Carboxylic Acid and Derivatives

Carboxylic acids combine a carbonyl group and a hydroxyl group on the same carbon, making them the highest-priority functional group tested on the MCAT. Three related functional groups — esters, amides, and anhydrides — are all built by modifying that same carboxylic acid structure. This article covers how to name all four, plus the complete functional-group priority order.

Key Takeaways

• Carboxylic acids (COOH) combine a carbonyl and hydroxyl group on the same, terminal carbon — the MCAT's highest-priority functional group, named with the suffix -oic acid (e.g., methanoic/formic acid, ethanoic/acetic acid, propanoic/propionic acid).

• Esters replace the acid's -OH with -OR, named alkyl-first-plus--oate (e.g., propyl ethanoate, methyl butanoate); prefix form is alkoxycarbonyl-.

• Amides replace the acid's -OH with -NR₂, named with the suffix -amide and N-substituents cited by an N- prefix (e.g., propanamide, N,N-dimethylethanamide); prefix form is carbamoyl-/amido-.

• Anhydrides form from two dehydrated carboxylic acids, named with the suffix -anhydride — symmetrical anhydrides use one acid name (e.g., ethanoic anhydride), mixed anhydrides list both acid names alphabetically (e.g., ethanoic propanoic anhydride).

• Full priority order: carboxylic acids > anhydrides > esters > amides > aldehydes > ketones > alcohols > alkenes > alkynes > alkanes — the highest-ranked group present takes the suffix; every other group is demoted to a prefix.

Carboxylic Acids — The Highest-Priority Functional Group

A carboxylic acid has a carbonyl group (C=O) and a hydroxyl group (-OH) attached to the same carbon (COOH). Like aldehydes, carboxylic acids are terminal, so their carbon is numbered 1. This combination makes carboxylic acids a "2-in-1" functional group, combining the reactivity of both the carbonyl and the hydroxyl. It's also the most oxidized functional group tested on the MCAT — which is why COOH outranks every other functional group in the naming priority order.

The carbonyl-hydroxyl combination lets carboxylic acids act as both hydrogen bond donors and acceptors, contributing to their high polarity and water solubility. Carboxylic acids are also acidic, due to resonance stabilization of their conjugate base, which makes them reactive with bases to form salts.

To name a carboxylic acid, use the suffix -oic acid.

Worked examples:

• Methanoic acid — one carbon. Common name: formic acid.

• Ethanoic acid — two carbons. Common name: acetic acid (the acid in vinegar).

• Propanoic acid — three carbons. Common name: propionic acid.

If a carboxylic acid is part of a molecule containing a higher-priority group, it's indicated with the prefix carboxy- instead of the -oic acid suffix.

Esters — Replacing the Hydroxyl with an Alkoxy Group

An ester is formed when the hydroxyl group of a carboxylic acid is replaced with an alkoxy group (-OR), where R is an alkyl or aryl group. This substitution makes esters less polar than carboxylic acids and more soluble in organic solvents. Esters are commonly formed through an acid-catalyzed reaction between a carboxylic acid and an alcohol, called esterification.

To name an ester, use the suffix -oate. The name has two parts: the alkyl group attached to the oxygen (from the alcohol) is named first, followed by the acid-derived portion with "-oic acid" replaced by "-oate."

Worked examples:

• Propyl ethanoate — the oxygen carries a three-carbon alkyl group (propyl); the carbonyl-attached portion is two carbons, which as an acid would be ethanoic acid, so it becomes ethanoate.

• Methyl butanoate — the oxygen carries a one-carbon alkyl group (methyl); the carbonyl-attached portion is four carbons, which as an acid would be butanoic acid, so it becomes butanoate.

If an ester is part of a molecule containing a higher-priority group, it's indicated with the prefix alkoxycarbonyl-.

Amides — Replacing the Hydroxyl with an Amino Group

An amide is formed when the hydroxyl group of a carboxylic acid is replaced with an amino group (-NR₂), where R is hydrogen or an alkyl group. This makes amides significantly less acidic than carboxylic acids, but gives them the ability to form strong hydrogen bonds. Amides are highly polar and biologically important — the peptide bonds holding proteins together are amide bonds.

To name an amide, use the suffix -amide. Substituents on the nitrogen are cited as N-substituents in the name.

Worked examples:

• Propanamide — a three-carbon chain with the -amide suffix and no nitrogen substituents.

• N,N-Dimethylethanamide — a two-carbon chain (ethanamide) with two methyl groups on the nitrogen, indicated by the N,N- prefix.

If an amide is part of a molecule containing a higher-priority group, it's indicated with the prefix carbamoyl- (or, equivalently, amido-).

Anhydrides — Two Carboxylic Acids, Dehydrated

An anhydride forms when two carboxylic acid molecules undergo a dehydration reaction, losing a molecule of water and leaving two carbonyl groups connected by a single oxygen. This structure makes anhydrides highly reactive and useful in synthesis — and highly sensitive to hydrolysis, which breaks them back down into their parent carboxylic acids.

To name an anhydride, add the suffix -anhydride to the name of the parent carboxylic acid(s).

• Symmetrical anhydrides (both sides from the same acid): replace "acid" with "anhydride" in the single parent acid's name. Worked example: ethanoic anhydride — both sides derived from ethanoic acid.

• Mixed anhydrides (sides from two different acids): list both acid names in alphabetical order, then add "anhydride." Worked example: ethanoic propanoic anhydride — one side from ethanoic acid, the other from propanoic acid ("ethanoic" comes before "propanoic" alphabetically).

Functional Group Suffixes and Prefixes — Full Priority Order

Every functional group covered in this chapter has a suffix (used when it's the highest-priority group present) and a prefix (used when it isn't):

This table also gives the full functional-group priority order, from highest to lowest: carboxylic acids > anhydrides > esters > amides > aldehydes > ketones > alcohols > alkenes > alkynes > alkanes. Whichever group sits highest in this order in a given molecule becomes the suffix; every other group present is demoted to a prefix.

Common MCAT Mistakes

• Reversing the order of an ester's two-part name. The alkyl group attached to the oxygen (from the alcohol) always comes first, followed by the acid-derived "-oate" portion — writing "ethanoate propyl" instead of "propyl ethanoate" reverses this.

• Forgetting the alphabetical rule for mixed anhydrides. When two different acids form an anhydride, their names must be listed in alphabetical order (e.g., "ethanoic propanoic anhydride," not "propanoic ethanoic anhydride").

• Misranking esters and amides against aldehydes and ketones. All four carboxylic acid derivatives covered here — anhydrides, esters, and amides — outrank aldehydes and ketones in the priority order, so a molecule with both an ester and a ketone always takes the -oate suffix, never -one.

• Using oxo- as the demoted prefix for a carboxylic acid. Oxo- is reserved for demoted aldehydes and ketones; a demoted carboxylic acid uses carboxy- instead.

MCAT-Style Concept Check

Question: A molecule contains both an ester group and an amide group. How is the molecule's IUPAC name constructed?

• A) The amide takes the suffix -amide, and the ester is indicated with the prefix alkoxycarbonyl-

• B) The ester takes the suffix -oate, and the amide is indicated with the prefix carbamoyl- (or amido-)

• C) Both groups are cited as suffixes together, joined by a hyphen

• D) Neither group can be named directly — only carboxylic acids can appear in an IUPAC name alongside another functional group

Answer: B

Explanation: In the functional-group priority order, esters outrank amides (carboxylic acids > anhydrides > esters > amides > ...). The higher-priority group present controls the parent name and takes the suffix — here, the ester takes -oate. The amide, no longer the suffix-defining group, is named instead with the carbamoyl- (or equivalently amido-) prefix at its locant. Option A reverses the priority order. Option C is not how IUPAC nomenclature works — only one group can hold the suffix position. Option D is false; molecules with multiple carboxylic acid derivatives are named routinely using this exact suffix/prefix rule.

FAQ

Why do carboxylic acids outrank every other functional group covered in this chapter?

A carboxylic acid combines a carbonyl and a hydroxyl group on the same carbon, making it the most oxidized functional group tested on the MCAT — that "2-in-1" structure is why COOH sits at the top of the priority order.

What's the structural difference between an ester and an amide?

Both are formed by replacing a carboxylic acid's hydroxyl group: an ester replaces it with an alkoxy group (-OR), while an amide replaces it with an amino group (-NR₂). The amide's nitrogen also lets it form strong hydrogen bonds and makes it far less acidic than the parent acid.

How do you name a mixed anhydride, where the two sides come from different acids?

List both parent acid names in alphabetical order, then add "anhydride" — for example, "ethanoic propanoic anhydride," where "ethanoic" precedes "propanoic" alphabetically.

What is the full functional-group priority order for this chapter's four groups plus aldehydes, ketones, alcohols, alkenes, alkynes, and alkanes?

From highest to lowest: carboxylic acids > anhydrides > esters > amides > aldehydes > ketones > alcohols > alkenes > alkynes > alkanes. Whichever group ranks highest in a given molecule takes the suffix; every other group present is demoted to a prefix.