How to Name a Molecule

Naming an organic compound looks complicated at first, but it is a mechanical process with just a few steps. This article walks through the whole path, including two math formulas that help a lot: the alkane general formula and the degree of unsaturation.

Step 1 — Count the carbons in the main chain

Find the longest continuous chain containing the highest-priority functional group. For a simple alkane with $n$ carbons, the number of hydrogens always follows the same formula:

$$C_nH_{2n+2}$$

For example, for $n=4$ (butane): $C_4H_{2(4)+2} = C_4H_{10}$ — matches butane's actual formula.

Butane (C₄H₁₀) — 4 carbons in a zigzag, each completing its valence with hydrogen (shown above each carbon)

Step 2 — Calculate the degree of unsaturation (if you don't already know it)

When you only have the molecular formula (say, from an exercise) and need to figure out how many double bonds, triple bonds, or rings the molecule has, you use the degree of unsaturation (also called the hydrogen deficiency index):

$$DoU = \frac{2C + 2 + N - H}{2}$$

where $C$ is the number of carbons, $N$ the number of nitrogens, and $H$ the number of hydrogens (halogens count as hydrogen in this formula, since they occupy an equivalent valence slot).

Example: for $C_4H_8$ (a butene), $DoU = \frac{2(4)+2-8}{2} = \frac{2}{2} = 1$ — one unsaturation, which could be a double bond or a ring. You need more information (or to draw the structure) to know which one.

Step 3 — Choose the right suffix

With the chain defined, the suffix depends on the highest-priority functional group present (see the full list in functional groups). Carboxylic acid always wins; halogen never becomes a suffix.

Step 4 — Number the way that gives the lowest locant

The golden rule: number the chain starting from the end that gives the lowest number to the main group. Only after breaking that tie do you look at the position of double/triple bonds, and last at the substituents.

Numbering from the left: the double bond falls on C1-C2 → but-1-ene (locant 1)

Numbering from the right: the double bond would fall on C3-C4 → higher locant, wrong

Step 5 — Assemble the final name

Put it all together in this order: substituent prefixes (alphabetical) + chain numerical prefix + unsaturation infix + main group suffix. For example, 3-chloro-2-methylpentan-1-ol has two substituents (chloro and methyl, alphabetically ordered), a 5-carbon chain (pentan), and the -ol suffix at position 1.

3-chloro-2-methylpentan-1-ol — a 5-carbon chain (numbered), with -OH on C1, methyl on C2, and chlorine on C3

Want to see this process in action? Build the molecule in the simulator and watch the name (and its full breakdown) get generated automatically.

Try it in the simulator →