JUNIOR COLLEGE CHEMISTRY DEPARTMENT EXPERIMENT 14 SECOND YEAR PRACTICAL Name: Group: Date: This practical will serve as (i) an introduction to aromatic chemistry and (ii) a revision of some of the reactions encountered in aliphatic chemistry You will be working with compounds AA to LL. 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. Some of the following exercises are interactive:you will have to devise tests and give answers to questions. After each test record your observations and inferences. Equations for the reactions occurring should be given wherever possible. 1. The liquid labeled AA, molecular formula C 7 H 8 O, has the following structural formula CH 2 OH This compound is commonly called Phenylmethanol. benzyl alcohol. Its systematic name is Carry out and describe tests to show that: (a) AA is highly unsaturated Test: Burn a small amount of substance AA burns with a very sooty, highly luminous flame The high level of sootiness indicates a high degree of unsaturation - possibly aromatic (b) The -OH group in AA is not attached directly to the aromatic ring: Test: Add a drop or two of AA to neutral FeCl 3(aq) No visible reaction was seen when these two reagents were mixed. The two reagents gave two separate layers. Lack of formation of a strongly coloured solution here eliminates the presence of a phenol i.e. a compound with an OH directly on the ring Expt 14 Page 1
(c) The -OH group in AA is attached to the aliphatic side chain : Test: Add a drop or two of AA to some solid PCl 5. Addition of AA to PCl 5 gave the release Gas released is HCl and its release on mixing of a gas which fumes in moist air. The with solid PCl 5 is a confirmation of the presence gas turns blue litmus papers red and of an OH group in AA (e.g. either an alcohol or intensifies NH 3(g) an acid). [Phenols tend to behave differently] (d) AA is not an acid : Test: Mix a sample of AA with some solid or aqueous Na 2 CO 3. No visible reaction was given when these two reagents were mixed Lack of reactivity with sodium carbonate rules out the presence of an acid. (e) AA is a primary alcohol : 2- Test: Oxidation with acidified Cr 2 O 7 (aq) should produce a colour change, from orange to green/blue, if the alcohol is a primary or a secondary one Colour of mixture turned from orange to Colour change is indicative of the presence of an green. Smell of almost also noted Oxidisable material or reducing agent e.g a 1 o or 2 o alcohol or an aldehyde. Smell of almonds suggests the formation of benzaldehyde an aldehyde, only possible when 1 o alcohols are oxidized. AA 1 and AA 2 are isomers of AA. AA 1 behaves differently from AA in tests (b), (c) and (e) while compound AA 2 is very unreactive. Suggest possible structural formulae and names for : COMPOUND AA 1 COMPOUND AA 2 OH O CH 3 CH 3 Name:..2-methylphenol... Name:..Methoxybenzene... Expt 14 Page 2
2. The molecular formula of the liquid BB is C 7 H 6 O. The characteristic odour of this compound resembles that of : bitter almonds. (a) Ignite a few drops of BB by placing on a crucible lid and heating them from above by the bunsen burner. Liquid BB burns with a very sooty, luminous flame The level of sootiness suggests a highly unsaturated compound possibly aromatic (b) Place about 1 cm 3 of 2,4 - DNPH. reagent in a test tube and add one drop of BB. An orange precipitate was seen to form. Formation of a precipitate with 2,4-DNPH Confirms the presence of a carbonyl compound e.g. an aldehyde or ketone (c) Place 1 cm 3 of the sodium hydrogen sulphite solution in a test tube and add one drop of BB. Set aside for about 30 minutes. A white crystalline solid precipitates out when the two colourless liquids are mixed The crystalline solid formed with the NaHSO 3 is the adduct and is seen to form when carbonyl compounds e.g. aldehydes or ketones are present.. (d) Prepare some Tollens reagent as follows: place about 2 cm 3 of silver nitrate solution in a test tube and add 2 drops of sodium hydroxide solution. Then add aqueous ammonia slowly until only a faint trace of precipitate remains. Divide this solution into two portions: use one portion for the following test and keep the other portion for test 3 (c) on page 5. To one portion of the Tollens reagent add 1 drop of BB and warm carefully in a water-bath if necessary. A silver mirror eventually forms on the inside of the test tube containing all the reagents The generation of a silver mirror from Tollens reagent is a confirmation of the presence of a reducing agent e.g. an aldehyde Expt 14 Page 3
In view of the results obtained in the above tests, suggest a structural formula and name for BB : CHO Name:...Benzalehyde... Explain briefly why a crust of white crystals form around the neck of the bottle where BB is in contact with air. Explanation:.Aerial oxidation with atmospheric oxygen turns the liquid benzadehyde to crystalline benzoic acid. Write a balanced equation for the reaction which would take place if some BB is heated with concentrated sodium hydroxide solution. Equation 2.C 6 H 5 CHO + OH - à C 6 H 5 CH 2 OH + C 6 H 5 COO -. What is the reaction called? Name of reaction : Cannizzaro. Give the structural formulae and names of another two organic compounds which undergo this reaction. H C O H Name:...methanal... CH 3 O CH 3 C C H CH 3 Name:..dimethylpropanal... Give the reagents and conditions required to convert compound BB to compound AA. Reagents: LiAlH 4. Conditions:.dry ether / heat. 3. The molecular formula of the liquid CC is C 8 H 8 O. Note the odour of this compound. (a) Place 2 drops of CC on a crucible lid and ignite by heating from above. CC burns with a very sooty and a very luminous flame A highly unsaturated compound, possibly an aromatic compound is indicated. (b) Place about 1 cm 3 of 2,4 - DNPH. reagent in a test tube and add one drop of CC Expt 14 Page 4
An orange precipitate was obtained when CC was added to 2,4-DNPH The presence of a carbonyl compound i.e. an aldehyde or ketone is suggested (c) Add 1 drop of CC to the portion of Tollens reagent prepared in test 2(d). Warm very carefully on a water-bath. No visible reaction. CC does not show reducing properties i.e. it is not an aldehyde and must therefore be a ketone In view of the results obtained in the above tests, suggest a name and structure for CC: O C CH 3 Name:.. Phenylethanone... (d) Carry out another test to confirm that CC has the structure you have chosen. Test: Test with Iodine in KI + dilute NaOH (aq). A pale yellow precipitate having an Pale yellow ppt, with an antiseptic smell is CHI 3.. antiseptic smell was seen to form Its formation is indicative of the presence of either CH 3 CHOH or CH 3 CO in the structure. The compound CC 1 is isomeric with CC. When the above tests were carried out with portions of CC 1 the same results as for CC were obtained in tests 3(a) and (b) but the results in tests (c) and (d) were different. Suggest a possible structural formula for CC 1. Expt 14 Page 5
CH 3 C O H Name:..2-methylbenzaldehyde.. 4. The molecular formula of solid DD is C 7 H 6 O 2. (a) Ignite a small quantity of DD on a crucible lid by first heating it until it melts and then continue heating it from above until it ignites. N.B. THIS IS THE WAY IN WHICH THIS TEST SHOULD BE CARRIED OUT WITH A SOLID SAMPLE. Solid DD, after heating strongly, burns with a very sooty and a very luminous The sooty flame indicates a highly unsaturated compound, possibly an aromatic compound... flame. (b) Place about 1 cm 3 of aqueous sodium carbonate in a test tube and add a small quantity of DD. Heat gently if no reaction occurs. Gentle warming gave effervescence. (Solid appeared to float on the liquid) The release of CO 2 from carbonates shows that DD has acidic properties The structural formula and name of DD is: COOH Name:...Benzoic acid... The following tests (c) and (d) illustrate a characteristic property of many aromatic acids. Expt 14 Page 6
(c) Shake a small amount of DD with distilled water. Note what happens, and keep mixture for the next test. No visible reaction. The solid remains Suspended in the aqueous layer Aromatic carboxylic acids (i.e. benzoic acid and its derivatives) are insoluble in cold water. (d) Heat the mixture from test (c) above until the water is almost boiling. Then cool back by immersing the test tube in ice-cold water. Boiling causes the white solid to dissolve. Cooling regenerates the solid (i.e. solid Benzoic acid and its derivatives are soluble in boiling water, insoluble in cold. recrystallises on cooling). (e) Place a spatula load of DD in a test tube and add about 1 cm 3 of distilled water. Then add aqueous sodium hydroxide dropwise until no further change occurs. Then acidify the mixture slowly with concentrated hydrochloric acid. Slow addition of NaOH to DD caused the Latter to dissolve. Addition of acid s Salts of benzoic acid are water soluble, even in the cold, but the free acid is not. reforms the solid. (f) Place some DD in a test tube and add about 2 cm 3 of distilled water. Then add 3 drops of aqueous ammonia and shake well. Allow to settle and then decant the clear solution into another test tube.(n.b. This procedure will give you a neutral solution of the compound DD). Then add a few drops of neutral iron(iii) chloride solution. A buff precipitate forms with neutral Benzoates give a buff ppt with FeCl 3 FeCl 3. What would be observed if the above test with neutral iron(iii) chloride was carried out with ammonium ethanoate instead of with ammonium salt of DD. Observation: Ethanoates (and methanoates for that matter) give a brown solution which produces a brown precipitate on boiling (g) Place a spatula load of DD in a test tube and add about 1 cm 3 of ethanol and a few drops of concentrated sulphuric acid. Heat carefully on a water bath for a few minutes. Allow to cool and Expt 14 Page 7
then pour the contents of the test tube into a small quantity of sodium carbonate solution in a small beaker. An ester smell is noted after neutralization of the excess acids Ester formation with an alcohol confirms the Presence of a carboxylic acid group in DD. What is the purpose of pouring the reaction mixture into sodium carbonate solution? Explanation: To remove the excess acids and make the ester smell more conspicuous (obvious) (h) Dissolve a small portion of DD in some warm distilled water. Then add a few drops of bromine water.. No visible reaction. Brown colour of bromine remains. Bromine deactivates the ring i.e. it lowers the ring s electron density and its reaction with electrophiles is slowed down. Expt 14 Page 8