Experiment 8AQualitative Analysis
QUALITATIVE ANALYSIS used to A set of procedures and techniques
identify and quantify the chemical composition of a given sample of a substance; Seeks to identify the substances in a sample, i.e. functional groups; Relies on certain chemical reactions to detect particular functional groups, such as alcohol, amine, aldehydes. A visible reaction indicates the presence of a functional group. The presence of Functional Groups is identified by reacting the compound to a reagent which gives a specific visible result.
QUALITATIVE ANALYSIS Four basic areas of inquiry are useful for identifying the nature of an unknown compound namely, physical properties of compounds, classification by solubility, elemental analysis, and classification test for functional groups. This experiment focuses on the fourth area of inquiry.
1. Reaction with Bromines in Methylene Chloride 2. Baeyer Test 3. Ignition Test
TEST FOR UNSATURATIO N
METHODOLOG Y
PREPARATION OF ACETYLENEPlace a piece of CaC2 lump in a 250-ml distilling flask. Fill a 125-ml separatory funnel with 50-ml water.
CaC2 + 2H2O
C2H2 + Ca(OH)2
TEST FOR UNSATURATIONReaction with Bromines in Methylene Chloride
METHODOLOG Y
3 drops of 2% SOLUTION OF BROMINE IN METHYLENE CHLORIDE
3 DROPS
ACETYLENE BENZENE
STYRENE HEXANE
RESULTS
AcetyleneTwo layers Clear Orange Evolution of gas
BenzeneDark orange-red
StyreneCloudy white
HexaneOrange red
Reaction with Bromines in Methylene
DISCUSSION
ELECTROPHILIC ADDITION (HALOGENATION)ALKENE
ALKYNE
Reaction with Bromines in Methylene
DISCUSSION
ELECTROPHILIC ADDITION (HALOGENATION) The decoloration of Bromine solution test and Baeyer test differentiate saturated from unsaturated hydrocarbons. In decoloration of bromine solution test, if the test compound is unsaturated, the orangebrown color of the aqueous bromine solution rapidly disappears. Bromine adds to each of carbon of the bond then the pi bond converts into a sigma bond and thus become saturated. As a result of this reaction, an OH group is added to each carbon at each end of a multiple bond. When this happens, the multiple bond becomes saturated.
Reaction with Bromines in Methylene
TEST FOR UNSATURATION
Baeyer Test
METHODOLOG Y
3 drops of 1% AQUEOUS SOLUTION OF KMnO4
3 DROPS
ACETYLENE BENZENE
STYRENE HEXANE
RESULTS
AcetyleneDark brown precipitate
BenzeneTwo layers Clear Violet
StyreneCloudy white Brown precipitate
HexaneTwo layers Clear Violet
DISCUSSION
REDOX REACTION
Potassium Permanganate (KMNO4) is an oxidizing agent. It is purple. Following the oxidation of an unsaturated compound, the Permanganate ion is reduced to Manganese Dioxide (MnO4), a brown precipitate.
TEST FOR UNSATURATION
Ignition Test
METHODOLOG YACETYLENE TOLUENE
STYRENE HEXANE
RESULTS
AcetyleneOrange & blue flameAl
XyleneOrange & blue flame Formation of sootAr
StyreneDark orange flame Formation of sootAr
HexaneOrange flameAl
DISCUSSION
HIGH DEGREES OF UNSATURATIONDifferentiate aliphatic substances from aromatic unsaturated hydrocarbons Highly unsaturated compounds such as aromatic compounds, give off a yellow sooty flame.The yellow color is due to the presence of carbon and the soot is credited to the deposition of carbon
1.Nitration 2.Le Rosen Test
TEST FOR AROMATICITY
TEST FOR AROMATICITY
Nitration
METHODOLOG Y
RESULTS
SMEL L COLO R
Naphthalene-like odor Light yellow Slightly immisicible
DISCUSSION
ELECTROPHILIC AROMATIC SUBSTITUTION (NITRATION)
TEST FOR AROMATICITY Le Rosen Test
METHODOLOG Y
5 drops Le Rosen reagent 5 drops xylene
INCLINE
RESULTS
COLOR 2 LAYERS
Pink Immisicible P-quinone
DISCUSSION
ELECTROPHILIC AROMATIC SUBSTITUTION (NITRATION)
Same as nitration; formaldehyde in place of HNO3
1. pH of Aqueous Solution 2. NaHCO3 Test 3. AgNO3 Test 4. Silver Acetylide Test
TEST FOR ACIDITY AND BASICITY
TEST FOR ACIDITY AND BASICITYpH of Aqueous Soln
METHODOLOG Y
pH PAPER
2 DROPS OF TEST SAMPLE DISSOLVED IN 20 DROPS WATER
20 mg TEST SAMPLE DISSOLVE D IN 20 DROPS 95% ETHANOL
ACETIC ACID PHENOL
ANILINE BENZOIC ACID
RESULTS
Acetic Acid 2-3 (4.76 at 25oC)
Phenol 4-5 (9.95)
Aniline 6-7 (27; Pkb 9.42)
Benzoic Acid 3 (4.21)
DISCUSSION
ACIDS AND BASES
TEST FOR ACIDITY AND BASICITY
NaHCO3 Test
METHODOLOG Y
20 drops 5% NAHCO3
2 DROPS OF TEST SAMPLE DISSOLVED IN 20 DROPS WATER
20 mg TEST SAMPLE DISSOLVE D IN 20 DROPS 95% ETHANOL
ACETIC ACID PHENOL
ANILINE BENZOIC ACID
SHAKE GENTLY
RESULTS
Acetic Acid Gas bubble Clear
Phenol Brownish red Dark precipitate
Aniline Two layers Aniline NaHCO3
Benzoic Acid Gas bubbles Benzoic acid crystals deposit on walls (Clear solution)
DISCUSSION
If the compound is soluble, then it is acidic. Solubility indicated by evolution of gas, bubbles and change in color. Carboxlyic acid is a strong acid. Phenol is a common weak acid. Negatively substituted phenols such as nitrophenols, aldehydrophenols, and polyhalophenols are sufficiently acidic to dissolve in 5% sodium bicarbonate.
TEST FOR ACIDITY AND BASICITY
AgNO3 Test
METHODOLOG Y
ACETIC ACID
PHENOL
BENZOIC ACID
ANILINE
Place 2 drops test sample and 20 drops 95% ethanol.
Add 20 drops 2% ethanolic AgNO3
Precipitate forms, add 5% HNO3 dropwise.
Precipitate does not form.
RESULTS
Acetic Acid No precipitate (White precipitate)
Phenol No precipitate
Aniline No precipitate
Benzoic Acid No precipitate (White precipitate)
DISCUSSION
Only acids will react to form the precipitate due to the departure of H from the molecule and replacing it with a silver ion. The ammoniacal silver ion reacts with acetylene to form C2Ag2 (Silver acetylide), which is the solid precipitate.
DISCUSSION
The presence of an acid is detected by the presence of replaceable H+ atoms and the formation of precipitates. The essence of this test is the reaction between carboxylic acids and silver nitrate. Silver nitrate reacts with carboxylic acids to form salts of the carboxylic acid. To verify the presence of the silver salt, dilute nitric acid should be added. Silver salts should dissolve in nitric acid.
TEST FOR ACIDITY AND BASICITY Silver Acetylide Test
METHODOLOG Y
20 drops 2% AgNO3 solution
1M NH3 dropwise until precipitate dissolves
Pour into test tube with acetylene gas. Stopper and shake
RESULTS
Precipitate
White to yellow
DISCUSSION
Silver acetylide is formed when acetylene is passed into an ammoniacal solution of silver nitrate. These substitution reactions which ethynes undergo to form compounds with metals are not occur with the alknes. These reactions can be used as tests to distinguish between acetylene and ethylene. When acetylene is passed through an ammonical solution of silver nitrate or cuprous chloride, at room temperature, precipitates of silver acetylide (white) or cuprous acetylide (red) are formed.
1. Beilstein Test 2. Alcoholic AgNO3 Test
TEST FOR ALKYLHALIDE S
TEST FOR ALKYLHALIDES
Beilstein Test
METHODOLOG Y
METHODOLOG Y
Benzylchloride
Dip the loop into 5 ml test tube containing 20 drops of test sample and bring to flame
ethylbromide
chlorobenzene
Ethylcholoride
Tert-butylchlorideGreen
RESULTS
(Green)
EthylbromideGreen
Sec-butylchlorideGreen
Ethyliodide(Green)
Benzylchloride ChlorobenzeneGreen Green
DISCUSSION
A positive test is indicated by a green flame caused by the formation of a copper halide. This test is little used nowadays; one reason why it is not popular is that it is possible to generate the highly toxic chloro-dioxins if the test material is a polychloroarene.
CuO + 2 R-X --> CuX2 + 2 R-O-?
TEST FOR ALKYLHALIDES Alcoholic AgNO3 Test
METHODOLOG Y
Ethylcholoride
Tert-butylchlorideGrayish white precipitate
RESULTS
(White layer)
EthylbromideGreen precipitate Clear layer
Sec-butylchlorideGrayish white precipitate
Ethyliodide(White layer)
ChlorobenzeneUnsoluble immiscible
DISCUSSION
ALIPHATIC HALIDE vs AROMATIC HALIDE this test can be used to differentiate aromatic halides from aliphatic halides. 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. Different rate if silver halide precipitation would be expected from halogen in each of these environments, namely, 3o>2o>1o Alkyl bromides and iodides react more rapidly than chlorides Aryl Halides are unreactive toward the test reagent.
DISCUSSION
ALIPHATIC HALIDE vs AROMATIC HALIDE
GUIDE QUESTIONS
What test/s could be used to differentiate saturated and unsaturated hydrocarbons. Explain.ANSWER The decoloration of Bromine solution test and Baeyer test differentiate saturated from unsaturated hydrocarbons. In decoloration of bromine solution test, if the test compound is unsaturated, the orange-brown color of the aqueous bromine solution rapidly disappears. Bromine adds to each of carbon of the bond then the pi bond converts into a sigma bond and thus become saturated. In the permanganate test, an unsaturated compound reduces the purple permanganate ion (MnO4-), forming a precipitate of brown manganese (IV) oxide (manganese oxide, MnO2). As a result of this reaction, an OH group is added to each carbon at each end of a multiple bond. When this happens, the multiple bond becomes saturated.
QUESTION #1
Do alkynes give all tests which alkenes exhibit? What tests could be used to differentiate them?ANSWER No. The silver acetylide test could be used to differentiate between an alkyne and an alkene. It only produces a positive test (formation of a silver precipitate) on alkynes.
QUESTION #2
Illustrate with equation the reaction of Br2 in methylene chloride with acetylene.ANSWER
ALKYNE
QUESTION #3
Does toluene behave in the same way like the other unsaturated hydrocarbons? Why?ANSWER In our case, we used xylene as a substitute for toluene. Xylene does not behave in the same way like other unsaturated hydrocarbons because of the presence of a benzene ring. This stable structure due to resonance is less reactive than the other saturated hydrocarbons. The shared electrons in the saturated bonds of the aromatic compound ring prevent aqueous bromine solution from reacting with them.
QUESTION #4
What is the role of concentrated sulfuric acid in the nitration of toluene?ANSWER Concentrated sulfuric acid acts as an electrophilic generator. It displaces weaker acids from their salts with nitric acid itself, sulfuric acid acts as both an acid and a dehydrating agent, forming nitronium ion, NO2+.
QUESTION #5
Show the mechanisms for nitration and Le Rosen reaction.ANSWER
QUESTION #6
Give the equation involved in the reaction of acetylene with ammoniacal silver nitrateANSWER
C2H2 + Ag(NH3)NO3 C2Ag2 + HNO3Only acids will react to form the precipitate due to the departure of H from the molecule and replacing it with a silver ion. The ammoniacal silver ion reacts with acetylene to form C2Ag2 (Silver acetylide), which is the solid precipitate.
QUESTION #7
Tollens reagent for the silver acetylide test should always be freshly prepared. Why?ANSWER It is because Tollen's reagent when left standing decomposes and forms a highly explosive precipitate-silver fulminate, which is dangerous in laboratory experiment.
QUESTION #8.
Explain why terminal hydrogen in acetylene is acidic when in fact the electronegativity values of hydrogen and carbon are almost the same.ANSWER Terminal alkynes are unusual hydrocarbons for they can be deprotonized using an appropriate base to generate carbanion. This is an acidic situation since terminal alkynes have an sp-hybridized orbital with 50% s character, meaning it is a highly electronegative orbital, the electron pair shared with H(terminal and directly attached to the unsaturation) is tightly held by the nucleus of C making it electrostatically stabilized. So, The H+ now, to relieve the tension can leave off the chain. Since this is a positive H, then terminal alkynes are acidic.
QUESTION #9
What test/s could be used to carboxylic acids from phenol. Explain.ANSWER The NaHCO3 test could be used. Carboxylic acids are soluble in sodium bicarbonate solution while phenols are not. A visible evolution of bubbles happens during the reaction of a carboxylic acid. Phenols do not. Ethanolic silver nitrate test could also be used. Carboxylic acids initiate the production of precipitate (silver carboxylate salt). Phenols do not.
QUESTION #10
Why is the order of reactivity of the alkylhalides toward alcoholic silver nitrate 3o>2o>1o?ANSWER Tertiary carbocations formed by the heterolysis of alkyl halide is more stable due to C-H hyperconjugation. The positive charge is distributed to a greater number of surrounding hydrogen atoms compared to the secondary and primary alkylhalides. The primary carbonium ion is the least stable since its positive charge is concentrated only to the carbon atom.
QUESTION #11
In the Beilstein test, the appearance of green flame indicates the presence of what compound?ANSWER The green flame is caused by the copper halide which is formed from the combustion reaction of copper (II) oxide (from heated copper wire) and the alkyl halide.
QUESTION #12
In the silver nitrate test for halides, explain why alcohol is used as the solvent for silver nitrate and not water.ANSWER Water is not used as the solvent because it is highly polar, considering that the reaction proceeds with a hydrocarbon. Ethanol, having both hydrophobic and hydrophilic portion, can considerably dissolve the polar nitrate and the nonpolar alkyl chain. Alkylhalides AgBr and AgCl form precipitates insoluble in ethanol and provides indicator for the alkylhalide test.
QUESTION #13
UNSATURATION
(+)
(-) No reaction. Color does not become clear
TESTS FOR
BROMINE IN METHYLENE CHLORIDE
Disappearance of orange-red color
UNSATURATION (REACTIVITY TO HALOGEN)
BAEYER TEST
Clear color; brown precipitate
Immiscible
UNSATURATION (REACTIVITY TO KMnO4) AROMATICITY/ HIGH DEGREE OF UNSATURATION
IGNITION TEST
A sooty yellow flame
No soot
ACIDITY / BASICITY
(+) pH OF AQ. SOLN --Solubility (evolution of gas, bubbles, change in color)
(-) ---
TESTS FOR ACIDITY/ BASICITY
NaHCO3
No change
ACIDITY
AgNO3
White precipitate
No formation of precipitate
ACIDITY (REACTIVITY TO AMMONIACAL SILVER ION) ALKYNE
SILVER ACETYLIDE TEST
Silver precipitate
No formation of precipitate
ALKYLHALIDE S
(+) BEILSTEIN TEST ALCOHOLIC AgNO3 TEST Green flame White-Gray precipitate
(-) Other color flame Insoluble
TESTS FOR Cl, Br, I but not F ALIPHATIC HALIDE
CONCLUSION
Identification of the functional groups gives the grounds for establishing the nature and full-identity of an organic compound. Positive results from the tests confirm the presence of a specific functional group. The limitations of the experiments are the fact that the true identity of the compound cannot be determined through the tests conducted. Only the functional groups present in the organic compound can be determined. Further investigations can be conducted to find out the real nature and components of the organic compound.
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