3.2 Alkanes and Alkane Isomers

Alkanes

Alkanes are the simplest hydrocarbons, built from only carbon and hydrogen atoms connected by single bonds. They follow the general formula $C_nH_{2n+2}$ and can be arranged as straight chains or branched structures. Understanding how alkanes are built and named gives you the foundation for working with every other functional group in organic chemistry.

Straight-chain vs. branched-chain alkanes

Straight-chain alkanes have all their carbon atoms connected in one continuous chain with no branches. Some common examples:

• Ethane: $C_2H_6$
• Propane: $C_3H_8$
• Butane: $C_4H_{10}$

Branched-chain alkanes have one or more carbon atoms attached to the main chain as side groups called alkyl groups. An alkyl group is just an alkane missing one hydrogen. The most common ones you'll encounter:

• Methyl ($CH_3$-): a one-carbon branch
• Ethyl ($CH_3CH_2$-): a two-carbon branch
• Propyl ($CH_3CH_2CH_2$-): a three-carbon branch

For example, isobutane (2-methylpropane) is a branched version of butane, and isopentane (2-methylbutane) is a branched version of pentane. Both have the same molecular formula as their straight-chain counterparts but different structures.

Every carbon in an alkane is $sp^3$ hybridized, which means each carbon has tetrahedral geometry with bond angles of approximately $109.5°$. Because all the bonds are single bonds, carbon atoms can rotate freely around them, allowing alkanes to adopt different conformations.

Constitutional isomers of alkanes

Constitutional isomers (also called structural isomers) share the same molecular formula but differ in how their atoms are connected. This isn't just a different shape or rotation; the bonds themselves go to different places.

The number of possible constitutional isomers grows rapidly as you add carbons:

• 1 to 3 carbons: no constitutional isomers possible (there's only one way to connect them)
• Butane ($C_4H_{10}$): 2 isomers
  • n-butane (straight chain)
  • isobutane, or 2-methylpropane (branched)
• Pentane ($C_5H_{12}$): 3 isomers
  • n-pentane (straight chain)
  • isopentane, or 2-methylbutane (one branch)
  • neopentane, or 2,2-dimethylpropane (two branches on the same carbon)
• Hexane ($C_6H_{14}$): 5 isomers
  • n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane

A useful exercise is to try drawing all the isomers for a given formula yourself. Start with the longest possible chain, then systematically shorten it by one carbon and place that carbon as a branch in every unique position. Keep going until you can't shorten the chain any further. Watch out for duplicates: two structures that look different on paper might actually be the same molecule flipped or rotated.

Alkane Nomenclature

IUPAC naming rules give every organic molecule one unambiguous name. Learning these rules for alkanes is worth the effort because the same system extends to every other class of organic compound you'll study.

IUPAC nomenclature for alkanes

For straight-chain alkanes, you just count the carbons and add the suffix -ane to the appropriate prefix:

• 1C: methane, 2C: ethane, 3C: propane, 4C: butane, 5C: pentane, 6C: hexane, 7C: heptane, 8C: octane, 9C: nonane, 10C: decane

For branched alkanes, follow these steps:

1. Find the longest continuous carbon chain. This determines the parent name. The longest chain isn't always drawn in a straight line, so trace carefully.
2. Number the chain starting from the end nearest to the first branch point. This gives substituents the lowest possible numbers.
3. Identify and name each substituent. Determine what alkyl group each branch is (methyl, ethyl, etc.) and note which carbon number it's attached to.
4. Handle multiple identical substituents with the prefixes di-, tri-, tetra-, etc. For example, two methyl groups become "dimethyl." Each one still gets its own locator number (e.g., 2,2-dimethyl or 2,3-dimethyl).
5. Assemble the name. List substituents in alphabetical order (ignoring the di-, tri- prefixes when alphabetizing), then add the parent chain name. Separate numbers from words with hyphens and numbers from numbers with commas.