6.1.3: CARBOXYLIC ACIDS & ESTERS

• acyl chlorides contain the functional group R-COCl where R is an alkyl group
• acyl chlorides are carboxylic acid derivatives where the -OH of the acid group has been replaced with a chlorine atom
• small acyl chlorides are fuming (visual gas or vapour released) colourless liquids. they are reactive, with the chlorine atom being substituted for other groups
• when naming acyl chlorides the suffix is -oyl chloride
• to make an acyl chloride, the -OH group on a carboxylic acid must be substituted for a chlorine atom

  one method is to use SOCl2 (liquid at room temp) which readily reacts with a carboxylic acid to make a desired product
• preparation of acyl chloride

  sulfur dioxide and hydrogen chloride gases are also made

  the acyl chloride is separated from the reaction mixture using distillation
• preparation of acyl chloride (e.g ethanoyl chloride) equation

  CH3COOH + SOCl2 -> CH3COCl + SO2 + HCl
• an acid anhydride is an acid derivative that is more reactive than a similar carboxylic acid. it is made by the removal of a molecule of water from 2 carboxylic acid molecules
• acid anhydrides will react with alcohols, including phenol and its derivatives to make an ester

  this method of ester production is not reversible and therefore has a higher yield than using a carboxylic acid

  the rate of reaction is still slow but can be increased by gently warming the reaction mixture
• hydrolysis is a chemical reaction where water breaks down another product. esters can undergo hydrolysis to form an alcohol; reverse of esterification. by varying the conditions, either carboxylic acids or carboxylate salts are formed as the other product
• when esters are refluxed with a catalyst of hot aqueous acids, such as dilute sulfuric acid or dilute hydrochloric acid, the ester will decompose reversibly into an alcohol and a carboxylic acid
• when an ester is refluxed with a hot aqueous alkali such as potassium hydroxide or sodium hydroxide, it will decompose into an alcohol and a carboxylate salt. this reaction is not reversible
• alkaline hydrolysis of esters is used to make soaps, therefore it is also called saponification (from the latin for soap - sapo)
• small carboxylic acids are very soluble in polar solvents like water. this is because hydrogen bonds can be formed between the carboxylic acid functional group and water
• as the hydrocarbon chain of a carboxylic acid increases in size, the solubility decreases

  this is because only the polar carboxylic acid functional group can form hydrogen bonds with the water

  so as more of the molecule becomes non-polar, then solubility will decrease
• carboxylic acids are weak acids as they partially ionise in solution releasing the H+ ion from the carboxylic acid group, forming a carboxylate ion
• reaction with metals

  carboxylic acids will react with metals above hydrogen in the reactivity series to make hydrogen and a metal salt

  sodium + ethanoic acid -> sodium ethanoate + hydrogen

  2Na + 2CH3COOH -> 2CH3COONa + H2
• reaction with metal oxides

  metal oxides react with acids and therefore can be classified as bases.

  magnesium oxide + methanoic acid -> magnesium methanoate + water

  MgO (s) + HCOOH (aq) -> (HCOO)2Mg(aq) + H2O (l)
• reaction with metal hydroxides

  group 1 metal hydroxides are soluble bases that release OH- (aq)

  potassium hydroxide + propanoic acid -> potassium propanoate + water

  KOH (aq) + CH3CH2COOH (l) -> CH3CH2COOK (aq) + H2O (l)
• reaction with metal carbonates

  metal carbonates are also bases

  sodium carbonate + methanoic acid -> sodium methanote + water + carbon dioxide

  Na2CO3 (s) + 2HCOOH (aq) -> 2HCOONa (aq) + H2O (l) + CO2 (g)
• carboxylic acids with an alcohol

  to prepare a small ester, an alcohol and a carboxylic acid are heated gently in the presence of a sulfuric acid catalyst

  esterification is a reversible reaction and has a slow rate of reaction
• uses of acyl chlorides as reagents in organic synthesis: esters

  acyl chlorides will react with alcohols to make an ester. this method of ester production is not reversible and therefore has a higher yield than using a carboxylic acid. the balanced chemical reaction shows the formation of an ester - ethyl ethanoate from an acyl chloride

  CH3COCl + CH3CH2OH -> CH3COOCH2CH3 + HCl
• alcohols can also react with carboxylic acids to make esters. like alcohols, phenols have an -OH group but they do not react easily with carboxylic acids

  therefore to make an ester from phenols the acyl chloride method must be used

  the reaction is violent and produces corrosive fumes of HCl
• uses of acyl chlorides as reagents in organic synthesis: carboxylic acids

  when a small acyl chloride such as ethanol chloride is added to water, it quickly hydrolyses to produce a carboxylic acid. this is a very exothermic reaction and misty fumes of HCl are given off

  CH3COCl + H2O -> CH3COOH + HCl
• uses of acyl chlorides as reagents in organic synthesis: primary amides

  when an acyl chloride reacts with ammonia, a primary amide is produced. to prepare ethanamide, ethanoyl chloride is added to a concentrated ammonia solution. this quickly produces a mixture of solid ammonium chloride and ethanamide - observed as white smoke. some of the products remain in a colourless solution

  CH3COCl + 2NH3 -> CH3CONH2 + NH4Cl
• uses of acyl chlorides as reagents in organic synthesis: secondary amides

  when an acyl chloride reacts with a primary amide, the product is a secondary amide, where the nitrogen has one hydrogen atom directly bonded to it. the nitrogen atom also has 2 organic groups attached and is often called an N-substituted amide
• uses of acyl chlorides as reagents in organic synthesis: secondary amides

  a white solid compound of N-ethylethanamide can be made from ethanoyl chloride and a cold concentrated solution of ethylamine

  CH3COCl + CH3CH2NH2 -> CH3CONHCH2CH3 + HCl