aldehydes & ketones classification tests the use of chemical classification tests, selected...

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

04/19/23 1


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

04/19/23 2


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!!

04/19/23 3


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

04/19/23 4


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

04/19/23 5


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

04/19/23 6


Boiling Point– BackgroundThe 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 levelAt 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 bar1 bar = 105 Pascals = 0.98692 atmospheres = 14.5038 psi (pounds per square inch) = 29.53 in Hg (inches of mercury) = 750.06 mm

04/19/23 7


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 tables04/19/23 8


04/19/23 9

Typical Distillation Setup


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

04/19/23 10


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

04/19/23 11


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

04/19/23 12


Conc H2SO4 Solubility

Compounds containing Nitrogen, Oxygen, Sulfur can be protonated in concentrated H2SO4 and are thus considered soluble

Alkenes AlkynesEthers NitroaromaticsNitrobenzene AmidesAlcohols KetonesAldehydes Esters

Not Soluble in H2SO4 (Inert Compounds)

Alkanes

Aromatic Hydrocarbons

Alkyl Halides

Aromatic Halides

04/19/23 13


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

04/19/23 14


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 Acid04/19/23 15


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 Holder04/19/23 16


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!04/19/23 17

Classification Test

Compound Observation +/-

Unknown (#)

Known Compound #1

Known Compound #2

Known Compound #3

Known Compound #4

Known Compound #5

The “+/-” column in the above table indicates whether the observed results indicated a positive or negative response of the tested compound to the test.


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.

04/19/23 18


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.

04/19/23 19


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.

04/19/23 20

.




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.

04/19/23 21




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

04/19/23 22


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 MORETo 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.

04/19/23 23



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.

04/19/23 24



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.

04/19/23 25




The Reaction

Most Aldehydes reduce Ammoniacal Silver Nitrate solution to give a precipitate of Silver metal (pronounced coating on test tube wall).

The Aldehyde is oxidized to a Carboxylic Acid.

04/19/23 26




Reagents:

Prepare the Tollens Reagent as follows: Soln A - Silver Nitrate (AgNO3)

Soln B - 10% Sodium Hydroxide (NaOH) 10% Ammonium Hydroxide (NH4OH)

04/19/23 27


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

04/19/23 28


Note: If your sample is insoluble in distilled water add a drop of 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.

04/19/23 29


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).04/19/23 30


Classification Tests (Con’t) Iodoform Test - Methyl Ketones

The Reaction

04/19/23 31



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

04/19/23 32


Classification Tests (Con’t) Iodoform Test - Methyl Ketones (Con’t) Procedure (Con’t)

Add 2 mL of 3 M (~10-12%) 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

04/19/23 33

aldehydes & ketones classification tests the use of chemical classification tests, selected...

Documents

atmospheric boiling

standard boiling point

atmospheric pressure

boiling pointif

unknown aldehyde

vapor pressure

unknown compoundliquid

ambiguous results