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Reactions of Aromatic Carbonyl Compounds with NaI and Bleach in Alcohol Solvents

Introduction

There are a number of methods available to perform electrophilic iodination on aromatic rings. Most methods involve using iodine and an oxidizing agent, such as nitric or periodic acids, and are not very environmentally friendly. One method, which is potentially more environmental friendly, uses NaI and bleach in an alcohol solvent (K. M. Doxsee and J. E. Hutchinson, Green Organic Chemistry, Brooks-Cole, 2004, 182-188). The bleach oxidizes the iodide to iodine (or iodonium ion), which then reacts with the aromatic ring to iodinate it.

We will react a variety of aromatic carbonyl-containing compounds and determine the structure of the product formed using IR and NMR spectroscopy. IR will tell us functional groups. If the aromatic ring iodinates, then we can use proton NMR to give us the coupling constants and peak positions to see the remaining hydrogens on the ring. We can use carbon NMR to tell us the number of different types of carbons in the product, and what types of carbons are present.

Sample Reaction Scheme

Iodination of Vanillin

Precautions and notes

Bleach is approximately 5.25% by weight NaOCl in water, which also contains some NaOH. Therefore, it is corrosive, so wear gloves, and if you spill any on you, wash it off quickly with lots of water. Methanol is toxic and flammable. Hydrochloric acid is also corrosive.

Procedure:

In your 100 mL round-bottomed flask, place 1.0 g (record what you actually use) of your carbonyl compound, 2.4 g of sodium iodide, and 30 mL of your assigned alcohol. Add a magnetic stir bar, and stir to dissolve the solids. While stirring, cool the flask in an ice-water bath for five minutes. Place 22 mL of bleach in an addition funnel, and suspend the funnel so the tip is inside the neck of the flask. Add the bleach dropwise to the reaction mixture over a period of at least 10 minutes. Remove the ice water bath, and allow the reaction to stir for an hour. Add 10 mL of 10% sodium thiosulfate solution in approximately 1 mL portions until the brown color disappears. Add dilute HCl (2 or 3M) in approximately 1 mL portions until the pH is less than 4. If a dark color reappears, add more sodium thiosulfate solution to get rid of it. Dilute your reaction with about 50 mL of water, cool the flask in ice, and suction filter the solid. Wash the solid with more water.

Recrystallize about a half a spatula full of the solid from an appropriate solvent of solvent mixture: your instructor will give you some suggestions.

Allow both your crude and recrystallized solids to dry, then weigh them and take melting point ranges. Hand in about 0.040g of your dry crude product in a test-tube labeled with your name and reaction product from your carbonyl compound in the NMR test-tube rack, and we will take NMR spectra of it for you.

Thin-Layer Chromatography (TLC) Analysis

TLC your products and starting materials on silica gel TLC plates. Use methyl t-butyl ether as the TLC solvent. Calculate Rf values for all of the spots on your plate.

Report Format

1.Title Page

  1. Descriptive title with between 15 & 25 words.
  2. Course and section number.
  3. Dates the experiment was performed.
  4. Your name.
  5. Tape your TLC plate on the bottom of the page. Clearly label each column of spots so I know what they are.

2.Body of the report

  1. Chemical equation for the reaction, with chemical structures, not just formulas.
  2. Important observations and possible interpretations. Color changes are important in this lab.
  3. Weight and melting point of the product.
  4. Calculation of theoretical and % yields. Show the calculations of the numbers of moles of all reactants from the grams you actually started with. Identify the limiting reagent.
  5. Organize your TLC data in a nice table of Rf values. Include a sample Rf value calculation.
  6. Include your IR spectra with your report, and interpret the major peaks.
  7. Include your proton NMR spectrum with your report. Interpret the spectrum by drawing the structure of the product on the spectrum, and drawing arrows from each peak in the spectrum to hydrogen(s) in the structure. Explain your reasoning.
  8. Include your carbon NMR spectrum with your report. Interpret the spectrum by drawing the structure of the product on the spectrum, and drawing arrows from each peak in the spectrum to carbon(s) in the structure. Explain your reasoning.

3.Questions

  1. What was the structure of the product of your reaction? How did you determine the structure of the product? Explain completely using all of your data. Why did this product form? Draw a mechanism to account for the formation of this product.
  1. Was your TLC solvent effective in separating your product from your starting material? How could you more effectively separate them? Explain your reasoning.

General Procedure For Thin-Layer Chromatography

Obtain a TLC plate, and draw a light pencil line across the width of the plate, about 1 cm from the edge. Place a number of light “tic marks” on the line, approximately equally spaced, corresponding to the number of samples you need to spot on your plate.

In separate test-tubes, dissolve tiny amounts of each of your samples in about 1 mL of acetone. Using a new spotting capillaryfor each sample, spot a tiny drop of the acetone solution of each sample on a separate “tic-mark”. Allow them to dry, then check the plate under a short-wave UV light, to see if you have visible spots for each sample. If not, spot the solution again on the same “tic-mark”. Do not spot any more times than needed - too much material may give results that are hard to interpret! Be sure to record what order you spotted your samples on the plate.

To prepare a developing chamber, obtain a piece of 11 cm filter paper, and fold it about 1 inch from the edge, to obtain a flat edge. Place the filter paper, flat edge down, in your 400 mL beaker. Obtain 10 mL of the TLC solvent, and pour it into the beaker, swirling the beaker to wet the filter paper with the solvent.

Carefully place the TLC plate in the developing chamber, spotted side down, trying not to splash the solvent on the plate. The level of solvent must be below the pencil line! Cover the beaker with a watch glass. The solvent will rise up through the stationary phase on the plate. When the solvent has risen to 1-2 cm from the top of the plate, remove the plate, and draw a light pencil line across the plate, at the level to which the solvent rose. Allow the solvent to evaporate (waving the plate in the air will speed this up), then look at the plate under the UV light. Circle all of the spots.

TLC PlateTLC Developing Chamber

You will identify the components of your unknown tablet by comparing the amounts the components traveled up the plate with the amounts the standards traveled. These amounts are reported as Rf (retention factor) values.

XDistance the spot traveled

Rf = --- = ------

YDistance the solvent traveled

Measure the distance a spot moves from the center of the spot.

Cleaning Up

When you are finished with the experiment, pour the TLC solvent in the appropriate “Recovered TLC Solvent” container. The filter paper may be thrown away in the trash can. Used spotting capillaries should be placed in the “Clean Broken Glass” container. Your acetone solutions you used to spot with may be flushed down the sink with lots of water. Be sure to keep the TLC plate, since you will be turning it in with your report.