3.4 Naming Alkanes

Alkane Nomenclature

Alkane nomenclature is the systematic method for naming carbon-based molecules. You identify the longest carbon chain, name any branches (substituents), and assemble the name following IUPAC rules for numbering and ordering. Mastering this process is essential because every other class of organic compounds builds on these same naming conventions.

Longest Carbon Chain Identification

The parent name of an alkane comes from its longest continuous chain of carbon atoms. "Continuous" means the carbons are connected in an unbroken sequence, but the chain doesn't have to be drawn in a straight line. Look carefully at branched structures because the longest chain sometimes bends or zigzags through the molecule.

Count the carbons in that longest chain and match the number to the corresponding prefix:

For chains longer than 10 carbons, use Greek-derived prefixes: undecane (11), dodecane (12), tridecane (13), tetradecane (14), pentadecane (15), and so on.

If the chain is unbranched (no substituents at all), the IUPAC name is just the parent name by itself. For example, a straight six-carbon chain is simply "hexane."

Alkyl Substituent Naming

Branches attached to the parent chain are called alkyl groups. They're named by replacing the "-ane" ending of the corresponding alkane with "-yl":

• Methyl (1 carbon), Ethyl (2 carbons), Propyl (3 carbons), Butyl (4 carbons), Pentyl (5 carbons)

Here's the step-by-step process for building the full IUPAC name:

1. Number the parent chain starting from the end nearest to the first branch point. The goal is to give substituents the lowest possible set of locant numbers. If both directions give the same first locant, compare the second locant, and so on, until you find a difference.

2. Assign a locant to each substituent based on the carbon it's attached to.

3. Group identical substituents using the prefixes di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), etc.

4. Alphabetize substituents by their base name. Ignore multiplying prefixes (di-, tri-, tetra-) when alphabetizing. For example, "ethyl" comes before "dimethyl" because you compare "e" to "m."

5. Assemble the name. Separate locant numbers from each other with commas. Place a hyphen between a number and a word. No spaces or hyphens between the last substituent name and the parent name.

Example: A six-carbon parent chain with methyl groups on carbons 2 and 3 is named 2,3-dimethylhexane.

Skeletal Structure Drawing

Skeletal (line-angle) structures are the shorthand organic chemists use most often. To convert an IUPAC name into a skeletal drawing:

1. Determine the parent chain length from the name (e.g., "pentane" = 5 carbons).
2. Draw the chain as a zigzag line. Each vertex (angle) and each line endpoint represents a carbon atom. Don't write out C or H atoms explicitly.
3. Attach substituents at the correct carbons. Count vertices from one end to find the right locant, then draw a line branching off that vertex.
4. Check tetravalency. Every carbon must have exactly four bonds. Any bonds not shown as lines to other carbons or substituents are understood to be C–H bonds.

Example: For 3-ethylpentane, draw a five-carbon zigzag. At the third vertex, draw a two-carbon branch extending off the chain.

A common mistake is miscounting carbons in the zigzag. Remember that both the starting endpoint and the ending endpoint count as carbons.

Structural Representations and Isomers

There are several ways to represent the same molecule, each showing different levels of detail:

• Structural formula: Shows every atom and every bond explicitly. Complete but takes up a lot of space.
• Condensed structural formula: Groups hydrogens with their parent carbon (e.g., $CH_3CH_2CH_3$ for propane). Carbon-carbon bonds are implied by the sequence of groups.
• Skeletal (line-angle) structure: The most compact form, described above.

Constitutional isomers are compounds that share the same molecular formula but differ in how their atoms are connected. For example, butane ($C_4H_{10}$) has two constitutional isomers: a straight chain (butane) and a branched chain (2-methylpropane). As the number of carbons increases, the number of possible constitutional isomers grows rapidly.

All alkanes are hydrocarbons, meaning they contain only carbon and hydrogen. Because alkanes have only single bonds between carbons, they're classified as saturated hydrocarbons.