Carboxylic acids and derivatives

What are esters?

• Derivatives of carboxcylic acids

How do you name an ester?

• Name right hand side of the molecule first which is the prefix e.g. Ethyl

• Name left hand side of the molecule next which is the suffix e.g. ethanoate

• Name is ethyl ethanoate

Intermolecular forces of esters

• Ester have weaker IM forces so they have lower BPs than carboxcylic acids and alcohols

• As they cannot form hydrogen bonds with each other as they lack a partially positive H (while alcohols and carboxcylic acids can)

Why can esters form hydrogen bonds with H2o ?

• Oxygen atoms in an ester have lone pairs of electrons that attracts the partially positive H of H2o

• Check notes for additional guidance.

What the name for forming ester and how are they formed ?

• Esterification

• Reaction between a carboxcylic acid +alcohol → ester + water

• Esterification is a reversible reaction

Explain how you can get your ester from esterification

• Get rid of the H from the OH of carboxcylic acid

• Get rid of OH from alcohol

• Add remaining structure of alcohol to the acid to get ester.

Conditions for an esterification reaction

• Reaction is quite slow so an acid catalyst is used (Typically H₂SO₄)

• And reaction is heated under reflux

• Esterification also called a condensation reaction

Why is esterification reversible?

• Ester can react with water to produce a carboxcylic acid and alcohols

• Known as hydrolysis of esters

In what 2 conditions can esters be hydrolysed in?

• Acid conditions

• Alkaline conditions

Reagent needed to hydrolyse an ester in acidic conditions

• Dilute HCL- (source of H+ catalyst)

• Heat under reflux

• Reaction is reversible

What do you get when ethyl propanoate is hydrolysed (in acidic conditions)

• Propanoic acid

• Ethanol

• (Whatever is the ..oate becomes the acid)

How is hydrolysis of an ester in alkaline conditions different?

• Not reversible

• Will form a carboxylate salt (and alcohol) instead of acid

• This reaction also called saponification

What are the products when ethylethanoate reacts with NaOH (hydrolysis in alkaline conditions)- How did we get the products?

• CH₃COO^-Na⁺ →Sodium ethanoate (carboxylate salt)

• CH₃CH₂OH → Ethanol

• For the salt get rid of the CH2CH3 of ethyl ethanoate and replace it with Na

• The CH2CH3 you got rid of becomes the alcohol

Why is saponification not a reversible reaction?

• Carboxylate salt is negatively charged.

• Negative charge is delocalised over the 2 oxygen atoms

• Making the carbonyl carbon less positively charged

• So it is resistant to attack by weak nucleophiles like alcohols

• (So the products don’t react to form the reactants)

Conditions for hydrolysis in alkaline conditions

• Reagent: Dilute NaOH

• Heat under reflux

• Excess NaOH → To ensure the seter is completely hydrolysed into its carboxylate salt and alcohol.

Uses for esters and why

• Solvents - as esters are polar

• Plasticisers- added to pure polymers to increase flexibility as esters reduce the IMF between the polymer chains

• Perfumes and food flavouring - due to pleasant smells and taste

• (Esters used in food flavouring and perfumes must be non toxic)

For esters to be used in perfumes what conditions must be met?

• Esters must be soluble in alcohol

• Must not react with water

• And must be volatile (can turn into gas/vapour)

What happens when we react glycerol and fatty acids?

• Glycerol (alcohol)

• Fatty acids (long chain carboxcylic acids)

• Produce fats and oils -which are naturally occurring esters (also called triglycerides)

Fatty acids can be ..

• Saturated

• Unsaturated

What are poly unsaturated fatty acids?

• Unsaturated fatty acids with more than one double bond

Properties of vegetable oil

• Have unsaturated hydrocarbon chains

• That are not straight, which means the chains cannot pack closely together

• Lower Vander Waals forces

• So veg oils have lower melting points and are liquids at room temperature

Why are the unsaturated hydrocarbon chains not straight in oils?

• Have a cis double bond

• Hydrogen atoms are on the same side of the double bond

• Creating a kink in the chain

• Kink prevents tight packing of the chains

Properties of fat

• Fats (animal fat) have saturated hydrocarbon chains

• That are straight and more uniform than oils

• They can pack together closely

• Higher Vander Waals forces

• High Melting points and are solids at room temperature

How can soap be produced?

• Animal fats and vegetable oils can be hydrolysed by heating them with NaOH

• Producing soap

Equation for the production of soap

How can vegetable oils be converted into biodiesel?

• Reacting oils with methanol

• And KOH as a catalyst

What is biodiesel?

• A mixture of fatty acid methyl esters

• (Made from reacting methanol with rapeseed oil)

Equation for production of biodiesel

Functional group of acyl/acid chlorides

and how to name them

• COCL (acyl group)

• Carbon attached to acyl group is always carbon number 1 and acyl group is always on the end

• Find the longest carbon chain and then add “oyl chloride” to the end

What can react with acyl chlorides?

• Water

• Ammonia

• Alcohol

• Primary amines

*In each reaction Cl is substituted for either an Oxygen or Nitrogen

What is produced when acyl chlorides react with the 4 reactants listed and what is the nature of the reaction?

• White misty fumes of hydrogen chloride gas

• Vigorous reaction

Acyl chloride+ H2O → ?

• Carboxcylic acid

• HCL (Hydrogen chloride gas)

Acyl chloride + NH3→?

• Amide (Cl of acyl chloride replaced with NH2)

• HCL

Acyl chloride+ Alcohol→?

• Ester

• HCL

Acyl chloride+ Primary amine→?

• N- substituted amide

• HCL

• E.g If the amine is CH₃NH₂

• The CL group of the acyl chloride is replaced with NHCH₃

What are acid anhydrides and how do you name them?

• A molecule made from 2 carboxcylic acids

• Identify the carboxcylic acid and replace “acid” with “anhydride”

What do acid anhydrides react with and what’s the nature of the reaction?

• H2O

• NH3

• Alcohols

• Primary amines

• Less vigorous

Acid anhydride + H2O → ?

• 2 carboxcylic acids

Acid anhydride+ NH3→ ?

• Amide

• Carboxcylic acid

• BUT if NH3 is in excess a carboxylate salt is produced instead of acid

• E.g instead of ethanoic acid you will get CH3COO^-NH4⁺

Acid anhydride + alcohol→ ?

• ester

• Carboxcylic acid

Acid anhydride + Primary amines→?

• N-substituted amide

• Carboxcylic acid

• E.g instead of ethanoic acid you get CH₃COO^-CH₃NH₃⁺ (methylammonium ethanoate)

What do both acid chlorides and acid anhydrides have, that allow them to go through nucleophilic addition elimination?

• A strong partial positive charge on carbon atom(s) in the carbonyl group(s)

• Which is susceptible to attack from nucleophiles

Nucleophilic addition-elimination mechanism for propanol chloride reacting with ethanol: part 1

• Ethanol is a nucleophile and attacks partially positive carbon

• Pair of electrons from double bond is then transferred to oxygen -double bond breaks

• Lone pair of electrons on the oxygen move to remake double bond

• Electrons from c-cl bond move to cl→ bond breaks

Nucleophilic addition-elimination mechanism for propanol chloride reacting with ethanol: part 2

• Chlorine that was removed in the previous step bonds with hydrogen atom in the hydroxyl group -HCL breaks off

• Electrons in the OH bond move to the positive O →Neutralising the charge

• Producing methyl propanoate and HCL

What is aspirin?

• An ester made by reacting ethanoic anhydride or ethanoyl chloride with salicylic acid

Why is ethanoic anhydride used instead of ethanoyl chloride in the production of aspirin?

• Ethanoic anhydride is less corrosive

• Does not produce harmful HCL gas

• Also cheaper and don’t react vigorously with water

Equation for ethanoic anhydride + salicylic acid to produce aspirin?

What is reflux/refluxing

• Technique used to heat a mixture of volatile liquids

• Reflux allows strong heating without losing volatile reactants and products

Reflux apparatus

• Liebig condenser has cold water running through the walls

• When hot evaporating substances hit the condenser they turn back into a liquid and return back to round bottomed flask to react further

When is distillation used

• To separate substances with different boiling points

• Gently heating the mixture will result in compounds separating out in order of boiling points

Why may a sample be re-distilled?

• In order to further purify it

• As it can still contain other substances if boiling points of those substances are very close to BP of desired compound

Why may a separation funnel be used?

• Separate immiscible liquids (liquids that don’t mix together)

• Purify an organic product

How a separation funnel works?

• Add sample into separating funnel→ add water to dissolve soluble impurities

• 2 layers will form → Top layer is impure product, bottom layer is aqueous containing water soluble impurities

• Drain aqueous layer by removing stopper

• Remove impure product from funnel and add into round bottomed flask

• Add anhydrous cacl2 (solid) →dehydrating agent and will remove aqueous substances still remaining

• Invert the flask and leave for 20/30 minutes

• Filter the solid drying agent

What is Buchner funnel used for

• Vacuum used to help separate liquid and solid components thoroughly.

How do you use Buchner funnel?

• Place filter paper disc in funnel and dampen slightly to make a seal

• pour reaction mixture into funnel and ensure vacuum source is on

• Vacuum creates a reduced pressure in flask and pulls the liquid through

• Solid will be left in Buchner funnel

• (In the case of aspirin the solid component is what we are after)

Why is recrystallisation used?

• To purify an impure solid

Steps for recrystallisation

• Dissolve impure solid in a minimum volume of hot solvent → Minimum volume maximises the amount that will recrystalise

• Hot filtration to remove insoluble impurities

• Allow solution to cool and crystalise → Can put in ice bath to maximise crystal formation. (soluble impurities stay dissolved)

• Use a vacuum (Buchner funnel) to filter the crystals

• Wash crystals with cold solvent to remove remaining soluble impurities stuck to the surface of crystals

• Dry crystals

How can you determine the purity of a liquid ?

• Measuring its BP via distillation

• Gently heat the sample and ,measure the temperature at which it distils using a thermometer → which will give you its BP

How do you know if a liquid contains impurities?

• Measured BP is higher than data book value

• If it boils over a range of temperatures instead of just 1

*Various organic products have the same BP→ so other techniques like mass spectrometry may need to be used

How do you determine the purity of a solid?

• Measuring its melting point

• Add sample of solid product into a capillary tube and place into the heating element of the MP apparatus

• Slowly increase temp until sample begins to melt

• There will be a temp range from when solid just starts to melt to when it fully melts

• Compare values to data book value.

How do you know if a solid contains impurities?

• Temp range the substance melts at is larger then data book range

• If MP is lower than data book value→ as pure solids have a regular ordered crystal lattice, impurities fit irregularly into the lattice and weaken IMF so MP is lower