Practical Organic chemistry Identification of organic compounds For Fourth Year chemistry Department
By
Dr.Baram AHMED Jaff Ph.D. organic chemistry at University of Strasbourg I-Louis Pasteur-2005
University of Sualiamni/College of Science Chemistry Department
2009
2010 1
Introduction The identification and characterization of the structures of unknown substances are an important part of organic chemistry. Although it is often possible to establish the structure of a compound on the basis of spectra alone (IR, NMR, etc.), the spectra typically must be supplemented with other information about the compound: physical state and properties (melting point, boiling point, solubility, odor, color, etc.), elemental analysis, and confirmatory tests for functional groups. In this experiment you will carry out several qualitative tests that will allow you to identify functional groups in organic molecules. You will then apply what you have learned by characterizing unknown organic compounds in terms of their functional group and solubility behavior. The functional groups you will examine include amines, alcohols, carboxylic acids, alkenes, alkanes, and alkyl halides.
Solubility and Functional Group Tests Each functional group has a particular set of chemical properties that allow it to be identified. Some of these properties can be demonstrated by observing solubility behavior, while others can be seen in chemical reactions that are accompanied by color changes, precipitate formation, or other visible effects. 1. Solubility Tests The solubility of an organic compound in water, dilute acid, or dilute base can provide useful information about the presence or absence of certain functional groups. A flowchart showing the sequence of solubility tests along with the appropriate conclusions is shown in Figure 1. Solubility in water: Most organic compounds are not soluble in water, except for low molecular-weight amines and oxygen-containing compounds. Low molecular-weight compounds are generally limited to those with fewer than five carbon atoms. • Carboxylic acids with fewer that five carbon atoms are soluble in water and form solutions that give an acidic response (pH < 7) when tested with litmus paper. • Amines with fewer than five carbons are also soluble in water, and their solutions give a basic response (pH > 7) when tested with litmus paper. • Ketones, aldehydes, and alcohols with fewer that five carbon atoms are soluble in water and form neutral solutions (pH = 7). 2
Solubility in NaOH: Solubility in 6M NaOH is a positive identification test for acids. A carboxylic acid that is insoluble in pure water will be soluble in base due to the formation of the sodium salt of the acid as the acid is neutralized by the base. Solubility in HCl: Solubility in 6M HCl is a positive identification test for bases. Amines that are insoluble in pure water will be soluble in acid due to the formation of an ammonium chloride salt. Organic Compound water insoluble
soluble test with pH paper
6M NaOH insoluble
soluble
acidic
6M HCl
insoluble
neutral soluble carboxylic carboxylic acids acids (high MW) (low MW)
aldehydes ketones alcohols (high MW) or alkanes alkenes alkyl halides (low or high MW)
basic
amines (high MW)
amines (low MW) aldehydes ketones alcohols (low MW)
Figure 1. The Solubility Test Flowchart
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4
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2-Silver Nitrate Test for Alkyl Halides and Carboxylic Acids The reaction of an alkyl halide with silver nitrate in ethanol will result in the formation of a white or yellow silver halide precipitate that is insoluble in nitric acid. This reaction quite often proceeds slowly, and occasionally slight warming is necessary.
RONO2 +
RX + AgNO3
AgX(s)
It is important to check if the precipitate is soluble in dilute nitric acid. Carboxylic acids form insoluble silver salts that precipitate, but these dissolve in nitric acid whereas the silver halides do not.
2. Beilstein Test for Halides Halogens can be detected easily and reliably by the Beilstein test. It is the simplest method for determining the presence of a halogen, but does not differentiate among chlorine, bromine, and iodine. A positive Beilstein test results from the production of a volatile copper halide when an organic halides is heated with copper oxide. The copper halide imparts a blue-green color to the flame.
3. Lucas Test for Alcohols This test depends on the appearance of an alkyl chloride as an insoluble second layer when an alcohol is treated with a mixture of hydrochloric acid and zinc chloride (Lucas reagent): ZnCl2
RCl + H2O
ROH + HCl
Primary alcohols do not react at room temperature; therefore, the alcohol is seen simply to dissolve. Secondary alcohols will react slowly, whereas tertiary, benzylic, and allylic alcohols react instantly.
4. Chromic Acid Test for Alcohols This test is based on the reduction of chromium(IV), which is orange, to chromium(III), which is green, when an alcohol is oxidized by the reagent. A change in color of the reagent from organce to green represents a positive test. Primary alcohols are oxidized by the reagent to carboxylic aicds; secondary alcohols are oxidized to ketones. Tertiary alcohols are not oxidized at all by the reagent. Hence, this reaction can be used to 6
distinguish tertiary alcohols form primary and secondary alcohols. Unlike the Lucas test, this test can be used with all alcohols, regardless of molecular weight and solubility.
H R
C
H2CrO4
H
R
C
OH
H
H2CrO4
R
O
C
OH
O
primary alcohol
H R
C
H2CrO4
R
R
C
OH
R
O
secondary alcohol
5. Bromination Test for Alkenes Bromine readily adds across the carbon-carbon double bond of an alkene to produce a dibromoalkane. A red solution of bromine in methylene chloride will quickly become colorless as the addition reaction with the alkene takes place. The general equation describing the test is: Br Br2
(red) Br
(colorless)
(colorless)
6. Potassium Permanganate Test for Alkenes This test is positive for double and triple bonds but not for aromatic rings. It depends on the conversion of the purple ion MnO4- to a brown precipitate of MnO2 following the oxidation of an unsaturated compound. 7
C
C
+ MnO4(purple)
C
C
OH
OH
+
MnO2 (brown)
Other easily oxidized compounds also give a positive test with potassium permanganate solution. These include aldehydes, some alcohols, phenols, and aromatic amines.
Ex 1. Method for preparing and collecting a gas less dense (lighter) than air by heating solid reactants. The less dense gas rises into, and displaces, the more dense air. This is called upward delivery. e.g. Heating a mixture of ammonium chloride and calcium hydroxide solids gives ammonia which has a very pungent odour! and turns red litmus blue. See also Ex 7. method. 2NH4Cl(s) + Ca(OH)2(s) ==> CaCl2(s) + 2H2O(l) + 2NH3(g) To make dry ammonia you need a U tube packed with granules of calcium oxide between the horizontal pyrex tube and the vertical inverted collection test tube.
Ex 2. Method for preparing and collecting a gas less dense (lighter) than air by reacting a liquid and a solid. The less dense gas rises into, and displaces, the more dense air. This is called upward delivery. e.g. A mixture of zinc and hydrochloric acid makes hydrogen. Hydrogen gives a squeaky pop! with a lit splint. See also methods Ex 5., Ex 6. and Ex 7. Zn(s) + 2HCl(aq) ==> ZnCl2(aq) + H2(g) 8
OR Ex 3. Method for preparing and collecting a gas more dense (heavier) than air by heating the reactants. The more dense gas sinks down into, and displaces, the less dense air. This is called downward delivery. eg Making nasty brown nitrogen dioxide by heating lead(II) nitrate crystals (thermal decomposition). The solid 'deflacrates', it crackles as the gas formed splits the crystals apart. See also method Ex 7. 2Pb(NO3)2(s) ==> 2PbO(s) + 4NO2(g) + O2(g)
Ex 4. Method for preparing and collecting a gas more dense (heavier) than air by reacting a solid and a liquid. The more dense gas sinks down into, and displaces, the less dense air. This is called downward delivery. Examples: (i) Calcium carbonate (limestone/marble chips) with hydrochloric acid makes carbon dioxide. Can also be done via Ex 6. but carbon dioxide is moderately soluble and does make 'carbonated water. See also Ex 8. for carbonate test.
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OR collected in boiling tube CaCO3(s) + 2HCl(aq) ==> CaCl2(aq) + H2O(l) + CO2(g) (ii) Sulphur dioxide from solid sodium metabisulphite or sodium sulphite and excess dilute hydrochloric acid. The nasty choking gas turns potassium dichromate(VI) paper from orange to green. Should be done in fume cupboard. Na2S2O5(s) + 2HCl(aq) ==> 2NaCl(aq) + H2O(l) + 2SO2(g) or Na2SO3(s) + 2HCl(aq) ==> 2NaCl(aq) + H2O(l) + SO2(g) (iii) Nasty acrid Hydrogen chloride is formed when conc. sulphuric acid is mixed with solid sodium chloride. Should be done in fume cupboard. NaCl(s) + H2SO4(l) ==> NaHSO4(s) + HCl(g) (iv) Chlorine from conc. sodium chlorate(I) and conc. hydrochloric acid. Very dangerous and should be done in a fume cupboard. NaClO(aq) + 2HCl(aq) ==> NaCl(aq) + H2O(l) + Cl2(g) All of these can be done via Ex. 5 or Ex 7. below.
Ex 5. Method for preparing and collecting a gas of any density by reacting a solid and a liquid at room temperature. e.g. making carbon
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dioxide Ex 4., chlorine Ex 4., hydrogen Ex 2., hydrogen chloride Ex 4., oxygen Ex 6., sulfur dioxide Ex 4.
Content
Ex 6. Method for preparing and collecting a gas of any density by reacting a solid-liquid, as long as the gas is not too soluble in water! (dissolving or reacting). All gases are less dense than air and will displace the water downwards. No good for soluble gases like ammonia, hydrogen chloride, nitrogen dioxide or sulphur dioxide. You can collect in inverted gas jar if bigger sample required. You have to watch for 'sucking back' effects. Examples: (i) Making oxygen from hydrogen peroxide solution using manganese dioxide catalyst. Oxygen has similar density to air so must be collected by methods Ex 5., Ex 6. or Ex 7. 2H2O2(aq) ==> 2H2O(l) + O2(g) (ii) Hydrogen, (iii) carbon dioxide and (iv) chlorine (moderately soluble, makes 'chlorine water')
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This is called collecting over water, or displacement of water or pneumatic trough collection.
Ex 7. Method for preparing and collecting a gas of any density by reacting a solid-liquid or heating solids. The angled boiling tube minimises the risk of contaminating the gas syringe with solids or liquids eg making ammonia or nitrogen dioxide. Its a smaller scale alternative to Ex 5. and using a Pyrex tube suitable for small scale heated experiments.
Ex 8. A simple way to test for a carbonate. Add acid to the suspected carbonate. Collect a sample of the gas in a teat pipette from just above the reaction mixture. Bubble the gas sample into calcium hydroxide solution (limewater) and a milky white confirms the gas is carbon dioxide formed from the original carbonate.
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Ex 9. Use of a U tube. This is useful if the dry gas is needed. It is inserted in the apparatus set-up between the reaction container and the gas collection system. It is packed with a solid water absorbing drying agent e.g. anhydrous calcium chloride (not for ammonia), calcium oxide (not acidic gases like sulphur dioxide, carbon dioxide and chlorine), anhydrous sodium sulphate. A dreschel bottle can also be used e.g. the gas is bubbled through concentrated sulphuric acid which will dry the gas. It cannot be used to dry alkaline gases like ammonia, with which it will react exothermically to form the solid salt ammonium sulphate.
U tube dreschel b Ex 10. A cracking experiment. This diagram outlines a way of demonstrating cracking larger hydrocarbons into smaller molecules. This is a thermal decomposition reaction catalysed by aluminium oxide (or broken porous pot). The gases produced can be tested with (i) a match! and (ii) bromine water, if it is decolorized from orange to colourless, then unsaturated alkenes were formed. This is another example of over water, or displacement of water collection. The dreschel bottle is to collect any sucked back water if the hot gasses cool and contract. Cold water on the hot Pyrex tube has very nasty effect! plus the risk of fire!
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I- Solubility test I-1 CHEMICAL identification TESTS I-1- A - Esters RCOOR' R = H, alkyl or aryl R' = alkyl or aryl There is no simple test for an ester. Usually a colorless liquid with a pleasant 'odor'. The ester can be reacted with saturated ethanolic hydroxylamine hydrochloride + 20% methanolic KOH and gently heated until boiling. Then mixture acidified with 1M HCl(aq) and FeCl3(aq) added drop wise. Deep red or purple colour formed. The test depends on the formation of a hydroxamic acid R-C(=NOH)OH which forms coloured salts with Fe3+(aq) ion. The reaction is also given by acid chlorides and acid anhydrides, and phenols give a purple colour with iron(III) chloride, so frankly, the test is not that good. This test is not likely to be expected
RCOCl ( or (RCO)2O) + PhOH + FeCl3 --Æ Purple colour I-1-B
Halogenoalkanes (haloalkanes) R-X where R = alkyl, X = Cl,
Br or I The halide is covalently bound (C-X bond), so the halogen X cannot react with the silver ion to form the ionic Ag+X-(s) precipitate until it is converted to the 'free' X- ionic form. Note that aromatic halogen compounds where the X is directly attached to the ring, do NOT readily hydrolyse in this way and no AgX ppt. will be seen. Aromatic C-X is a stronger bond than aliphatic C-X.
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(i) Warm a few drops of the haloalkane with aqueous ethanolic silver nitrate solution, the ethanol increases the solubility of the immiscible haloalkanes. (ii) Gently simmering a few drops with aqueous NaOH (may need to add ethanol to increase solubility and reaction rate). Add dilute nitric acid followed by aqueous silver nitrate solution.
(i) Observe colour of precipitate and the effect of ammonia solution on it (for rest of details see the (i) notes for chloride, bromide and iodide tests above in inorganic) (ii) see the (i) notes as above for more details. (i) AgNO3 + RX ==> R-NO3? + AgX(s) (ii) The sodium hydroxide converts the halogen atom into the ionic halide ion in a hydrolysis reaction. RX(aq) + NaOH(aq) ==> ROH(aq) + NaX(aq) then Ag+(aq) + X-(aq) ==> AgX(s) The addition of dilute nitric acid prevents the precipitation of other silver salts or silver oxide (e.g. Ag2O forms if solution alkaline).
I-1-C Hydro acids
Hydrogen sulphide H2S
Test gas with damp lead(II) ethanoate paper (old name lead acetate). Rotten egg smell of hydrogen sulphide gas and the H2S gas turns lead(II) ethanoate paper black. Hydrogen sulphide gives sulphide ions in water, so Pb2+(aq) + S2-(aq) => PbS(s) The gas is formed when acids react with sulphides.
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Hydrogen bromide HBr and Hydrogen iodide HI As above. In water they are hydrobromic acid and hydriodic acid. as above but cream HBr or yellow HI precipitate as above - combination of acid and halide ion tests
HCl Hydrogen chloride gas, in water forms hydrochloric acid. i) Blue litmus. (ii) Apply a drop of silver nitrate on the end of a glass rod (i) Litmus turns red (ii) A white precipitate with silver nitrate. (i) Strongly acid gas. (ii) In water forms chloride ions - hence precipitate with silver nitrate, see chloride test.
Hydroxide ion i.e. an alkali which forms the OH- ion in water
(note: to completely identify alkalis you need to test for the cation e.g. sodium for NaOH etc.) (i) Litmus or universal indicator or pH meter. (ii) Add a little of an ammonium salt. (i) It turns litmus blue, variety of colours univ. ind. dark green - violet for weak - strong. (ii) If strongly alkaline ammonia should be released, see ammonia test for rest of details (i) A pH meter gives a value of more than 7, the higher the pH number the stronger the alkali, the higher the OH- concentration, (ii) ammonia gas is evolved: (ii) Ammonia released from the salt. NH4+(aq) + OH-(aq) ==> NH3(g) + H2O(l) 16
Iodide ion I(i) If iodide soluble, add dilute nitric acid and silver nitrate solution. (ii) If insoluble salt can heat with conc. sulphuric acid, (ii) get purple fumes of iodine and very smelly hydrogen sulphide, (iii) if soluble, add lead(II) nitrate solution. (iii) If iodide soluble, add lead(II) nitrate solution. (iv) Yellow precipitate of silver iodide insoluble in concentrated ammonia. (v) purple vapour and rotten egg smell!, (iii) a yellow precipitate forms (vi) Yellow precipitate of lead(II) iodide. (vii) Ag+(aq) + I-(aq) ==> AgI(s) , any soluble silver salt + any soluble iodide ==> silver iodide precipitate, (viii) iodide ion is oxidised to iodine and the sulphuric acid is reduced to 'rotten eggs' smelly hydrogen sulphide, (x) insoluble lead(II) iodide formed Pb2+(aq) + 2I-(aq) ==> PbI2(s)
Iodine (i) solid or (ii) solution
A dark coloured solid. (i) Gently heat the solid. (ii) Test aqueous solution or solid with starch solution (i) Gives brilliant purple vapour. (ii) A blue black colour with starch solution. (i) Iodine forms a distinctive coloured vapour. (ii) Forms a blue-black complex with starch and in biology the test is used to detect starch with iodine solution.
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Iodoform test
The formation of CHI3, triiodomethane (or old name 'iodoform') . NaOH(aq) is added to a solution of iodine in potassium iodide solution until most of the colour has gone. The organic compound is warmed with this solution. A yellow solid is formed with the smell of an antiseptic, CHI3, triiodomethane, melting point 119oC. This reaction is given by ethanol CH3CH2OH, ethanal CH3CHO and all ketones with a '2-one' structure R-CO-CH3 Its a combination of halogenation and oxidation and is not a definitive test for anything, it just indicates a possible part of a molecules structure.
MISCELLANEOUS CATION TESTS: (i) Lead(II) ion
(i) add potassium iodide solution => yellow precipitate (i) Pb2+(aq) +2I-(aq) ==>PbI2(s) lead(II) iodide ppt.
Nitrate ion or nitrate(V) ion NO3-
(i) Boil the suspected nitrate with sodium hydroxide solution and fine aluminium powder (Devarda's Alloy) (ii) Add iron(ii) sulphate solution and then conc. sulphuric acid (the 'brown ring' test) (i) the fumes contain ammonia, which turns red litmus blue, see ammonia test details (ii) Where the liquids meet a brown ring forms (i) The aluminium powder is a powerful reducing agent and converts the nitrate ion, NO3-, into ammonia gas, NH3 (ii) NO complex of iron(II) formed
Nitrogen (IV) oxide or nitrogen dioxide NO2
There is no simple relatively unambiguous test. The other common orange-brown gas is bromine. Its a nasty orange-brown gas.Its a strong oxidising agent. 18
Dissolved in water it gives a solution of nitrite and nitrate ions. The other common brown gas is bromine and the solution of nitrogen dioxide shouldn't give a cream ppt. with silver nitrate solution.
Phenols (OH group is attached directly to aromatic ring). R-OH, where
R is aryl e.g. C6H5OH Add a few drops of iron(III) chloride solution to a little of the phenol in water. Usually gives a purple colour see also test for primary aromatic amines - use it in reverse starting with a known primary aromatic amine!)
Sulphate ion or sulphate(VI) ion SO42-
To a solution of the suspected sulphate add dilute hydrochloric acid and a few drops of barium chloride or nitrate solution. A white precipitate of barium sulphate. Ba2+(aq) + SO42-(aq) ==> BaSO4(s)
Any soluble barium salt + any soluble sulphate forms a white dense barium sulphate precipitate.
Sulphide ion S2-
In test (ii) dangerous hydrogen sulphide is formed. (i) If soluble, add a few drops lead(II) ethanoate solution. (ii) If solid, add dil. HCl(aq) acid, test smelly gas with damp lead(II) ethanoate paper (old name lead acetate). (i) Black ppt. of lead sulphide. (ii) Rotten egg smell of hydrogen sulphide and the H2S gas turns lead(II) ethanoate paper black. (i) Pb2+(aq) + S2-(aq) => PbS(s) (ii) MS(s) + 2H+(aq) => M2+(aq) + H2S(g) (e.g. M = Pb, Fe, Cu, Ni etc.) Then reaction (i) above occurs on the lead(II) ethanoate paper (old name lead acetate).
Sulphur dioxide gas SO2
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Freshly made potassium dichromate(VI) paper paper changes from orange to green the dichromate(VI) ion, Cr2O72-(aq) is reduced to the green Cr3+(aq) ion
Water liquid H2O
Easy to get these colour changes muddled! (i) Add a few drops to white anhydrous copper(II) sulphate. (ii) Dip in dry blue cobalt chloride paper . (i) Turns from white to blue. (ii) Turns from blue to pink. (i) Blue hydrated copper(II) crystals or solution formed (ii) Pink hydrated cobalt ion formed [Co(H2O)6]2+ • •
•
•
•
•
•
•
PLEASE NOTE: Sometimes a precipitate (ppt) initially forms with a limited amount of a reagent, it may then dissolve in excess of reagent to give a clear solution. Both observations will be crucial for a positive id. There are no tests specific to a compound e.g. (i) there is no test for calcium chloride, but there are tests for the calcium ion and the chloride ion. (ii) similarly, in organic tests, all you can do is identify a functional group i.e. a particular bit of molecular structure. The first tests in the 'inorganic' section are typical of GCSE Science level, but finally these overlap and extend into those needed for GCE Advanced AS or A2 level. In the organic section, only the alkene test is in GCSE double award science, but some others might be found in a full single or co-ordinated triple award GCSE syllabus. These days more emphasis is given to modern spectroscopic methods of analysis such as NMR, Infrared, Mass spectrometry, Atomic Emission etc. Quite correctly, though updating A level chemistry is intellectually challenging at times, it isn't always as much fun! The methods described give no recipe details or risk assessment, just basically what is needed, what you see and what you can or cannot deduce. Consult teacher, 'practical' text books and Hazcards before attempting any analysis. Most tests involve 'standard' chemical reactions and few tests are totally specific so observations should be viewed in context, i.e. is this a realistic deduction in that particular situation? Please remember each syllabus has its own 'list' of required tests - so do not 'over learn' - check out what is needed! 20
These notes are for written examinations only. • They describe and explain the basic methods used. • THEY DO NOT GIVE A FULL RISK ASSESSMENT OF THE SITUATION! • Students should be supervised in doing all these experiments and some will be teacher demonstrations. •
•
Teachers should do a full risk assessment consulting Hazard Data and larger samples can be prepared using 'flat bottomed' flasks and gas jars for demonstrations e.g. burning things in oxygen or chlorine.
INFRARED SPECTROSCOPY P URDUE UNIVERSITY INSTRUMENT VAN PROJECT CLASSIFICATION OF ORGANIC COMPOUNDS In this experiment we shall explore some of the methods employed in the classification of organic compounds. These will include tests for specific functional groups and the identification of functional groups from infrared spectra. Functional Groups: The emphasis in this experiment is on the methods of organic qualitative analysis. Nevertheless, a brief discussion of the properties and molecular structures of the substances to be tested should prove useful. Organic compounds are often defined as those substances containing carbon; most contain hydrogen as well. Atoms and groups of atoms attached to the hydrocarbon chain are referred to as functional groups. One of the first steps in the characterization of an organic compound is the identification of its functional groups by specific chemical tests. A list of some of the most common functional groups together with the properties associated with them follows: 1. Hydroxyl (-OH) group. There are two broad classes: a) Alcohols (R-OH), in which the hydroxyl group is attached to a chain of carbon atoms. Because of hydrogen bonding involving the -OH group, alcohols are higher boiling and more water soluble than hydrocarbons of similar molecular weight. Common alcohols are methyl alcohol (CH3OH), sometimes referred to as wood alcohol; ethyl alcohol (CH3CH2OH), which is found in alcoholic beverages; and isopropyl alcohol (CH3CH(OH)CH3), which is rubbing alcohol. b) Phenols (C6H5OH). When the hydroxyl group is attached directly to a benzene ring, the result is a phenol. Phenols are more acidic than alcohols, and the +
anions resulting from the loss of H form colored complexes with transition metal cations. 2. Carbonyl (R-C=O) group. These compounds are obtained from the mild oxidation of alcohols. They are intermediate between alcohols and hydrocarbons in boiling point and water solubility. Sugars contain both hydroxyl and carbonyl groups. There are two classes of carbonyls: a) Aldehydes (R-C=O) have a hydrogen atom attached directly to the carbonyl carbon. Perhaps the most common aldehyde is formaldehyde (H2C=O), 21
the compound responsible for the characteristic odor of so many Biology labs. Aldehydes are the active ingredients in a number of perfumes and flavors, including vanilla. They are INFRARED SPECTROSCOPY P URDUE UNIVERSITY INSTRUMENT VAN PROJECT easily oxidized to carboxylic acids, a property which may be used to distinguish them from ketones. b) Ketones (R-C=O) In ketones there is no hydrogen attached to the carbonyl group. The most common ketone is acetone (CH3)2C=O. 3. Carboxylic Acids (R-C-OH) are characterized by the presence of the carboxylic group (-C-OH). Like alcohols, they have relatively high boiling points and a tendency toward water solubility. Like other acids, they have a sour taste and react with sodium bicarbonate solution to release carbon dioxide gas. The most common carboxylic acid is acetic acid (CH3COOH), the active ingredient in vinegar. Aspirin is also a carboxylic acid. 4. Unsaturated hydrocarbons are hydrocarbons containing double and/or triple bonds between carbon atoms. Unlike saturated hydrocarbons (all single bonds) these substances react rapidly with halogens by addition across the double (or triple) bond. PRECAUTIONS. Until you have identified your unknown you will not know to which hazard it belongs. Therefore, treat all unknowns as if they were corrosive, carcinoegnic, and toxic until you prove otherwise. PROCEDURE: In your laboratory notebook, make a table listing the various functional groups. Space would be provided next to this list to record the observations for a positive test, the observations for a negative test, the observations for your unknown, and the conclusion as to whether the functional group was present or absent. Part 1. Identification of the Unknown Functional Groups. Obtain an unknown compound and record the number. It is possible that more than one functional group will be present in the unknown so all the tests should be done. TESTS FOR FUNCTIONAL GROUPS: HYDROXYL GROUP: Add 3 drops of ceric nitrate solution to three small test tubes or depressions in a spot plate. To the first test tube or well add 2 drops of the known alcohol. 2 INFRARED SPECTROSCOPY P URDUE UNIVERSITY INSTRUMENT VAN PROJECT To the second add 2 drops of the saturated hydrocarbon (which will be negative test) and to the third add 2 drops of your unknown. Let them stand for 2 minutes. Note the colors and record your observations.
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CARBONYLS: (Both aldehydes and ketones give positive results.) Add 10 drops of dinitrophenylhydrazine reagent (DNP) to four small test tubes or depressions in a spot plate. To the first test tube or well add 1 drop of the known ketone. To the second add one drop of the known aldehyde. To the third add one drop of the saturated hydrocarbon (which will be a negative test) and to the fourth add one drop of your unknown. Record your observations. ALDEHYDES: To a clean test tube add 5 drops of 0.1M silver nitrate (AgNO3) solution and 2 drops of 6 M ammonia. Add 2 or 3 drops of the known aldehyde and mix the contents thoroughly. Allow the test tube to stand in a hot water bath for a few minutes. A silver mirror should be deposited on the walls of the test tube if an aldehyde is present. If your unknown tested positive for a carbonyl with the DNP reagent, repeat the above test using your unknown in place of the aldehyde in order to establish whether or not it is an aldehyde or a ketone. WASH THE TEST TUBE IMMEDIATELY. This reaction may produce silver fulminate, a powerful detonator when dry. KETONES: If your test for the carbonyl group was positive, but the test for an aldehyde is negative, your unknown is a ketone. CARBOXYLIC ACIDS: Place a few drops of the known carboxylic acid in a small test tube or the well of a spot plate. In a second test tube or well place a few drops of the saturated hydrocarbon (which will give a negative test.) In a third test tube or well place a few drops of your unknown. To each test tube or well add a few drops of 10% sodium bicarbonate solution. Record your results. UNSATURATED HYDROCARBONS: Place a few drops of the known unsaturated hydrocarbon in a small test tube or the well of a spot plate. In a second test tube or well place a few drops of the saturated hydrocarbon (which will give a negative test.) In a third test tube or well place a few drops of your unknown. To each test tube or well add one drop of Br2/TTE solution. Record your observations. ALTERNATIVE TEST FOR UNSATURATED HYDROCARBONS: Place a few drops of the known unsaturated hydrocarbon in a small test tube or the well of a spot plate. 3 INFRARED SPECTROSCOPY P URDUE UNIVERSITY INSTRUMENT VAN PROJECT In a second test tube or well place a few drops of the saturated hydrocarbon (which will give a negative test.) In a third test tube or well place a few drops of your unknown. To each test tube or well add one drop of ethyl alcohol and one drop of 2% potassium permanganate (KMnO4). Record your observations.
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SATURATED HYDROCARBON: If all the above tests are negative, your unknown is a saturated hydrocarbon. CONCLUSION: On the basis of the above experimental information, what functional groups seem to be present in your unknown? INFRARED SPECTRUM: Obtain an infrared spectrum of your unknown. Follow the instructions which come with the instrument. On your copy of the infrared spectrum, mark the regions of ALL the functional groups listed on the table of CHARACTERISTIC INFRARED ABSORPTION BANDS. It is just as important to note that a particular functional group is not present as it is to note that one is present. On the basis of your infrared spectrum, what functional groups are present in your unknown? If your experimental results and your spectral results do not agree, redo them to see if an error has been made. Identify the functional groups present in the unknown. LAB WRITTEN BY: PRU PHILLIPS
4 INFRARED SPECTROSCOPY P URDUE UNIVERSITY INSTRUMENT VAN PROJECT 5 CHARACTERISTIC INFRARED ABSORPTION BANDS 1
GROUP WAVENUMBER (cm- ) ─────────────────────────────────────────────────────────── -OH (hydrogen bonded alcohol) 3200-3400 H C=C (aromatic or olefinic C-H) 3010-3040 -C-H (saturated C-H) 2840-2980 O -C-OH (carboxylic acid) 1700-1725 O -C-H (aldehyde) 1720-1740 O -C- (ketone) 1705-1725 C=C (simple olefin) 1620-1680 aromatic ring 1450-1600 (several bands) -C-H (saturated C-H, bending mode) 1350-1475 -C-O-(ethers,, alcohols and esters) 1060-1410
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INFRARED SPECTROSCOPY P URDUE UNIVERSITY INSTRUMENT VAN PROJECT 6 TEACHERS GUIDE This is a basic straightforward experiment which could be done at any level, even Physical Science. CURRICULUM INTEGRATION During an Organic Chemistry Unit STUDENT TIME The tests for functional groups will take about 30 minutes. The infrared spectrums will take longer. Allow 2 class periods. MATERIALS Reagents should be in dropper bottles. Organic liquids are both volatile and smelly. Furthermore they will react with plastic. The following amounts would be made for 4 classes of 20 students. 100 mL ceric ammonium nitrate solution Dissolve 20 g of ceric ammonium nitrate in 100 mL of 2M HNO3. 100 mL dinitrophenylhydrazine reagent (DNP) Dissolve 3.0 g of 2,4-dinitrophenylhydrazine in 15 mL of 18 M H2SO4. This solution is added slowly, with constant stirring, to a solution of 20 mL water *
and 70 mL 95% ethanol. The solution is mixed thoroughly and filtered. *
The 95% ethanol must be specially denatured with isopropyl alcohol. Plain denatured alcohol will contain methyl ethyl ketone which will precipitate with the 2,4-dinitrophenylhydrazine. 100 mL of 0.1 M Silver Nitrate Dissolve 1.7 g of AgNO3 in 100 mL of solution 100 mL of 6M NH3 100 mL of 10% Sodium Bicarbonate Solution 100 mL of Br2 in TTE or 100 mL of 2% KMnO4 and 100 mL of ethyl alcohol
Report Sheet: Identification of Unknown Organic Compounds
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Report Sheet: Identification of Unknown Organic Compounds Name(s) __________________________________________
Date ____________________
A. Characterizing Known Compounds Carefully record your observations. Accurately describe each positive test and include factors such as reaction time, color change, precipitate formation, and the need for heating, stirring, or shaking. 1. Solubility Tests Indicate which compounds are soluble in water, and for those that are soluble, indicate whether the solution is acidic, neutral, or basic. 2. Silver Nitrate Test Observations for 1-bromobutane:
Observations for other compounds:
What happened when HNO3 was added to each precipitate?
3. Beilstein Test Observations for unknown:
Observations for benzoic acid
Lucas Test Observations for 1-butanol:
Observations for 2-butanol:
Obsercatuibs for t-butyl alcohol:
4. Chromic Acid Test 26
Observations for 1-butanol:
Observations for 2-butanol:
Obsercatuibs for t-butyl alcohol:
5. Bromination Test Observations with cyclohexene:
Observations with cyclohexane:
6. Permanganate Test Observations with cyclohexene:
Observations with toluene:
Characterizing Unknown Compounds Unknown numbers: (a) ____________ (b) ____________ Test Solubility in water and pH of the solution
Solubility in 6M NaOH (if needed)
Observations
Conclusions
(a)
(a)
(b)
(b)
(a)
(a)
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Solubility in 6M HCl (if needed)
Silver nitrate test
Beilstein test
Lucas test
Chromic Acid test
Bromination Test
Permanganate Test
(b)
(b)
(a)
(a)
(b)
(b)
(a)
(a)
(b)
(b)
(a)
(a)
(b)
(b)
(a)
(a)
(b)
(b)
(a)
(a)
(b)
(b)
(a)
(a)
(b)
(b)
(a)
(a)
Indicate the functional group present in the unknown: (a) Unknown number ______: __________________________ (b) Unknown number ______: __________________________
From its solubility in water, is the unknown of low molecular weight? (Answer yes, no, or test was indecisive.) (a) Unknown number ______: __________________________ 28
(b) Unknown number ______: __________________________ 3. What class of organic compound with less than five carbon atoms dissolves in water to form a slightly acidic solution? Draw an equation to represent this reaction. 4. What class of organic compound with less than five carbon atoms dissolves in water to form a slightly basic solution? Draw an equation to represent this reaction. 5. What class of organic compound with more than five carbon atoms is insoluble in water but readily dissolves in base? Draw an equation to represent this reaction. 6. Draw the reaction between cyclohexene and bromine in CCl4. what type of reaction is this? 7. What type of reaction is the reaction between an alkyl halide and alcoholic silver nitrate? How would you expect the structure of the alkyl halide to affect the rate of this reaction? 8. You have now proposed a functional group present in your unknown. Name two methods that could be used to confirm your conclusion. You can visit this site for more information: http://a-s.clayton.edu/cclower/CHEM2412L/Organic%20Functional%20Groups%20Lab.doc
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