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Organic acids, bases and esters
Carboxylic acids as weak organic acids and amines as weak organic bases, how to tell them apart, and esterification: naming esters, making them under reflux in the school lab, increasing the yield, and purifying the product.
The Kakadu plum, gubinge to the Bardi and Nyul Nyul people of the Kimberley, has been eaten and used as a medicine for thousands of years. It turns out to be the richest natural source of vitamin C ever measured, often more than 50 times the concentration in an orange. Vitamin C is an organic acid. So is the citric acid in its flesh, the ethanoic acid in vinegar and the lactic acid in yoghurt. Organic bases are just as common: they give fish its smell. And when an organic acid meets an alcohol, it makes an ester, the family of compounds behind the smell of bananas, pineapples and pears.
Organic acids
Carboxylic acids are weak acids: in water only a small fraction of molecules donate the H⁺ from their –COOH group (about 1% for 0.1 mol/L ethanoic acid).
CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq) $K_a = 1.74 \times 10^{-5}$
The ethanoate ion is stabilised because its negative charge is shared over both oxygen atoms. Carboxylic acids show typical acid reactions, which is how you recognise them:
- with bases: CH₃COOH + NaOH → CH₃COONa + H₂O (sodium ethanoate)
- with carbonates and hydrogencarbonates (fizzing): CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
- with reactive metals: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
Electronegative atoms near the carboxyl group pull electron density away and make the acid stronger: chloroethanoic acid is about 75 times stronger than ethanoic acid. Alcohols contain O–H too, but they are not acidic in water, because the alkoxide ion has no way to spread its charge.
Organic bases
Amines are weak bases. Like ammonia, the nitrogen has a lone pair of electrons that can accept a proton from water:
CH₃NH₂(aq) + H₂O(l) ⇌ CH₃NH₃⁺(aq) + OH⁻(aq)
Amines react with acids to form salts: CH₃NH₂ + HCl → CH₃NH₃⁺Cl⁻ (methylammonium chloride). These salts are ionic, non-volatile and odourless, which is why a squeeze of lemon juice takes the fishy smell off seafood: the citric acid converts volatile amines into salts. Many medicines that contain amine groups are sold as hydrochloride salts because they dissolve better in water. Amides, by contrast, are almost neutral: the nitrogen's lone pair is drawn towards the neighbouring C=O, so it is not available to accept a proton.
| Organic acid (carboxylic acid) | Organic base (amine) | |
|---|---|---|
| Functional group | –COOH | –NH₂, –NH–, –N< |
| Brønsted–Lowry role | proton donor | proton acceptor (lone pair on N) |
| Aqueous solution | pH below 7; blue litmus turns red | pH above 7; red litmus turns blue |
| With NaHCO₃ | fizzes, releasing CO₂ | no reaction |
| With HCl | no reaction | forms an ammonium salt |
| Typical smell | sharp or rancid (vinegar, butter) | fishy, ammonia-like |
Esters
An ester forms when a carboxylic acid reacts with an alcohol. The OH from the acid and the H from the alcohol's OH leave as water, so this is a condensation reaction, called esterification. It is slow and reversible.
CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O (conc. H₂SO₄ catalyst, heat under reflux)
Naming: the alkyl group from the alcohol comes first, then the acid name with -oic acid changed to -oate. Ethanol + ethanoic acid gives ethyl ethanoate (nail-polish remover). Methanol + butanoic acid gives methyl butanoate (apple). Pentan-1-ol + ethanoic acid gives pentyl ethanoate (pear and banana). Ethanol + methanoic acid gives ethyl methanoate (rum and raspberry). When reading an ester formula, the part bonded to the C=O carbon came from the acid.
Esters have no O–H, so they can't hydrogen bond to each other. That gives them lower boiling points than the acids and alcohols they are made from, which makes them volatile and fragrant.
Name CH₃CH₂COOCH₃ and the acid and alcohol used to make it.
Making an ester in the lab
Mix the carboxylic acid and the alcohol in a round-bottom flask, add a few drops of concentrated sulfuric acid and some boiling chips, and heat under reflux for 30–60 minutes.
- Why heat? Esterification is very slow at room temperature; heating increases the rate.
- Why reflux? The reactants and the ester are volatile and flammable. A vertical condenser cools the vapours so they condense and run back into the flask, so the mixture can be heated for a long time without losing material or releasing flammable vapour.
- Why sulfuric acid? It is a catalyst that speeds up both directions. Because it is concentrated and hygroscopic, it also absorbs a little of the water formed, nudging the equilibrium towards the ester.
- Safety: heat with a heating mantle or water bath, never a flame; never stopper the top of the condenser, or pressure builds up; water enters the condenser at the bottom and leaves at the top so the jacket stays full; boiling chips prevent violent "bumping"; conc. H₂SO₄ is corrosive, so wear goggles and gloves.
Increasing the yield
With equal amounts of acid and alcohol, the equilibrium yield is only about two-thirds. By Le Chatelier's principle, using an excess of the cheaper reactant (usually the alcohol), or removing water or ester as it forms, shifts the equilibrium to the right. With three moles of ethanol per mole of ethanoic acid, the yield rises to about 90%.
Purifying the ester
The flask now contains ester, water, unreacted acid and alcohol, and sulfuric acid.
- Cool the mixture and pour it into a separating funnel with sodium carbonate solution. This neutralises the acids (it fizzes, so vent the funnel often) and turns them into water-soluble salts. Unreacted small alcohols also dissolve in the water.
- Let the layers separate. The ester is less dense than water and doesn't mix with it, so it forms the top layer. Drain off and discard the lower aqueous layer.
- Dry the ester layer with anhydrous magnesium sulfate or calcium chloride, then filter.
- Distil, collecting the fraction that boils at the ester's boiling point (77 °C for ethyl ethanoate).
- Make ethyl ethanoate with a 1 : 1 ratio under reflux with the catalyst. What yield does it level off at?
- Take the catalyst out. Does the final yield change, or only how fast you get there?
- Change to 3 : 1 alcohol to acid. What is the new yield? Use Le Chatelier's principle to explain.
- Heat in an open beaker instead of under reflux. What happens to the amount of ester after a while, and why?
- Make pentyl ethanoate and methyl butanoate. What does each smell of?
Test yourself
Which ester is formed from propan-1-ol and ethanoic acid?
Alcohol part first (propyl), then the acid as -oate (ethanoate).
The main purpose of heating under reflux in esterification is to
The condenser returns vapours to the flask.
Which observation distinguishes propanoic acid from propan-1-amine?
Acids release CO₂ from hydrogencarbonates; amines are bases.
The crude ester is washed with sodium carbonate solution to
Na₂CO₃ reacts with H₂SO₄ and the carboxylic acid, forming water-soluble salts.
Label the reflux apparatus.
Put the steps for making and purifying ethyl ethanoate in order.
Carboxylic acids are acids because they only partly ionise in water. Amines act as bases because the nitrogen has a lone that accepts a proton. An acid and an alcohol react to form an ester and water in a reaction. Concentrated sulfuric acid acts as a . Using an excess of alcohol increases the of ester.
Ethyl butanoate (pineapple flavour) is made in a school laboratory from butanoic acid and ethanol. Write the equation, and explain the choice of conditions used to make the ester and to obtain a good yield.
CH₃CH₂CH₂COOH + CH₃CH₂OH ⇌ CH₃CH₂CH₂COOCH₂CH₃ + H₂O
Esterification is slow at room temperature, so the mixture is heated, and a few drops of concentrated sulfuric acid are added as a catalyst to increase the rate of both the forward and reverse reactions. Because the acid is hygroscopic it also absorbs some of the water formed. Ethanol and the ester are volatile and flammable, so the mixture is heated under reflux: vapours condense in the vertical condenser and return to the flask, allowing heating for 30–60 minutes without losing reactants or product, and with a heating mantle or water bath rather than a flame. The reaction reaches equilibrium with only about a two-thirds yield from equal amounts, so an excess of ethanol, the cheaper reactant, is used. By Le Chatelier's principle, increasing the concentration of a reactant shifts the equilibrium to the right, increasing the yield of ethyl butanoate.