Carboxylic acids around us Ethanedioic acid (oxalic acid) Ethanoic acid (acetic acid) 2-hydroxypropanoic acid (lactic acid) Methanoic acid (formic acid) 3-hydroxypentanedioicacid – 3 carboxylic acid (citric acid)

The carboxylic acid functionality They contain a carboxyl functional group COOH The bonds are in a planar arrangement And include a carbonyl (C=O) group and a hydroxyl (O-H) group They are also isomeric with esters

The carboxylic acid functionality They form a homologous series, which, when more carbon atoms are present can demonstrate structural isomerism.

Naming carboxylic acids Acids are named according to standard IUPAC rules But, many are still known under their trivial names, some having been called after characteristic properties or their origin. Formula Systematic name (trivial name) origin of name HCOOH methanoic acid formic acid latin for ant CH3COOH ethanoic acid acetic acid latin for vinegar C6H5COOH benzenecarboxylic acid benzoic acid from benzene

Physical properties Boiling point: Increases as Mr increases. This is a result of increased London (van der Waals) forces. Carboxylic acids have high boiling points for their relative mass. This is as a result of: Inter-molecular hydrogen bonding due to polar O—H bonds. AN EXTREME CASE... DIMERISATION N.B: The increased inter-molecular forces mean more energy is required to separate individual molecules. HYDROGEN BONDING

Physical properties Solubility: Carboxylic acids are soluble in organic solvents. They are also soluble in water due to hydrogen bonding Small ones dissolve readily in cold water. As mass increases, the solubility decreases. Benzoic acid is fairly insoluble in cold water but soluble in hot water HYDROGEN BONDING

Chemical properties of carboxylic acids ACIDITY As a weak acid: RCOOH + H2O(l) RCOO¯(aq) + H3O+(aq) Formation of salts: RCOOH + NaOH(aq) RCOO¯Na+(aq) + H2O(l) The acid can be liberated from its salt by treatment with a stronger acid. e.g. RCOO¯ Na+(aq) + HCl(aq) RCOOH + NaCl(aq) This method of forming a soluble salt followed by acidification to produce the acid again is often used to separate acids from a mixture.

Preparation of carboxylic acid Oxidation of aldehydes (from 1° alcohols): RCHO + [O] RCOOH Hydrolysis of esters: RCOOR + H2O RCOOH + ROH Hydrolysis of acyl chlorides: RCOCl + H2O RCOOH + HCl Hydrolysis of nitriles: RCN + 2H2O RCOOH + NH3 Hydrolysis of amides: RCONH2 + H2O RCOOH + NH3

Key reactions: Neutralisation to produce salts Sodium carbonate RCOOH + Na2CO3 RCOO-Na+ + NaHCO3 2RCOOH + Na2CO3 RCOO-Na+ + H2O + CO2 Sodium hydrogencarbonate RCOOH + NaHCO3 RCOO-Na+ + H2O + CO2 N.B: This can be used to determine the amount of citric acid in fruit. How?

Key reactions: Chlorination of carboxylic acids Involves replacing the OH with a Cl. The product is an acyl chloride. The reagent is phosphorus pentachloride (PCl5) DRY (dry ether) conditions are required to prevent hydrolysis of PCl5 CH3COOH + PCl5 CH3COCl + POCl3 + HCl N.B: Could also use thionyl chloride (SOCl2) producing SO2 instead of POCl3 PCl5 not usually used as the POCl3 product is very reactive. If alcohol is present, this will chlorinate the alcohol too. If primary amide present, this will oxidise it into the nitrile. A very reactive entity made. If thionyl chloride used, products are unreactive and do cause potential problems to other functional groups that may be present. POCl3 is caustic and toxic – adding to it not being used in industry.

Key reactions: Reduction of carboxylic acids The product is an alcohol. The reagent is Lithium aluminium hydride (LiAlH4) DRY (dry ether) conditions are required to prevent hydrolysis of LiAlH4 RCOOH + 4[H] RCH2OH + H20

Key reactions: Esterification The reagents are an alcohol + strong acid catalyst (e.g conc. H2SO4 ) Reflux conditions are required to produce an ester Example prep: Preparation of ethyl ethanoate CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l) ethanol ethanoic acid ethyl ethanoate water N.B: Conc.H2SO4 is a dehydrating agent. It removes water causing the equilibrium to move to the right so increasing the ester yield

Naming esters Basically, an organic group is substituted in for the H in a carboxylic acids The first part of the name comes from the alcohol. Second part then comes from the acid e.g. methyl ethanoate CH3COOCH3 Key point about esters: Made from carboxylic acids or acyl chlorides. They are unreactive compared with acids and acyl chlorides Esters are structural isomers of carboxylic acids ETHYL METHANOATE

Uses of esters Despite being fairly chemically unreactive, esters are useful as ... • flavourings apple 2-methylbutanoate pear 3-methylbutylethanoate banana 1-methylbutylethanoate pineapple butylbutanoate rum 2-methylpropylpropanoate • solvents nail varnish remover - ethyl ethanoate • plasticisers in polymer preparation (PVC vs uPVC)