Properties PAG7
6.1.3 Carboxylic Acids and Esters Solubility in Water The smaller carboxylic (up to C4) acids dissolve in water in all proportions but after this the solubility rapidly reduces. They dissolve because they can hydrogen bond to the water molecules. Acidity The carboxylic acid are only weak acids in water and only slightly dissociate, but they are strong enough to displace carbon dioxide from carbonates. CH3CO2H(aq) CH3CO2 – (aq)+ H+ (aq) Delocalisation The carboxylic acid salts are stabilised by delocalisation, which makes the dissociation more likely. delocalised H3C C O OH H3C C O O The 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. H3C C O O The pi charge cloud has delocalised and spread out. The delocalisation makes the ion more stable and therefore more likely to form. Strength of carboxylic acids C C O OH H H Cl CH2 C O OH H3C CH2 C O O delocalised H3C delocalised Increasing chain length pushes electron density on to the COOion, making it more negative and less stable. This make the acid less strong. Alkyl groups electron releasing Chlorine electron withdrawing Electronegative chlorine atoms withdraw electron density from the COOion, making it less negative and more stable. This make the acid more strong. Propanoic acid less acidic than ethanoic acid chloroethanoic acid more acidic than ethanoic acid N Goalby chemrevise.org C C O O Cl H H H3C C O O H O H δ − H δ − δ − δ + δ + δ + : δ + Salt formation reactions of carboxylic acids Carboxylic acids can form salts with metals, alkalis and carbonates. acid + metal (Na) salt + hydrogen CH3CO2H + Na CH3CO2 -Na+ + H2 acid + alkali (NaOH) salt + water CH3CO2H + NaOH CH3CO2 -Na+ + H2O acid + carbonate (Na2CO3 ) salt + water + CO2 2CH3CO2H + Na2CO3 2CH3CO2 -Na+ + H2O + CO2 The effervescence caused by production of CO2 with carboxylic acids with solid Na2CO3 or aqueous NaHCO3 can be used as a functional group test for carboxylic acids. Oxidation of methanoic acid Carboxylic acids cannot be oxidised by using oxidising agents but methanoic acid is an exception as its structure has effectively an aldehyde group C O O H H C O O H + [O] H O It forms carbonic acid (H2 CO3 ) which can decompose to give CO2
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6.1.3 Carboxylic acids and esters
Properties of carboxylic acids (a) explanation of the water solubility of carboxylic acids in terms of hydrogen bonding (b) reactions in aqueous conditions of carboxylic acids with metals and bases (including carbonates, metal oxides and alkalis) Comparison of acidity of different carboxylic acids not required.
Esters
Esterification Carboxylic acids react with alcohols, in the presence of a strong sulphuric acid catalyst, to form esters and water. C C C O O C H H H H H H H H Esters have two parts to their names, eg methyl propanoate. The bit ending in –yl comes from the alcohol that has formed it and is next to the single bonded oxygen. The bit ending in –anoate comes from the carboxylic acid and includes the C in the C=O bond. Carboxylic Acid + Alcohol Ester + water H+ The reaction is reversible. The reaction is quite slow and needs heating under reflux, (often for several hours or days). Low yields (50% ish) are achieved. An acid catalyst (H2SO4 ) is needed. H C + + H2O 3 C O OH C C O O C C H H H H H H H H H+ C O H H H C H H H CH3CO2H + CH3CH2OH CH3CO2CH2CH3 + H2O Ethanoic acid Ethanol Ethyl Ethanoate Oxidation of methanoic acid Carboxylic acids cannot be oxidised by using oxidising agents but methanoic acid is an exception as its structure has effectively an aldehyde group C O O H H C O O H + [O] H O It forms carbonic acid (H2 CO3 ) which can decompose to give CO2 N Goalby chemrevise.org Change in functional group: acid anhydride ester Reagent: alcohol Conditions: room temp. (RCO)2O + CH3CH2OH RCO2CH2CH3 +RCO2H CH3 C O O CH3 C O C C O O C C H H H H H H H + CH3CH2OH H + CH3CO2H Esterification using acid anhydrides The acid anhydrides are more reactive than carboxylic acids. The reaction is not reversible and a higher yield is achieved. Hydrolysis of esters Esters can be hydrolysed and split up by either heating with acid or with sodium hydroxide. i) with acid reagents: dilute acid (HCl) conditions: heat under reflux This reaction is the reverse reaction of ester formation. When an ester is hydrolysed a carboxylic acid and an alcohol are formed. CH3CH2CO2CH2CH3 + H2O CH3CH2CO2H + CH3CH2OH ethyl propanoate H+ This reaction is reversible and does not give a good yield of the products. ii) with sodium hydroxide reagents: dilute sodium hydroxide conditions: heat under reflux CH3CH2CO2CH3 + NaOH CH3CH2CO2 – Na+ + CH3OH methyl propanoate sodium propanoate methanol The carboxylic acid salt product is the anion of the carboxylic acid. The anion is resistant to attack by weak nucleophiles such as alcohols, so the reaction is not reversible. This reaction goes to completion.
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6.1.3 Carboxylic acids and esters
Esters (c) esterification of: (i) carboxylic acids with alcohols in the presence of an acid catalyst (e.g. concentrated H2SO4) (ii) acid anhydrides with alcohols (d) hydrolysis of esters: (i) in hot aqueous acid to form carboxylic acids and alcohols (ii) in hot aqueous alkali to form carboxylate salts and alcohols
Acyl chloride
Acyl Chlorides CH3 C O Cl ethanoyl chloride Acyl chlorides are much more reactive than carboxylic acids The Cl group is classed as a good leaving groups (to do with less effective delocalisation.) This makes acyl chlorides much more reactive than carboxylic acids and esters CH3COOH + SOCl2 CH3COCl + SO2 + HCl Formation of acyl chloride from a carboxylic acid Reaction: carboxylic acid acyl chloride Reagent: SOCl2 Conditions: room temp H3C C O OH H3C C O Cl + SOCl2 + SO2 + HCl Sulphur dichloride oxide (thionyl chloride) SOCl2 is a liquid Reaction with water Change in functional group: acyl chloride carboxylic acid Reagent: water Conditions: room temp. RCOCl (l) + H2O RCO2H + HCl (g) CH3 C O Cl + H2O CH3 C O OH + HCl (g) Observation: Steamy white fumes of HCl are given off Change in functional group: acyl chloride ester Reagent: alcohol Conditions: room temp. RCOCl (l) + CH3CH2OH RCO2CH2CH3 + HCl (g) CH 3 C O Cl + CH3CH2OH + HCl Observation: Steamy white fumes of HCl are given off Reaction with alcohol C C O O C C H H H H H H H H This reaction for making esters is much better than using carboxylic acids as the reaction is much quicker and it is not a reversible reaction Reaction with ammonia Change in functional group: acyl chloride primary amide Reagent: ammonia Conditions: room temp. RCOCl (l) +2NH3 RCONH2 + NH4Cl (s) CH3 C O Cl + 2NH3 + NH4Cl (s) Observation: white smoke of NH4Cl is given off CH3 C O NH2 Reaction with primary amines Change in functional group: acyl chloride secondary amide Reagent: primary amine Conditions: room temp. RCOCl +2CH3NH2 RCONHCH3 + CH3NH3 +ClCH3 C O Cl + 2CH3NH2 + CH3NH3 +ClN-methylethanamide C NH CH3 Reaction with phenol. Change in functional group: acyl chloride ester Reagent: phenol Conditions: room temp. RCOCl (l) + C6H5OH RCO2C6H5 + HCl (g) CH 3 C O Cl + HCl Observation: Steamy white fumes of HCl are given off Reaction with phenol Phenols do not easily form esters with carboxylic acids but do so readily with acyl chlorides
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6.1.3 Carboxylic acids and esters
Acyl chlorides (e) the formation of acyl chlorides from carboxylic acids using SOCl 2 (f) use of acyl chlorides in synthesis in formation of esters, carboxylic acids and primary and secondary amides. Including esterification of phenol, which is not readily esterified by carboxylic acids.
Credits:Neil Goalby