A-level Chemistry: 3.3.9 Carboxylic Acids and Derivatives

-yl –oate

-oyl chloride

Partially dissociate into carboxylate ion and H+ ion

Equation: CH₃COOH ⇌ CH₃COO⁻ + H⁺

Displayed formula: CH₃–C(=O)–OH ⇌ CH₃–C(=O)–O⁻ + H⁺

Carbon dioxide (CO₂)

Esters

-COO-

• Esterification

• Made by heating carboxylic acid with alcohol in presence of strong acid catalyst

Concentrated sulfuric acid

Equation: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

Displayed formula: CH₃–C(=O)–OH + CH₃–CH₂–OH ⇌ CH₃–C(=O)–O–CH₂–CH₃ + H–OH

ethyl ethanoate

1st part = alcohol

2nd part = carboxylic acid

• Perfumes
  

  • Have sweet swell

• Food flavourings
  

  • Flavour drinks and sweets

• Solvents
  

  • Glues, drinking inks

• Plasticisers

• Are polar liquids so lots of polar organic compounds will dissolve in them

• Got low boiling points = evaporate easily from mixtures

Added to plastics during polymerisation to make plastic more flexible

Over time, plasticiser molecules escape and plastic becomes brittle and stiff

Hydrolysis

Using water to split up a substance is very slow, so an acid or alkali is often used to speed it up

• Acid Hydrolysis

• Base Hydrolysis

• Reflux ester with dilute acid e.g. hydrochloric or sulfuric

  • Splits ester in acid and alcohol

• Refluxing ester with dilute alkali e.g. sodium hydroxide

• Get carboxylate ion and an alcohol

Equation: CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH

Displayed formula: CH₃–C(=O)–O–CH₂–CH₃ + H–OH ⇌ CH₃–C(=O)–OH + CH₃–CH₂–OH

Equation: CH₃COOCH₂CH₃ + OH⁻ → CH₃COO⁻ + CH₃CH₂OH

Displayed formula: CH₃–C(=O)–O–CH₂–CH₃ + OH⁻ → CH₃–C(=O)–O⁻ + CH₃–CH₂–OH

long chain carboxylic acids

Esters = fats and oils

Fatty acids can be saturated (no double bonds) or unsaturated (C=C double bonds)

fatty acid

They give them many of their properties

Have mainly saturated hydrocarbon chains

Have unsaturated hydrocarbon chains

• Saturated hydrocarbon chains fit neatly together = increase van der Waals forces between them

• Means you need higher temperature to melt them

• Double bond means mean chains are bent & don't pack together well = ↓ van der Waals forces

• Easier to melt

Here’s the displayed formula for an ester formed between glycerol and three fatty acid molecules (a triglyceride):

Displayed formula:

HO–CH₂–CH(OH)–CH₂–OH + 3 R–COOH → CH₂–O–CO–R

• By heating ester with sodium hydroxide

• Produces glycerol, fatty acids and soap = sodium salt

Vegetable oils can't be burned directly in engines

• Involves reacting them methanol, using potassium hydroxide as a catalyst

• Get mixture of methyl esters of fatty acids

  • = biodiesel

Equation: Vegetable oil (triglyceride) + 3 CH₃OH → 3 CH₃–(CH₂)ₙ–COOCH₃ + CH₂OH–CHOH–CH₂OH

Displayed formula: CH₂–O–CO–R

COCl

• Water

• Alcohols

• Ammonia

• Primary amines

A vigorous reaction with cold water, producing a carboxylic acid

A vigorous reaction at room temperature, producing an ester

A violent reaction at room temperature, producing an amide

A violent reaction at room temperature, producing an N-substituted amide

Cl is substituted by oxygen or nitrogen group & misty fumes of hydrogen chloride are given off

Acid Anhydrides

2 identical carboxylic acid molecules

take away 'acid' and add 'anhydride'

Almost the same as those of acyl chlorides but reactions a less vigorous & get carboxylic acid formed instead of HCl

Nucleophilic Addition-Elimination

Both chlorine and oxygen draw electrons towards themselves, so carbon has slight positive charge

H2O:

:NH3

ester

Made by reacting salicylic acid with ethanoic anhydride or ethanoyl chloride

• Cheaper than ethanoyl chloride

• Safer to use than ethanoyl chloride as it's less corrosive, reacts more slowly with water, and doesn't produce dangerous hydrogen chloride fumes

If product is insoluble in water, can use separation to remove any impurities that do dissolve in water

1. Once reaction to form product is completed, pour the mixture into separating funnel & add water

2. Shake funnel and then allow it to settle

   • Organic layer and aqueous layer (which contains any water soluble impurities) are immiscible (they don't mix) so separate out into 2 distinct layers

3. Can open tap and run each layer off into a separate container

Solvent extraction

| (similar separation method)

1. Take organic solvent in which the product is more soluble in than it is in water

2. Add it to impure product solution & shake well

3. Product will dissolve into organic solvent = leaving impurities dissolved in water

4. Solvent containing product can be run off using separating funnel

∵ organic layer will end up containing trace amounts of water

1. Add anhydrous salt (e.g. magnesium sulfate or calcium chloride)

2. When you first add salt to organic layer = clumps together

   • Keep adding drying agent until it disperses evenly when you swirl the flask

3. Filter mixture to remove solid drying agent

   • Pop a piece of filter paper into a funnel that's in a flask and pour mixture into filter paper

Salt is used as drying agent = binds to any water present to become hydrated

• Can remove some of these by washing the product

  • (Adding another liquid and shaking)

• ∵ product can be contaminated with leftover reagents or unwanted side products

• Aqueous sodium hydrogencarbonate can be added to impure product in solution to remove acid from it

• Acid reacts with sodium hydrogencarbonate to give CO2 gas and organic product can be removed using separating funnel

By distillation

• Works by gently heating a mixture in distillation apparatus

• Substances will evaporate out of mixture in order of increasing boiling point

1. Connect a condenser to round bottomed flask containing your impure product in solution

2. Place thermometer in neck of flask so bulb sits next to entrance to condenser

   • Temperature on thermometer will show boiling point of substance that's evaporating at any given time

3. Heat impure product

4. When product you want to collect boils, place a flask at open end of condenser to collect your pure product

Many organic chemicals are flammable

Recrystallisation

1. Dissolve impure solid in minimum hot solvent

   • Don't add too much solvent = want saturated solution of impure product

2. Filter hot solution through heated funnel

   • To remove any insoluble impurities

3. Leave solution to cool down slowly

   • Crystals of product will form as it cools

4. Filter mixture under reduced pressure

   • To remove liquid containing soluble impurities from crystals

5. Wash crystal with ice-cold solvent

   • To remove any soluble impurities from their surface

6. Leave your purified crystals to dry

Pour mixture into filter paper lined Büchner funnel (flat bottomed funnel with holes in base) that's sitting in side-arm flask attached to vacuum line

• As solution cools, solubility of product falls

• Reaches point where it can't stay in solution = forms pure crystals

Melting and boiling points

specific

Melting point is lowered and boiling point is raised

Melting and boiling will occur across a wide range of temperatures

Melting point apparatus

1. Pack small sample of solid into glass capillary tube and place it inside the heating element

2. Increase the temperature until sample turns from solid to liquid

3. Measure a melting range (range of temperatures from where solid begins to melt to where it has melted completely)

4. Can look up the melting point of substance in data books and compare it to your measurements

5. Impurities in sample will lower the melting point and broaden the melting range

sharply

Sweet smell/oily layer

Want saturated solution of impure product

To remove any insoluble impurities

More crystals of product will form as it cools/decreases solubility of solid

To remove liquid containing soluble impurities from crystals

To remove any soluble impurities from their surface