IR/NMR LG Problems

1
Lecture 14

• IR/NMR LG Problems
• Ch 8 Column Chromatography
• Lecture Problem 5 Due Next Lecture
• This week in lab
• Ch 7 TLC, PreLab Due
• Quiz 4
• Spec Unknown Progress Check
• Next week in lab
• Ch 8 PreLab Due
• Quiz 5 (last quiz given in lab)

2
Lab Guide Problem 11.53(f)

Propose a structure that is consistent with the
following data C3H6O2 (HDI 1) 1H NMR 1.27
ppm (t, 3H) 2.66 ppm (q, 2H) 10.95 ppm (s,
1H) IR 1715 cm-1 3500-3000 cm-1
Possible compound(s)

?
?
?
Solution
Double check structure. Make sure splitting
patterns and ppm values make sense as well as
integration values. Also, make sure the numbers
and types of atoms match the MF!
3
NMR IR Problem

Propose a structure that is consistent with the
following data C4H7ClO2 1H NMR 1.5 ppm (t,
3H) 4.0 ppm (s, 2H) 4.3 ppm (q, 2H) IR 1740
cm-1 1200 cm-1
Possible compound(s)


Solution
Double check structure. Make sure splitting
patterns and ppm values make sense as well as
integration values. Also, make sure the numbers
and types of atoms match the MF!
4
Ch 8 Column Chromatography Experiment

• 2 Lab Periods
• 1st You will perform your first synthetic
  experiment - the oxidation of fluorene to
  fluorenone - and will monitor the reaction by
  TLC.
• 2nd  You will separate the fluorene - fluorenone
  mixture by column chromatography monitored by
  TLC.

5
Ch 8 Column Chromatography Experiment

• PreLab
• Be sure to include the reaction mechanism with
  conditions, limiting reagent identified,
  theoretical yield of fluorenone, in the Diagrams
  of Special Apparatus/Reactions section.

6
Oxidation of Fluorene to Fluorenone

• Fluorene can be oxidized by oxygen in the air.
• Use a phase-transfer catalyst, Starks catalyst.
• Allow the reaction to go half-way to completion
  - monitor by TLC could take 5 to 30 minutes.
• Work-up liquid/liquid extraction
• Purification column chromatography

7
Phase Transfer Catalysis - How does this work?
Oxidation mechanism given in class.
8
Hints on 1st Synthetic Expmt Fluorene?Fluorenone

• Be very careful with the 10 M NaOH!  It will burn
  you badly if it touches your skin.
• Reaction times will vary - Stirring the reaction
  VIGOROUSLY speeds up the reaction and shortens
  time.
• You are monitoring the reaction by TLC until it
  is approximately half-way complete.  You will
  judge this based upon UV intensity of the S. M.
  and Prod. spots on the developed TLC plate. 
• Run TLC 5 minutes after reaction starts.
• As with the TLC experiment, check your TLC plate
  under the UV lamp for spot intensity BEFORE
  developing, BUT be sure toluene is evaporated!
• Be sure to label anything you leave in the hood
  to evaporate or dry.

9
Column Chromatography

• Pack column with alumina (dry pack or slurry
• method). Alumina will be combined with
• hexanes. Allow alumina to settle - avoid
• air bubbles.
• Add the sample - by pipette. Sample will be
• dissolved in a minimal amount of solvent.
• Sample should be a small narrow band on
• the alumina.
• Elute your sample.
• Collect fractions in small Erlenmeyers. Test
  each
• fraction by TLC. Combine like fractions.
• As the mobile phase drains, replenish with fresh
• mobile phase.
• Never let the column go dry! Always have mobile
• phase above the top of the column!

10
Hints on Column Chromatography of
Fluorene/Fluorenone

• Flash chromatography (use of nitrogen gas on top
  of the column) can be used to help push the
  mobile phase through the column more quickly. Do
  flash the whole time.
• Once the fluorene has come off the column, switch
  to a more
• polar mobile phase such as 50 dichloromethane
  in hexanes to
• elute the fluorenone more quickly.