Naming Alkanes

Alkanes are the plain vanilla of organic chemistry - just carbon and hydrogen, no double bonds, no functional groups, no drama. That is exactly why they are the best place to learn the full IUPAC naming process. Every rule you internalize here transfers directly to alkenes, alkynes, alcohols, ketones, and every compound the MCAT can throw at you. Master alkane naming, and half the nomenclature battle is already won.

The goal of this section is simple: given a structure, you should be able to write its IUPAC name in under thirty seconds. Given a name, you should be able to draw the structure in the same time. We build that speed with a five-step recipe that never changes.

The Five-Step Recipe

Every alkane name is built the same way. Follow these steps in order, every time:

1. Find the longest continuous carbon chain - this becomes the parent chain (covered in Section 1.2).
2. Number the chain from the end nearest the first substituent - to give the lowest set of locants.
3. Identify and name every substituent - each branch gets its own prefix (methyl, ethyl, propyl, etc.).
4. Alphabetize the substituents - ignore multiplier prefixes (di, tri) when alphabetizing.
5. Assemble: locant-substituent-locant-substituent-parent chain.

Let us walk through each step with a real example.

Step 1: The Parent Chain Sets the Suffix

Count the carbons in the longest continuous chain. The count gives you the root name, and the suffix -ane confirms it is an alkane (single bonds only).

Step 2: Number to Give Lowest Locants

Always number the chain so the substituents get the lowest possible set of numbers. If both directions give the same first locant, compare the second locant, then the third, and so on. This is called the first point of difference rule.

Example: a six-carbon chain with methyl groups on carbons 2 and 4 (numbered from the left) would be numbered 3 and 5 from the right. The set {2, 4} beats {3, 5} at the first point of difference, so we number from the left.

If the numbering is a tie at every locant, alphabetical order of the substituents breaks the tie - the substituent that comes first alphabetically gets the lower number.

Step 3: Name the Substituents

Every branch off the parent chain is a substituent. Name each substituent by counting its carbons and changing the alkane ending from -ane to -yl.

• -CH₃ = methyl (from methane)
• -CH₂CH₃ = ethyl (from ethane)
• -CH₂CH₂CH₃ = propyl (from propane)
• -CH(CH₃)₂ = isopropyl (common) or propan-2-yl (IUPAC)
• -C(CH₃)₃ = tert-butyl (common) or 2-methylpropan-2-yl (IUPAC)

For this chapter, we will use the common names (isopropyl, sec-butyl, tert-butyl) since those are the names the MCAT uses in passages. Full systematic names for branched substituents appear in Section 1.10.

Step 4: Alphabetize, Ignoring Multipliers

When a molecule has multiple types of substituents, they must appear in the name in alphabetical order by substituent name. Importantly, the multiplier prefixes (di-, tri-, tetra-) do NOT count for alphabetization.

• “3-ethyl-2,2-dimethylpentane” - ethyl (e) comes before methyl (m), even though “dimethyl” starts with d.
• “4-ethyl-3-isopropyloctane” - ethyl (e) comes before isopropyl (i).

Hyphenated prefixes like sec- and tert- are also ignored when alphabetizing, but iso- and neo- are part of the name and DO count for alphabetization order.

Step 5: Assemble the Name

Put it all together in the format:

locant-substituent-locant-substituent-parent chain

A complete worked example for the molecule with structure:

    CH₃    CH₃
    |      |
CH₃-CH-CH₂-CH-CH₂-CH₃

Following the recipe:

1. Longest chain = 6 carbons → hexane.
2. Number from the left so methyl groups land on C2 and C4 (set {2,4} not {3,5}).
3. Substituents = two methyl groups → dimethyl.
4. Only one substituent type, no alphabetization needed.
5. Assemble: 2,4-dimethylhexane.

Notice the punctuation: numbers are separated from each other by commas, and from letters by hyphens.

Handling Multiple Identical Substituents

When the same substituent appears more than once, use the multiplier prefixes: di- (2), tri- (3), tetra- (4), penta- (5), and so on. Each position still needs its own locant number.

• Two methyl groups = “dimethyl” with two locants: “2,3-dimethyl…”
• Three chloro groups = “trichloro” with three locants: “1,2,3-trichloro…”

When Substituents Have Substituents (Complex Branches)

If a substituent itself has branches, name the whole branch in parentheses and number its own carbons starting from the point of attachment to the main chain.

Example: a 4-carbon branch with a methyl group on its second carbon is named (2-methylpropyl) - also known by its common name isobutyl. In context, the name might look like “4-(2-methylpropyl)octane.”

This rule keeps long, complex names unambiguous. Section 1.10 drills deeper into these complex substituent names, but most MCAT questions stop at the common-name level (isopropyl, isobutyl, sec-butyl, tert-butyl, neopentyl).

Common Naming Traps

1. Not finding the longest chain. A 2D structure can hide the longest chain because of how it is drawn. Always count every possible path.
2. Wrong numbering direction. Students often number from the end closest to the “drawn left side” out of habit. The rule is lowest locants, regardless of visual orientation.
3. Forgetting to alphabetize. Even if it looks weird (like “ethyl” appearing before “methyl”), alphabetical order is the law.
4. Miscounting multiplier prefixes. “Trimethyl” needs three locants, not one. Missing a locant is a common error.
5. Ignoring the parent chain tie-breaker. When two chains are the same length, pick the one with more substituents. If they tie on substituents too, use lowest locants.

Worked Examples

Example 1: A 7-carbon chain with a methyl on C3 and an ethyl on C5.

Alphabetize: ethyl (e) before methyl (m). But check numbering first. Numbered from one end: ethyl-5, methyl-3 (set {3,5}). From the other end: ethyl-3, methyl-5 (set {3,5}). Tie. Go to alphabetical tie-breaker: ethyl should get the lower number, so ethyl at 3 wins. The name is 3-ethyl-5-methylheptane.

Example 2: A 6-carbon chain with two methyls on C2 and one methyl on C4.

Substituents: three methyls at positions 2, 2, and 4 → “2,2,4-trimethyl”. Name: 2,2,4-trimethylhexane (famously the high-octane component of gasoline).

Example 3: A 5-carbon chain with a bromo on C1 and a chloro on C3.

Alphabetize: bromo (b) before chloro (c). Numbering: bromo at 1, chloro at 3 (set {1,3}) or bromo at 5, chloro at 3 (set {3,5}). Set {1,3} wins. Name: 1-bromo-3-chloropentane.

What the MCAT Actually Asks

You will rarely be asked to name a compound from scratch in an AAMC exam. What you will see is a passage that references compounds by name and expects you to:

1. Recognize the structural implications of the name (chain length, branches, functional group).
2. Predict reactivity based on the structure implied by the name.
3. Compare two similarly named compounds to identify isomerism or polarity differences.

Practice going from name to structure (reverse nomenclature) as much as structure to name. The MCAT rewards the latter skill more than the former.