Transcript 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:
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
Slayden - p. 73 – 76

Pavia

Web Notes: http://classweb.gmu.edu/jschorni/chem318
- p. 491 – 496
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Aldehydes & Ketones Classification Tests
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Overview
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Identification of Aldehyde/Ketone Unknown
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Liquid Unknown – Purification & B.P. (Simple
Distillation)
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Solid Unknown – Melting Point
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Solubility Relative to Water & Sulfuric Acid
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Density Relative to Water

Infrared (IR) & NMR Spectroscopy

Chemical Classification Tests

Chromic Acid
- Aldehydes
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Tollens Reagent- Aldehydes
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Iodoform Test - Methyl Ketones
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Lab Report Notes
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Procedures:

Title – Be Concise Ex. Vacuum Filtration,
Recrystallization, etc.

Materials & Equipment – 2 Columns in list (bullet)
form
Note: 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!!
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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
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Overview
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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
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IR & NMR Spectra
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Chemical Tests
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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
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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
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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
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Typical Distillation Setup
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Simple Distillation – Procedure
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Set up Simple Distillation apparatus (previous lside)
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Use 25 mL or 50 mL Distillation flask
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Place a Corundum or Teflon boiling chip in the flask
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Start gentle water flow through condenser
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Put a waste receiving container (small beaker) into an
ice water bath – especially for low boiling liquids.
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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
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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
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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
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Conc H2SO4 Solubility
 Compounds containing Nitrogen, Oxygen, Sulfur can
be protonated in concentrated H2SO4 and are thus
considered soluble
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Alkenes
Alkynes
Ethers
Nitroaromatics
Nitrobenzene
Amides
Alcohols
Ketones
Aldehydes
Esters
Not Soluble in H2SO4 (Inert Compounds)
Alkanes
Aromatic Hydrocarbons
Alkyl Halides
Aromatic Halides
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Procedure (Con’t)
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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
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Observe the Color, Odor and Physical State of your
unknown
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Procedure (Con’t)
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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
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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
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Repeat for Concentrated Sulfuric Acid
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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
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Procedure (Con’t)
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Infrared Spectroscopy
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Insert Plate Holder in IR Spectrophotometer

Press “Scan”, check for “4” scans in monitor
window; Press “Execute.”
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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!
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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.
Classification Test
Compound
Observation
+/-
Unknown (#)
Known Compound #1
Known Compound #2
Known Compound #3
Known Compound #4
Known Compound #5
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The “+/-” column in the above table indicates whether
the observed results indicated a positive or negative
response of the tested compound to the test.
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Classification Tests (Con’t)
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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.
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Positive Test - Green precipitate (Chromous
Sulfate) with loss of orange color in reagent.
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Classification Test (Con’t)
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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.
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Classification Test (Con’t)
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Chromic Acid Test – Aldehydes (Con’t)
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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.
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Classification Test (Con’t)
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Chromic Acid Test – Aldehydes (Con’t)
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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
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Classification Tests (Con’t)
 Chromic Acid Test – Aldehydes (Con’t)
 Test Procedure

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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.
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Classification Test (Con’t)
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Tollens Test – Aldehydes
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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.
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Classification Test (Con’t)
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Tollens Test – Aldehydes (Con’t)
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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.
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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.
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Classification Test (Con’t)
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Tollens Test – Aldehydes (Con’t)
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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.
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Classification Tests (Con’t)
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Tollens Test – Aldehydes (Con’t)
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Reagents:
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Prepare the Tollens Reagent as follows:

Soln A - Silver Nitrate (AgNO3)

Soln B - 10% Sodium Hydroxide (NaOH)

10% Ammonium Hydroxide (NH4OH)
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Classification Tests (Con’t)
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Tollens Test – Aldehydes (Con’t)
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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
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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.
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
1,2-Di-Methoxyethane
Dimethyl Ether)
(Ethylene Glycol

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.
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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).
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Classification Tests (Con’t)

Iodoform Test - Methyl Ketones
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The Reaction
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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
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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
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