Expt' 28'2 part E'1

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• Expt. 28.2 part E.1
• Qualitative Analysis of Aldehydes and Ketones

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Qualitative Analysis

• Goals
• to develop observation skills
• to further develop lab technical skills
• to use deductive reasoning
• to gain an appreciation for scientists of yore
• General approach
• run tests on known compounds along with your
  unknown to determine if you have an aldehyde or
  ketone
• make two solid derivatives of the unknown and
  determine melting points
• use MP information to identify the unknown

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Prelab

• Read
• p. 537-542, background and tests for aldehydes
  and ketones
• p. 551-553, how to select derivatives
• p. 561, 563, 565-566, spot tests for aldehydes
  and ketones (C2, C3, C6, C11)
• p. 568-569, how to make derivatives for aldehydes
  and ketones (D4-D7)
• p. 574, Part E.1
• Look over
• Appendix A, tables 3 and 4

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Prelab

• Notebook
• Title, purpose, procedural reference
• No table of physical properties for reactants
• Changes to procedure
• work in pairs for tests. Each student will make a
  derivative.
• solubility tests will not be done
• 2,4-DNP test will not be done

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During Lab

• work in pairs
• measure physical properties
• run chemical tests
• identify functional group
• select and make derivatives
• Notebook
• Unknown number
• Names of comparison compounds you use
• aldehyde benzaldehyde
• ketone 2-butanone
• All observations and results for tests and
  derivatives

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After Lab

• Determine MP of each derivative (wait 24 hrs)
• Analyze all observations
• Decide on the identity of the unknown
• Complete the datasheet

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Typical Qualitative Analysis Steps

• Purify the unknown
• Determine the physical properties
• Examine the physical state
• Perform solubility tests
• Identify the functional groups
• Choose a derivative to make
• Prepare and purify the derivative
• Identify the unknown

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Step 1 Purify

• Liquid Distillation
• Solid Recrystallization
• Note This will not be necessary for this weeks
  lab.

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Step 2 Determine the Physical Properties

• Liquid
• Boiling Point
• correct boiling point for atmospheric pressure
  and tell instructor the result
• Refractive Index
• correct for temperature
• Solid
• Melting Point
• tell instructor the result

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Step 3 Examine the Physical State

• Color
• Odor
• Crystal shape and size
• Viscosity
• Anything else you notice

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Step 4 Perform Solubility Tests

• Water Short-chain molecules (lt 5 C) or
  molecules with polar groups that can hydrogen
  bond
• HCl Amines
• NaOH Phenols and carboxylic acids
• NaHCO3 carboxylic acids
• H2SO4 everything that has oxygens or nitrogens
• Will solubility tests be helpful to distinguish
  ketones from aldehydes?
• P. 538-539
• We wont be doing these

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Step 5 Identify Functional Groups

• DNP Test All ketones and aldehydes reaction with
  2,4-DNP

Positive test formation of yellow, orange, or
red precipitate p. 540 You already know you have
an aldehyde or ketone. So we wont run it as a
test.
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Identify Functional Groups (continued)

• Tests for Aldehydes

• Positive chromic acid test may look brown
  rather than blue-green ppt, but will be a change
  from orange
• Positive Tollens test formation of silver
  mirror on side of test tube (Do NOT store test
  tube possible explosion!)
• Clean silver mirror with 6M HNO3

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Identify Functional Groups (continued)
Iodoform Test test for methyl carbonyls

• Positive test for methyl ketones formation of
  yellow ppt (iodoform)
• What is the only aldehyde that would give a
  positive iodoform test?
• acetaldehyde (ethanal)

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Chemical Tests

• Run tests on known aldehyde and ketone and your
  unknown simultaneously.
• You need to know what positive and negative test
  results look like
• You can run the tests in any order you like
• pick a station that is not busy

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Reaction Chart
Complete this as part of the prelab. Write
positive or negative in each of the boxes.
Use qualifiers for iodoform test.
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Create Flow Chart
Unknown
XXX

-
Aldehyde or Ketone
XXX
-

aldehyde
ketone
XXX
XXX
-
-


aldehyde
methyl ketone
not a methyl ketone
Fill in the XXXs with the name of an appropriate
test.
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Step 6 Choose Derivative

• Use appropriate table (ketones or aldehydes)
• Find possible unknown
• 5-6 degrees above observed melting point for
  solid
• 5 degree on either side of observed boiling point
  for liquid

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Derivatives (continued)

• Choose derivative that
• has different melting point from that of unknown
• melts above 50 degrees but below 250 degrees
• differs by at least 10 degrees from other
  possible unknowns of the same derivative

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Derivative Examples
Observed corrected boiling point 98
C Chemical test results ketone
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Derivative Examples
Observed melting point 54 56 C Chemical
test results aldehyde
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Derivative Reagents
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Step 7. Prepare and Purify the Derivative

• D4, p.568, preparation of 2,4-DNP derivatives
• D5, p.568, preparation of semicarbazone
  derivatives
• D6, p. 568, preparation of oxime derivatives
• D7, p. 569, preparation of
  p-nitrophenylhydrazone and
  phenylhydrazone derivatives
• Should recrystallize at least once. Derivative
  must be pure to get a sharp melting point

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Step 8. Identify the Unknown

• Based on the results from steps 1-7
• In case of conflicting/inconclusive data, make an
  educated guess.
• Be able to defend your choice based on the
  evidence obtained.