Aldehydes and Ketones

Aldehydes and ketones are two closely related functional groups, both built around a carbonyl group — a carbon atom double-bonded to an oxygen atom. What separates them isn't the carbonyl itself, but where it sits in the molecule. This article covers how to name both, plus the prefixes used when a higher-priority functional group is also present.

Key Takeaways

• Both aldehydes and ketones contain a carbonyl group (C=O); carbonyl position is what distinguishes them.

• Aldehydes have a terminal carbonyl (bonded to at least one hydrogen), named with the -al suffix (e.g., methanal/formaldehyde, ethanal/acetaldehyde, propanal/propionaldehyde).

• Ketones have an internal carbonyl (bonded to two carbons), named with the -one suffix (e.g., 2-propanone/acetone, but-3-en-2-one).

• When a higher-priority functional group is present, both aldehydes and ketones are named with the prefix oxo- instead of their usual suffix; ketones can alternately use keto-.

• Worked example: 4-oxopentanoic acid (levulinic acid) shows a carboxylic acid outranking a ketone, demoting the ketone to a prefix.

The Carbonyl Group — What Aldehydes and Ketones Share

Both aldehydes and ketones contain a carbonyl group (C=O): a carbon atom double-bonded to an oxygen atom. The position of this carbonyl group determines which of the two functional groups a compound is classified as.

Aldehydes — Naming the Terminal Carbonyl

Aldehydes have the carbonyl group at the end of the parent chain, with the carbonyl carbon bonded to at least one hydrogen atom. To name an aldehyde, replace the parent chain's -e ending with the suffix -al.

Because the carbonyl carbon is always at the end of the chain, it's always carbon 1 — so no locant number is needed in the name.

Worked examples:

• Methanal — the simplest aldehyde, one carbon. Common name: formaldehyde.

• Ethanal — two carbons. Common name: acetaldehyde.

• Propanal — three carbons. Common name: propionaldehyde.

If the aldehyde is part of a molecule that also contains a higher-priority functional group, it's no longer named with the -al suffix. Instead, it's indicated with the prefix oxo-.

Ketones — Naming the Internal Carbonyl

Ketones have the carbonyl group located within the parent chain, with the carbonyl carbon bonded to two other carbon atoms (rather than a hydrogen). To name a ketone, use the suffix -one.

Worked examples:

• 2-Propanone — commonly known as acetone.

• But-3-en-2-one — a ketone with both a carbonyl and a carbon-carbon double bond in the same molecule (common name: methyl vinyl ketone). The ketone outranks the double bond in the functional-group seniority order established in "IUPAC Naming Conventions," so it gets the lower locant (2) and the double bond takes the higher one (3).

As with aldehydes, the prefix oxo- is used for a ketone carbonyl when a higher-priority functional group is present. Ketones may also sometimes be indicated with the alternate prefix keto-.

The Oxo- and Keto- Prefixes — When a Higher-Priority Group Is Present

Both aldehydes and ketones use oxo- as a substituent prefix when a higher-priority functional group (such as a carboxylic acid) controls the parent name and takes the suffix instead. Ketones specifically also have an older, still-used alternate prefix: keto-.

Worked Example: Naming a Molecule with a Ketone and a Carboxylic Acid

Consider a five-carbon chain that is a carboxylic acid at one end, with a ketone (carbonyl) at carbon 4.

• Highest-priority group: carboxylic acids outrank ketones in the functional-group seniority order, so the carboxylic acid controls the parent name and suffix (-oic acid), and the ketone must be named as a prefix instead of getting the -one suffix.

• Numbering: the carboxylic acid carbon is numbered 1 (as the highest-priority, terminal group), placing the ketone's carbonyl at carbon 4.

• Prefix: since the ketone is not the suffix-defining group here, it's indicated with oxo- at its locant.

Complete name: 4-oxopentanoic acid — also known by its common name, levulinic acid (and, using the alternate ketone prefix, sometimes written 4-ketopentanoic acid).

This demonstrates the priority rule directly: even though the molecule contains a ketone, the higher-priority carboxylic acid takes the suffix, and the ketone is demoted to a prefix.

Common MCAT Mistakes

• Adding a locant number to a simple aldehyde name. Because the carbonyl carbon in an aldehyde is always carbon 1, no locant is needed for the -al suffix itself — writing "1-propanal" instead of "propanal" is a common but unnecessary addition.

• Forgetting that a ketone's carbonyl carbon needs a locant. Unlike aldehydes, a ketone's carbonyl can sit at more than one internal position, so its locant must be stated (e.g., 2-propanone, not just "propanone" once the chain is long enough to have more than one possible position).

• Using keto- for an aldehyde. The alternate prefix keto- is specific to ketones — an aldehyde demoted by a higher-priority group always uses oxo-, never keto-.

• Assuming a ketone always outranks other functional groups. As the levulinic acid example shows, a carboxylic acid outranks a ketone in the seniority order — the ketone loses the -one suffix and becomes an oxo- prefix whenever a higher-priority group like a carboxylic acid is present.

MCAT-Style Concept Check

Question: A six-carbon chain has a carboxylic acid at one end and a ketone at carbon 3. Which of the following correctly identifies how the ketone is named in the final IUPAC name?

• A) With the -one suffix, since ketones always take the suffix position over other functional groups

• B) With an oxo- prefix at carbon 3, since the carboxylic acid outranks the ketone and controls the suffix

• C) With a keto- prefix, because keto- is required whenever a carboxylic acid is present in the molecule

• D) The ketone cannot be named in the same molecule as a carboxylic acid

Answer: B

Explanation: Carboxylic acids outrank ketones in the functional-group seniority order, so the carboxylic acid controls the parent name and takes the -oic acid suffix. The ketone, no longer the suffix-defining group, is named instead with the oxo- prefix at its locant — exactly as in the 4-oxopentanoic acid (levulinic acid) worked example. Option A reverses the seniority order. Option C incorrectly treats keto- as mandatory rather than as an optional alternate prefix that's available for ketones regardless of what other groups are present. Option D is false — molecules with both a ketone and a carboxylic acid are common and are named using this exact prefix rule.

FAQ

What's the structural difference between an aldehyde and a ketone?

Both contain a carbonyl group (C=O), but an aldehyde's carbonyl carbon sits at the end of the chain and is bonded to at least one hydrogen, while a ketone's carbonyl carbon sits within the chain and is bonded to two other carbon atoms.

Why doesn't an aldehyde's name need a locant number?

Because the carbonyl carbon in an aldehyde is always at the end of the parent chain, it's always carbon 1 — so there's no ambiguity to resolve with a locant, and the -al suffix is written without one.

What's the difference between the oxo- and keto- prefixes?

Oxo- is the prefix used for either an aldehyde or a ketone carbonyl when a higher-priority functional group takes the suffix instead. Keto- is an older alternate prefix available only for ketones, not aldehydes.

Why would a ketone be named with a prefix instead of the -one suffix?

When a molecule also contains a functional group that outranks ketones in the seniority order — a carboxylic acid, for example — that higher-priority group takes the suffix, and the ketone is demoted to the oxo- prefix at its locant.