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Aldehydes & Ketones Classification Tests

Aldehydes & Ketones Classification Tests. The use of Chemical Classification Tests, Selected Physical Properties, NMR, and IR to Identify an Unknown Aldehyde or Ketone References : Slayden - p. 73 – 76 Pavia - p. 491 – 496 Web Notes: http://classweb.gmu.edu/jschorni/chem318.

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Aldehydes & Ketones Classification Tests

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  1. Aldehydes & Ketones Classification Tests The use of Chemical Classification Tests, Selected Physical Properties, NMR, and IR to Identify an Unknown Aldehyde or Ketone References: • Slayden - p. 73 – 76 • Pavia - p. 491 – 496 • Web Notes: http://classweb.gmu.edu/jschorni/chem318

  2. Aldehydes & Ketones Classification Tests • Overview • Identification of Aldehyde/Ketone Unknown • Liquid Unknown – Purification & B.P. (Simple Distillation) • Solid Unknown – Melting Point • Solubility Relative to Water & Sulfuric Acid • Density Relative to Water • Infrared (IR) & NMR Spectroscopy • Chemical Classification Tests • Chromic Acid - Aldehydes • Tollens Reagent - Aldehydes • Iodoform Test - Methyl Ketones

  3. Aldehydes & Ketones Classification Tests • Lab Report Notes • Procedures: • Title – Be Concise Ex. Vacuum Filtration, Recrystallization, etc. • Materials & Equipment – 2 Columns in list (bullet) formNote: include all reagents and principal equipment • Description of Procedure: • Use list (bullet) form • Concise, but complete descriptions • Use your own words – Don’t copy book!!

  4. Aldehydes & Ketones Classification Tests • Lab Report Notes (Con’t) • Results • Use table (see slide 12) to present Classification test results • Repeat each test for the known compounds until you get the expected result • Summary • Summarize ALL experimental results (knowns & unknown) and computed results • Analysis & Conclusions • Discuss the results you obtained for your unknown • The analysis of the IR & NMR spectra should be more than a summary of the spectra. It should explain how the various absorptions, signals, splitting patterns, etc. lead to the identity of the compound

  5. Aldehydes & Ketones Classification Tests • Overview • Physical Properties • If you get a liquid unknown you will do a Simple Distillation to purify sample and determine its Boiling Point • If you get a solid unknown you will just do a Melting Point • Physical Characteristics (describe purified sample) • Solubility/Density relative to Water & Sulfuric Acid • Refractive Index • IR & NMR Spectra • Chemical Tests

  6. Aldehydes & Ketones Classification Tests • Organic Lab – Unknowns, Purification, Boiling Point • Several experiments in Chem 315/318 (Org Lab I & II) involve the identification of an unknown compound • Liquid samples that students receive in Lab may contain some impurities in addition to the unknown compound that could produce ambiguous results when determining the chemical or physical properties of the compound • Simple Distillation is used to purify the sample by separating the pure compound that comes over in a narrow temperature range – corresponding to its boiling point – from impurities that have boiling points either lower than or higher than the compound

  7. Aldehydes & Ketones Classification Tests • Boiling Point– Background The normal boiling point (also called the atmospheric boiling point or the atmospheric pressure boiling point) of a liquid is the temperature at which the vapor pressure of the liquid is equal to 1 atmosphere (atm), the atmospheric pressure at sea level At that temperature, the vapor pressure of the liquid becomes sufficient to overcome atmospheric pressure and allow bubbles of vapor to form inside the bulk of the liquid. The standard boiling point is now (as of 1982) defined by IUPAC as the temperature at which boiling occurs under a pressure of 1 bar 1 bar = 105 Pascals = 0.98692 atmospheres = 14.5038 psi (pounds per square inch) = 29.53 in Hg (inches of mercury) = 750.06 mm

  8. Aldehydes & Ketones Classification Tests • Boiling Point – Background (con’t) Note: The temperature range you obtain for your boiling point may be inaccurate for three (3) reasons 1. The atmospheric pressure in the lab may not be: 1 bar (0.98692 atm) 2. The thermometers used in the lab may not reflect the actual temperature 3. The thermal inefficiency of the glassware used for the boiling point determination may result in a lower than expected measured value by as much as 2 – 5oC You should take this potential temperature differential into account when you compare your measured results with the list of possible unknowns in lab manual tables

  9. Aldehydes & Ketones Classification Tests Typical Distillation Setup

  10. Aldehydes & Ketones Classification Tests • Simple Distillation – Procedure • Set up Simple Distillation apparatus (previous lside) • Use 25 mL or 50 mL Distillation flask • Place a Corundum or Teflon boiling chip in the flask • Start gentle water flow through condenser • Put a waste receiving container (small beaker) into an ice water bath – especially for low boiling liquids. • Begin heating sample Note: The sample may appear to be boiling, but the actual boiling point is not reached until the temperature of the boiling liquid and the vapor surrounding the thermometer bulb reach equilibrium. At this point the vapor will start to condense in the condenser

  11. Aldehydes & Ketones Classification Tests • Simple Distillation - Procedure • Note the temperature when the distillate begins to drip into the waste receiving container • Continue to collect distillate in the waste container until the temperature begins to level off • Remove the waster container and begin collecting the distillate in a small clean Erlenmeyer flask • Note the temperature when you start to collect the purified sample • Continue to collect the sample until the temperature begins to rise again (it may not change before the all of the sample has come over) • Note the temperature just before the temperature begins to change • The first and last temperatures recorded in the narrow boiling range represent the boiling point range of your sample

  12. Aldehydes & Ketones Classification Tests • Water Solubility • Compounds with < 5 carbons containing oxygen, nitrogen, sulfur are soluble • Compounds with 5-6 carbons containing oxygen, nitrogen, sulfur are borderline soluble • Branching alkyl chains result in lower melting/boiling points and increased water solubility • Increased N, O, S to carbon ratio increases solubility

  13. Aldehydes & Ketones Classification Tests • Conc H2SO4 Solubility • Compounds containing Nitrogen, Oxygen, Sulfur can be protonated in concentrated H2SO4 and are thus considered soluble Alkenes Alkynes Ethers Nitroaromatics Nitrobenzene Amides Alcohols Ketones Aldehydes Esters • Not Soluble in H2SO4 (Inert Compounds) Alkanes Aromatic Hydrocarbons Alkyl Halides Aromatic Halides

  14. Aldehydes & Ketones Classification Tests • Procedure (Con’t) • Physical Properties (Con’t) • Determine the Refractive Index of the sample if it is a liquid • Correct the Ref Index for temperature Note: Thermometers in lab may be inaccurate Instructor will determine and post the room temperature using an accurate thermometer • Observe the Color, Odor and Physical State of your unknown

  15. Aldehydes & Ketones Classification Tests • Procedure (Con’t) • Physical Properties (con’t) • Density / Solubility of Unknown relative to Water & Sulfuric Acid • Place 4-5 drops of the compound in a test tube containing 2 mL of Distilled Water • Stopper top of test tube with your gloved thumb and shake the test tube vigorously • Observe whether the compound dissolves in the reagent, floats on top of the reagent, or sinks to the middle or bottom of the reagent • Note: If your Unknown is soluble in Water, you cannot make a statement relative to its density • Repeat for Concentrated Sulfuric Acid

  16. Aldehydes & Ketones Classification Tests • Procedure (Con’t) • Infrared Spectroscopy • Liquid Unknown • Place 2-3 drops of liquid unknown on a salt plate • Cover the sample with the second salt plate and place in the Plate Holder • Place Salt Plates in Plate Holder • Solid Unknown • Dissolve small amount of solid sample in 1-2 mL Acetone • Place 3-4 drops on Salt Plate and allow to evaporate • Cover the sample with the second salt plate and place in the Plate Holder

  17. Aldehydes & Ketones Classification Tests • Procedure (Con’t) • Infrared Spectroscopy • Insert Plate Holder in IR Spectrophotometer • Press “Scan”, check for “4” scans in monitor window; Press “Execute.” • In your report, record the Principal Absorptions found in the Spectra in the results section of IR procedure. • NMR Spectroscopy • The NMR spectra for your unknown will be handed out • In your report, record the Principle Signals found in Spectra • Note: Trust the Spectra Over the Classification Tests!

  18. Aldehydes & Ketones Classification Tests • Procedure (Con’t) • Classification Tests • Presentation of Results • In the results section of each Classification Test Procedure, insert a table similar to the one below to record your results. The “+/-” column in the above table indicates whether the observed results indicated a positive or negative response of the tested compound to the test.

  19. Aldehydes & Ketones Classification Tests • Procedure (Con’t) • Classification Tests • 2,4 – Dinitrophenylhydrazine Aldehydes & Ketones Note: Do not do this test unless instructed to do so • Background • Most Aldehydes and Ketones give a positive test with 2,4 – Dinitrophenylhydrazine.

  20. Aldehydes & Ketones Classification Tests • Classification Tests (2,4-dimethylphenyl hydrazine) • Positive Test - Yellow to Red Precipitate. • Unconjugated Ketones give a yellow precipitate. • Highly conjugated Ketones give a red precipitate. • Slight precipitation usually indicates impurities,possibly some Allylic or Benzylic Alcohols thathave been oxidized to Aldehydes or Ketones. • The Reaction Yellow to Red ppt Acetone 2,4-Dinitrophenylhydrazine 2,4-Dinitrophenyldimethylhydrazone

  21. Aldehydes & Ketones Classification Tests • Classification Tests (Con’t) • 2,4 – Dinitrophenylhydrazine (con’t) • Test Procedure: • If sample is a solid, dissolve about 10 mg of sample in minimum of one of the following: • 95% Ethanol • 1,2-Di-Methoxyethane (Ethylene Glycol Dimethyl Ether) • bis-2-Ethoxyethyl Ether (Diethylene Glycol Diethyl Ether)

  22. Aldehydes & Ketones Classification Tests • Classification Tests (Con’t) • 2,4 – Dinitrophenylhydrazine (con’t) • Test Procedure (Con’t) • Place one drop of sample into test tube with 1 mL of reagent. • Stopper top of Test Tube with your thumb and shake vigorously. • Test may take up to 15 minutes and require some gentle heating in a water bath. • Positive test is yellow to red precipitate

  23. Aldehydes & Ketones Classification Tests • Classification Tests (Con’t) • Chromic Acid Test – Aldehydes • Aldehydes (Carbonyl group) are oxidized to Carboxylic Acid. • Cr+6 in Chromic Acid (orange) is reduced to Cr+3 (green) Note: Most Ketones do not test positive because they are not as easily oxidized as Aldehydes. • Positive Test - Green precipitate (Chromous Sulfate) with loss of orange color in reagent.

  24. Aldehydes & Ketones Classification Tests • Classification Test (Con’t) • Chromic Acid Test – Aldehydes (Con’t) • Aliphatic Aldehydes turn cloudy within 5 seconds and form the precipitate within 30 seconds. • Aromatic Aldehydes take from 30 – 120 seconds to form a precipitate. • In a negative test there is usually no precipitate. Occasionally a precipitate may form; but the reagent color remains orange. Note: Primary & Secondary Alcohols also give positive Chromic Acid test; therefore, test for Aldehydes only after a positive identification of the Carbonyl group has been made.

  25. Aldehydes & Ketones Classification Tests • Classification Test (Con’t) • Chromic Acid Test – Aldehydes (Con’t) • The Reaction This is an Oxidation/Reduction (REDOX) reaction in which Chromium +6 is reduced (gains electrons) to Chromium +3 and the Aldehyde is oxidized by gaining an oxygen and loosing electrons. .

  26. Aldehydes & Ketones Classification Tests • Classification Test (Con’t) • Chromic Acid Test – Aldehydes (Con’t) • The Reaction (Con’t) • The Chromic Acid Reagent is produced from Chromium (+6) Oxide in concentrated Sulfuric Acid (H2SO4). • The Sulfuric Acid also supplies the Sulfate ion (SO4-), which reacts with the reduced Cr+3 to form Chromous Sulfate, a green precipitate

  27. Aldehydes & Ketones Classification Tests • Classification Tests (Con’t) • Chromic Acid Test – Aldehydes (Con’t) • Test Procedure • Dissolve 1 drop of liquid sample or 10 mg solid sample in 1 mL reagent-grade Acetone. • To the sample solution add 1 or 2 drops of the Chromic Acid reagent, a drop at a time, while shaking the mixture. Note: Use 1 or 2 drops of Chromic Acid, NO MORE To much reagent will mask the green color. Also note that the sample is dissolved in acetone, a ketone. If your unknown happens to be acetone, you are dissolving your unknown in additional acetone. This does not present a conflict with the test for the presence of an Aldehyde by Chromic Acid.

  28. Aldehydes & Ketones Classification Tests • Classification Test (Con’t) • Tollens Test – Aldehydes • Background • Positive Test – Formation of a Silver mirror on the wall of test tube. • Most Aldehydes reduce Ammoniacal Silver Nitrate to give a precipitate of Silver metal (the silver mirror). • The mirror on the walls of the test tube is quite pronounced. • The Aldehyde is oxidized to a Carboxylic Acid.

  29. Aldehydes & Ketones Classification Tests • Classification Test (Con’t) • Tollens Test – Aldehydes (Con’t) • Background (Con’t) • Ordinary Ketones do not give a positive test, although some results from ketones may indicate a slight silverish look, but little, if any, will be on the wall of the test tube. Compare with a good test from a known Aldehyde. • Use this test only after it has been determined that the compound is either an Aldehyde or a Ketone, i.e., positive test for Carbonyl group.

  30. Aldehydes & Ketones Classification Tests • Classification Test (Con’t) • Tollens Test – Aldehydes (Con’t) • The Reaction • Most Aldehydes reduce Ammoniacal Silver Nitrate solution to give a precipitate of Silver metal (pronounced coating on test tube wall) • Under basic conditions (NaOH & NH3), the Aldehyde is oxidized to a Carboxylic Acid salt

  31. Aldehydes & Ketones Classification Tests • Classification Tests (Con’t) • Tollens Test – Aldehydes (Con’t) • Reagents: • Prepare the Tollens Reagent as follows: • Soln A - Silver Nitrate (AgNO3) • Soln B - 10% Sodium Hydroxide (NaOH) • 10% Ammonium Hydroxide (NH4OH)

  32. Aldehydes & Ketones Classification Tests • Classification Tests (Con’t) • Tollens Test – Aldehydes (Con’t) • Procedure • In a 150 mL beaker combine 6 mL Soln A with 6 mL Soln B (solid Ag2O forms) • Add NH4OH in 2 mL increments until the solid Ag2O dissolves forming a clear solution. • In a test tube add 1 drop of liquid sample or 10 mg of solid sample to 2-3 mL of Tollens Reagent

  33. Aldehydes & Ketones Classification Tests Note: You may be instructed to first add your sample to a minimal amount of one of the following before adding to the Tollens reagent. • 1,2-Di-Methoxyethane (Ethylene Glycol Dimethyl Ether) • bis-2-Ethoxyethyl Ether (Diethylene Glycol Diethyl Ether) • If a reaction is not immediate, warm the mixture in a water bath (60-70oC) for several minutes. • A positive test is the formation of a distinct Silver mirror on the sides of the test tube.

  34. Aldehydes & Ketones Classification Tests • Classification Tests (Con’t) • Iodoform Test - Methyl Ketones • Background • Methyl Ketones are the most common type of compounds to give a positive Iodoform test. • Acetaldehyde (CH3CHO) and Secondary Alcohols with the alpha Hydrogen Carbon atom next to the Hydroxyl Carbon atom also give a positive test. This type of Secondary Alcohol is relatively easily oxidized to a Methyl Ketone. • A positive test is the formation of a pale yellow precipitate (Iodoform – CHI3) when the sample is treated with a basic solution of Iodine. • The other product of the reaction is the Sodium salt of a Carboxylic Acid (RCOONa).

  35. Aldehydes & Ketones Classification Tests • Classification Tests (Con’t) • Iodoform Test - Methyl Ketones • The Reaction

  36. Aldehydes & Ketones Classification Tests • Classification Tests (Con’t) • Iodoform Test - Methyl Ketones (Con’t) • The Reagents • Iodine – Potassium Iodide + Iodine + Water • 10% Sodium Hydroxide • Procedure • Use a large (15 x 125 mm) test tube • If the substance to be tested is water soluble, dissolve 6 drops of liquid sample or 75 mg of solid in 2 mL distilled water • If the sample is insoluble in water dissolve it in 2 ml of 1,2-Di-Methoxyethane

  37. Aldehydes & Ketones Classification Tests • Classification Tests (Con’t) • Iodoform Test - Methyl Ketones (Con’t) • Procedure • Add 2 mL of 3 M sodium hydroxide (NaOH) • Slowly add 3 mL of the Iodine Solution • Stopper the test tube and shake vigorously • A positive test will result if the brown color disappears and a yellow iodoform solid precipitates out of solution

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