Carboxylic acids and derivatives

In a nutshell

Carboxylic acids are weak acids built around the carboxyl group, -COOH. Their "derivatives" (esters, acyl chlorides, acid anhydrides and amides) are all made by swapping the -OH of the acid for something else.

The single idea that ties the whole topic together: a carbonyl carbon carries a small positive charge, so it is attacked by nucleophiles. That one pattern explains esterification, ester hydrolysis, soap, biodiesel and every acylation reaction here.

Assumed knowledge: Alcohols, Organic analysis.

Core content

The functional groups

Every compound in this topic is a carbonyl (C=O) with a different group attached to the carbonyl carbon.

An ester name has two parts: the -yl part comes from the alcohol and the -oate part from the acid. So ethyl ethanoate is made from ethanol and ethanoic acid, but ethyl butanoate (from butanoic acid) is a different compound from butyl ethanoate (from ethanoic acid).

Carboxylic acids are weak acids

A carboxylic acid only partly dissociates in water, so it is a weak acid:

CH₃COOH(aq) ⇌ CH₃COO^-(aq) + H⁺(aq)

Even though they are weak, carboxylic acids are strong enough to liberate carbon dioxide from carbonates. This is the standard functional-group test for a carboxylic acid: add a carbonate or hydrogencarbonate and look for effervescence (bubbles of CO₂).

2CH₃COOH(aq) + Na₂CO₃(aq) ⟶ 2CH₃COONa(aq) + H₂O(l) + CO₂(g)

Esterification

A carboxylic acid and an alcohol react, warmed with a strong acid catalyst (concentrated H₂SO₄), to give an ester and water:

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

Three things the marks hinge on:

• The reaction is reversible (use ⇌, not an arrow), so the yield is only moderate.
• You must include the water as a product. Leaving out H₂O is the most common equation error here.
• The acid is a catalyst, not a reactant, so it does not appear in the equation.

Uses of esters

Esters are volatile, sweet-smelling and largely unreactive, which makes them useful as:

• solvents for polar organic substances (for example in glues and printing inks),
• plasticisers, which sit between polymer chains and let them slide, making the plastic more flexible,
• perfumes, and
• food flavourings.

Hydrolysis of esters

Hydrolysis is the reverse of esterification: the ester is split by water. It is done by heating under reflux, and the conditions decide the products.

Acid hydrolysis (dilute acid, for example HCl, heat under reflux) is reversible and gives back the carboxylic acid and the alcohol:

CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH

Alkaline hydrolysis (dilute NaOH, heat under reflux) goes to completion and gives the carboxylate salt plus the alcohol:

CH₃COOCH₂CH₃ + NaOH ⟶ CH₃COO^-Na⁺ + CH₃CH₂OH

Alkaline hydrolysis goes to completion because the product is the carboxylate ion, which is not attacked by the alcohol, so the reaction cannot reverse. Using excess NaOH makes sure all the ester reacts.

Fats, oils and soap

Vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol) with long-chain carboxylic (fatty) acids. A single fat molecule is a triester: three fatty-acid chains esterified to one glycerol.

Heating a fat or oil with aqueous alkali (alkaline hydrolysis, also called saponification) breaks all three ester links to give:

• soap, the salts of the long-chain carboxylic acids, and
• glycerol (propane-1,2,3-triol).

A soap molecule works because it has two ends: the ionic CO₂^- head is attracted to water (hydrophilic), while the long hydrocarbon tail mixes with grease (hydrophobic), so grease and water can be washed away together.

Biodiesel

Biodiesel is a mixture of methyl esters of long-chain carboxylic acids.

It is made by reacting a vegetable oil (a triester of glycerol) with methanol in the presence of a catalyst (a strong alkali such as NaOH or KOH). The three fatty-acid chains are transferred from glycerol onto methanol, giving three methyl ester molecules and releasing glycerol.

Note the contrast with soap: react a fat with alkali and you get soap (the carboxylate salt); react it with methanol and a catalyst and you get biodiesel (the methyl ester). Confusing these two is a classic exam trap.

Acylation: acyl chlorides and acid anhydrides

Acyl chlorides and acid anhydrides are the most reactive of the derivatives, because Cl and the OOCR group are both good leaving groups. They carry out the same functional-group changes; the only difference is the by-product:

• an acyl chloride releases HCl (misty/steamy white fumes),
• an acid anhydride releases a carboxylic acid (here ethanoic acid), with no HCl fumes.

Each of four nucleophiles (water, an alcohol, ammonia and a primary amine) attacks the carbonyl carbon and ends up bonded to it, displacing the leaving group:

With ammonia and with amines, two molecules of the nucleophile are needed: one forms the amide, and a second mops up the HCl (or the acid) as an ammonium salt. For the ammonia reaction, the mark requires NH₄Cl as the product, not HCl.

Using an acyl chloride or anhydride is a far better way to make an ester than esterification: the reaction is fast, happens at room temperature, and goes to completion (it is not reversible).

The nucleophilic addition-elimination mechanism

For acyl chlorides, you must be able to outline the mechanism with each of the four nucleophiles. It always has the same two stages, which is why it is called nucleophilic addition-elimination:

1. Addition. A lone pair on the nucleophile is donated to the δ+ carbonyl carbon. The C=O double bond becomes a C-O single bond, putting a negative charge on the oxygen. This gives a tetrahedral intermediate.
2. Elimination. The C-O bond reforms the C=O double bond, pushing out the Cl^- ion. A proton is then lost from the group that attacked.

Still don't get it? · why it is called addition-elimination

Think of the carbonyl carbon as a crowded bus seat with a double-decker passenger (the C=O) and a chloride passenger sitting next to it.

Stage 1, addition: a new passenger (the nucleophile) squeezes on. There is no room for a double-decker and a newcomer, so the top deck of the C=O folds down onto the oxygen, which now carries the leftover negative charge. The carbon is briefly bonded to four things at once. That "squeezing a new group on" is the addition.

Stage 2, elimination: the oxygen pushes its charge back up to rebuild the C=O double deck, and that shove kicks the chloride passenger off the bus as Cl-. Throwing a group off is the elimination.

In exam words: a nucleophile with a lone pair is donated to the delta-positive carbonyl carbon (addition), the C=O reforms and Cl- leaves (elimination), then a proton is lost. Name the mechanism in full: nucleophilic addition-elimination.

Making aspirin: anhydride versus acyl chloride

Aspirin is made industrially by acylating 2-hydroxybenzoic acid (salicylic acid): its -OH group is turned into an ester using ethanoic anhydride.

Although ethanoyl chloride would do the same job, industry chooses ethanoic anhydride because it is:

• less readily hydrolysed by moisture in the air, so it is easier to store and handle,
• less violent / less dangerous to react (it reacts less vigorously),
• less corrosive, because it produces ethanoic acid rather than corrosive HCl fumes, and
• cheaper and more easily recycled.

Required practical 10: preparing and purifying organic solids and liquids

RP10 is two preparations: a pure organic solid (for example aspirin) and a pure organic liquid (for example an ester).

Purifying an organic solid, recrystallisation (the order of the steps earns the marks):

1. Dissolve the impure solid in the minimum volume of hot solvent to make a saturated solution.
2. Hot filter to remove insoluble impurities (hot, so crystals do not form in the filter).
3. Cool the filtrate (in ice) so the pure product crystallises; soluble impurities stay in solution because there is too little of them to saturate it.
4. Filter under reduced pressure (Buchner/suction filtration) to collect the crystals.
5. Wash the crystals with a little cold solvent, then dry them.

Testing purity, melting point: measure the melting point and compare with a data-book value.

• A pure solid melts sharply at the data-book temperature.
• An impure solid melts lower and over a range of temperatures.
• Heat slowly near the melting point so the reading is accurate.

Purifying an organic liquid (for example an ester made by reflux): run the crude liquid through a separating funnel to wash out impurities (such as acid), dry it with a drying agent (an anhydrous salt), then purify by distillation, collecting the fraction at the product's boiling point.

Worked examples

Worked example 1: percentage yield of aspirin

5.00 g of 2-hydroxybenzoic acid (M = 138.0 g mol^-1) is reacted with excess ethanoic anhydride. The aspirin produced (M = 180.0 g mol^-1) has a mass of 5.90 g. Calculate the percentage yield.

The equation is 1 mol acid to 1 mol aspirin, and the acid is the limiting reagent (the anhydride is in excess).

Moles of 2-hydroxybenzoic acid: $n = \frac{m}{M} = \frac{5.00}{138.0} = 0.03623\ \text{mol}$

The mole ratio is 1:1, so the maximum (theoretical) moles of aspirin is also 0.03623 mol. Its theoretical mass: $m = n \times M = 0.03623 \times 180.0 = 6.522\ \text{g}$

Percentage yield compares the actual mass with this theoretical mass: $\text{percentage yield} = \frac{5.90}{6.522} \times 100 = 90.5\%$

The answer is given to 3 significant figures, matching the 3 sig figs of the data (5.00 and 5.90).

Where marks are lost: dividing by the M_r of the anhydride or of the wrong compound; and, when an answer comes out just over 100%, "correcting" it by flipping the fraction. Put the actual mass on top and the theoretical mass on the bottom, whatever the result.

Worked example 2: building a saponification equation

Write the equation for the complete alkaline hydrolysis of the triester (fat) below, where each of the three chains is the stearate chain C₁₇H₃₅COO.

Work it out in steps rather than guessing the product:

1. A fat is a triester, so it has three ester links, and each needs one NaOH: the left side is the fat + 3NaOH.
2. Each ester link breaks to give one carboxylate salt, so you get three molecules of the soap, 3C₁₇H₃₅COO^-Na⁺.
3. The three chains were all held on one glycerol, so the other product is one molecule of propane-1,2,3-triol (glycerol), C₃H₅(OH)₃.

$\text{fat} + 3\text{NaOH} \rightarrow 3\,\text{C}_{17}\text{H}_{35}\text{COO}^-\text{Na}^+ + \text{C}_3\text{H}_5(\text{OH})_3$

Where marks are lost: writing a covalent O-Na bond in the soap (it is the ionic salt C₁₇H₃₅COO^- Na⁺); forgetting the 3 in front of NaOH and the salt; and adding an extra hydrogen to the middle carbon of glycerol.

Common exam mistakes

• Defining a nucleophile as something that just "has a lone pair". The mark needs a lone pair of electrons that it can donate; examiners report students leave out "donate".
• Explaining the attack without saying the carbonyl carbon is δ+. You are expected to state the polarising effect of both the oxygen and the chlorine atoms that makes the carbon electron-deficient.
• Drawing a negative charge as well as a lone pair on a neutral nucleophile (water or an alcohol). A neutral O donates a lone pair and becomes positive, so it carries no minus sign.
• Showing the C=O breaking before the nucleophile attacks. The curly arrow must start from the nucleophile's lone pair, and every curly arrow must start at a lone pair or a bond.
• Calling the mechanism "acylation", "esterification" or "electrophilic addition-elimination". The full name is nucleophilic addition-elimination.
• For ethanoyl chloride + ammonia, giving HCl as the product instead of NH₄Cl, and forgetting that two NH₃ are needed.
• Leaving water out of the esterification equation, or using an arrow instead of ⇌ for the reversible reaction.
• Confusing ethyl butanoate with butyl ethanoate: the -yl is the alcohol part, the -oate is the acid part.
• Answering a biodiesel question in terms of alkaline hydrolysis to make soap. Biodiesel is made by reaction with methanol to give methyl esters, not with alkali to give salts.
• In a saponification equation, drawing a covalent bond from O to Na in the soap, or getting the number of molecules wrong (three salts, one glycerol, 3NaOH).
• Saying ethanoic anhydride is chosen for aspirin only because it is cheaper. When a question says "other than cost", give a chemical reason: it is less readily hydrolysed, less violent, or produces no corrosive HCl fumes.
• Recrystallisation out of order, for example washing before filtering, or omitting the hot filtration and the final wash and dry.
• Treating reflux as a separation method. Reflux returns boiled-off organic vapours to the flask so the mixture can be heated without losing volatile reagents; it does not separate anything.

Key definitions

• Weak acid - an acid that only partially dissociates (ionises) in aqueous solution.
• Nucleophile - an electron-pair donor: a species with a lone pair of electrons that it can donate to an electron-deficient (δ+) carbon.
• Esterification - the reaction of a carboxylic acid with an alcohol, in the presence of an acid catalyst, to form an ester and water.
• Hydrolysis - the breaking down of a molecule by reaction with water (or aqueous acid or alkali).
• Nucleophilic addition-elimination - a mechanism in which a nucleophile adds to the carbonyl carbon (the C=O becomes C-O), and then the C=O reforms and the leaving group (Cl^-) is eliminated.
• Saponification - the alkaline hydrolysis of a fat or oil to produce soap (salts of long-chain carboxylic acids) and glycerol.
• Soap - the salt of a long-chain carboxylic (fatty) acid.
• Biodiesel - a mixture of methyl esters of long-chain carboxylic acids, made by reacting vegetable oils with methanol in the presence of a catalyst.

Specification

• I can draw and recognise the structures of carboxylic acids, esters, acyl chlorides, acid anhydrides and amides.
• I can state that carboxylic acids are weak acids but will liberate CO₂ from carbonates, and use this as a test.
• I can write the equation for a carboxylic acid reacting with an alcohol, with an acid catalyst, to form an ester.
• I can give common uses of esters (solvents, plasticisers, perfumes, food flavourings).
• I can state that vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol).
• I can write equations for the hydrolysis of esters in acid and in alkaline conditions, naming the products.
• I can explain how alkaline hydrolysis of fats and oils gives soap and glycerol.
• I can state that biodiesel is a mixture of methyl esters of long-chain carboxylic acids, made from vegetable oils and methanol with a catalyst.
• I can write equations for the reactions of water, alcohols, ammonia and primary amines with acyl chlorides and acid anhydrides.
• I can outline the nucleophilic addition-elimination mechanism of acyl chlorides with water, alcohols, ammonia and primary amines.
• I can give the industrial advantages of ethanoic anhydride over ethanoyl chloride in making aspirin.
• I can prepare and purify a pure organic solid (recrystallisation and melting point) and a pure organic liquid (Required practical 10).

Related notes

• Alcohols
• Organic analysis
• Aldehydes and ketones (3.3.8): the nucleophilic addition the carbonyl group does when there is no leaving group.
• Amines (3.3.11): the primary amines used as nucleophiles here, and the amides formed.
• Polymers (3.3.12): esterification and amide formation scaled up into polyesters and polyamides.