:

cyclopropane

cyclobutane

cyclopentane

etc.

Now when substituents are added to the rings the names are also rather straightforward. The addition of a single group, such as methyl, requires no numbers of course because all positions on a ring are equivalent. Everyone then knows that methylcyclobutane is

methylcyclobutane

However, if another substituent is added we need to be more specific about exactly where it is located on the ring. We follow the IUPAC rules and specify that we must end up with the lowest numbers involving the substitutions. Consider the following compound for example,

Done correctly:

cycloalk1

Done incorrectly:

cycloakwrong

We start at one methyl group and we could call this either 1,4-dimethylcyclobutane or 1,2- dimethylcyclobutane. Using the smallest numbers rule, we call this 1,2 dimethylcyclobutane. A little practice with this nomenclature should allow you to name such compounds easily. You should also be familiar with the very condensed structures that look like stick and ring drawings as shown above.

ASSIGNMENTS

Draw the structure for 1,1-dibromo-3,2-dichloro-4-methylheptane.
Draw the structure for 1,3-dimethylcyclopentane
Work the problems in the book appropriate for this section
Work the problems from the assignment booklet appropriate for this section.

QUIZ TWO

After you have studied this material and practiced some problems, take quiz two. If you score at least 80 on the test then you are ready to continue to the next section.