Organic Chemistry · Aldehydes and Ketones: Nucleophilic Addition Reactions

Naming Aldehydes and Ketones

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Aldehydes and ketones are the two simplest carbonyl (C=O) families. In an the carbonyl carbon carries at least one hydrogen — the group is written R-CHO — and the carbonyl sits at the end of a chain. In a the carbonyl carbon is bonded to two carbon groups, R-CO-R', so it sits inside a chain. Naming them is a two-step game: find the longest chain that contains the carbonyl, then number it so the carbonyl gets the lowest . Aldehydes take the suffix -al (the CHO carbon is automatically C1), and ketones take -one (the C=O carbon gets the lowest possible number). This topic also covers common names (formaldehyde, acetaldehyde, acetone), carbonyl priority when other functional groups compete, and the special "" rule for rings.

Why this matters

Nomenclature is the grammar of organic chemistry: every reaction in this chapter — hydration, cyanohydrin formation, Grignard addition, acetal formation, the Wittig reaction — is described as a transformation of a named carbonyl compound. Getting the name right means getting the structure right — and structure drives reactivity (aldehydes are more reactive than ketones toward nucleophiles, a fact that shows up in every mechanism that follows). In health-science contexts carbonyl names appear constantly: acetone (a ketone body in diabetic ketoacidosis), formaldehyde and glutaraldehyde (disinfectants), and retinal (vitamin A's aldehyde form). Naming carbonyls fluently lets you read the rest of this chapter — and the medical literature — without friction.

The college version

Core Concepts

Aldehydes: the -al suffix

Select the longest continuous chain that includes the CHO carbon, and change the parent alkane's ending to -al: CH3CH2CH2CHO is butanal, CH3CH(CH3)CHO is 2-methylpropanal. The aldehyde carbon is always C1, so numbering starts there. Two CHO groups on one chain give -dial (e.g., pentanedial, an industrial disinfectant). When the aldehyde attaches directly to a ring, the ring is the parent and the suffix is -carbaldehyde: cyclohexanecarbaldehyde, and benzaldehyde for benzene. When the CHO group is a substituent on a molecule with a higher-priority group, it is called formyl-. Common names you must recognize: formaldehyde (methanal), acetaldehyde (ethanal), and benzaldehyde.

Ketones: the -one suffix

For a ketone, select the longest chain containing the carbonyl and change the ending to -one, numbering so the C=O carbon gets the lowest locant: CH3COCH3 is propan-2-one (propanone; common name acetone), CH3COCH2CH3 is butan-2-one (common: methyl ethyl ketone, MEK), and CH3CH2COCH2CH3 is pentan-3-one. Two carbonyls give -dione (e.g., pentane-2,4-dione, a classic diketone). Cyclic ketones keep the ring name plus -one: cyclohexanone, cyclobutanone. Common names worth knowing: acetophenone (C6H5COCH3, methyl phenyl ketone) and benzophenone (C6H5COC6H5, diphenyl ketone).

Functional group priority

When several functional groups are present, only one provides the suffix; the rest become prefixes. The carbonyl outranks alcohols, amines, and alkenes, and aldehydes outrank ketones: a molecule with both is named as an aldehyde, with the ketone as an prefix — CH3COCH2CH2CHO is 4-oxopentanal, not a pentanone. An alcohol loses to either carbonyl: HOCH2CH2CHO is 3-hydroxypropanal. Aldehyde priority also governs numbering, so the CHO carbon is C1 even when a ketone sits nearby.

Aldehydes vs ketones: what the name reveals about reactivity

The name reveals the substitution pattern at the carbonyl carbon, which drives reactivity. An aldehyde has one carbon substituent and one small hydrogen; a ketone has two carbon substituents. Sterically the aldehyde carbonyl is more open to attack; electronically, alkyl groups donate electron density, stabilizing the carbonyl's partial positive charge and making it less electrophilic. Both effects make aldehydes more reactive toward nucleophilic addition — a rule governing every reaction in this chapter.

Counting degrees of unsaturation

Every C=O contributes one degree of unsaturation. For acetone, C₃H₆O:

$DBE = 2(3) + 2 - 6 2 = 1$

That single unit is the carbonyl π bond — a useful cross-check when you convert a common name to a formula and want to verify the structure.

Common Confusions

Do not confuse	With	Difference
Aldehyde, -al (`RCHO`)	Alcohol, -ol (`RCH2OH`)	One letter changes the whole group: carbonyl at chain end vs C–OH; a common reading error on exams
Ketone, -one	Aldehyde, -al	Ketone carbonyl is inside the chain (needs a locant); aldehyde is at the end (always C1, no locant needed)
Acetone (propanone)	Acetaldehyde (ethanal)	Acetone is a ketone, acetaldehyde an aldehyde — different functional groups, different reactivity
"3-methylbutanal"	"3-methylbutan-3-ol"	The aldehyde carbon is C1 by definition; don't renumber to favor the substituent
Formyl- (prefix)	-al (suffix)	Formyl is used when the CHO group is a substituent on a higher-priority functional group; -al is the suffix when CHO is the principal group
Aryl ketone (acetophenone)	Benzaldehyde	Acetophenone is C₆H₅COCH₃ (ketone); benzaldehyde is C₆H₅CHO (aldehyde) — the classic aromatic pair

Eli explains

The same idea, in plain words

Explain it like I’m 10

An aldehyde is a carbon with a double bond to oxygen, a hydrogen, and one other group — it lives at the end of the line, like the last car of a train. A ketone is the same double bond but with two other groups — it lives in the middle, like a middle car. Naming is just counting: find the longest train of carbons that includes the double-bonded carbon, then number from the end that gives it the smallest number. Aldehydes end in "-al" and ketones end in "-one".

Worked example

Example 1: Name `CH3CH2CH(CH3)CHO`

Step 1 — find the parent. The longest chain containing the CHO carbon has four carbons (butanal parent).

Step 2 — number. The CHO carbon is C1, so the chain runs C1–C4.

Step 3 — locate substituents. The methyl branch is on carbon 2:

$Name: 2-methylbutanal$

Check. The carbonyl carbon must be C1 — numbering from the other end would put the methyl at C3, but the aldehyde rule overrides.

Example 2: Name `(CH3)2CHCOCH3`

Step 1 — find the parent. The longest chain containing the C=O has four carbons: butan-?-one.

Step 2 — number for lowest locant. Number from the right so the carbonyl is C2:

$butan-2-one with a methyl at C3$

Step 3 — assemble. The methyl branch sits on carbon 3: 3-methylbutan-2-one. Common-name cross-check: this is the ketone body precursor analog pattern — the carbonyl is an "inside" group, so it must carry a locant.

Example 3: From common name to structure and formula — acetone

Problem. Write the structure of acetone and confirm its formula C₃H₆O with the DBE test.

Structure. Acetone = propan-2-one, a three-carbon chain with the C=O on the middle carbon: CH3COCH3.

Formula count. C: 3; H: 6 (two CH₃ groups); O: 1 → C₃H₆O.

DBE check.

$DBE = 2(3) + 2 - 6 2 = 1$

Exactly one degree of unsaturation, accounted for by the C=O π bond — the structure and formula are consistent.

Key takeaways

Aldehyde: R-CHO, suffix -al, CHO carbon is always C1.
Ketone: R-CO-R', suffix -one, number to give C=O the lowest locant.
Two carbonyls: -dial (aldehydes) and -dione (ketones).
CHO on a ring: -carbaldehyde (cyclohexanecarbaldehyde, benzaldehyde).
Priority: aldehyde > ketone > alcohol > amine; a ketone in an aldehyde's name becomes oxo-.
Common names to memorize: formaldehyde (methanal), acetaldehyde (ethanal), acetone (propanone), MEK (butan-2-one), benzaldehyde, acetophenone, benzophenone.
Aldehydes are more reactive than ketones toward nucleophiles (less steric hindrance + less electron donation from only one alkyl group).
C=O counts as one degree of unsaturation.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Name CH3CH2CH2CHO and explain why the CHO carbon needs no locant.
Show answer
Butanal. The aldehyde carbon is by definition C1 (the chain is numbered from the CHO end), so no locant is written for the suffix.
Give the IUPAC name of acetone and of MEK.
Show answer
Acetone = propan-2-one (propanone); MEK = butan-2-one (methyl ethyl ketone).
A molecule contains both an aldehyde and a ketone. Which group provides the suffix, and how is the other named?
Show answer
The aldehyde provides the suffix (-al); the ketone is named as an oxo- prefix, e.g., 4-oxopentanal.
Name the compound formed by attaching a CHO group directly to a cyclohexane ring.
Show answer
Cyclohexanecarbaldehyde (a ring-attached aldehyde uses the -carbaldehyde suffix).
How many degrees of unsaturation does butan-2-one (C₄H₈O) have, and what accounts for it?
Show answer
DBE = (2(4) + 2 - 8)/2 = 1, accounted for by the C=O π bond.
Why are aldehydes generally more reactive toward nucleophilic addition than ketones?
Show answer
The aldehyde carbonyl carbon has only one alkyl group: less steric hindrance to nucleophile approach, and less electron donation into the carbonyl, leaving the carbon more electrophilic.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Aldehyde: A carbonyl group with one H and one carbon group, R-CHO, at a chain end
Ketone: A carbonyl group with two carbon groups, R-CO-R', inside a chain
Carbonyl group: A carbon double-bonded to oxygen, C=O
Locant: The number locating a substituent or group on the parent chain
Oxo-: Prefix naming a ketone when an aldehyde outranks it
Carbaldehyde: Suffix for an aldehyde attached directly to a ring

Sources & references

openstax.org — Organic Chemistry

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