Carboxylic Acids and Derivatives

Section 1 of 19Carboxylic acids

Carboxylic acids

Functional group

• Functional group the carboxyl group, –COOH, with general formula C_nH_2n+1COOH (RCOOH)

Naming

• Naming alkane stem + "oic acid", with the carboxyl carbon always carbon 1, so no number is needed, such as propanoic acid

Acidity

• Acidity they partially ionise, so they are weak acids

Test

• Test they react with sodium carbonate or hydrogencarbonate to give carbon dioxide: 2RCOOH + Na₂CO₃ → 2RCOO^-Na⁺ + CO₂ + H₂O

Section 2 of 19Esters: preparation and naming

Preparation

• Preparation a condensation reaction between an alcohol and a carboxylic acid with a concentrated strong acid catalyst, called esterification

Condensation

• Condensation a reaction that eliminates a small molecule, here water

Naming

• Naming replace the -oic acid ending of the acid with -oate, and put the alkyl group from the alcohol first, such as propyl ethanoate

Note

• Note the name is written backwards from the structure and comes from the alcohol and acid used

Section 3 of 19Uses of esters: fragrances and test

Uses of esters: fragrances and test

Fragrances and flavourings

• Fragrances and flavourings esters smell fruitier and sweeter than aldehydes and ketones, are responsible for many natural scents,
  • and are used as artificial fruit flavours,
  • such as ethyl methanoate for raspberry

Complex mixtures

• Complex mixtures natural aromas and tastes often come from mixtures of esters

Test for an alcohol (or acid)

• Warm the unknown with ethanoic acid and concentrated sulfuric acid.
• Remove excess acid with warm aqueous sodium carbonate.
• A sweet, fruity smell confirms the alcohol, as the warm solution makes the ester evaporate.

Section 4 of 19Uses of esters: plasticisers and solvents

Uses of esters: plasticisers and solvents

Plasticisers

• PVC is strong and rigid and makes drainpipes and guttering.
• Adding an ester plasticiser makes it soft and pliable, for cling film.
• The ester penetrates between chains, increasing their separation, weakening the polar effects of the C–Cl bond and reducing rigidity so chains slide.

Solvents

• Ethyl ethanoate is a common solvent with low toxicity, low volatility (boiling point 77 °C) and low cost.
• It is used in glues, fragrances and nail varnishes.

Section 5 of 19Naturally occurring esters

Triglycerides

• Triglycerides animal fats and olive oil are tri-esters of glycerol and fatty acids (long-chain carboxylic acids such as stearic acid, CH₃(CH₂)₁₆COOH)
  • (propane-1,2,3-triol)

Formation

• Formation one mole of glycerol with three moles of fatty acid and an acid catalyst gives the triglyceride, with three moles of water eliminated

Section 6 of 19Acid hydrolysis of esters

Hydrolysis

• Hydrolysis the reverse of esterification, useful for making biodegradable plastics

Conditions

• Conditions heating under reflux with dilute acid such as sulfuric acid

Result

• Result an equilibrium mixture of ester, carboxylic acid and alcohol, so hydrolysis is incomplete

Example

• CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH

Section 7 of 19Alkaline hydrolysis of esters

Conditions

• Conditions heating under reflux with dilute alkali such as sodium hydroxide

Result

• Result an irreversible reaction that goes to completion, giving a carboxylate salt and an alcohol

Acidification

• Acidification the carboxylate ion needs an acid such as HCl to be protonated to the carboxylic acid

Example

• CH₃CH₂COOCH₃ + NaOH → CH₃CH₂COONa + CH₃OH

	Acid hydrolysis	Alkaline hydrolysis
Extent	Equilibrium, incomplete	Irreversible, complete
Conditions	Reflux with dilute HCl or H2SO4	Reflux with dilute NaOH
Products	Carboxylic acid and alcohol	Carboxylate salt and alcohol

Section 8 of 19Soap and biodiesel

Soap and biodiesel

Soap (saponification)

• Vegetable oils and animal fats are hydrolysed with aqueous sodium hydroxide.
• The products are glycerol and the sodium salts of fatty acids, which are soaps.

Biodiesel

• It is made from renewable vegetable oils rather than petrochemicals.
• Triglycerides are converted to methyl esters (FAME), which are less viscous, by transesterification with methanol.
• Acids (protonating the carbonyl) or alkalis (deprotonating methanol) can catalyse it, and NaOH or KOH is more common.
• The reaction is reversible, so excess methanol drives it right, giving yields up to 98%.
• Glycerol is the by-product.

Section 9 of 19Acyl derivatives

Acyl chlorides

• Acyl chlorides carboxylic acid derivatives in which –OH is replaced by Cl, named with the suffix -oyl chloride

Acid anhydrides

• Acid anhydrides derivatives in which –OH is replaced by an alkanoate group, named with the suffix -oic anhydride

Amides

• Amides compounds with the –CONR₂ group, made from an acyl chloride and ammonia or an amine

Section 10 of 19Nucleophilic addition–elimination of acyl chlorides

Nucleophilic addition–elimination of acyl chlorides

Reactions

• Reactions hydrolysis, reaction with alcohols to form esters, and reaction with ammonia and primary amines to form amides

Mechanism type

• Mechanism type the nucleophile adds across the C=O bond and then a small molecule, HCl, is eliminated

Hydrolysis

• Hydrolysis an acyl chloride gives a carboxylic acid and HCl
• Step 1 (addition): the lone pair on water’s oxygen attacks the carbonyl carbon.
• Step 2 (elimination): HCl is eliminated.
• Examplepropanoyl chloride gives propanoic acid and HCl.

Reaction

• Reaction an acyl chloride reacts with an alcohol to give an ester and HCl

Mechanism

• Mechanism the alcohol acts as a nucleophile

Steps

• Additionthe lone pair on the alcohol oxygen attacks the carbonyl carbon.
• EliminationHCl is eliminated.

Section 11 of 19Formation of amides

Reaction

• Reaction an acyl chloride reacts with ammonia or a primary amine in a condensation reaction

Mechanism

• Mechanism a lone pair on nitrogen attacks the carbonyl carbon (addition), and then a chloride ion is eliminated, forming HCl

Fate of HCl

• It is neutralised by a second molecule of ammonia or amine in excess.
• This forms an ammonium salt such as NH₄Cl or CH₃NH₃Cl.

Two molecules needed

• Two molecules needed the first forms the amide and the second neutralises the HCl

Section 12 of 19Examples of amide formation

Reactants	Products
CH3CH2COCl + 2NH3	CH3CH2CONH2 (propanamide, a primary amide) + NH4Cl
CH3COCl + 2CH3NH2	CH3CONHCH3 (a substituted amide) + CH3NH3Cl
CH3COCl + 2CH3CH2NH2	CH3CONHCH2CH3 + CH3CH2NH3Cl

• Summary
  • HCl is never seen as a separate product, because excess ammonia or amine neutralises it

Section 13 of 19Mechanism of amide formation

• Nucleophile
  • the nitrogen of ammonia or a primary amine

Mechanism of amide formation

Nucleophilic addition: the nitrogen lone pair attacks the carbonyl carbon.

Elimination: a chloride is lost and HCl forms.

Acid–base reaction: the HCl immediately reacts with excess ammonia or amine to give an ammonium chloride.

Section 14 of 19Acylation and the manufacture of aspirin

Uses of acylation

• Uses of acylation pharmaceuticals such as aspirin, and textiles such as cellulose acetate

Aspirin

• Aspirin salicylic acid is reacted with ethanoic anhydride to give aspirin and ethanoic acid

Equation

• HOOCC₆H₄OH + (CH₃CO)₂O → HOOCC₆H₄OCOCH₃ + CH₃COOH

Why anhydrides are preferred to acyl chlorides

• They are cheaper to produce.
• They are less reactive and the reactions are easier to control.
• The by-product is the less corrosive carboxylic acid, instead of corrosive HCl gas.

Acyl chloride drawback

• Acyl chloride drawback reactions with water or alcohols are fast and exothermic, giving rapid corrosive HCl

Section 15 of 19Required Practical 10: apparatus and aim

Required Practical 10: apparatus and aim

Aim

• Aim to prepare a pure organic solid (aspirin) and test its purity

Preparation apparatus

• Preparation apparatus salicylic acid, 100 cm³ conical flask, 10 cm³ measuring cylinder, ethanoic anhydride, concentrated sulfuric acid, 400 cm³ beaker, tripod, gauze and Bunsen burner, thermometer, 250 cm³ beaker, Buchner funnel, filter paper, stirring rod, wash bottle and spatula

Recrystallisation and purity apparatus

• 25 cm³ measuring cylinder, boiling tube, ethanol, 100 cm³ conical flask, kettle, balance to 2 decimal places, thermometer (0–250 °C), melting point apparatus and tubes, watch glass and rubber ring

Section 16 of 19Required Practical 10: preparation and recrystallisation

Required Practical 10: preparation and recrystallisation

Preparation

• Add 6.00 g of salicylic acid, 10 cm³ of ethanoic anhydride and 5 drops of concentrated sulfuric acid to a conical flask.
• Swirl and hold in a water bath at about 60 °C for about 20 minutes.
• Cool and add the contents to 75 cm³ of cold water, so the aspirin crystallises.
• Recover the aspirin by Buchner filtration and leave to dry.

Recrystallisation

• Measure 15 cm³ of ethanol into a boiling tube.
• Half-fill a beaker with water at 75 °C and stand the boiling tube in it.
• Add the crude aspirin without scraping the filter paper, and stir until it dissolves.
• Pour the solution into about 40 cm³ of water in a 100 cm³ conical flask and warm gently until the solution is complete.
• Cool in an ice bath so crystals form, scratching the inside of the vessel if needed.
• Filter with a Buchner funnel, dry, and record the mass.

Section 17 of 19Required Practical 10: melting point test and evaluation

1

Crush a sample with a spatula to a powder.

2

Fill three melting point tubes to a depth of 0.5 cm.

3

Place them in the melting point apparatus.

4

Heat the oil bath gently and note the temperature at which the solid melts.

5

Repeat with the other tubes, heating more slowly as the approximate value is approached.

6

Deduce the melting point from the readings.

Purity

• Purity compare the melting point with the data book value of 136 °C, because impurities lower the melting point and a close match suggests purity

Reporting

• Reporting quote a melting point as a range, ±, and cite a data book value

Yield

• Yield calculate the theoretical yield from 6.00 g of salicylic acid, and then the percentage yield

Example

• 6.00 g of salicylic acid (M_r 138.1) gives 7.82 g of aspirin (M_r 180.1), and 4.13 g obtained is a 52.8% yield

Section 18 of 19Key terms

Term	Meaning
Esterification	The reaction of an alcohol with a carboxylic acid to form an ester and water.
Saponification	Alkaline hydrolysis of fats to make soap.
Transesterification	The conversion of one ester into another, as in making biodiesel.
Acyl chloride	A carboxylic acid derivative with Cl in place of OH.
Triglyceride	A tri-ester of glycerol and fatty acids.
Plasticiser	An additive that makes a polymer more flexible.

Term	Meaning
Amide	A compound with the –CONR2 group.
Acylation	A reaction that introduces an acyl group.
Recrystallisation	Purifying a solid by dissolving it in hot solvent and cooling to re-form crystals.
Buchner filtration	Vacuum filtration using a Buchner funnel.
Melting point range	The temperatures over which a solid melts, used to assess purity.

Section 19 of 19Exam technique

1

Writing the ester name in structural order — the alcohol part comes first in the name.

2

Saying acid hydrolysis goes to completion — it is an equilibrium.

3

Forgetting to acidify after alkaline hydrolysis — the product is a carboxylate until acidified.

4

Saying acyl chloride hydrolysis is slow — it is fast and exothermic.

1

Letting HCl accumulate in amide formation — it is neutralised by excess ammonia or amine.

2

Using acyl chlorides for aspirin because they are cheaper — anhydrides are cheaper and safer.

3

Scraping the filter paper when transferring crude aspirin — this contaminates the product.

4

Saying a high melting point proves purity — compare it with the data book value, since impurities lower it.