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Shriner, Hermann, Morril, Curtin and Fuson The Systematic Identification of Organic Compounds J. Wiley and sons, INC. 8 th E dn. 2004. Identification of Unknown. Identification of Unknown. Preliminary Examination, Physical Properties, and Elemental Analysis . Physical State Color Odor
Procedure (b) for Nitrogen. Add two drops of a 10% ammoniumpolysulfide solution to 2 mL of the sodium fusion filtrate, and evaporate the mixture to dryness in a steam bath or hot-water bath. Add 5 mL of a 5% hydrochloric acid solution, and warm the solution. Filter the solution. Add three drops of 5% ferric chloride solution to the filtrate. The presence of a red color indicates that nitrogen was present in the original unknown.
Procedure (c) for Nitrogen Using 5% sodiumhydroxide solution, adjust the pH of 1 mL of the sodium fusion filtrate to pH 13. Add two drops of saturatedferrousammoniumsulfate solution and two drops of 30% potassiumfluoride solution. Boil gently for 30 sec. Acidify the hot solution carefully by adding 30% sulfuric acid dropwise until the ironhydroxide just dissolves. The appearance of the characteristic precipitate of Prussianblue (dark blue)indicates the presence of nitrogen.
If no precipitate is observed but a blue or greenish-blue solution is obtained, then the original sodium decomposition was not complete.
Procedure (d) for Bromine and/or Iodine, or Chlorine. Acidify 3 mL of the sodium fusion filtrate with 10% sulfuric acid and boil the solution for a few minutes. Cool the solution to room temperature. Add 1 mL of methylenechloride, followed by a drop of 5.25% sodiumhypochlorite. View the color of the solution against a white background. The production of a purple color in the bottom methylene chloride layer indicates the presence of iodine.
2NaI + CI2(H2O) 2NaCI + I2(CH2CI2) (purple).
Add 5.25% sodium hypochlorite, drop by drop. Shake the solution after each addition. The purple will gradually disappear and be replaced by a reddish-brown color if bromine is present.
I2(CH2CI2) + Cl2(H2O) 2ICI
2NaBr + Cl2(H2O) 2NaCl + Br2(CH2Cl2)
B: Aliphatic amines with eight or more carbons; anilines (only one phenyl group attached to nitrogen); some ethers.
MN: Miscellaneous neutral compounds containing nitrogen or sulfur and having more than five carbon atoms.
N: Alcohols, aldehydes, ketones, esters with one functional group and more than five but fewer than nine carbons, ethers, epoxides, alkenes. alkynes, some aromatic compounds (especially those with activating groups).
I: Saturated hydrocarbons, haloalkanes, aryl halides, other deactivated aromatic compounds, diaryl ethers.
The diamides of dicarboxylic acids constitute another group of compounds for which the melting point is a valuable index of the forces present in the crystals. Urea (mp132°C) is watersoluble. On the other hand, ethanediamide (oxalic acid diamide) has a quite highmeltingpoint of 420°C and a lowsolubility in water. Substitution of methyl groups for the hydrogen atoms of the amide group lowers the melting point by reducing intermolecular hydrogen bonding and increases the solubility in water; N,N' -dimethylethanediamide and N,N,N',N'-tetramethylethanediamide are water soluble. Hexanediamide is water insoluble, whereas its N,N,N',N'-tetramethyl derivative is water soluble.
2,4,6-tribromophenol is a sufficiently strong acid to dissolve in 5% sodium bicarbonate solution. The increase in acidity can be explained by inductive effects.
Similar electronic influences affect the basicity of amines. Aliphatic amines in aqueous solution have basicity constants, %, of about 1 X 10-3 or 1 X 10-4, which is approximately the basicity constant of ammonia, 1 X 10-5. Introduction of a conjugated phenyl group5 lowers the basicity by about 6 orders of magnitude, for example, aniline has a %of5 X10-10• The phenyl ring stabilizes the free amine by resonance and also decreases the basicity of nitrogen inductively.
A secondphenyl substituent decreases the basicity to such an extent that the amine is no longer measurably basic in water. For example, diphenylamine is insoluble in 5% hydrochloric acid solution. Substitution of a nitro group on the phenyl ring of aniline lowers the base strength because this electron-withdrawing group destabilizes the anilinium ion, the conjugate acid, while stabilizing the free base.
Part of the effect of the two methyl groups, in reducing the acidity of the nitrophenol, seems to be due to steric inhibition of resonance in the anion. Thus, structure A requires coplanarity or near coplanarity of all of the nitro group's atoms and the aromatic ring. Such coplanarity is inhibited by the presence of the methyl groups in the ion.
Disubstituted amides (RCONR2) of sufficiently high molecular weight to be water insoluble are soluble in 5% hydrochloric acid solution. Simple amides (RCONH2 and most monosubstituted amides (RCONHR) are neutral compounds. N-Benzylacetamide, however, is a basic compound.
Amines may undergo reaction with 5% hydrochloric acid solution to form insoluble hydrochlorides, which may lead to errors in classification. For example, certain aryl-amines, such as l-aminonaphthalene, form hydrochlorides that are sparingly soluble in 5% hydrochloric acid solution. By warming the mixture slightly and diluting it with water, it may make the compound soluble. The appearance of a solid will indicate if the amine has undergone a change. In order to decide doubtful cases, the solid should be separated and its melting point compared with that of the original compound. A positive halogen test with alcoholic silver nitrate would indicate formation of a hydrochloride.
Another useful technique is to dissolve the suspected base in ether and treat that with 5% hydrochloric acid solution with shaking. Formation of a solid at the interface of the two layers indicates the presence of a basic amine.
Although 1,3-dicarbonyl compounds are approximately as acidic as the phenols, the rate of proton removal from carbon may be relatively slow; as a result, the rate of solution of such compounds may be so slow that they appear to be, at first, insoluble in 5% base.
Imides are soluble in 5% sodium hydroxide solution but not in 5% sodium bicarbonate solution. A 4-nitrophenyl group makes the -CONH-function weakly acidic in aqueous solution. Thus 4-nitroacetanilidedissolves in 5% sodium hydroxide solution but not in 5%sodium bicarbonate solution. Sulfonamides show the same solubility trends in base as 4-nitroacetanilide. Oximes, which have a hydroxyl group attached to a nitrogen atom, display similar solubility behavior.
Cold, concentrated sulfuric acid is used with neutral, water insoluble compounds containing no elements other than carbon, hydrogen, and oxygen. If the compound is unsaturated, is readily sulfonated, or possesses a functional group containing oxygen, it will dissolve in cold, concentrated sulfuric acid.
Alkanes, cycloalkanes, and their halogen derivatives are insoluble in sulfuric acid.
Simple aromatic hydrocarbons and their halogen derivatives do not undergo sulfonation under these conditions and are also insoluble in sulfuric acid. However, the presence of two or more alkyl groups on the aromatic ring permits the compound to be sulfonated quite easily. Therefore, polyalkylbenzenes such as 1,2,3,5-tetramethylbenzene (isodurene) and 1,3,5-trimethylbenzene (mesitylene) dissolve rather readily in sulfuric acid.
Many secondary and tertiary alcohols are dehydrated readily by concentrated sulfuric acid to give olefins which subsequently undergo polymerization. The resulting polymers are insoluble in cold, concentrated sulfuric acid and will form a distinct layer on top of the acid. Benzyl alcohol and other similar alcohols react with concentrated sulfuric acid, resulting in a colored precipitate.
Cautiously add a few drops or a few crystals of the unknown compound to 1 mL of water and touch the test tube to see if heat is evolved. If the test tube is warm, then the test is positive for an acid anhydride or an acyl halide, since water will react with these compounds to form the carboxylic acid with the evolution of heat. Add 1mL of methanol to dissolve the sample. Pour the solution into 1 mL of a saturated solution of sodium bicarbonate. Evolution of carbon dioxide gas is a positive test for the presence of the product carboxylic acid.
(a)Preliminary Test: Dissolve a drop or a few crystals of the compound to be tested in 1 mL of 95% ethanol and add 1 mL of 1 M hydrochloric acid. Note the color produced when one drop of 5% ferric chloride solution is added to the solution. If a definite orange, red, blue, or violet color is produced, the following test for the acyl group is not applicable and should be omitted. Too much hydrochloric acid prevents the development of colored complexes of many phenols and all enols.
(b) Hydroxamic Acid Formation from Anhydrides, Acyl Halides, and Esters
Heat to boiling a mixture of one drop or about 40 mg of the compound, 1 mL of 0.5 M hydroxylamine hydrochloride in 95% ethanol, and 0.2 mL of 6 M sodium hydroxide. After the solution has cooled slightly, cautiously add 2 mL of 1 M hydrochloric acid. Anhydrides, acyl halides, and esters react with hydroxylamine to form the hydroxamic acid, as indicated in the above equations. If the solution is cloudy, add 2 mL of 95%
ethanol. Observe the color produced when one drop of 5% ferric chloride solution is added. If the color caused by the drop of ferric chloride solution does not persist, continue to add the ferric chloride solution drop wise until the observed color permeates the entire test solution. Compare the color with that produced in (a). A positive test will be a distinct burgundy or magentacolor of the ferric hydroxamate complex, which is formed upon the reaction of the hydroxamic acid with the ferric chloride. Compare the color of this solution with the yellow observed when the original compound is tested with ferric chloride in the presence of acid.
Add one drop or 30 mg of the compound dissolved in a minimum amount of propylene glycol to 2 mL of a 1 M hydroxylamine hydrochloride solution in propylene glycol. Add 1 mL of 1 M potassium hydroxide and boil the mixture gently for 2 min. Allow the mixture to cool to room temperature and add 0.5-1 mL of a solution of 5% alcoholic ferric chloride. A red to violet color constitutes a positive test. Yellow colors indicate negative tests, and brown colors or precipitates are indeterminate.
(d) Hydroxamic Acid Formation from Aromatic Primary Amides
Place 50 mg of the unknown in 5 mL of water. Add 0.5 mL of3% hydrogen peroxide and two drops of5% ferric chloride solution. Heat the solution to boiling. The hydrogen peroxide reacts with the aromatic amide to form the hydroxamic acid, which then reacts with the ferric chloride to form ferric hydroxamate complex. The characteristic magenta color should develop if the compound is an aromatic primary amide.
(e) Hydroxamic Acid Formation from Sulfonic Acids and Sulfonyl Chlorides.
To prepare the sulfonyl chloride from the sulfonic acid, combine five drops of thionyl chloride and 100 mg of the sulfonic acid in a test tube and heat in boiling water for 1 min. Allow the test tube to cool. To the test tube, add 0.5 mL of a saturated solution of hydroxylamine hydrochloride in methanol. Add a drop of acetaldehyde. The sulfonyl chloride undergoes the reaction with the hydroxylamine to form an intermediate, which, when treated with the acetaldehyde, forms the hydroxamate acid. Add dropwise a solution of 2 M potassium hydroxide in methanol until the solution is slightly basic when checked with pH paper. Heat the solution to boiling and then allow it to cool. Acidify the mixture by dropwise adding 0.5 M hydrochloric acid, until blue litmus paper turns red. Add a drop of 5% ferric chloride solution. Ferric chloride converts the hydroxamic acids to the ferric hydroxamate complex. The magenta color of the ferric hydroxamate complex is a positive result.
Sulfonyl chlorides can be treated directly with the hydroxylamine hydrochloride. Neutralize the salts of sulfonic acids with hydrochloric acid, then evaporate to dryness. Treat the residue with thionyl chloride as described above.
Warm a mixture of 0.20 g of the compound, 0.40 mL of absolute ethanol, and 0.20 mL of concentrated sulfuric acid over a steam bath or hot-water bath for 2 min. Pour the mixture slowly into an evaporating dish containing 2 mL of saturated sodium bicarbonate solution. A second layer should be formed. Carefully smell the mixture. The presence of a sweet, fruity smell in the product, where no such smell existed in the original unknown, indicates that the original compound was a carboxylic acid and the acid was esterified. Large-molecular-weight carboxylic acids produce esters that are odorless.
Another method of detecting such an active hydrogen is by adding acetyl chloride to the alcohol to form the ester (Experiment 6, p. 264), which is less dense than the aqueous layer.
Another method of testing for the presence of the alcoholic hydrogen involves ceric ammonium nitrate (Experiment 7, p. 265). The yellow ceric ammonium nitrate forms a red organometallic compound with alcohols. Positive results are obtained from alcohols of 10 or fewer carbons.
The Jones oxidation, in conjunction with the sodium metal test and the Lucas test, may be used to differentiate among primary (10), secondary (2°), and tertiary (3°) alcohols. The Jones oxidation (Experiment 8, p. 268) only detects the presence of a hydroxyl substituent that is on a carbon bearing at least one hydrogen. Thus, only primary and secondary alcohols are oxidized to the corresponding carboxylic acids and ketones. Tertiary alcohols are not oxidized under these conditions. As the alcohol is oxidized, the
solution changes from an orange-red color from the Cr+6 ion, to a blue to greencolor from the Cr3+ ion.
Substrates that easily give rise to cationic character at the carbon bearing the hydroxyl group undergo the Lucas test (Experiment 9, p. 269) readily. Therefore, only secondary (2°) and tertiary (3°) alcohols form the alkyl halide, which appears as a second liquid layer, tertiary alcohols being the most reactive. Primary (1°) alcohols undergo reaction with the zinc chloride and hydrochloric acid either negligibly slowly or not at all.
The iodoform test (Experiment 11, p. 273) gives positive results for secondary alcohols in which a methyl group is attached to the carbon bearing the hydroxyl group. This type of alcohol is oxidized to a methyl ketone and under these basic conditions it forms a triiodo intermediate, which is then oxidized to the sodium salt of the acid and iodoform. Iodoform is a foul-smelling yellow precipitate.
Only primary and secondaryamines will react with acetyl chloride to form amides.
(b) For Water-Insoluble Compounds Add 2 mL of dioxane4 to 1 mL of the ceric ammonium nitrate reagent. If a red color develops or if the solution becomes colorless, purify the dioxane. If the mixture remains yellow or is only a light orange-yellow, it may be used to test water-insoluble compounds. Divide the 6 mL of the solution in half, reserving 3 mL for observation as a control. To the other 3 mL of the dioxane containing reagent, add four to five drops of a liquid unknown or 0.1-0.2 g of a solid. Mix thoroughly and make the same observations as in (a).
(c) Alternate Experiment for Water Insoluble Compounds To prepare the reagent, dissolve 0.43 g of ceric ammonium nitrate in 2 mL of acetonitrile. Add about 0.1 g of the unknown compound to the reagent in a test tube. Stir the mixture with a glass rod and heat just to boiling. In 1-6 min the color will change from yellow to red. Even cholesterol, C27H45OH, gives a red to orange color. The red color disappears as oxidation of the alcohol group takes place.
Negative tests are indicated by the absence of the red complex with retention of the yellow color of the reagent.
(a) For Neutral Aldehydes Add a drop or a few crystals of the compound to 1 mL of the Bogen or Grammercy indicator-hydroxylamine hydrochloride reagent. Note the color change. If no pronounced change occurs at room temperature, heat the mixture to boiling. A change in color from orange to red constitutes a positive test.
Place 2 mL of Schiffs reagent in a test tube and add two drops or a few crystals of the unknown. Shake the tube gently, and observe the color that is developed in 3-4 min. Aldehydes react with Schiffs reagent to form a complex which has a wine-purple color.
Secondary amines, when treated with benzenesulfonyl chloride, yield the sulfonamides, which precipitate from the solution. Acidification of the solution does not dissolve the sulfonamide.
Tertiary amines undergo reaction with benzenesulfonyl chloride to produce quaternary ammonium sulfonate salts, which yield sodium sulfonates and insoluble tertiary amines in basic solution. Acidification of the reaction mixture results in the formation of sulfonic acids and soluble amine salts.
If a residue is formed and it is insoluble in acid, the original unknown is a secondary
mine. Secondary amines undergo reaction with benzenesulfonyl chloride to precipitate the sulfonamide, and acidification does not result in any change.
If a residue is present that is soluble in acid, it indicates that the residue is the unreacted tertiary amine.
Tertiary amines undergo reaction with benzenesulfonyl chloride to produce quaternary ammonium sulfonate salts, which yield sodium sulfonates and water-insoluble tertiary amines in basic solution. Acidification of the reaction mixture results in the formation of sulfonic acids and water-soluble amine salts. Any solid that is formed should be isolated and purified, and its melting point should be compared against the original amine. If the amount of the solid is in sufficient quantity, it may be saved and used as a derivative for that unknown.
If a pale yellow oil or low-melting solid, which is the N-nitrosoamine, is formed with no evolution of gas, the original amine is a secondary amine. The oil or solid is isolated and treated under conditions of the Liebermann nitroso reaction (c) to provide confirmation of the presence of the N-nitrosoamine.
An immediate positive test for nitrous acid with no evolution of gas indicates the presence of a tertiary aliphatic amine. The tertiary aliphatic amine is simply protonated to form a soluble salt under these conditions and does not react with the nitrous acid.
The reaction of a tertiary aromatic amine (an aniline) with nitrous acid produces a dark-orange solution or an orange crystalline solid, which is the hydrochloride salt of the C-nitrosoamine. Treating 2 mL of the solution with 10% sodium hydroxide or sodium carbonate solution will produce the bright-green or -blue nitrosoamine base. The nitrosoamine base can be isolated, purified, and characterized.
Add 0.05 g of the N-nitrosoamine, 0.05 g of phenol, and 2 mL of concentrated sulfuric acid to a test tube and warm gently for 20 sec. Cool the solution slightly. A blue color should develop, which changes to red when the solution is poured into 20 mL of ice water. Add 10% sodium hydroxide until the mixture is alkaline, and the blue color is produced again.
The N-nitrosoamine liberates nitrous acid in the presence of sulfuric acid. The nitrous acid then undergoes reaction with phenol to yield the yellow 4-nitrosophenol
(quinone monoxime). The blue color observed in this reaction is due to phenolin- dophenol formed from the reaction of the initially produced 4-nitrosophenol (quinone
monoxime) with excess phenol. This reaction is characteristic of phenols in which an
ortho or para position is unsubstituted.
Esters are cleaved by hydroiodic acid (Experiment 34, p. 318) to form an alkyl iodide and a carboxylic acid. The alkyl iodide is treated with mercuric nitrate to yield mercuric iodide, which is an orange color.
Ethers can be detected by the iodine test (Experiment 33, p. 317); as the ether undergoes reaction with iodine, the color of the solution changes from purple to tan.
Treatment of an ether with hydroiodic acid results in the cleavage of the ether (Experiment 34. p. 318). The alkyl iodide product is heated and its vapors undergo reaction with mercuric nitrate to form mercuric iodide, which is orange.
In both the silver nitrate test (Experiment 35, p. 320) and the sodium iodide test (Experiment 36, p. 323), the halide is displaced from the alkyl halide to form an insoluble salt. The reaction of the alkyl halide with silver nitrate yields a silver halide precipitate.
The sodium iodide test can be used to test for the presence of bromine or chlorine. Sodium halide precipitates from the solution.
I. The following water-soluble compounds give an immediate precipitate with aqueous silver nitrate.
1. Amine salts of halogen acids.
3. Carbonium halides.
4. Low-molecular-weight acid chlorides. Many of these are easily hydrolyzed by water and produce the halide ion.
Water-insoluble compounds fall roughly into three groups with respect to their behavior toward alcoholic silver nitrate solutions.
1. Compounds in the first group give an immediate precipitation at room temperature.
2. The second group includes compounds that react slowly or not at all at room temperature but give a precipitate readily at higher temperatures.
3. A final group is made up of compounds that are usually inert toward hot alcoholic silver nitrate solutions.
Addition across the double bond occurs in both the bromine test (Experiment 37, p. 326) and the potassium permanganate test (Experiment 38, p. 328). Since the ionic characters of the bromine and potassium permanganate reactions are very different, there is some complementary character between the two tests. For example, some alkenes bearing electron-withdrawing groups undergo rapid reaction with potassium permanganate but often slow or negligible reaction with bromine. Bromine adds across the carbon-carbon double bond, with dissipation of the brown-redbromine color.
Alkenes are oxidized to 1,2-diols with potassium permanganate, with reduction of the manganese from +7, which is purple, to +4, which is brown.
A positive test for a carbon-carbon double bond or triple bond involves the disappearance of the bromine color without the evolution of hydrogen bromide. If the bromine color is discharged and hydrogen bromide is evolved, then substitution has occurred on the unknown. Hydrogen bromide gas can be detected by placing moistened bluelitmus paper across the mouth of the test tube and noting whether it turns red, indicating the presence of an acidic gas.
Discharge of the bromine color accompanied by the evolution of hydrogen bromide, indicating that substitution has occurred, is characteristic of many compounds. In this category are enols, many phenols, and enolizable compounds.
Ketones, like other carbonyl compounds, may exhibit an induction period because the hydrobromic acid liberated acts as a catalyst for the enolization step of the bromination.
Aromatic amines are exceptional because the first mole of hydrogen bromide is not evolved but reacts with the amine to produce a salt. For this reason this reaction could be mistaken for Simple addition.
Bromine Test (left) No reaction, saturated, (right) Reaction, unsaturated
The reaction of aromatic compounds with azoxybenzene and aluminum chloride
gives rise to 4-arylazobenzene, with the color of the solution or precipitate indicating
particular functional groups (Experiment 40, p. 336).
Aromatic compounds react with chloroform and aluminum chloride to produce
triaryl carbocations in a variety of colors, dependent upon the functional groups on the aryl ring (Experiment 41, p. 337).
The triaryl carbocations are in solution as Ar3C+ AlCl4- salts and are responsible for the colors observed.
This test must be done in a hood. Place 0.5 mL of 20% fuming sulfuric acid (hazardous) in a clean, dry test tube, and add 0.25 mL or 0.25 g of the unknown. Shake the mixture vigorously, and allow it to stand for a few minutes. A positive test for the presence of an aromatic ring is complete dissolution of the unknown, evolution of heat, and minimal charring.
Benzenes and aryl halides
The reaction of hydroxylamine hydrochloride with ketones (Experiment 13, p. 280) produces oximes and results in the liberation of hydrogen chloride, which can be detected by an indicator.
Nitriles. along with many other compounds. give a positive hydroxamic acid test (Experiment 2c. p. 254). The hydroxamic acid is detected with ferric chloride to form the ferric hydroxamate complex. which has a burgundy or magenta color.
Of the nitro compounds, only tertiary aliphatic nitro compounds and aromatic nitro compounds are reduced by zinc and ammonium chloride (Experiment 43, p. 342) to the hydroxylamine. The hydroxylamine is then detected by the formation of metallic silver in the Tollens test (Experiment 15, p. 283).
The number of nitro groups on an aromatic ring can be determined by the reaction of the unknown with sodiumhydroxide (Experiment 44, p. 342). In the reaction with sodium hydroxide, the mononitro aromatic compounds yield no color change, dinitroaromatic compounds produce a bluish-purplecolor, and trinitro aromatic compounds give a red color. The color of the solution is due to a Meisenheimer complex.
Phenols undergo reaction with the yellowceric ammonium nitrate (Experiment 7,
p. 265) to produce brown or black products.
A modification of Liebermann's test (Experiment 20d, p. 297) can be used to test for the presence of a phenol. A blue intermediate is formed, which changes to red when diluted and blue when the solution is made basic.