33. Carboxylic acids and derivatives

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33.1 Carboxylic acids

An alkylbenzene with at least one benzylic hydrogen can be oxidised by hot alkaline KMnO4. After acidification, the side chain is converted to a carboxylic acid attached directly to the ring.

The carbon skeleton beyond the benzylic carbon is removed during vigorous oxidation, so different alkylbenzenes can give the same benzoic-acid ring product.

Methylbenzene + hot alkaline KMnO4, then dilute acid -> benzoic acid. The methyl carbon becomes the carboxyl carbon.

Do not oxidise the aromatic ring itself or stop at an alcohol; this is a strong side-chain oxidation followed by acidification.

A carboxylic acid is converted to the corresponding acyl chloride by replacing the -OH of its -COOH group with Cl. The carbon skeleton and carbonyl group remain unchanged: RCOOH becomes RCOCl.

Chlorinating reagent	Required condition	General equation and by-products
PCl3	heat	3RCOOH + PCl3 -> 3RCOCl + H3PO3
PCl5	no additional condition specified	RCOOH + PCl5 -> RCOCl + POCl3 + HCl
SOCl2	no additional condition specified	RCOOH + SOCl2 -> RCOCl + SO2 + HCl

With any of the three reagent choices, ethanoic acid, CH3COOH, gives ethanoyl chloride, CH3COCl. Keep the acid's root name and change the ending from -oic acid to -oyl chloride.

Only PCl3 is paired with heat in the official outcome. Do not write the same by-products for all three reagents, and do not replace a hydrogen on the hydrocarbon chain: the functional-group change is -COOH -> -COCl.

Most carboxylic acids resist further oxidation under ordinary syllabus conditions, but methanoic acid and ethanedioic acid are named exceptions. Recognise the acid first, then choose only its permitted oxidant and products.

$\ce{HCOOH + [O] -> CO2 + H2O}$

$\ce{HOOCCOOH + [O] -> 2CO2 + H2O}$

Acid	Oxidising reagent	Products	Distinguishing observation
methanoic acid	Fehling's reagent, warmed	CO2 + H2O	brick-red Cu2O precipitate
methanoic acid	Tollens' reagent, warmed	CO2 + H2O	silver mirror or grey silver deposit
methanoic acid	acidified KMnO4	CO2 + H2O	purple solution decolourises
methanoic acid	acidified K2Cr2O7	CO2 + H2O	orange solution turns green
ethanedioic acid	warm acidified KMnO4	CO2	purple solution decolourises and CO2 is evolved

Do not generalise the Fehling's or Tollens' result to all carboxylic acids. Ethanedioic acid requires warm acidified KMnO4 in this objective, while methanoic acid is the one accepted with all four listed oxidising systems.

The more stable and less basic the conjugate base after H+ loss, the stronger the parent O-H acid. The decisive difference is where the negative charge can be placed and how attached groups affect it.

Parent acid	Conjugate base	Charge distribution and stability	Relative acidity
carboxylic acid, RCOOH	carboxylate, RCOO-	charge is equally delocalised over two electronegative O atoms; most stabilised	strongest of the three
phenol, C6H5OH	phenoxide, C6H5O-	charge is delocalised from O into the aromatic ring, but some is placed on less electronegative C atoms	intermediate
alcohol, ROH	alkoxide, RO-	charge remains localised on one O and an alkyl group donates electron density toward it; least stabilised	weakest of the three

$\text{carboxylic acid} > \text{phenol} > \text{alcohol}$

A carboxylic acid is strong enough to react with aqueous carbonate and release CO2. Phenol does not do so under ordinary conditions, and an alcohol is weaker still. This behaviour is consistent with, rather than a substitute for, the conjugate-base explanation.

Delocalisation alone is not enough to rank the ions: carboxylate is especially stabilised because both main charge-bearing atoms are oxygen. Do not claim phenoxide is more stable merely because its charge can spread across more atoms.

Chlorine withdraws electron density through sigma bonds by its negative inductive effect. This stabilises the carboxylate conjugate base, makes recombination with H+ less favourable, and therefore increases acidity.

Structural comparison	Acidity trend	Reason
add Cl atoms beside -COOH	ethanoic acid < chloroethanoic acid < dichloroethanoic acid < trichloroethanoic acid	more electron-withdrawing Cl atoms reinforce conjugate-base stabilisation
move one Cl nearer -COOH	3-chloropropanoic acid < 2-chloropropanoic acid	inductive withdrawal is stronger over fewer sigma bonds and weakens with distance

When comparing chlorine-substituted acids, first hold the carbon skeleton constant, then count chlorine atoms and compare their distance from the carboxyl group. More chlorine and shorter distance both predict the stronger acid.

This is an inductive effect transmitted through sigma bonds, not chlorine's directing effect in an aromatic substitution. A chlorine atom does not need to be attached to an aromatic ring to change carboxylic-acid strength.

33.2 Esters

An alcohol reacts with an acyl chloride to form an ester. The alcohol supplies the alkyl-O part and the acyl chloride supplies the acid-derived carbonyl part; HCl is released.

Name the two fragments before writing the product. The reaction is usually rapid at room temperature and is not the reversible equilibrium route used for direct acid/alcohol esterification.

Ethanol + ethanoyl chloride -> ethyl ethanoate + HCl. Phenol + benzoyl chloride -> phenyl benzoate + HCl.

Do not swap the two ester names or write water as the by-product; acyl-chloride esterification produces hydrogen chloride.

33.3 Acyl chlorides

A carboxylic acid is converted to the corresponding acyl chloride when the -OH in RCOOH is replaced by Cl. The accepted chlorinating choices are PCl3 with heat, PCl5, or SOCl2.

Reagent	Condition	Overall reaction
PCl3	heat	3RCOOH + PCl3 -> 3RCOCl + H3PO3
PCl5	no additional condition specified	RCOOH + PCl5 -> RCOCl + POCl3 + HCl
SOCl2	no additional condition specified	RCOOH + SOCl2 -> RCOCl + SO2 + HCl

Ethanoic acid, CH3COOH, gives ethanoyl chloride, CH3COCl, with any of the three reagent choices. The root name stays eth- and the ending changes from -oic acid to -oyl chloride.

Only PCl3 is explicitly paired with heat in this outcome. Acyl chlorides are RCOCl, not alkyl chlorides RCl; preserve the carbonyl group and use the reagent-specific by-products.

At room temperature, an acyl chloride reacts at its carbonyl carbon. Replace Cl by the atom bearing the nucleophile's lone pair, then identify the new carboxylic-acid derivative; HCl is produced in every specified overall reaction.

Reactant with RCOCl at room temperature	Main organic product	General overall equation
water, H2O	carboxylic acid	RCOCl + H2O -> RCOOH + HCl
alcohol, R'OH	ester	RCOCl + R'OH -> RCOOR' + HCl
phenol, C6H5OH	phenyl ester	RCOCl + C6H5OH -> RCOOC6H5 + HCl
ammonia, NH3	primary amide	RCOCl + NH3 -> RCONH2 + HCl
primary amine, R'NH2	N-substituted amide	RCOCl + R'NH2 -> RCONHR' + HCl
secondary amine, R'2NH	N,N-disubstituted amide	RCOCl + R'2NH -> RCONR'2 + HCl

In practice, excess ammonia or amine can neutralise the HCl to form an ammonium or alkylammonium chloride salt. This acid capture does not change the identity of the amide formed from the first nucleophile molecule.

Do not omit phenol or secondary amines from the reaction set. Tertiary amines have no N-H bond and are not included in this amide-forming objective; the specified reactants are ammonia, primary amines and secondary amines.

Water, alcohols, phenol, ammonia and primary/secondary amines all supply a lone pair to the electrophilic carbonyl carbon of RCOCl. The attacking atom is O for water/alcohol/phenol and N for ammonia/amines.

Draw a curly arrow from the nucleophile lone pair to the carbonyl carbon, and a second arrow from the C=O pi bond to O. This addition gives a tetrahedral intermediate with O-. 2) Draw an arrow from the O- lone pair back to reform C=O, and an arrow from the C-Cl bond to Cl, eliminating Cl-. 3) Transfer the proton from the newly attached O-H or N-H group; HCl is formed or captured by excess base.

$\ce{RCOCl + HNu -> RCONu + HCl}\quad(\ce{Nu}=\text{O- or N-centred group})$

Incoming nucleophile	Attacking atom	Group retained in product	Product family
H2O	O	-OH	carboxylic acid
R'OH or C6H5OH	O	-OR' or -OC6H5	ester
NH3	N	-NH2	primary amide
R'NH2 or R'2NH	N	-NHR' or -NR'2	substituted amide

Addition is only the first step: the tetrahedral intermediate then eliminates chloride and the carbonyl is restored. Do not draw direct SN2 displacement at the carbonyl carbon or leave the final product with a permanently single C-O bond.

$\text{ease of hydrolysis: acyl chloride} > \text{alkyl chloride} > \text{aryl chloride}$

Chloride type	Local electronic structure	Hydrolysis pathway and conditions	Why it has this position
acyl chloride, RCOCl	carbonyl O and Cl withdraw electron density, making the carbonyl C strongly delta-positive	water attacks the carbonyl at room temperature; vigorous addition-elimination	highly electrophilic reaction centre and Cl- leaves as C=O reforms
alkyl chloride, RCl	C-Cl is a polar single bond at an sp3 carbon, but the carbon is less electrophilic	aqueous OH- and heating/reflux are normally required for nucleophilic substitution	water alone is a weaker nucleophile and attack is less favourable
aryl chloride, ArCl	a Cl lone pair overlaps with the aromatic pi system; C-Cl gains partial double-bond character at an sp2 carbon	does not readily undergo ordinary hydrolysis; much harsher conditions are needed	strengthened C-Cl bond is difficult to break

Compare the actual reaction centre, not chlorine alone. Acyl-chloride hydrolysis attacks C=O and then expels chloride; alkyl-chloride hydrolysis substitutes at saturated carbon; aryl chloride is protected by pi-system overlap and a strengthened C-Cl bond.

Do not explain all three with one SN1 or SN2 mechanism. Acyl chlorides use addition-elimination, alkyl chlorides use nucleophilic substitution, and aryl chlorides resist ordinary substitution because their C-Cl bonding is different.

33. Carboxylic acids and derivatives

Exam analysis

Most tested topics

33.1 Carboxylic acids

Oxidise an alkylbenzene side chain to benzoic acid

Three chlorinating reagents convert RCOOH into RCOCl

Methanoic and ethanedioic acids are exceptional further-oxidation cases

Conjugate-base stability gives carboxylic acid > phenol > alcohol

More and nearer chlorine substituents make a carboxylic acid stronger

33.2 Esters

Prepare esters from alcohols and acyl chlorides

33.3 Acyl chlorides

Prepare acyl chlorides from carboxylic acids with chlorinating agents

The nucleophile determines the product of an acyl-chloride reaction

All five acyl-chloride reaction families share addition-elimination

Hydrolysis becomes harder from acyl chloride to alkyl chloride to aryl chloride