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JUNIOR COLLEGE CHEMISTRY DEPARTMENT SECOND YEAR PRACTICAL EXPERIMENT 15 Name: __________________________ Group: _________ Date: ___________ This practical continues the study of the aromatic series of compounds. It will illustrate some common reactions of a few chosen important aromatic compounds. You will be working with compounds EE to HH. Always use small samples unless otherwise specified and take great care to avoid droppers of reagent bottles getting interchanged. Slight contamination can lead to completely wrong results. ___________________________________________________________________________________ OH 5. The compound EE is phenol : It consists of colourless deliquecent crystals which turn pink due to atmospheric oxidation. PHENOL IS A VERY CORROSIVE SUBSTANCE. AVOID ALL CONTACT WITH THE SKIN AND DO NOT SPILL ANY OF THE SOLID ON THE BENCHES. Note the characteristic odour of phenol ; this resembles carbolic 3 Place a few crystals of phenol in a test tube and add about 4 cm of distilled water. Shake well and use portions of the solution for the following tests. (a) To a portion of the solution add 2 drops of Universal indicator. OBSERVATIONS INFERENCES Solid phenol dissolves only slightly in Acidic character is seen. Phenol (PhOH) ionizes water. Solution has a pH of ~ 5 -6 in water to a higher degree than H2O PhOH(aq) à H+(aq) + PhO-(aq) (b) Add some of the phenol solution to a little sodium carbonate solution and warm gently. CARE OBSERVATIONS INFERENCES The phenol dissolves but no gas Phenol though acidic, is not acidic enough to release CO2 from CO32- i.e. In acidity H2CO3 > (effervescence) is seen PhOH > H2O > ROH PhOH(aq) + CO32-(aq) à PhO-(aq) + HCO3-(aq). (c) Add some drops of neutral iron(III)chloride solution to a portion of the solution of phenol. OBSERVATIONS INFERENCES The addition of neutral FeCl3 to phenol Phenols give a brightly coloured solution with (EE) gave a bright purple/ violet solution FeCl3.(Test is used to distinguish between alcohols i.e. ROH and phenols i.e. PhOH / ArOH The OH group is an activating group. Explain the meaning of this term. Expt 15 Page 1 Explanation: .An activating group is one which increases the electron density on the aromatic ring and as a result the ring becomes more reactive towards electrophiles. (Reactivity is greater than that of the parent i.e. benzene.) When aqueous bromine is added to solutions of such compounds, the tribromoderivative is formed as a white crystalline precipitate. 3 (d) Add about 1 cm of bromine water top a portion of the solution of phenol. OBSERVATIONS INFERENCES The brown colour of Br2(aq) disappears The decolourisation of bromine water, which is and a white precipitate is obtained followed by the formation of a white precipitate, is indicative of the presence of an activated (benzene) ring e.g. a phenol, phenylamine or a derivative of these. OH OH Br Br (aq) + + 3Br 2(aq) Br 3HBr (s) (aq) 6. The molecular formula of the compound FF is C7H6O3 ; it has 2 functional groups attached to the aromatic ring. When carrying out tests (b), (c)(i),(ii) with compound FF it would be useful to compare the results obtained with those of the tests with compounds DD and EE. KEEP IN MIND THE FACT THAT FF HAS TWO FUNCTIONAL GROUPS. (a) Ignite a small portion of FF. Do this by placing some FF on a crucible lid: then heat from underneath until the solid melts and then apply the flame from above. OBSERVATIONS INFERENCES FF burns with aluminous and very sooty A highly sooty flame suggests a high degree of flame unsaturation – aromatic compound likely. 3 (b) Place some FF in a test tube and add about 2 cm of aqueous sodium carbonate. If nothing happens in the cold warm gently. OBSERVATIONS INFERENCES The water insoluble FF was first Effervescence with CO32- confirms the presence of dissolved by warming. Addition of an acidic group (This also explains the Na2CO3(aq) gave a gentle effervescence. insolubility of FF in cold water – a property of The gas turned lime-water milky. aromatic carboxylic acids. Expt 15 Page 2 (c). Dissolve a small portion of FF in water and to separate portions of the solution add: (i) a few drops of neutral iron(III)chloride solution. OBSERVATIONS INFERENCES A brightly coloured solution was obtained Formation of the brightly coloured solution with when FeCl3 was mixed with a sample of FF shows that the latter is (also) a phenol i.e. has FF an OH group directly attached to the ring. (ii) a few drops of bromine water. OBSERVATIONS INFERENCES The brown Br2(aq) colour is discharged Observations indicative of the presence of an and a white precipitate forms activated ring i.e. a phenol, phenylamine or one of their derivatives (Formula here rules out the amine. See also equation (i) at the end of this practical) (d) Heat a small portion of FF with an approximately equal quantity of soda-lime. Note the odour of the product. CARE. OBSERVATIONS INFERENCES A carbolic smell was produced when FF Carbolic smell indicates the formation of phenol was heated strongly with (solid) sodalime – a result of a decarboxylation reaction. Hence FF must be a carboxylated phenol. i.e. FF is a hydroxybenzoic acid 3 (e) Place a small portion of FF in a test tube , add about 1 cm of methanol and a few drops of concentrated sulphuric acid. Warm the mixture in a water bath for a few minutes. Allow to cool and then pour the contents of the test tube into some aqueous sodium carbonate in small beaker. OBSERVATIONS INFERENCES After heating FF in CH3OH + c. H2SO4, Smell is that for methyl 2-hydroxybenzoate – and adding the cooled mixture to Na2CO3 a highly characteristic smell. FF must therefore a strong smell of surgical spirit/Oil of be 2-hydroxybenzoi c acid wintergreen/“Vicks” was noted. Expt 15 Page 3 Suggest a name and structure for FF: OH COOH Name:..2-hydroxybenzoic acid........ In view of the two functional groups present in FF comment on the results obtained in tests: u (c)(i) - N.B. Keep in mind that both functional groups in FF react with neutral iron(III)chloride Comment: The deactivating COOH would have given a buff precipitate with FeCl3 while the phenol gives a strongly coloured solution. FF, with both types of groups, gives the formation of a brightly coloured solution. The activating group chemistry i.e. that of the OH dominates over that of the deactivating group, the COOH u (c)(ii) - N.B. Keep in mind that one of the functional groups present in FF is activating and the other deactivating. Comment: For c(ii), ArCOOH + Br2 would give a slow decolourisation reaction while the ArOH gives an instant decolourisation with the subsequent formation of a white precipitate. F, having both groups, gives an immediate decolourisation followed by the formation of a white precipitate. Thus the activating group reaction is, once again, dominant over that of the deactivating group 7. The molecular formula of compound GG is C9H8O2 (a) Ignite a small portion of the solid GG. OBSERVATIONS INFERENCES GG burns with a luminous, very sooty Degree of sootiness suggests a highly unsaturated flame compound – aromatic compound likely (b) Mix some GG with an approximately equal amount of solid sodium carbonate in a test tube. then add some distilled water and warm gently IF nothing happens in the cold. OBSERVATIONS INFERENCES When a suspension of GG in water was Release of CO2 from CO32-.indicates the presence heated, then mixed with Na2CO3(aq) , of an acidic group i.e. COOH/ SO3H (if formula effervescence was observed. Gas turned prmits) lime-water milky 3 (c) Place some GG in a test tube and add about 1 cm of ethanol and a few drops of concentrated sulphuric acid. Warm the mixture for a few minutes in a water- bath , allow to cool and then pour the contents of the test tube into some sodium carbonate solution. OBSERVATIONS INFERENCES Expt 15 Page 4 The addition of the cooled mixture of GG Fruity smell ( from an ester), by reaction with an + C2H5OH + H2SO4 to aqueous Na2CO3 alcohol confirms the presence of a carboxylic gave the release of a fruity smelling acid group in GG vapour (d) Dissolve some GG in hot water and divide the solution into two portions. (i) to the first portion add a few drops of bromine water. NOTE CAREFULLY WHAT HAPPENS. OBSERVATIONS INFERENCES Br2 is decolourised – mixture goes Decolorisation suggests either an activated ring slightly cloudy. White ppt?. and/ or an unsaturation on a side-chain (ii) to the other portion add a few drops of sodium carbonate solution followed by 2 drops of dilute potassium manganate(VII) solution. OBSERVATIONS INFERENCES The purple MnO4- in aq. CO32- turns to Colour change shows that an oxidisable material a brown precipitate on the addition of FF such as an alkene (i.e. an unsaturation on a side chain) is present In view of the results obtained in the above tests, complete the following structure of GG: H H C H C H C COOH C H COOH 8. The molecular formula of HH is C7H6O2. (a) Ignite 2 drops of the liquid on a crucible lid. OBSERVATIONS INFERENCES HH burns with a very sooty flame. Its Sootiness in flame indicates a high degree of flame is also fairly luminous unsaturation i.e. HH is likely to be an aromatic compound 3 (b) Add 1 drop of HH to about 1 cm of neutral iron(III)chloride. OBSERVATIONS A highly coloured solution is seen when INFERENCES A phenol (or a derivative) is indicated HH is mixed with FeCl3 3 (c) Add 1 drop of HH about 1 cm of bromine water. Expt 15 Page 5 OBSERVATIONS INFERENCES The bromine instantly decolourised and Reaction with Br2 suggests and confirms the a white precipitate was formed presence of an activated ring e.g. a phenol, a phenylamine or one of their derivatives. 3 (d) Place 1 cm of 2,4 - D.N.P.H reagent in a test tube and add 1 drop of HH. OBSERVATIONS INFERENCES Mixing 2,4-DNPH with a sample of HH Formation of a precipitate with 2,4-DNPH gave an orange/yellow precipitate confirms that the unknown compound is either an aldehyde or a ketone i.e. it has a carbonyl group in its structure 3 (e) Place 1 cm of silver nitrate solution in a test tube and add 1 drop of aqueous sodium hydroxide. Then add aqueous ammonia dropwise until only a faint trace of precipitate remains. Add 1drop of HH to the mixture. OBSERVATIONS INFERENCES A silver mirror was produced when HH HH is a reducing agent since only these can give was treated with Tollen’s reagent a silver mirror with Tollen’s. Aldehydes are reducing agents (and have a carbonyl group as seen in test (d) above)…(See eqn. (ii) below).. N.B. RINSE THE TEST TUBE WITH DILUTE NITRIC ACID AFTER YOU FINISH THE ABOVE TEST. Suggest possible structural formulae for: (i) HH : (ii) The main organic products obtained in reactions (c) and (d) : OH H C OH O Br O 2N Br OH H C N N H Name:. 2-hydroxybenzaldehyde. Br Expt 15 Page 6 NO2 (i) OH OH COOH Br Br2 Br + CO2 Br (ii) OH OH OH CHO Br2 Br COOH Br Br2 + CO2 Br Expt 15 Page 7