JUNIOR COLLEGE CHEMISTRY DEPARTMENT EXPERIMENT 15 SECOND YEAR PRACTICAL. Name: Group: Date:

Transcription

1 JUNIOR COLLEGE CEMISTRY DEPARTMENT EXPERIMENT 15 SECOND YEAR PRACTICAL 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. 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. 5. The compound EE is phenol : It consists of colourless deliquecent crystals which turn pink due to atmospheric oxidation. PENOL IS A VERY CORROSIVE SUBSTANCE. AVOID ALL CONTACT WIT TE SKIN AND DO NOT SPILL ANY OF TE SOLID ON TE BENCES. Note the characteristic odour of phenol ; this resembles carbolic Place a few crystals of phenol in a test tube and add about 4 cm 3 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. Solid phenol dissolves only slightly in water. Solution has a p of ~ 5-6 Acidic character is seen. Phenol (Ph) ionizes in water to a higher degree than 2 O Ph (aq) à + (aq) + PhO - (aq) (b) Add some of the phenol solution to a little sodium carbonate solution and warm gently. CARE The phenol dissolves but no gas Phenol though acidic, is not acidic enough to (effervescence) is seen release CO 2 from CO 2-3 i.e. In acidity 2 CO 3 > Ph > 2 O > R 2- Ph (aq) + CO 3 (aq) à PhO - - (aq) + CO 3 (aq). (c) Add some drops of neutral iron(iii)chloride solution to a portion of the solution of phenol. The addition of neutral FeCl 3 to phenol (EE) gave a bright purple/ violet solution Phenols give a brightly coloured solution with FeCl 3.(Test is used to distinguish between alcohols i.e. R and phenols i.e. Ph / Ar The group is an activating group. Explain the meaning of this term. Expt 15 Page 1

2 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. (d) Add about 1 cm 3 of bromine water top a portion of the solution of phenol. The brown colour of 2(aq) disappears and a white precipitate is obtained The decolourisation of bromine water, which is 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. (aq) + 3 2(aq) (s) + 3 (aq) 6. The molecular formula of the compound FF is C 7 6 O 3 ; 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 TE FACT TAT FF AS 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. FF burns with aluminous and very sooty flame A highly sooty flame suggests a high degree of unsaturation aromatic compound likely. (b) Place some FF in a test tube and add about 2 cm 3 of aqueous sodium carbonate. If nothing happens in the cold warm gently. The water insoluble FF was first dissolved by warming. Addition of Na 2 CO 3(aq) gave a gentle effervescence. The gas turned lime-water milky. Effervescence with CO 2-3 confirms the presence of an acidic group (This also explains the insolubility of FF in cold water a property of aromatic carboxylic acids. Expt 15 Page 2

3 (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. A brightly coloured solution was obtained when FeCl 3 was mixed with a sample of FF Formation of the brightly coloured solution with FF shows that the latter is (also) a phenol i.e. has an group directly attached to the ring. (ii) a few drops of bromine water. The brown 2(aq) colour is discharged and a white precipitate forms Observations indicative of the presence of an 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) eat a small portion of FF with an approximately equal quantity of soda-lime. Note the odour of the product. CARE. A carbolic smell was produced when FF was heated strongly with (solid) sodalime Carbolic smell indicates the formation of phenol a result of a decarboxylation reaction. ence FF must be a carboxylated phenol. i.e. FF is a hydroxybenzoic acid (e) Place a small portion of FF in a test tube, add about 1 cm 3 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. After heating FF in C 3 + c. 2 SO 4, Smell is that for methyl 2-hydroxybenzoate and adding the cooled mixture to Na 2 CO 3 a strong smell of surgical spirit/oil of a highly characteristic smell. FF must therefore be 2-hydroxybenzoi c acid wintergreen/ Vicks was noted. Expt 15 Page 3

4 Suggest a name and structure for FF: CO 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 CO would have given a buff precipitate with FeCl 3 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 dominates over that of the deactivating group, the CO 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), ArCO + 2 would give a slow decolourisation reaction while the Ar 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 C 9 8 O 2 (a) Ignite a small portion of the solid GG. GG burns with a luminous, very sooty flame Degree of sootiness suggests a highly unsaturated 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. When a suspension of GG in water was heated, then mixed with Na 2 CO 3(aq), effervescence was observed. Gas turned Release of CO 2 from CO 3 2-.indicates the presence of an acidic group i.e. CO/ SO 3 (if formula prmits) lime-water milky (c) Place some GG in a test tube and add about 1 cm 3 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. Expt 15 Page 4

5 The addition of the cooled mixture of GG Fruity smell ( from an ester), by reaction with an + C SO 4 to aqueous Na 2 CO 3 alcohol confirms the presence of a carboxylic gave the release of a fruity smelling vapour acid group in GG (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 WAT APPENS. 2 is decolourised mixture goes slightly cloudy. White ppt?. Decolorisation suggests either an activated ring 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. The purple MnO 4 - in aq. CO 3 2- turns to a brown precipitate on the addition of FF Colour change shows that an oxidisable material 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: C C C C CO CO 8. The molecular formula of is C 7 6 O 2. (a) Ignite 2 drops of the liquid on a crucible lid. burns with a very sooty flame. Its flame is also fairly luminous Sootiness in flame indicates a high degree of unsaturation i.e. is likely to be an aromatic compound (b) Add 1 drop of to about 1 cm 3 of neutral iron(iii)chloride. A highly coloured solution is seen when A phenol (or a derivative) is indicated is mixed with FeCl 3 (c) Add 1 drop of about 1 cm 3 of bromine water. Expt 15 Page 5

6 The bromine instantly decolourised and a white precipitate was formed Reaction with 2 suggests and confirms the presence of an activated ring e.g. a phenol, a phenylamine or one of their derivatives. (d) Place 1 cm 3 of 2,4 - D.N.P. reagent in a test tube and add 1 drop of. Mixing 2,4-DNP with a sample of gave an orange/yellow precipitate Formation of a precipitate with 2,4-DNP confirms that the unknown compound is either an aldehyde or a ketone i.e. it has a carbonyl group in its structure (e) Place 1 cm 3 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 to the mixture. A silver mirror was produced when was treated with Tollen s reagent is a reducing agent since only these can give 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 TE TEST TUBE WIT DILUTE NITRIC ACID AFTER YOU FINIS TE ABOVE TEST. Suggest possible structural formulae for: (i) : (ii) The main organic products obtained in reactions (c) and (d) : C O O 2 N C N N NO 2 Name:. 2-hydroxybenzaldehyde. Expt 15 Page 6

7 (i) CO 2 + CO 2 (ii) CO CO CO 2 Expt 15 Page 7