AS & A Level Chemistry · Area 1 · AS Level Content

Carboxylic acids and derivatives

A carboxylic acid carries the carboxyl group, -COOH, and is acidic because the carboxylate ion left behind is stabilised by delocalisation of the negative charge over both oxygen atoms. This topic covers the routes that make a carboxylic acid, its reactions with metals, alkalis and carbonates, esterification and ester hydrolysis, and reduction to a primary alcohol.

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Syllabus coverage

  • Recall the reactions by which carboxylic acids can be produced: the oxidation of primary alcohols and aldehydes with acidified K2Cr2O7 or acidified KMnO4 and refluxing
  • Recall the production of a carboxylic acid by the hydrolysis of a nitrile with dilute acid or dilute alkali followed by acidification
  • Recall the production of a carboxylic acid by the hydrolysis of an ester with dilute acid or dilute alkali and heat followed by acidification
  • Describe the redox reaction of a carboxylic acid with reactive metals to produce a salt and H2(g)
  • Describe the neutralisation reaction of a carboxylic acid with alkalis to produce a salt and H2O(l)
  • Describe the acid-base reaction of a carboxylic acid with carbonates to produce a salt and H2O(l) and CO2(g)
  • Describe the esterification of a carboxylic acid with an alcohol, using concentrated H2SO4 as catalyst
  • Describe the reduction of a carboxylic acid by LiAlH4 to form a primary alcohol
  • Recall the reaction (reagents and conditions) by which esters can be produced: the condensation reaction between an alcohol and a carboxylic acid with concentrated H2SO4 as catalyst
  • Describe the hydrolysis of esters by dilute acid and by dilute alkali and heat

Part of the AS & A Level Chemistry syllabus 9701.

The reactivity of the carboxylic acid derivativesEvery derivative has an acyl group, R-C=O, joined to a group that can leave. The better that leaving group, the faster the hydrolysis.reactivity towards nucleophiles falls down the list1Acyl chlorideRCOClROClleaving groupCl-a very weak base, sothe best leaving groupRCOCl + H2O -> RCOOH + HClWater at room temperature, or just moist air: the reaction is violent, andclouds of hydrogen chloride gas escape.The C=O bond is strongly polarised by the electronegative chlorine, so thecarbon is very electrophilic.2Acid anhydride(RCO)2OROOORleaving groupRCOO-a weak base, so agood leaving group(RCO)2O + H2O -> 2 RCOOHWater at room temperature, or on warming: fast, but less violent than anacyl chloride.Both halves of the anhydride become a carboxylic acid, so no atoms arewasted.3EsterRCOOR'ROOR'leaving groupR'O-a strong base, so apoor leaving groupRCOOR' + H2O <-> RCOOH + R'OHHeat under reflux with dilute hydrochloric acid, or with aqueous sodiumhydroxide. With water alone it is slow and reversible.Hydrolysis with aqueous sodium hydroxide gives the sodium salt of the acidand the alcohol, and it goes to completion.4AmideRCONH2RONH2leaving groupNH2-a very strong base,the poorest of allRCONH2 + H2O + H+ -> RCOOH + NH4+Heat under reflux with dilute hydrochloric acid, or with aqueous sodiumhydroxide: the slowest of the four reactions.With aqueous sodium hydroxide the products are the sodium carboxylate saltand ammonia.Reactivity falls in the order acyl chloride > acid anhydride > ester > amide, and so does the ease of hydrolysis. The order follows thebase strength of the leaving group: the weaker the base, the more stable the group is on its own, and the easier it leaves. An acyl chlorideand an anhydride also react with an alcohol to give an ester, and with an amine to give an amide; the ester reacts far more slowly.
The carboxyl group -COOH and the compounds derived from it: the carboxylic acid, the carboxylate salt formed with an alkali or a carbonate, the ester formed with an alcohol and concentrated sulphuric acid, and the primary alcohol obtained by reduction with LiAlH4.

The carboxyl group, and why a carboxylic acid is acidic

A carboxylic acid contains the carboxyl group, written -COOH. It is a carbonyl group, C=O, joined to a hydroxyl group, -OH, on the same carbon atom, and it is that combination which makes the group behave as an acid. The carboxyl carbon is always C-1 of the chain, so the name ends in -oic acid: methanoic acid, HCOOH; ethanoic acid, CH3COOH; propanoic acid, CH3CH2COOH; butanoic acid, CH3CH2CH2COOH. The hydrocarbon chain behind the group is not acidic and takes no part in the reactions of the acid, although it does control solubility in water.

In water a carboxylic acid is only partly ionised, and that is what makes it a weak acid. The molecules set up an equilibrium in which a small fraction of them donate a proton to a water molecule: CH3COOH + H2O <-> CH3COO- + H3O+. What is left when the proton goes is the carboxylate ion, CH3COO-, and in that ion the negative charge is delocalised over both oxygen atoms instead of sitting on one of them. Spreading a charge lowers the energy of the ion, so the carboxylate ion is unusually stable and the equilibrium lies much further to the right than it would if the charge had nowhere else to go.

The same argument puts the acids in order of strength. A carboxylic acid is a stronger acid than phenol, phenol is stronger than water, and water is stronger than an alcohol. Ethanol also has an O-H bond, but when it loses its proton the resulting ethoxide ion, CH3CH2O-, carries the negative charge on a single oxygen atom with no C=O group next to it to help. That ion is much less stable, so ethanol releases a proton far less readily. This is not a fine distinction: ethanoic acid reacts with sodium carbonate and ethanol does not.

The physical properties follow from hydrogen bonding. Short-chain carboxylic acids such as methanoic and ethanoic acid are completely miscible with water, because the -COOH group forms hydrogen bonds to water molecules, but solubility falls as the chain lengthens, so butanoic acid is far less soluble and long-chain acids behave like oils. In the pure liquid and in the vapour, two molecules pair up through a pair of hydrogen bonds to form a dimer. That is why carboxylic acids boil at temperatures well above those of alcohols and aldehydes of similar size, and why the apparent relative molecular mass measured in the vapour is about twice the formula value.

AcidFormulaSalt formed with NaOHAnion
Methanoic acidHCOOHSodium methanoateHCOO-
Ethanoic acidCH3COOHSodium ethanoateCH3COO-
Propanoic acidCH3CH2COOHSodium propanoateCH3CH2COO-
Butanoic acidCH3CH2CH2COOHSodium butanoateCH3CH2CH2COO-
-COOH is the acid group. The charge left behind when it loses a proton is delocalised over both oxygen atoms, and that stabilised carboxylate ion is why an acid beats an alcohol by a long way.
The reactivity of the carboxylic acid derivativesEvery derivative has an acyl group, R-C=O, joined to a group that can leave. The better that leaving group, the faster the hydrolysis.reactivity towards nucleophiles falls down the list1Acyl chlorideRCOClROClleaving groupCl-a very weak base, sothe best leaving groupRCOCl + H2O -> RCOOH + HClWater at room temperature, or just moist air: the reaction is violent, andclouds of hydrogen chloride gas escape.The C=O bond is strongly polarised by the electronegative chlorine, so thecarbon is very electrophilic.2Acid anhydride(RCO)2OROOORleaving groupRCOO-a weak base, so agood leaving group(RCO)2O + H2O -> 2 RCOOHWater at room temperature, or on warming: fast, but less violent than anacyl chloride.Both halves of the anhydride become a carboxylic acid, so no atoms arewasted.3EsterRCOOR'ROOR'leaving groupR'O-a strong base, so apoor leaving groupRCOOR' + H2O <-> RCOOH + R'OHHeat under reflux with dilute hydrochloric acid, or with aqueous sodiumhydroxide. With water alone it is slow and reversible.Hydrolysis with aqueous sodium hydroxide gives the sodium salt of the acidand the alcohol, and it goes to completion.4AmideRCONH2RONH2leaving groupNH2-a very strong base,the poorest of allRCONH2 + H2O + H+ -> RCOOH + NH4+Heat under reflux with dilute hydrochloric acid, or with aqueous sodiumhydroxide: the slowest of the four reactions.With aqueous sodium hydroxide the products are the sodium carboxylate saltand ammonia.Reactivity falls in the order acyl chloride > acid anhydride > ester > amide, and so does the ease of hydrolysis. The order follows thebase strength of the leaving group: the weaker the base, the more stable the group is on its own, and the easier it leaves. An acyl chlorideand an anhydride also react with an alcohol to give an ester, and with an amine to give an amide; the ester reacts far more slowly.
The carboxyl group -COOH and the family of compounds built from it: the carboxylic acid itself, its carboxylate salt, its ester formed with an alcohol, and the primary alcohol obtained by reducing it.

Making carboxylic acids by oxidation

Primary alcohols and aldehydes are oxidised to carboxylic acids by heating them with excess acidified potassium dichromate(VI), K2Cr2O7/H2SO4, or acidified potassium manganate(VII), KMnO4/H2SO4, under reflux. The alcohol is oxidised in two steps, primary alcohol -> aldehyde -> carboxylic acid, and the aldehyde is never seen because it is attacked again as soon as it is formed.

Reflux means boiling the mixture in a flask with a condenser clamped vertically above it, so that vapour condenses and runs straight back into the flask. Nothing is lost from the mixture, so a volatile alcohol or aldehyde cannot escape and the excess oxidant carries the oxidation all the way to the acid. If the aldehyde is what you want, you set the apparatus for distillation instead and heat gently: ethanal boils near room temperature and is far more volatile than ethanol, so it distils out of the oxidising mixture before the oxidant can touch it again. Choosing reflux or distillation is choosing the product.

The oxidising agent is reduced as it works, and that colour change is the evidence for the reaction. Dichromate(VI) ions are orange and chromium(III) ions are green, so an orange mixture turns green. Manganate(VII) ions are purple and are reduced in acidic solution to almost colourless manganese(II) ions, so the purple colour is decoloured. The ionic half-equations are Cr2O7^2- + 14H+ + 6e- -> 2Cr3+ + 7H2O and MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O. In equations for the organic change it is simplest to show the oxygen picked up from the oxidising agent as [O]: CH3CH2OH + 2[O] -> CH3COOH + H2O, and CH3CHO + [O] -> CH3COOH.

Only primary alcohols and aldehydes can be taken to a carboxylic acid this way. A secondary alcohol is oxidised once, to a ketone, and stops there, because oxidising a ketone further would mean breaking a carbon-carbon bond. A tertiary alcohol is not oxidised at all by these reagents. When a question names one oxidising agent rather than the other, quote the colour change that belongs to it: orange to green for dichromate(VI), purple to colourless for manganate(VII).

Starting materialReagent and conditionsProductObservation
Propan-1-olacidified K2Cr2O7, heat under refluxPropanoic acidOrange turns green
Propanalacidified KMnO4, heat under refluxPropanoic acidPurple turns colourless
Propan-1-olacidified K2Cr2O7, heat gently and distilPropanalOrange turns green; distillate collected
Propan-2-olacidified K2Cr2O7, heat under refluxPropanoneOrange turns green
2-methylpropan-2-olacidified K2Cr2O7, heat under refluxNo reactionStays orange
Reflux to make the acid, distil to catch the aldehyde. Under reflux the aldehyde is oxidised on to the acid before it ever leaves the flask.

Making carboxylic acids by hydrolysis: nitriles and esters

A nitrile, R-CN, is hydrolysed to a carboxylic acid by heating it with dilute acid, such as dilute hydrochloric or dilute sulphuric acid. The nitrogen is turned into an ammonium ion, so the products are the acid and an ammonium salt: CH3CH2CN + 2H2O + HCl -> CH3CH2COOH + NH4Cl. With dilute alkali instead, such as dilute sodium hydroxide solution and heat, the product is the carboxylate salt, because the alkali neutralises the acid as it forms: CH3CH2CN + NaOH + H2O -> CH3CH2COONa + NH3. That mixture then has to be acidified before the free carboxylic acid appears: CH3CH2COONa + HCl -> CH3CH2COOH + NaCl.

Nitriles matter because they are how you make an acid with a longer carbon chain. A halogenoalkane heated with potassium cyanide in ethanol gives a nitrile containing one more carbon atom than the halogenoalkane, and hydrolysing that nitrile gives a carboxylic acid with one more carbon atom than you started from. Oxidation of an alcohol can never do that: it changes the functional group but leaves the carbon skeleton alone.

Esters are the other hydrolysis route, and they give back exactly the acid they were made from. Heat the ester with dilute acid, or with dilute alkali, and hydrolyse it; if the alkali was used, acidify the mixture afterwards. With dilute acid the hydrolysis is the reverse of esterification and is reversible: CH3COOC2H5 + H2O <-> CH3COOH + C2H5OH. The reacting mixture settles at an equilibrium containing ester, water, ethanoic acid and ethanol, so the conversion is never complete unless one of the products is removed or a large excess of water is used. With dilute alkali and heat the reaction goes to completion, because the sodium hydroxide converts the acid into its salt the moment it is formed and removes it from the equilibrium: CH3COOC2H5 + NaOH -> CH3COONa + C2H5OH. The immediate product is therefore the salt, and the free acid only appears after acidification.

All four of these conversions are worth holding side by side, because exam questions mix them freely: a route that changes the functional group (oxidation, reduction), a route that lengthens the chain (cyanide then hydrolysis), and a route that takes the acid back out of an ester (hydrolysis).

Route to a carboxylic acidReagents and conditionsThe point to remember
Oxidation of a primary alcoholacidified K2Cr2O7 or KMnO4, heat under refluxThe aldehyde formed first is oxidised straight on to the acid
Oxidation of an aldehydeacidified K2Cr2O7 or KMnO4, heat under refluxThe carbon chain is unchanged
Hydrolysis of a nitriledilute acid and heat, or dilute alkali and heat then acidifyThe chain has already been lengthened by one carbon using KCN
Hydrolysis of an esterdilute acid and heat, or dilute alkali and heat then acidifyThe reverse of esterification; the alkali route goes to completion
Nitrile or ester, if you hydrolyse it with alkali you must acidify before you can write down the carboxylic acid. Until then you have only the salt.

Reactions of the carboxyl group: metals, alkalis and carbonates

A carboxylic acid reacts with a reactive metal such as magnesium, zinc or sodium in a redox reaction that gives a salt and hydrogen: 2CH3COOH + Mg -> (CH3COO)2Mg + H2. The metal is oxidised to its 2+ ion while the hydrogen of the -COOH group is reduced to H2 gas. You see steady effervescence as the metal dissolves, and a lighted splint held in the gas gives a squeaky pop, which identifies the hydrogen. Because ethanoic acid is a weak acid its concentration of hydrogen ions is low, so this reaction is noticeably slower than the same reaction with hydrochloric acid of equal concentration.

With an alkali, the acid is neutralised to a salt and water: CH3COOH + NaOH -> CH3COONa + H2O. In ionic terms the reaction is H+ + OH- -> H2O, which is exactly what happens when a solution of ethanoic acid is titrated against sodium hydroxide solution using phenolphthalein. The salts are named by swapping the -oic acid ending for -oate: ethanoic acid gives ethanoate, propanoic acid gives propanoate, methanoic acid gives methanoate.

With a carbonate, the acid gives a salt, water and carbon dioxide: 2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2. The mixture fizzes briskly, and the gas turns limewater milky, which is the standard test for carbon dioxide. This is the quickest chemical test that separates a carboxylic acid from an alcohol or a phenol: sodium carbonate solution gives no reaction at all with ethanol, and phenol does not fizz either, because neither is acidic enough to displace carbon dioxide from a carbonate.

One pattern runs through all three reactions. The carboxylic acid behaves as a proton donor, so two molecules of acid are needed for each carbonate ion, which accepts two protons, and for each magnesium atom, which loses two electrons. Writing CH3COOH + Na2CO3 -> CH3COONa + H2O + CO2, with the wrong ratio, is one of the commonest ways to lose marks on an otherwise correct equation.

ReagentProducts from ethanoic acidWhat you observe
Magnesium ribbonMagnesium ethanoate and hydrogenSteady effervescence, the metal disappears, lighted splint pops
Sodium hydroxide solutionSodium ethanoate and waterNo effervescence; the mixture warms and the pH rises towards 7
Sodium carbonate solutionSodium ethanoate, water and carbon dioxideBrisk effervescence; the gas turns limewater milky
Fizzing with sodium carbonate means a carboxylic acid. Alcohols and phenols do not react with carbonates, so this single test tells the three apart.

Esterification: the condensation reaction with an alcohol

An ester is made by esterification, a condensation reaction between a carboxylic acid and an alcohol in which the two molecules join together and a molecule of water is eliminated: CH3COOH + C2H5OH <-> CH3COOC2H5 + H2O. The reaction is slow on its own, so a few drops of concentrated sulphuric acid are added as a catalyst, and the mixture is warmed or heated under reflux. The concentrated sulphuric acid also absorbs water, and removing a product shifts an equilibrium to the right, so more ester is formed than the reversible equation on its own would suggest.

Esters are named with the alcohol part first and the acid part second, the acid ending changing from -oic acid to -oate. Ethanol with ethanoic acid gives ethyl ethanoate; methanol with propanoic acid gives methyl propanoate; propan-1-ol with methanoic acid gives propyl methanoate. The ester link is -COO-, and it is this same link, repeated many times along a chain, that forms a polyester such as PET.

Esters are usually volatile, sweet-smelling liquids, and they are much less soluble in water than the acids they came from, because an ester has no O-H group to hydrogen bond with water. Those two properties explain their uses: ethyl ethanoate and pentyl ethanoate (better known as amyl acetate, which smells of pears) are solvents in glues, printing inks and nail varnish remover, while short-chain esters supply the characteristic smell of many fruits. Fats and oils are natural esters of glycerol with long-chain carboxylic acids, and the smell of a fruit comes from the mixture of esters it contains.

The reaction is reversible, so the yield is never 100 per cent. Practical ways to improve it are to use an excess of the cheaper reactant, usually the alcohol, and to remove the water as it is formed, either by using concentrated sulphuric acid as a dehydrating agent or by distilling the ester out of the mixture.

AlcoholCarboxylic acidEsterFormula
MethanolEthanoic acidMethyl ethanoateCH3COOCH3
EthanolEthanoic acidEthyl ethanoateCH3COOC2H5
Propan-1-olEthanoic acidPropyl ethanoateCH3COOC3H7
EthanolPropanoic acidEthyl propanoateCH3CH2COOC2H5
Alcohol first, acid second, -oate at the end. Ethanol and ethanoic acid give ethyl ethanoate: swap the two halves round and the name is wrong.

Hydrolysis of esters: dilute acid against dilute alkali

Ester hydrolysis is the reverse of esterification, and the reagent you choose decides whether it ever finishes. With dilute acid and heat, for example dilute hydrochloric or dilute sulphuric acid, the ester splits back into the carboxylic acid and the alcohol: CH3COOC2H5 + H2O <-> CH3COOH + C2H5OH. The acid is only a catalyst and does not remove the carboxylic acid as it is formed, so the reaction is reversible and the mixture comes to an equilibrium containing all four substances. You can push the position of equilibrium to the right by using a large excess of water, but you cannot drive it to completion.

With dilute alkali and heat, such as sodium hydroxide solution, the hydrolysis goes to completion: CH3COOC2H5 + NaOH -> CH3COONa + C2H5OH. The reason is simply that sodium hydroxide neutralises the ethanoic acid the moment it is produced, converting it into sodium ethanoate. Removing a product from a reversible reaction pulls the equilibrium over to the right, and here the acid is removed as fast as it appears, so the reaction runs until the ester is used up. The product of alkaline hydrolysis is therefore the carboxylate salt, not the free acid; add dilute acid afterwards if the carboxylic acid itself is wanted.

The most important example of alkaline hydrolysis is soap making. Oils and fats are triglycerides, which are esters of glycerol with three long-chain carboxylic acids such as palmitic and stearic acid. Boiling them with concentrated sodium hydroxide solution hydrolyses all three ester links and gives glycerol together with the sodium salts of the long-chain acids. Those sodium carboxylates are soap, and they clean because each molecule has two ends with different jobs: the long hydrocarbon tail dissolves in grease and oil, while the ionic carboxylate head dissolves in water, so the grease is lifted off the surface and carried away with the rinse water. The old name for this hydrolysis is saponification.

Fats and vegetable oils can also be hydrolysed with acid, and that route gives the free long-chain carboxylic acids directly, which are then used as starting materials for soaps and detergents. The choice between acid and alkali hydrolysis is therefore a choice between wanting the free acid and wanting the salt.

Hydrolysis with dilute acidHydrolysis with dilute alkali
Reagents and conditionsdilute HCl or H2SO4, heatdilute NaOH, heat
ProductsCarboxylic acid and alcoholSodium carboxylate (a salt) and alcohol
Does it go to completion?No: reversible, an equilibrium mixture formsYes: the alkali neutralises the acid and pulls the reaction over
To obtain the free acidIt is already thereAcidify the mixture afterwards
Acid hydrolysis is reversible and stops at an equilibrium; alkaline hydrolysis goes to completion because the alkali removes the acid as its salt. Acidify afterwards to recover the carboxylic acid.

Reduction to a primary alcohol, and choosing a route

The carboxyl group can also be reduced. Lithium tetrahydridoaluminate, LiAlH4, in dry ether reduces a carboxylic acid all the way to a primary alcohol: CH3COOH + 4[H] -> CH3CH2OH + H2O. The reaction is a reduction because the oxidation state of the carboxyl carbon falls as the C=O double bond is turned into a -CH2OH group. LiAlH4 has to be used in dry ether because it reacts vigorously, sometimes violently, with water, so the apparatus must be dry, water must be kept out of the reaction, and there must be no flame near the ether.

LiAlH4 is a far stronger reducing agent than sodium tetrahydridoborate, NaBH4, the reagent used to reduce aldehydes and ketones. NaBH4 will not reduce a carboxylic acid or an ester at all: the carboxyl carbon is bonded to two oxygen atoms and needs the more powerful reagent. This is a favourite exam distinction, so learn it as a pair. To turn an aldehyde or a ketone into an alcohol, NaBH4 in water or aqueous ethanol is enough; to turn a carboxylic acid into a primary alcohol, the answer is LiAlH4 in dry ether.

The routes now fit together into a map. To go from a primary alcohol or an aldehyde to a carboxylic acid with the same number of carbon atoms, oxidise it under reflux with acidified dichromate(VI) or acidified manganate(VII). To make an acid with one carbon atom more than a halogenoalkane, heat it with potassium cyanide in ethanol and then hydrolyse the nitrile. To turn a carboxylic acid back into a primary alcohol, reduce it with LiAlH4 in dry ether. And to move between an acid and an ester, condense with an alcohol and concentrated sulphuric acid, or hydrolyse with dilute acid or dilute alkali.

Two warnings about the map. Oxidation and reduction only change the oxidation level of the functional group and never change the number of carbon atoms, so if a question asks for a product with more carbon atoms the answer must involve the cyanide step. And the hydrolysis of a nitrile or an ester in alkali gives the salt, so read the question carefully: if it asks for the acid, remember the acidification step.

Starting materialReagents and conditionsProduct
Primary alcohol or aldehydeacidified K2Cr2O7 or KMnO4, heat under refluxCarboxylic acid with the same number of carbon atoms
Carboxylic acidLiAlH4 in dry etherPrimary alcohol
Carboxylic acid and an alcoholconcentrated H2SO4 as catalyst, warmEster and water
Esterdilute acid and heat (reversible), or dilute alkali and heat then acidifyCarboxylic acid and alcohol
HalogenoalkaneKCN in ethanol, then hydrolyse the nitrile and acidifyCarboxylic acid with one more carbon atom
LiAlH4 in dry ether takes a carboxylic acid to a primary alcohol; NaBH4 cannot. Keep the two reducing agents apart, and keep water away from the LiAlH4.

Key terms you must be able to define

Carboxylic acid
A compound containing the carboxyl group, -COOH, in which a carbonyl group and a hydroxyl group are attached to the same carbon atom; it is a weak acid because it donates a proton to water to leave the stabilised carboxylate ion.
Carboxylate ion
The negative ion, RCOO-, left when a carboxylic acid loses a proton; it is stabilised because the negative charge is delocalised over both oxygen atoms, and it is the reason a carboxylic acid is a much stronger acid than an alcohol.
Ester
A compound containing the ester link -COO-, formed when a carboxylic acid condenses with an alcohol and a molecule of water is eliminated; named with the alcohol part first and the acid part changed to -oate.
Esterification
The condensation reaction between a carboxylic acid and an alcohol with concentrated sulphuric acid as catalyst, forming an ester and water; it is reversible, so the yield is improved by removing water or using an excess of one reactant.
Condensation reaction
A reaction in which two molecules join together with the elimination of a small molecule, usually water; esterification is a condensation, and so is the formation of a polyester such as PET.
Hydrolysis
The breaking of a bond by reaction with water; heating an ester with dilute acid hydrolyses it reversibly to the carboxylic acid and alcohol, while heating it with dilute alkali hydrolyses it to the carboxylate salt and alcohol and goes to completion.
Reflux
Boiling a reaction mixture with a condenser fitted vertically so that vapour condenses and runs back into the flask; used with acidified K2Cr2O7 or acidified KMnO4 to oxidise a primary alcohol all the way to a carboxylic acid.
Nitrile
A compound containing the group -CN; it is made from a halogenoalkane and potassium cyanide, so it has one more carbon atom, and hydrolysis with dilute acid or dilute alkali (then acidification) turns it into a carboxylic acid.
Lithium tetrahydridoaluminate
LiAlH4, a strong reducing agent used in dry ether, which reduces a carboxylic acid to a primary alcohol; it is much stronger than NaBH4, which reduces only aldehydes and ketones.

Applied to the real world

Cambridge sets 9701 in named contexts - an industrial plant, a laboratory, a hospital, a car battery, a water treatment works. Use these as models for how to apply the chemistry, then write your own.

In a laboratory

Ethanol is oxidised to ethanoic acid by heating it under reflux with excess acidified potassium dichromate(VI). The orange solution turns green as the dichromate(VI) ion is reduced to the green chromium(III) ion. The condenser is mounted vertically so that ethanol and ethanal vapour condense and fall back into the oxidising mixture instead of escaping, which is what guarantees that the oxidation goes past the aldehyde all the way to the acid.

In an industrial plant

Most ethanoic acid is made by the carbonylation of methanol rather than by oxidising ethanol. Methanol reacts with carbon monoxide over a rhodium catalyst (the Monsanto process) or an iridium catalyst (the Cativa process) to give ethanoic acid, and both routes are prized for being selective and high-yielding from cheap starting materials. The carbon monoxide comes from natural gas or coal.

In a soap works

Oils and fats are triglycerides, esters of glycerol with three long-chain carboxylic acids such as palmitic and stearic acid. Boiling them with concentrated sodium hydroxide solution hydrolyses every ester link and gives glycerol plus the sodium salts of those long-chain acids. Those sodium carboxylates are soap, and they work because the long hydrocarbon tail dissolves in grease while the carboxylate head dissolves in water, so the grease is lifted off and rinsed away.

In a food factory

Ethanoic acid is the acid of vinegar, made when the ethanol in wine or cider is oxidised by Acetobacter bacteria. In food production it is the acid used to lower the pH of pickles and sauces so that spoilage microorganisms cannot grow, and its salts are used as preservatives: sodium ethanoate is added to crisps and calcium propanoate to bread, where it slows the growth of mould.

In the home

Ethyl ethanoate and pentyl ethanoate are the esters you meet in solvents and flavourings. Pentyl ethanoate, still often called amyl acetate, smells strongly of pears and is used in fruit flavourings and in nail varnish remover, and ethyl ethanoate is a solvent in glues, printing inks and paint thinners. They are chosen because they dissolve oils and resins, evaporate quickly and have a far less unpleasant smell than the solvents they replace.

Flashcards

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Q1What is the functional group of a carboxylic acid, and how does the name end?
The carboxyl group, -COOH: a carbonyl group and a hydroxyl group on the same carbon atom. The name ends in -oic acid, and the carboxyl carbon is C-1.
Q2Give the reagents and conditions for oxidising a primary alcohol to a carboxylic acid.
Acidified potassium dichromate(VI), K2Cr2O7/H2SO4, or acidified potassium manganate(VII), KMnO4/H2SO4, with excess oxidant, heated under reflux.
Q3How do you stop the oxidation of a primary alcohol at the aldehyde?
Heat gently and distil the aldehyde out of the mixture as it forms. It boils well below the alcohol, so it leaves the flask before the excess oxidant can attack it again.
Q4What colour change shows that acidified potassium dichromate(VI) is oxidising an alcohol?
Orange to green, as the dichromate(VI) ion is reduced to the green chromium(III) ion. Acidified manganate(VII) is decolourised from purple.
Q5How is a nitrile converted into a carboxylic acid?
Heat it with dilute acid to give the acid and an ammonium salt, or with dilute alkali to give the carboxylate salt and ammonia and then acidify the mixture.
Q6Why is the nitrile route useful for making a carboxylic acid?
Because a halogenoalkane heated with KCN in ethanol gives a nitrile with one more carbon atom, so hydrolysis gives a carboxylic acid with a longer chain than the halogenoalkane you started from.
Q7Write the equation for ethanoic acid reacting with magnesium.
2CH3COOH + Mg -> (CH3COO)2Mg + H2. Effervescence is seen, and the gas gives a squeaky pop with a lighted splint.
Q8Write the equation for ethanoic acid reacting with sodium carbonate, and state what is seen.
2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2. Brisk effervescence of carbon dioxide, which turns limewater milky.
Q9Which single test tells a carboxylic acid apart from an alcohol or a phenol?
Adding sodium carbonate solution: the carboxylic acid fizzes strongly because carbon dioxide is released, while an alcohol and a phenol give no reaction.
Q10Give the reagents and conditions for making an ester.
A carboxylic acid and an alcohol warmed or heated under reflux with concentrated sulphuric acid as catalyst; the reaction is a reversible condensation that also produces water.
Q11How is an ester named?
The alcohol part comes first and the acid part second with its ending changed to -oate: ethanol with ethanoic acid gives ethyl ethanoate, ethanol with propanoic acid gives ethyl propanoate.
Q12What is the difference between the acid and the alkaline hydrolysis of an ester?
Dilute acid and heat is reversible and gives the carboxylic acid and the alcohol in an equilibrium mixture. Dilute alkali and heat goes to completion and gives the carboxylate salt and the alcohol, because the alkali removes the acid as its salt; acidify afterwards to obtain the acid.
Q13Which reagent reduces a carboxylic acid to a primary alcohol, and which reagent will not?
LiAlH4 in dry ether reduces it, giving a primary alcohol. NaBH4 will not reduce a carboxylic acid, only an aldehyde or a ketone.
Q14Why is ethanoic acid a stronger acid than ethanol?
Because the carboxylate ion left behind is stabilised by delocalisation of the negative charge over both oxygen atoms, whereas the ethoxide ion carries its charge on a single oxygen atom and is much less stable.
0 of 14 marked as known

Exam-style questions

8 questions, 26 marks. Write your answer, then reveal the mark scheme and tick each point you actually made. Answering honestly here is what makes the rest of the site useful.

StateAO12 marks
Butan-1-ol can be converted into butanoic acid. State the reagent and the condition needed for this conversion.

Mark scheme — tick each point you actually made

Examiner tip: The condition carries a mark here. 'Reflux' is the word the examiner wants, because a primary alcohol heated under reflux with excess oxidant is oxidised past the aldehyde all the way to the acid.
DescribeAO12 marks
An aldehyde is warmed with acidified potassium manganate(VII). Describe what is seen and name the type of organic product formed.

Mark scheme — tick each point you actually made

Examiner tip: Say that the colour is lost, not that it turns green. Green belongs to dichromate(VI) being reduced to chromium(III); manganate(VII) in acidic solution becomes almost colourless.
ExplainAO23 marks
Explain why ethanoic acid is a much stronger acid than ethanol.

Mark scheme — tick each point you actually made

Examiner tip: The mark is for delocalisation over both oxygen atoms. Statements about the number of hydrogen atoms or the size of the molecule score nothing.
SuggestAO23 marks
Propanenitrile is heated separately with (i) dilute hydrochloric acid and (ii) dilute sodium hydroxide solution. Explain why, in case (ii), the mixture must be acidified before propanoic acid can be obtained.

Mark scheme — tick each point you actually made

Examiner tip: Write the species you actually have at each stage. Alkaline hydrolysis gives the salt, and only the acidification step converts it into the acid, so an answer that is satisfied with sodium propanoate misses the point.
CalculateAO23 marks
A 25.0 cm3 sample of vinegar, which is a solution of ethanoic acid, was titrated with 0.100 mol dm-3 sodium hydroxide solution. The volume required to reach the end point was 22.50 cm3. (a) Calculate the concentration of ethanoic acid in the vinegar in mol dm-3. (b) Calculate the mass concentration of ethanoic acid in the vinegar in g dm-3. (Ar: C = 12.0, H = 1.0, O = 16.0)

Mark scheme — tick each point you actually made

Examiner tip: Convert the volumes from cm3 to dm3 before you divide. 25.0 cm3 is 0.0250 dm3, not 25 dm3, and the answer should be given to three significant figures.
StateAO24 marks
Ethanoic acid reacts with (i) magnesium ribbon and (ii) sodium carbonate solution. For each reaction, state the gas produced and name the organic salt formed.

Mark scheme — tick each point you actually made

Examiner tip: Keep the two salts apart: the metal in the salt comes from the reagent that attacked the acid, so magnesium gives magnesium ethanoate and sodium carbonate gives sodium ethanoate. Both reactions also produce water.
CompareAO24 marks
Ethyl ethanoate is heated with dilute hydrochloric acid in one experiment and with dilute sodium hydroxide solution in another. Compare the products of the two hydrolyses and explain why only one of them goes to completion.

Mark scheme — tick each point you actually made

Examiner tip: Both halves are needed: the products of each hydrolysis, and the equilibrium reason. Remember that the alkaline product is the salt unless the mixture is acidified afterwards.
DescribeAO35 marks
Ethyl ethanoate can be prepared in the laboratory from ethanol and ethanoic acid. Describe how the reaction is carried out and how a pure sample of the ester is obtained.

Mark scheme — tick each point you actually made

Examiner tip: Describe the work-up in order: neutralise, separate, dry, distil. Marks are lost by dissolving the ester in water at the start of the work-up, because that reverses the hydrolysis and sends the ester back to the acid.

Exam technique for this topic

  • When a question asks for the reagents and conditions for the oxidation of an alcohol to a carboxylic acid, give all three parts: acidified potassium dichromate(VI) (or acidified potassium manganate(VII)), excess oxidant, and heat under reflux. 'Dichromate' without 'acidified', or reflux replaced by 'warming', loses a mark.
  • Match the colour change to the oxidising agent you name: dichromate(VI) goes orange to green and manganate(VII) goes purple to colourless. Quoting the wrong pair is a straight loss, and the change for manganate(VII) is easy to get wrong because it is decolourised, not turned to a colour.
  • Watch the reagent in an ester hydrolysis question. Dilute acid gives the carboxylic acid and the alcohol in a reversible equilibrium; dilute alkali gives the salt and the alcohol and goes to completion. If the question names the sodium salt, write sodium ethanoate and not ethanoic acid, and remember the acidification step when the free acid is asked for.
  • Naming an ester is easy marks that are easily thrown away: the alcohol part comes first and the acid part ends in -oate, so ethanol and propanoic acid give ethyl propanoate. Keep the carbon figures straight too: propanoate has three carbon atoms in the acid chain.
  • Balance every acid equation by counting protons. Each carboxylic acid molecule supplies one proton, so two are needed for Mg, and two for each carbonate ion: 2CH3COOH + Mg -> (CH3COO)2Mg + H2, and 2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2.
  • In an explanation of acid strength, the mark is for the delocalisation of the negative charge over both oxygen atoms in the carboxylate ion. Saying the acid 'has a double bond', or 'is a bigger molecule', or 'is more polar' scores nothing, however true the words sound.

Why students lose marks here

A student wroteA student wrote: 'Heating propan-1-ol with acidified potassium dichromate(VI) under reflux gives propanal.'
Why it loses marksPropanal is what you get when the product is distilled out of the mixture as it forms. Under reflux nothing escapes, so the propanal that is made first stays in contact with the excess oxidant and is oxidised again, and the final product is the carboxylic acid.
The answer that scoresUnder reflux with excess acidified dichromate(VI) the oxidation runs through both steps, primary alcohol -> aldehyde -> carboxylic acid, so propan-1-ol gives propanoic acid. To stop at the aldehyde you heat gently and distil the propanal off as it is formed, because an aldehyde boils well below the alcohol it came from.
A student wroteA student wrote: 'Ethyl ethanoate hydrolyses in dilute sodium hydroxide solution to give ethanoic acid and ethanol.'
Why it loses marksThe dilute alkali reacts with the carboxylic acid as it is formed, converting it into its salt. The free acid is not the product of alkaline hydrolysis, and this is the point at issue whenever a question specifies alkali rather than acid.
The answer that scoresEthyl ethanoate with dilute sodium hydroxide and heat gives sodium ethanoate and ethanol, and the hydrolysis goes to completion because the alkali removes the carboxylic acid from the equilibrium as its salt. To obtain ethanoic acid itself the mixture must be acidified afterwards. Acid hydrolysis, by contrast, is reversible and gives ethanoic acid and ethanol in an equilibrium mixture.
A student wroteA student wrote: 'Carboxylic acids are weaker acids than alcohols, because they contain more hydrogen atoms that the molecule can lose.'
Why it loses marksAcid strength depends on how readily the proton is released, and that in turn depends on how stable the ion left behind is, not on the number of hydrogens. The carboxylate ion is stabilised by delocalisation of the negative charge over both oxygen atoms, which makes the carboxylic acid much stronger, not weaker.
The answer that scoresA carboxylic acid is a stronger acid than an alcohol. Ethanoic acid donates a proton to water to give the ethanoate ion, in which the negative charge is delocalised over both oxygen atoms; ethanol gives the ethoxide ion, in which the charge sits on a single oxygen atom, so that ion is far less stable and the equilibrium lies much further to the left. That is why ethanoic acid fizzes with sodium carbonate and ethanol does not react with it.
A student wroteA student wrote: 'Ethanoic acid is reduced to ethanol by warming it with sodium tetrahydridoborate, NaBH4.'
Why it loses marksNaBH4 is a mild reducing agent and will not reduce a carboxylic acid, whose carbon is bonded to two oxygen atoms and needs a stronger reagent. NaBH4 reduces aldehydes and ketones only.
The answer that scoresEthanoic acid is reduced to ethanol, a primary alcohol, by lithium tetrahydridoaluminate, LiAlH4, in dry ether: CH3COOH + 4[H] -> CH3CH2OH + H2O. Dry ether is essential because LiAlH4 reacts violently with water, so the apparatus must be dry and no flame may be used near the ether. NaBH4 would be the correct answer only for an aldehyde or a ketone.
A laboratory bench set up for the reduction of a carboxylic acid: the dry apparatus, the addition of LiAlH4 in dry ether, and the condenser used to heat the mixture under reflux with water carefully excluded.

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