Priority and Numbering

When a molecule has only one functional group, naming is straightforward - that group gets the suffix, and you are done. But most biologically relevant molecules have multiple functional groups. An amino acid has both a carboxylic acid and an amine. A sugar has multiple hydroxyl groups and an aldehyde or ketone. A fatty acid has a carboxylic acid, maybe some double bonds, and possibly a hydroxyl. How do you decide which group “wins” the suffix?

The answer is the IUPAC priority hierarchy - a VIP list that ranks every functional group from most important to least important. The highest-priority group gets the suffix. Every other group becomes a prefix.

The VIP List at a Party

The Priority Table (Highest to Lowest)

This table lists functional groups from highest to lowest IUPAC priority. The group highest on this list that is present in the molecule gets the suffix. Everything below it becomes a prefix.

How Priority Affects Naming - A Worked Example

Consider a molecule with four carbons, an -OH at carbon 3, and a -COOH at carbon 1.

Step 1: Identify all functional groups: carboxylic acid (-COOH) and alcohol (-OH).

Step 2: Determine which has higher priority. Carboxylic acid (priority 1) beats alcohol (priority 6). Carboxylic acid gets the suffix.

Step 3: The suffix is “-oic acid.” The parent chain is four carbons = butanoic acid.

Step 4: The alcohol group becomes a prefix: “hydroxy-” at carbon 3.

Final name: 3-hydroxybutanoic acid.

Notice that the -OH group does not get the “-ol” suffix. It becomes “hydroxy-” because a higher-priority group (carboxylic acid) has already claimed the suffix.

Numbering Rules in Detail

Once you know which group gets the suffix, numbering follows a clear hierarchy of tie-breaking rules:

Rule 1: Give the principal (suffix-determining) functional group the lowest possible locant. This is the most important rule. If the carboxylic acid can be at C1 or C5, it goes to C1.

Rule 2: Give multiple bonds (C=C or C≡C) the next lowest locant. If the suffix group is already placed, give the double or triple bond the lowest possible number.

Rule 3: Give substituent prefixes the lowest set of locants. Compare the locant sets position by position. At the first point of difference, the lower number wins.

Rule 4: If there is still a tie, give the lower locant to the substituent that comes first alphabetically.

Examples of Priority-Based Naming

Example 1: 5-aminopentan-2-ol

• Both -OH and -NH2 are present
• Alcohol (-ol) has higher priority than amine (-amine) in the IUPAC system
• The suffix is “-ol” (alcohol at C2)
• The amine becomes the prefix “amino-” at C5

Wait - actually, let’s check: is alcohol higher priority than amine? Looking at the table: alcohol is priority 6, amine is priority 7. Yes, alcohol outranks amine. So the suffix is “-ol.”

Example 2: 4-oxopentanoic acid

• Both a carboxylic acid and a ketone are present
• Carboxylic acid has highest priority, so suffix = “-oic acid”
• The ketone becomes the prefix “oxo-” at C4
• Parent chain is five carbons = pentanoic acid
• Full name: 4-oxopentanoic acid

Example 3: 3-hydroxyhexanal

• Both an aldehyde and an alcohol are present
• Aldehyde has higher priority than alcohol, so suffix = “-al”
• The alcohol becomes the prefix “hydroxy-” at C3
• Parent chain is six carbons = hexanal
• Full name: 3-hydroxyhexanal

The “Lowest Set of Locants” Rule

This rule is often tested on the MCAT because it requires careful comparison. When two numbering schemes give different locant sets, compare them number by number from left to right:

Numbering A: Substituents at 2, 3, 6
Numbering B: Substituents at 2, 4, 5

Compare: 2 vs. 2 (tie) → 3 vs. 4 → Numbering A wins because 3 < 4.

The comparison stops as soon as you find a difference. You do not add up the locants (that is a common misconception). You compare position by position.

Groups That Are Always Prefixes

Some groups never get a suffix because they are never the “principal” group. They always appear as prefixes: