AQA A-Level Chemistry - Carboxylic Acids and Derivatives Questions and Answers

AQA A-Level Chemistry - Carboxylic Acids and Derivatives Questions and Answers Carboxyl group -COOH Naming Carboxylic Acids - Suffix (-oic acid) always at end of name, and chain - Numbering of carbons always starts at COOH end - Suffix (-dioic acid) implies carboxylic acid groups on both ends Acidity of Carboxylic Acids - Weak acids in water - slightly dissociate - Strong enough to displace CO2 from carbonates - E.g. CH3CO2H(aq) ⇌ CH3CO2-(aq)+ H+(aq) Solubility of Carboxylic Acids - Smaller carboxylic acids (up to 4 carbons) dissolve in water in all proportions - After four carbons, solubility rapidly reduces - Solubility dependant on ability to produce hydrogen bonds Delocalisation of Carboxylic Acids - Carboxylic acid salts are stabilised by delocalisation, which makes the dissociation more likely - Pi charge cloud delocalised and spread out, ion more stable and likely to form - Delocalised ion has equal C-O bond lengths - If delocalisation did not occur, the C=O bond would be shorter than the C-O bond Strength of Carboxylic Acids - Increasing chain length pushes electron density on to the COO ion, making it more negative and less stable, hence weaker - ∴ Propanoic acid is less acidic than ethanoic acid - Electronegative Cl atoms withdraw electron density from the COO- ion, making it less negative and more stable, hence stronger - ∴ Chloroethanoic acid is more acidic than ethanoic acid Acid + Metal - Acid + metal → salt + hydrogen - E.g 2CH3CO2H + 2Na → 2CH3CO2-Na+ + H2 Acid + Alkali - Acid + alkali → salt + water - E.g. CH3CO2H + NaOH → CH3CO2-Na+ + H2O Acid + Carbonate - Acid + carbonate → salt + water + CO2 - 2CH3CO2H + Na2CO3 → 2CH3CO2-Na+ + H2O + CO2 Test for Carboxylic Acids Effervescence caused by production of CO2 with carboxylic acids with solid Na2CO3 or aqueous NaHCO3 Oxidation of Carboxylic Acids - Carboxylic acids cannot be oxidised using oxidising agents - However, methanoic acid is an exception as its structure has effectively an aldehyde group - Methanoic acid forms carbonic acid (H2CO3 ) which can decompose to give CO2 Esterification - Carboxylic acids react with alcohols, in the presence of a strong acid catalyst - Forms ester (RCOOR′) and water - Carboxylic Acid + Alcohol ⇌ Ester + water - Reaction is reversible. - Reaction is quite slow and needs heating under reflux, often for several hours or days - Low yields (50% ish) achieved - An acid catalyst (H2SO4) needed Naming Esters - Esters have two parts to their names - E.g. methyl propanoate - Suffix (-anoate) comes from the carboxylic acid and includes the C in the C=O bond - Part (-yl) comes from alcohol that has formed it and is next to the single bonded oxygen Solubility of Esters - Almost insoluble in water - Do not form hydrogen bonds - No hydrogen bonded to a highly electronegative atom - Bonus: lower bpt than the hydrogen-bonded carboxylic acids they came from Uses of Esters - Sweet smelling compounds that can be used in perfumes and flavourings - Solvents for polar organic substances - E.g. Ethyl ethanoate is used as a solvent in glues and printing inks - Plasticisers for polymers (inc. flexibility of polymers) Hydrolysis of Esters - 2 Methods Esters can be hydrolysed and split up by either heating with acid or with sodium hydroxide. Hydrolysis of Ester - Heating with Acid - Reagents: dilute acid (HCl) - Conditions: heat under reflux - Reverse reaction of ester formation - Carboxylic acid and an alcohol are formed - Reaction is reversible and does not give a good yield of the products Hydrolysis of Ester - Heating with Sodium Hydroxide - Reagents: dilute sodium hydroxide - Conditions: heat under reflux - Reaction goes to completion - Carboxylic acid salt product is the anion of the carboxylic acid - The anion is resistant to attack by weak nucleophiles e.g. alcohols, so the reaction is not reversible. Glycerol - Propane-1,2,3-triol - Alcohol produced by the hydrolysis of triglycerides - Can also be a by-product during the manufacture of soap and biodiesel Solubility of Glycerol Glycerol forms hydrogen bonds very easily and is readily soluble in water Uses of Glycerol - Cosmetics (multiple uses) - Solvent for flavours and colourings in food - Plasticiser & preservative in glues How are fats / oils derived from glycerol? - Fats and oils are esters of glycerol and long chain carboxylic acids (fatty acids) - Likewise, vegetable oils and animal fats can be hydrolysed to give soap, glycerol and long chain carboxylic (fatty) acids How can long chain carboxylic fatty acids, produced by hydrolysis of fats, act as soaps? - Polar CO2- end is hydrophilic and mixes with water - Long non-polar hydrocarbon chain is hydrophobic and mixes with grease - This allows the grease and water to mix and wash away Biodiesel - A mixture of methyl esters of long chain carboxylic acids - Vegetable oils convert into biodiesel when reacted with methanol, in presence of a strong alkali catalyst Why can the production of biodiesel from vegetable oils be seen as carbon neutral? Any CO2 given off when biofuel is burnt would have been extracted from the air by photosynthesis when the plant grew Why can the production of biodiesel from vegetable oils be seen as not carbon neutral? Energy needed to: - Irrigate plants - Extract the oil - Heat reaction with methanol mixture - Process the fuel This energy is most likely derived from fossil fuels Also, does not take into account effect on land available for food production Acyl Chlorides Derivatives of carboxylic acids, with formula RCOCl Acid Anhydrides Derivatives of carboxylic acids, with formula R(CO)O(CO)R' Compare reactivity of acyl chlorides with acid anhydrides - Acid anhydrides have a similar reactivity to acyl chlorides - ∴ Bring about the same changes in functional groups. - Main difference is the by-products - Acyl chlorides mostly give off HCl - Acid anhydrides give off RCOOH Why are acyl chlorides and acid anhydrides much more reactive than carboxylic acids and esters? - Cl and O(CO)R' groups are classed as good leaving groups, due to less effective delocalisation - Leaving group: breaks away from molecule taking a lone pair, and breaking bond between it and molecule What is the main mechanism for acyl chlorides and acid anyhdrides, at room temperature? Nucleophilic addition-elimination mechanism What is the difference between an amine, and an amide? - Amines have no carbonyl groups attached to the nitrogen atom - Amides have a carbonyl group attached to a nitrogen atom What is the main product when acyl chlorides / acid anyhdrides react with water? Carboxylic Acid What is the main product when acyl chlorides / acid anyhdrides react with an alcohol? Ester What is the main product when acyl chlorides / acid anyhdrides react with ammonia? Primary Amide What is the main product when acyl chlorides / acid anyhdrides react with primary amines? Secondary Amide In which nucleophilic addition-elimination reactions are steamy white fumes given off? - Acyl chloride with water, to produce carboxylic acid and HCl - Acyl chloride with alcohol, to produce ester and HCl - Note: steamy white fumes are those of HCl What is the advantage of reacting acyl chlorides/acid anhydrides with alcohol, rather than reacting carboxylic acids with alcohol, to produce esters? Reaction is much quicker and is not reversible Why is white smoke given off when acyl chlorides react with ammonia? White smoke of NH4Cl produced How is aspirin produced in the lab? - Made from 2-hydroxybenzoic acid which contains a phenol group (-OH attached to benzene ring) - Phenol group is turned into an ester by reacting it with ethanoic anhydride Why is ethanoic anhydride used instead of acid chlorides, when producing aspirin? - Cheaper - Less corrosive - Less vulnerable to hydrolysis - Less dangerous to use What are the six stages in purifying an organic solid via recrystallisation e.g. crude aspirin? 1 - Dissolve the impure compound, in a minimum volume of hot (near boiling) solvent 2 - Hot filter solution through fluted (folded) filter paper quickly 3 - Cool the filtered solution by inserting beaker in ice 4 - Carry out suction filtration with a Buchner flask, to separate out crystals 5 - Wash the crystals with distilled water 6 - Dry the crystals between absorbent paper For the recrystallisation process, what is an appropriate solvent, for stage 1? - Will dissolve both the compound and impurities when hot - One in which the compound itself does not dissolve well in when cold For the recrystallisation process, why is a minimum volume of hot water used, for stage 1?? - To obtain saturated solution - To enable crystallisation on cooling For the recrystallisation process, what is the purpose of hot filtration, during stage 2? - To remove any insoluble impurities - Heat prevents crystals reforming during filtration In recrystallisation, what happens to the crystals and soluble impurities in stage 3, when the filtered solution is cooled? - Crystals will reform - Use of ice will increase the yield of crystals - Soluble impurities remain in solution because they are present in small quantities / solution is not saturated List three ways in which yield is lost in the recrystallisation process - Crystals lost when filtering or washing - Some product stays in solution after recrystallisation - Unwanted side reactions may occur Describe appearance of crystals formed, when aspirin is purified - Large needle-like crystals - Lighter in colour How do we test for degree of purity, in purified samples? Determine the melting point, or melting range, of the sample What happens to melting point / range when sample is very pure? Melting point is sharp and equal to data book value What happens to melting point / range when sample contains impurities (inc. solvent from recrystallisation process) ? - Melting point lowered - Sample melts over a range of several °C