3.3.9 Carboxylic Acids Flashcards
Esters products
Vegetable oils and animal fats propane-1,2,3-triol (glycerol)
Common uses of esters
Solvents, plasticisers, perfumes, food flavourings
Esters hydrolysed in acid/alkaline conditions to form
Alcohols and carboxylic acids
Salts of carboxylic acids
Vegetable oils and animal fats can be hydrolysed
What are the conditions + products
Under alkaline conditions to give salt and glycerol
What is biodiesel + how is it produced
Mixture of methyl esters of long chain carboxylic acids
Produced by reacting vegetable oils with methanol with catalyst
Acid anhydride
R-C(=O)-O-C(=O)-R
Acyl chloride structure
-COCl
Amide structure
R-C(=O)-N-(R)2
Advantages of ethanoic anhydride over ethanoyl chloride in manufacture of aspirin
Add to salicylic acid
Cheaper
Safer
Less corrosive
Reacts more slowly with water
Doesn’t produce dangerous hydrogen chloride fumes
After the solid benzoic acid has been filtered off, it can be purified. Describe the method that the student should use to purify the benzoic acid.
Dissolve crude product in hot solvent/water
of minimum volume
Filter (hot to remove insoluble impurities)
Cool to recrystallise
Filter under reduced pressure
with Buchner/Hirsch apparatus wash (with cold solvent)
dry
State a simple chemical test that distinguishes the propanoic acid from the propan-1-ol
Add sodium carbonate
Acid-bubbles
Alcohol-no visible reaction
Compare solubility of carboxylic acids and esters
Both polar
Carboxylic acids more soluble
Can form hydrogen bonds with water
Esters can’t
As chain increases, solubility decreases - polarity decreases
Compare the melting point of carboxylic acids and esters
Carboxylic acids is higher
Contains hydrogen bonding
Esters only contain Van der Waals
Smell of carboxylic acids and esters
Acids- vinegar
Esters- fruity/sweet, used in food flavourings and perfume
Stability of carboxylic acids
In the carboxylate ion, the negative charge delocalises its electrons
Stabilising the ion
Reaction of carboxylic acids with metal/metal carbonate
Metal carboxylate (salt) + hydrogen
Metal carboxylate (salt) + water + CO2
Reaction of carboxylic acid with base
Ionic salt + water
E.g. CH3COOCH3 + NaOH —>
CH3COO-Na+ + H2O
Reactivity of acid chlorides and anhydrides
Reactive
Polar bonds
Large differences in electronegativity of C-Cl and C=O
Are acid chlorides or acid anhydrides more reactive ?
Acid chlorides
C-Cl bond has greater electronegativity difference
3 factors affecting how easily acylation occurs
Size of delta + charge on C
How easily Z is lost E.g. Cl
How good nucleophile is at donating lone pair
Give a physical properties of an ester
Volatile
Common esters
Fats and oils
What happens to the negative charge on carboxylate ions in terms of electrons
Electrons delocalised so charge is shared across whole carboxylate group
What catalyst is needed for the formation of esters from alcohols and carboxylic acids
Concentrated strong acid
What catalyst is needed for hydrolysis of esters
Dilute strong acid
Base hydrolysis advantage
Reaction goes to completion due to neutralisation by base - more product in the mixture than acid catalysed hydrolysis
What alcohol forms the esters that make up animal and vegetable oils
Glycerol
Difference between oil and fats
Oils- liquid at room temp, usually unsaturated
Fats- solid at room temp, usually saturated
How do carboxylate ions act as detergent
Long hydrocarbon chain of carboxylate ion mixes with grease, COO^- mixes with water
Uses of glycerol
Pharmaceutical + cosmetic products
Solvent in medicine and food
Plasticisers
Making biodiesel
Equations + conditions
NaOH catalyst, 60°C
Lipids + 3CH3OH —> 3 methyl esters + glycerol
What is transesterification
Converting one type of ester to another
What kind of crops is biodiesel made from
Rapeseed oil or soybean oil
How is the reaction mixture of biodiesel purified and separated
Centrifuge
Remove remainder with water
Add acid to neutralise excess alkali catalyst
Solid soap formed
Problems with biodiesel
Crops that could be used to make food are used to make fuel
2 acid derivatives
Acyl chlorides
Acid anhydrides
Name a commercially important acylation reaction
Manufacture of aspirin
Conditions needed to form methyl esters from acid anhydride/acyl chloride
React with methanol and heat gently under reflux
When purifying by recrystallisation, why is the minimum volume of hot solvent used
Saturated solution created
As many crystals will fall out of solution as possible when cooled
Why is the solution filtered hot when purifying by recrystallisation
Remove insoluble impurities and ensure that the crystals don’t form in filter paper
Why is the solution cooled in an ice bath when purifying by recrystallisation
Ensure as many crystals as possible fall out of solution (increase yield)
Why are the crystals washed with cold water when purifying by recrystallisation
Remove soluble impurities
Why is Büchner flask used during recrystallisation
Air goes through sample not just round it
Why might percentage yield not by 100% during recrystallisation- 3 reasons
Product lost during filtration, during, weighing
Not all transferred between apparatus
Product left dissolved in solution- doesn’t recrystallise
Some left on filter paper
Sample still wet
Suggest why an electric heater is used rather than a Bunsen burner in this hydrolysis.
Many organic compounds / the ester / ethanol are flammable
State why reflux is used in this hydrolysis.
Reflux allows reactant vapours (of volatile organic compounds) to be returned to the reaction mixture / does not allow any reactant vapour to escape
Suggest why sodium benzoate is soluble in cold water but benzoic acid is insoluble in
cold water
Sodium benzoate soluble because it is ionic
Benzoic acid insoluble because: despite the polarity of the COOH group / ability of COOH to form H-bonds, the benzene ring is non-polar
