A carboxylic acid and an alcohol join to make a sweet-smelling ester and water, slowly and reversibly. Hydrolyse the ester with acid and you get back an equilibrium mixture; hydrolyse it with alkali and it goes all the way, which is exactly how fats become soap.
An ester, RCOOR′, forms when a carboxylic acid reacts with an alcohol, heated with a few drops of concentrated H2SO4 as a catalyst; water is the other product and the reaction is reversible. Esters are named alkyl alkanoate: the alcohol part first, then the acid part, so CH3COOC2H5 is ethyl ethanoate. Acid hydrolysis (dilute acid, heat under reflux) is the reverse of esterification and reaches equilibrium. Alkaline hydrolysis with NaOH(aq) under reflux goes to completion, giving the carboxylate salt and the alcohol, because the carboxylate ion cannot react with the alcohol. Fats and oils are triesters of glycerol (propane-1,2,3-triol); boiling them with NaOH is saponification and gives soap. Esters are used as flavourings, perfumes and solvents.
Pick an acid and an alcohol in the simulator and watch the ester assemble. Follow the oxygen atom from the alcohol into the ester link.
| Acid | Alcohol | Ester | Name | Use or smell |
|---|---|---|---|---|
| ethanoic acid | ethanol | CH3COOC2H5 | ethyl ethanoate | solvent in glues and nail varnish remover |
| propanoic acid | methanol | CH3CH2COOCH3 | methyl propanoate | fruity smell |
| butanoic acid | ethanol | CH3CH2CH2COOC2H5 | ethyl butanoate | pineapple flavouring |
| ethanoic acid | pentan-1-ol | CH3COOC5H11 | pentyl ethanoate | pear or banana flavouring |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the name of CH3CH2COOCH3?
What is the role of concentrated sulfuric acid in esterification?
Which products form when ethyl ethanoate is heated under reflux with NaOH(aq)?
Which describes a fat?
Reflux the same ester with dilute acid and with sodium hydroxide in the simulator, and compare how far each reaction goes.
| Acid hydrolysis | Alkaline hydrolysis | |
|---|---|---|
| Reagent | dilute HCl or H2SO4 (H+ catalyst) | NaOH(aq), used up in the reaction |
| Conditions | heat under reflux | heat under reflux |
| Products from ethyl ethanoate | CH3COOH + C2H5OH | CH3COONa + C2H5OH |
| Extent | reversible, equilibrium mixture | goes to completion |
Build a triglyceride in the simulator, compare a saturated fat with an unsaturated oil, then boil it with sodium hydroxide.
Name the alcohol part first: CH3COOC2H5 is ethyl ethanoate, and HCOOCH3 is methyl methanoate. Concentrated H2SO4 is a catalyst in esterification, but in alkaline hydrolysis the NaOH is a reactant that is used up. Alkaline hydrolysis gives the salt (RCOO−Na+), not the free acid, until you acidify. One fat molecule uses three NaOH and gives three soap molecules.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Why does alkaline hydrolysis of an ester go to completion, while acid hydrolysis does not?
7.40 g of propanoic acid (Mr 74.0) reacts with excess methanol to give 5.72 g of methyl propanoate (Mr 88.0). What is the percentage yield?
8.90 g of glyceryl tristearate (Mr 890) is saponified with excess NaOH. What mass of sodium stearate (Mr 306) forms?
Ethanoic acid reacts with ethanol in which the oxygen atom is the isotope 18O. Where does the 18O end up?
This skill is now recorded on your profile. Keep the chain going.