MHT-CET Chemistry · Aldehydes, Ketones and Carboxylic Acids

Nomenclature and Classification of Aldehydes, Ketones and Carboxylic Acids

Aldehydes (CₙH₂ₙO, R–CHO) and ketones (R–CO–R') are named -al and -one; carboxylic acids are -oic acids and carry the highest IUPAC priority, so in a polyfunctional compound COOH is the parent, then CHO, then OH — and the common acids are learnt by the number of COOH groups they carry.

All Aldehydes, Ketones and Carboxylic Acids notes MHT-CET Chemistry strategy

Why this matters

24 PYQs, none HARD — recall with one rule. Eight ask which named acid is mono-, di- or tricarboxylic (citric is the only tricarboxylic; valeric and caproic are mono; malonic, succinic, glutaric, adipic, phthalic are di); eight are an IUPAC name from a drawn benzene ring carrying COOH, CHO, OH or CH₃; eight are the general formula, which ketone is simple, which aldehyde smells of butter, which boils highest. Three cards.

Concept 1 of 3

The Named Acids: Mono-, Di- and Tricarboxylic

Intuition

Count the COOH groups behind the common name. The straight-chain monoacids run formic, acetic, propionic, butyric, valeric, caproic (C1–C6). The diacids run oxalic, malonic, succinic, glutaric, adipic (C2–C6), plus phthalic on a benzene ring. Citric acid is the one tricarboxylic acid the paper uses. Salicylic, o-toluic and benzoic are monoacids on a ring.

Definition

Monocarboxylic: formic (methanoic), acetic, propionic, butyric, valeric (pentanoic), caproic (hexanoic); acrylic acid = prop-2-enoic acid; benzoic, salicylic (2-hydroxybenzoic), o-toluic (2-methylbenzoic).
Dicarboxylic: oxalic (ethanedioic), malonic (propanedioic), succinic (butanedioic), glutaric (pentanedioic), adipic (hexanedioic); phthalic (benzene-1,2-dicarboxylic).
Tricarboxylic: citric acid (2-hydroxypropane-1,2,3-tricarboxylic).
Pairs the paper keys: butyric + caproic (both mono); catechol-style traps: malonic + propionic (di + mono), valeric + succinic (mono + di).

Common name	IUPAC name	COOH groups
Formic / acetic / propionic	Methanoic / ethanoic / propanoic	1
Butyric / valeric / caproic	Butanoic / pentanoic / hexanoic	1 Valeric and caproic are MONO — the planted 'dicarboxylic' options.
Oxalic / malonic / succinic	Ethanedioic / propanedioic / butanedioic	2
Glutaric / adipic	Pentanedioic / hexanedioic	2
Phthalic	Benzene-1,2-dicarboxylic	2 The one aromatic acid on the list that is NOT mono.
Citric	2-Hydroxypropane-1,2,3-tricarboxylic	3 The only tricarboxylic acid asked.
Benzoic / salicylic / o-toluic	Benzoic / 2-hydroxybenzoic / 2-methylbenzoic	1

C1–C6 mono: F-A-P-B-V-C; C2–C6 di: O-M-S-G-A.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Aldehydes, Ketones and Carboxylic AcidsEASY

Which among the following is NOT dicarboxylic acid?

[Q80 · 16th May Shift 1 · 2023]

Hearing 'valeric' and 'glutaric' as the same kind of name

The -ic ending says nothing about the count. Valeric (C5) and caproic (C6) are straight-chain MONOacids; glutaric (C5) and adipic (C6) are the DIacids of the same length.

Drill 7 more on the named acids: mono-, di- and tricarboxylic

Concept 2 of 3

General Formula, Simple Ketones, and the Physical Facts

Intuition

An aldehyde or ketone has one C=O and no ring, so its formula is CₙH₂ₙO; an acid carries a second oxygen, CₙH₂ₙO₂. A ketone is 'simple' when both groups on the carbonyl are the same (benzophenone), 'mixed' otherwise (acetophenone, butanone). Boiling points climb with chain length: hexanal above pentanal above propanal.

Definition

Aldehydes and ketones: $\text{C}_n\text{H}_{2n}\text{O}$ . Carboxylic acids: $\text{C}_n\text{H}_{2n}\text{O}_2$ .
Simple (symmetrical) ketone: $\text{C}_6\text{H}_5\text{COC}_6\text{H}_5$ benzophenone, propanone, pentan-3-one. Mixed: acetophenone $\text{C}_6\text{H}_5\text{COCH}_3$ , butanone, pentan-2-one.
Identify the ketone among ethyl ethanoate (ester), acetophenone, N-methylphthalimide (imide), methyl salicylate (ester): acetophenone.
Odours and uses: butyraldehyde — buttery, margarine flavouring; benzaldehyde — almonds; cinnamaldehyde — cinnamon; vanillin — vanilla.
Boiling point rises with molar mass: ethanal < propanal < butanal < valeraldehyde < hexanal. Carbonyls boil above alkanes and ethers, below alcohols and acids.

General formulas

\text{aldehyde / ketone: } \text{C}_n\text{H}_{2n}\text{O};\qquad \text{acid: } \text{C}_n\text{H}_{2n}\text{O}_2

Worked example

Classify pentan-3-one, pentan-2-one and 4-methylpentan-2-one as simple or mixed ketones, and give the formula of the first.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Aldehydes, Ketones and Carboxylic AcidsEASY

Which among the following is a simple ketone?

[Q87 · 13th May Shift 2 · 2024]

Calling acetophenone a simple ketone

Phenyl and methyl are different groups — acetophenone is mixed. The simple one on the list is always benzophenone.

Drill 7 more on general formula, simple ketones, and the physical facts

Concept 3 of 3

IUPAC Priority: COOH Beats CHO Beats OH

Intuition

Pick the highest-ranking group as the parent and turn the rest into prefixes: COOH → benzoic acid with 'formyl' and 'hydroxy' prefixes; CHO → benzaldehyde with 'hydroxy'; C=O → -one with 'hydroxy' or 'chloro'. Then number from the parent group for the lowest locant set, breaking ties alphabetically.

Definition

Priority: $-\text{COOH} > -\text{CHO} > \text{>C=O} > -\text{OH} > -\text{NH}_2 > \text{C≡C}$ . Among CHO, OH, NH₂ and C≡C the principal group is CHO.
Ring with CHO, OH, CH₃ all meta: 3-hydroxy-5-methylbenzaldehyde (not 3-formyl-5-methylphenol).
Ring with COOH, OH at 3, CH₃ at 4: 3-hydroxy-4-methylbenzoic acid. COOH with ethyl and OH at 3,5: 3-ethyl-5-hydroxybenzoic acid (alphabetical tie-break). COOH, CH₃ ortho, CHO: 5-formyl-2-methylbenzoic acid.
Cyclohexanone with Cl next to C=O and CH₃ two carbons on: 2-chloro-4-methylcyclohexanone (C=O is C1, substituents alphabetical).
$\text{CH}_3\text{CH(OH)COCH}_2\text{CH}_3$ : ketone outranks OH → 2-hydroxypentan-3-one, formula C₅H₁₀O₂, OH on an sp³ carbon.

Priority and prefixes

\text{COOH (parent)} \to \text{CHO as 'formyl'};\quad \text{CHO (parent)} \to \text{OH as 'hydroxy'};\quad \text{C=O (parent)} \to \text{OH as 'hydroxy'}

Worked example

Name

\text{OHC-CH}_2\text{-CH(OH)-CH}_3

and a benzene ring carrying COOH at C1 and CHO at C4.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Aldehydes, Ketones and Carboxylic AcidsMODERATE

What is IUPAC name of the following compound?

[Q66 · 16th May Shift 1 · 2023]

Naming the compound as a phenol because the OH is on the ring

Ring OH is a phenol only when nothing outranks it. With CHO or COOH present the OH is a hydroxy prefix; 'formyl-phenol' and 'carboxy-phenol' names are the planted options.

Drill 7 more on iupac priority: cooh beats cho beats oh

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

General Formula, Simple Ketones, and the Physical Facts
General formulas
$\text{aldehyde / ketone: } \text{C}_n\text{H}_{2n}\text{O};\qquad \text{acid: } \text{C}_n\text{H}_{2n}\text{O}_2$
IUPAC Priority: COOH Beats CHO Beats OH
Priority and prefixes
$\text{COOH (parent)} \to \text{CHO as 'formyl'};\quad \text{CHO (parent)} \to \text{OH as 'hydroxy'};\quad \text{C=O (parent)} \to \text{OH as 'hydroxy'}$

Reference tables (1)

The Named Acids: Mono-, Di- and Tricarboxylic7 rows

Common name	IUPAC name	COOH groups
Formic / acetic / propionic	Methanoic / ethanoic / propanoic	1
Butyric / valeric / caproic	Butanoic / pentanoic / hexanoic	1 Valeric and caproic are MONO — the planted 'dicarboxylic' options.
Oxalic / malonic / succinic	Ethanedioic / propanedioic / butanedioic	2
Glutaric / adipic	Pentanedioic / hexanedioic	2
Phthalic	Benzene-1,2-dicarboxylic	2 The one aromatic acid on the list that is NOT mono.
Citric	2-Hydroxypropane-1,2,3-tricarboxylic	3 The only tricarboxylic acid asked.
Benzoic / salicylic / o-toluic	Benzoic / 2-hydroxybenzoic / 2-methylbenzoic	1

C1–C6 mono: F-A-P-B-V-C; C2–C6 di: O-M-S-G-A.

Watch out for (3)

Hearing 'valeric' and 'glutaric' as the same kind of name
→ The Named Acids: Mono-, Di- and Tricarboxylic
Calling acetophenone a simple ketone
→ General Formula, Simple Ketones, and the Physical Facts
Naming the compound as a phenol because the OH is on the ring
→ IUPAC Priority: COOH Beats CHO Beats OH

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