The correct order of acid strength is:

CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH

CH₃COOH > CH₂ClCOOH > CHCl₂COOH > CCl₃COOH

CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH

CH₂ClCOOH > CCl₃COOH > CH₃COOH > CHCl₂COOH

All four have the same strength

o-toluic acid is stronger than benzoic acid even though methyl is electron-releasing. This is due to:

The ortho effect — steric twisting breaks conjugation of COOH with the ring

A stronger inductive effect at ortho

The ortho effect — steric twisting breaks conjugation of COOH with the ring

Hydrogen bonding with the solvent

Methyl being electron-withdrawing at ortho

Ketones are less reactive than aldehydes towards nucleophilic addition because:

Two alkyl groups both donate electrons to the carbonyl carbon and crowd it sterically

Their carbonyl bond is stronger

Two alkyl groups both donate electrons to the carbonyl carbon and crowd it sterically

Ketones have no carbonyl oxygen

The Rosenmund reduction of an acid chloride stops at the aldehyde because:

The palladium catalyst is deliberately poisoned with BaSO₄ and sulphur to limit its activity

Aldehydes cannot be reduced further by any reagent

The palladium catalyst is deliberately poisoned with BaSO₄ and sulphur to limit its activity

Hydrogen gas is used up entirely in the first step

Acid chlorides are unreactive towards hydrogen

Aldehydes, Ketones and Carboxylic Acids

The carbonyl group is the busiest functional group in organic chemistry, and its reactivity has exactly one source: the carbon is electron-poor because oxygen has pulled the pi electrons towards itself. Every reaction in this chapter is a nucleophile arriving at that carbon, or a base removing the slightly acidic hydrogen on the carbon next door. Once you can see the molecule as a positive site with an acidic neighbour, the aldol, the Cannizzaro, the haloform and the cyanohydrin reactions all stop being separate facts and become variations on two moves.

Nucleophilic Addition to the Carbonyl

Aldehydes and ketones are made by oxidising alcohols, by ozonolysis, by Rosenmund reduction of an acid chloride (which stops at the aldehyde because the palladium catalyst is poisoned with BaSO₄ and sulphur), by Stephen reduction of a nitrile, and by Friedel–Crafts acylation for aryl ketones. Their central reaction is nucleophilic addition, and the order of reactivity is HCHO > other aldehydes > ketones.

🎯 Why ketones are less reactive than aldehydes, twice over

Two independent effects point the same way. Electronic: a ketone has two alkyl groups pushing electrons towards the carbonyl carbon, which reduces its positive character and makes it less attractive to a nucleophile; an aldehyde has only one such group and formaldehyde has none. Steric: the two alkyl groups also physically crowd the carbon, so an incoming nucleophile has a harder approach. Both effects grow with substitution, which is exactly why formaldehyde is the most reactive carbonyl of all and why di-tert-butyl ketone is almost inert. When a question asks you to rank carbonyls, count the alkyl groups — that single count captures both effects at once.

The additions and condensations to know
Reagent	Product	Use
HCN	Cyanohydrin	Adds one carbon; route to hydroxy acids
NaHSO₃	Bisulphite adduct	Purification, since the adduct crystallises
RMgX then H₂O	Alcohol, one class higher	The main carbon–carbon bond-forming tool
NH₂OH	Oxime	Characterisation by melting point
2,4-DNP	Orange hydrazone	Detection of any carbonyl
Alcohol with dry HCl	Acetal	Protecting the carbonyl during another step

The Named Reactions and What Each Requires

Which carbonyl does which reaction
Reaction	Requirement	Example and outcome
Aldol condensation	At least one alpha hydrogen	Acetaldehyde gives 3-hydroxybutanal, then crotonaldehyde
Cannizzaro reaction	No alpha hydrogen	HCHO with conc. NaOH disproportionates to methanol and formate
Iodoform / haloform	A CH₃CO– group	Acetone and acetaldehyde give CHI₃; benzaldehyde does not
Tollens / Fehling	An aldehyde	Silver mirror or red Cu₂O; ketones give nothing
Clemmensen / Wolff–Kishner	Any carbonyl	Reduces C=O all the way to CH₂
HVZ reaction	A carboxylic acid with an alpha hydrogen	Alpha-halogenation; fails for HCOOH

🎯 Why the aldol and the Cannizzaro are mutually exclusive

Both reactions start with a base and a carbonyl, but they need opposite structures — which is why no compound does both. An aldol requires the base to remove an alpha hydrogen and form a carbanion that then attacks a second carbonyl. If there is no alpha hydrogen, that route is closed, and the base has to attack the carbonyl carbon directly instead; the resulting species passes a hydride to a second molecule, so one molecule is oxidised to the acid and the other reduced to the alcohol — the Cannizzaro disproportionation. So "does this aldehyde have an alpha hydrogen?" decides which reaction you get, and the two classic no-alpha-hydrogen aldehydes are formaldehyde and benzaldehyde. This is also the cleanest way to remember that benzaldehyde undergoes Cannizzaro but not aldol.

⚠️ Formic acid behaves like an aldehyde, and that breaks three rules

HCOOH has a hydrogen attached directly to the carbonyl carbon, which is structurally what makes an aldehyde an aldehyde. Three consequences follow, all examinable. It gives a positive Tollens test and reduces Fehling's solution, unlike every other carboxylic acid. It decolourises KMnO₄, again unlike acetic acid. And it fails the HVZ reaction, because that requires an alpha hydrogen and formic acid has no alpha carbon at all. So a question that lists four acids and asks which reduces Tollens' reagent is asking whether you noticed the structural peculiarity of the first member of the series.

Carboxylic Acids and Their Strength

Carboxylic acids are made by oxidising primary alcohols or aldehydes, by hydrolysing nitriles or esters, and by carbonating a Grignard reagent with CO₂. They are far more acidic than alcohols or phenols because the carboxylate ion delocalises its charge over two equivalent oxygens, which is a much better arrangement than phenoxide's partial delocalisation into a ring. Hence the ordering: carboxylic acid > carbonic acid > phenol > water > alcohol.

What raises and lowers acid strength
Change	Effect on acidity	Example
Electron-withdrawing group nearby	Increases — it stabilises the anion	CCl₃COOH ≫ CHCl₂COOH > CH₂ClCOOH > CH₃COOH
Electron-releasing alkyl group	Decreases	HCOOH > CH₃COOH > CH₃CH₂COOH
Withdrawing group moved further away	Effect weakens fast — induction dies out	2-chlorobutanoic > 3-chloro > 4-chloro
Withdrawing group on a benzene ring	Increases, most at ortho and para	p-nitrobenzoic > benzoic > p-methoxybenzoic

🧠 Every ortho-substituted benzoic acid is stronger than benzoic acid itself

This looks wrong at first, because an electron-releasing group such as methyl ought to weaken an acid — and at the para position it does. But at the ortho position any substituent, releasing or withdrawing, twists the carboxyl group out of the plane of the ring, which breaks its conjugation with the ring and leaves the anion better stabilised relative to the acid. The steric effect outweighs the electronic one. So o-toluic acid is stronger than benzoic acid while p-toluic acid is weaker, and this ortho effect is one of the most reliably examined anomalies in the chapter — a question that ranks the three toluic acids is testing precisely this.

Derivatives interconvert in a fixed order of reactivity towards nucleophiles: acid chloride > anhydride > ester > amide, which is the order of how good the leaving group is and how much the group already donates electrons back to the carbonyl. The practical rule is that you can always go down the list easily but must go up it via the acid chloride. Note too that decarboxylation of a sodium salt with soda lime removes one carbon, while the HVZ reaction halogenates the alpha carbon and so opens the route to amino acids.

Key takeaways

The carbonyl carbon is electron-poor; every reaction is a nucleophile there or a base at the alpha hydrogen.
Reactivity is HCHO > other aldehydes > ketones, from electronic and steric effects together.
Aldol needs an alpha hydrogen; Cannizzaro needs none — so the two are mutually exclusive.
The iodoform test needs a CH₃CO– group, and Tollens needs an aldehyde — different requirements entirely.
Formic acid gives a positive Tollens test and fails HVZ, because it has a carbonyl H and no alpha carbon.
Acid strength: acid > carbonic acid > phenol > water > alcohol, all from conjugate-base stability.
The ortho effect makes every ortho-substituted benzoic acid stronger than benzoic acid itself.
Derivative reactivity: acid chloride > anhydride > ester > amide — easy to go down, hard to go up.

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

Which compound undergoes the Cannizzaro reaction?
1. Acetaldehyde
2. Benzaldehyde
3. Propanal
4. Acetone
Show answer
Answer: B — Benzaldehyde
The Cannizzaro reaction requires an aldehyde with no alpha hydrogen, so the base cannot form a carbanion and instead attacks the carbonyl directly, leading to disproportionation into an alcohol and a carboxylate. Benzaldehyde has its carbonyl attached to a ring with no alpha C–H, so it qualifies; acetaldehyde and propanal both have alpha hydrogens and undergo the aldol condensation instead. Acetone is a ketone and does neither reaction under these conditions.
Which acid gives a positive Tollens test?
1. Acetic acid
2. Formic acid
3. Benzoic acid
4. Propanoic acid
Show answer
Answer: B — Formic acid
Formic acid uniquely has a hydrogen bonded directly to its carbonyl carbon, which is structurally an aldehyde arrangement, so it can be oxidised further and reduces Tollens' reagent to a silver mirror. It also decolourises KMnO₄ for the same reason. Conversely it fails the HVZ reaction, since it has no alpha carbon at all — the same structural peculiarity working in the other direction.
The correct order of acid strength is:
1. CH₃COOH > CH₂ClCOOH > CHCl₂COOH > CCl₃COOH
2. CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH
3. CH₂ClCOOH > CCl₃COOH > CH₃COOH > CHCl₂COOH
4. All four have the same strength
Show answer
Answer: B — CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH
Each chlorine withdraws electron density inductively, which stabilises the carboxylate anion and therefore strengthens the acid, so acidity rises steadily with the number of chlorines — trichloroacetic acid is about ten thousand times stronger than acetic acid. Electron-releasing alkyl groups do the opposite, which is why formic acid is stronger than acetic and acetic stronger than propanoic.
o-toluic acid is stronger than benzoic acid even though methyl is electron-releasing. This is due to:
1. A stronger inductive effect at ortho
2. The ortho effect — steric twisting breaks conjugation of COOH with the ring
3. Hydrogen bonding with the solvent
4. Methyl being electron-withdrawing at ortho
Show answer
Answer: B — The ortho effect — steric twisting breaks conjugation of COOH with the ring
Any ortho substituent, whatever its electronic nature, forces the carboxyl group out of the ring plane; the loss of conjugation destabilises the undissociated acid more than the anion, so the acid ionises more readily. This steric effect outweighs the electronic one, which is why p-toluic acid is weaker than benzoic acid while o-toluic acid is stronger — the same methyl group, opposite outcomes by position.
Which compound gives a positive iodoform test?
1. Benzaldehyde
2. Acetone
3. Propanal
4. Formaldehyde
Show answer
Answer: B — Acetone
The iodoform test requires a methyl group attached to a carbonyl, that is a CH₃CO– unit, which acetone has. Propanal has an ethyl group next to its carbonyl rather than a methyl, and benzaldehyde and formaldehyde have no alkyl group at all. Note that the requirement here is quite different from that of Tollens' reagent, which needs an aldehyde — the two tests probe different structural features and neither implies the other.
Ketones are less reactive than aldehydes towards nucleophilic addition because:
1. Their carbonyl bond is stronger
2. Two alkyl groups both donate electrons to the carbonyl carbon and crowd it sterically
3. Ketones have no carbonyl oxygen
4. Ketones are always aromatic
Show answer
Answer: B — Two alkyl groups both donate electrons to the carbonyl carbon and crowd it sterically
Two effects act together and both weaken reactivity. Electron donation from the alkyl groups reduces the positive character of the carbonyl carbon, and their bulk physically hinders the approach of the nucleophile. Since both scale with the number of alkyl substituents, formaldehyde with none is the most reactive carbonyl of all, and counting alkyl groups is enough to rank any set.
The most reactive acid derivative towards nucleophilic substitution is:
1. Amide
2. Ester
3. Anhydride
4. Acid chloride
Show answer
Answer: D — Acid chloride
The order is acid chloride > anhydride > ester > amide, set by how readily the attached group leaves and how much electron density it donates back to the carbonyl. Chloride is a weak base and a good leaving group and donates almost nothing, whereas the nitrogen of an amide donates strongly and stabilises the carbonyl. The practical consequence is that you can convert downwards freely but must route upwards through the acid chloride.
The Rosenmund reduction of an acid chloride stops at the aldehyde because:
1. Aldehydes cannot be reduced further by any reagent
2. The palladium catalyst is deliberately poisoned with BaSO₄ and sulphur to limit its activity
3. Hydrogen gas is used up entirely in the first step
4. Acid chlorides are unreactive towards hydrogen
Show answer
Answer: B — The palladium catalyst is deliberately poisoned with BaSO₄ and sulphur to limit its activity
An unpoisoned palladium catalyst would take the reduction on to the alcohol, so the catalyst is deliberately deactivated with barium sulphate and a trace of sulphur, leaving it just active enough to stop at the aldehyde. This is a rare case of catalyst poisoning being used as a synthetic tool rather than being a nuisance, and that inversion of the usual role is what makes it examinable.