REACTIONS OF FUNCTIONAL GROUPS OF ORGANIC COMPOUNDS WITH BIOCHEMICAL IMPORTANCE

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1 REACTIONS OF FUNCTIONAL GROUPS OF ORGANIC COMPOUNDS WITH BIOCHEMICAL IMPORTANCE Key words: hydroxy compounds, oxo (carbonyl) compounds, alcohols, aldehydes, ketones, oxidation of alcohols, addition reaction of carbonyl group, acids, bases, phenols, carboxylic acids, salts, esters, amines, amides, complex compounds. Reagents: 1. Methanol!POISON!: 2. Ethanol 3. 4% formaldehyde solution 4. Salicylic acid 5. Oxalic acid 6. 70% phenol solution 7. Urea 8. Potassium dichromate 9. Concentrated sulphuric acid!caustic! 10. Schiff s reagent (fuchsine solution decolourised with sodium hydrogensulphite) 11. Sodium hydroxide!caustic! 2 mol/l 12. Concentrated ammonia 13. Hydrochloric acid diluted 1:1 14. Calcium chloride solution 0.1 mol/l 15. Iron (III) ch loride solution 0.05 mol/l 16. Copper sulfate solution 0.02 mol/l 1

2 Exercise 1 Oxidation of primary alcohols and aldehydes Primary alcohols are oxidized to Carbonyl group compounds - aldehydes. In practice it was used for detection of ethanol in breath acidified orange potassium dichromate (Cr(VI)) is reduced to green colored solution (Cr(III)) in presence of alcohol (which is oxidized to acetaldehyde). For aldehyde detection is used Schiff s reagent (Exercise 1.1). This reagent form colored compound in presence of aldehydes to gives so called Schiff s bases (aldimine; exercise 1.2) The aldehyde group is present in the entire spectrum of biologically significant organic molecules (carbohydrates). To functional detection is used usually the Fehling test. Cupric tartrate in an alkaline solution reacts with the aldehyde to form the corresponding acid, cuprous oxide and sodium tartrate (exercise 1.2). Fehling reagent is always prepared just before use by mixing the solutions Fehling I (copper sulfate) and Fehling II (sodium hydroxide and potassium sodium tartrate) in a 1:1 ratio Reaction of aldehydes with Schiff s reagent Perform this reaction used methanol and ethanol respectively. Put a small quantity of potassium dichromate into a test tube and add very carefully 5 drops of concentrated sulphuric acid (use a plastic pipette). Add l ml of an alcohol and mix carefuly. Heat the each test tube in the boiling water bath. Put 1 drops of Schiff s reagent (use a plastic pipette) on a strip of the filter paper. Detect the arising aldehyde holding the prepared strip with Shiff s reagent at the top of the tube. The red-violet colour indicates the presence of an aldehyde. Write the chemical equation of oxidation of methanol (a) and ethanol (b): a) b) 2

3 1.2 - Reduction of Fehling s solution Dark blue Fehling s solution contains complex copper (II) cations. Aldehydes are strong enough to reduce them to copper(i) oxide which is visible as an insoluble red precipitate. Copper(I) does not form any complex ion. Tartaric acid is aliphatic hydroxyl acid that easily forms complexes with metal ions. Insoluble copper hydroxide turns to solution after adding tartaric acid because resulting complex compound prevents the reaction of copper(ii) with hydroxide anions. After that, copper(ii) in complex with tartaric acid is easily reduced to copper(i). In first test tube prepare 2 ml of aqueous solution of tartaric acid In second test tube mix 1 ml of the copper sulphate with 1 ml of sodium hydroxide solution Mix Fehling s solutions I and II (l : l, total volume about 1 ml). Add 2 drops of formaldehyde. Heat gently in the boiling water bath. The appearance of a red precipitate of copper(i) oxide indicates the presence of the aldehyde. 3

4 Exercise 2 Solubility of organic acids and their salts 2.1 Salicylic acid (2-hydroxybenzoic acid) The salicylic acid (precursor for the production of acetylsalicylic acid - aspirin) is only slightly soluble in water. After the addition of sodium hydroxide solution it dissolves, due to the formation of water-soluble sodium salts. Try to dissolve a small quantity of the acid in 1 ml of water. Its solubility in water is limited. Add 2 ml of sodium hydroxide solution - the suspension of the acid is dissolved. Write the reaction in the ionic form: 4

5 2.2 Oxalic acid (ethanedioic acid) Calcium oxalate is insoluble in water but soluble in inorganic acids. Due to the formation of insoluble calcium oxalate, oxalic acid is toxic. Oxalate calcium may be present in the urine, where easily forms urinary stones. a) Dissolve a small quantity of oxalic acid in water. Alkalize it with the solution of ammonia (use a plastic pipette), check by a ph strip (a). Add calcium chloride solution in drops - white precipitate is forming (b). Arisen calcium oxalate can be dissolved in diluted hydrochloric acid, the reaction occurs under ph 5.0 (c). Try to write the reactions in the ionic form: b) c) 5

6 Exercise 3 Formation of complexes 3.1 The reaction with iron(iii) chloride Most of phenols (aromatic hydroxy compounds) react with a water solution of FeCl 3 forming complex iron(iii) ions. Phenol forms with FeCl 3 violet coloured solution, so called Uffelmann s reagent. This reagent was used to detect lactic acid in the gastric juice. Some hydroxy acids tend to form complexes as well. Because iron(iii) is bound to lactic acid (2-hydroxypropanoic acid) stronger (chelating ligand) than to phenol, a process of ligand replacement occurs. Pour 1 ml of phenol solution into a test tube and add 2 drops of the water solution of iron(iii) chloride to prepare violet coloured Uffelmann s reagent. Using plastic pipette add lactic acid drop wise (shaking the test tube) until the violet colouring disappears (complex of lactic acid with iron(iii) ion is yellow). Perform the reaction of iron(iii) chloride with salicylic acid (2- hydroxybenzoic acid) in the same way. In this case, dissolve a few crystals of salicylic acid in 1 ml of ethanol). The red-violet solution of a complex is formed. Explain differences between colors of complexes 6

7 Exercise 4 Urea 4.1 Hydrolysis Urea, diamide of carbonic acid, is very well soluble in water. During alkaline hydrolysis, urea is cleaved, like other amides, into ammonia and the alkaline salt of the original acid, in this case sodium carbonate. Dissolve a small quantity of urea in 1 ml of water. Add 2 ml of sodium hydroxide solution and mix well. Heat in the boiling water bath some min. Wet the indicator strip and detect the arising ammonia at the top of the tube (blue colour). Describe the reaction in the form of the chemical equation: 7