Nitration of Halobenzenes and Thin Layer Chromatography
Purpose: Nitrate a halobenzene and analyze the products by Thin Layer Chromatography
(TLC) and PMR Spectroscopy.
Reading Preparation for the following week's lab: Read Mayo, et al. pp. 122-124 and
148-150, and McMurry Ch.16.1-16.7.
Procedure: Add a spin vane and 2 ml of a prepared 1:1 mixture of concentrated
nitric acid (16 molar) and concentrated sulfuric acid in a 5 mL conical vial.
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Safety note: Both nitric and sulfuric acids are very corrosive and cause
serious burns. Treat these acids with caution!
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Attach the air condenser to the vial with a screw cap and an O-ring. Add 0.5 mL of
halobenzene (obtain unknown from stockroom) with a pipet extending through the condenser;
stir the reaction well. Add another 0.5 mL of the halobenzene five minutes later. Stir the
reaction in a basin of hot water, maintained at 80oC, for 15 minutes. What
evidence of a reaction have you observed?
Cool the vial to room temperature, remove the condenser and pour/scrape the reaction
mixture into a 20 mL beaker containing 5 mL of water with 5 g ice. Break up any
lumps with a small spatula. (The experiment can be interrupted at this point.)
The product mixture can now be separated into a solid fraction and an oil fraction.
Isolate the crude solid product by suction filtration with a Hirsch funnel attached to a
clean test tube with sidearm. Wash the filter cake with 3-5 mL of ice water with
aspiration. Allow the ice water to drain through the crystals under suction and
continue the aspiration until the solid is nearly dry. Weigh this crude solid product
before purifying it by recrystallization in the next step. Transfer the oil from the
sidearm test tube to a smaller test tube for subsequent TLC analysis. What product(s) have
possibly formed.
Transfer the crude solid from the filtration to a 25 mL Erlenmeyer flask and add 1 mL of
95% ethyl alcohol. Heat this mixture to boiling on the steam bath in order to dissolve the
crude product. Set the flask aside and allow the contents to cool slowly to room
temperature. Isolate the nearly pure crystals of nitrated product by suction filtration
(clean Hirsch funnel.) Wash the crystals with ice cold alcohol, allowing the washes to
drain into the filter flask. Dry the crystals completely. When the crystals
have dried obtain their weight and melting point. (Be sure to include all your
solid product in a sealed plastic bag with your notebook upon submission of your
report.) About 0.6 g of crystals may be expected. From the melting point,
which isomer is it? Obtain a PMR spectrum (see "additional common
practices...") of the crystalline product in deuterated chlorofrom. Make
spectroscopic assignments of the important peaks and comment on the purity of your
sample. What is your pure product?
The filtrate contains water and oil. Be certain the oil has come through the Hirsch
funnel or your yield of oil will be low. The oil will be analyzed by TLC (thin layer
chromatography, Mayo pp. 148-150) to identify the possible number of products formed.
Using a Pasteur pipet transfer only the oil from the two-phase mixture into a 3 ml conical
vial; add a small spatula-full of anhydrous Na2SO4 (What does
anhydrous mean?) to remove traces of water from the oil (termed the mother liquor).
Let the mother liquor dry for a few minutes and then transfer it with a Pasteur filter
pipet (Mayo, et al. p 41) to a pre-weighed, clean, dry vial; reweigh, and save for TLC
analysis.
Thin Layer Chromatography (TLC): Each student will carry out a TLC and report an
Rf value for each spot obtained and estimate the ratio of the nitrohalobenzenes
obtained. As always, consult your lab text (Mayo pp. 148-150) for background
information on TLC.
Prepare several micropipet spotters: snap off and discard the closed ends of several
melting point capillary tubes. With both hands, rotate the tube in a flame until the glass
softens. Remove the glass from the flame and immediately pull the tube into a fine
capillary, approximately 30 cm long. Finally, snap off most of the fine part,
approximately 6 cm from each open end. Two spotters can be made from each melting point
tube. By dipping the narrow end of the spotter into a solution, approximately 2 µL of
liquid will rise into the capillary. This may take some practice and will vary
depending on the nature of the solution you are spotting.
Obtain a 3½ x 10 cm TLC sheet which consists of a layer of silica gel spread over a
plastic sheet (Mayo, et al. p. 148). Pencil in a faint starting line 1 cm from the edge of
the plate (Fig. 5.53). Place a tiny spot (the smaller and more
concentrated, the better) of your oil product mixture by tapping the tip of the
spotter on the line. Allow the solvent to evaporate and re-tap the same point. Similarly
place another spot from a solution of a small amount of the crystalline product dissolved
in 1 drop of chloroform on another point on the line. Place the plate in a jar or beaker
containing 0.25 cm of a 9:1 (v:v) hexane:chloroform solution (Fig. 5.54 - the TLC plate
should be more upright), cover the jar or beaker and observe this solution rise on the
plate. (Now might be a good time to get the melting point of the crystalline product while
the TLC is developing.) When the solvent front has crept to within 1-2 cm from the top,
remove the plate and immediately mark the point where the solvent front reached its
highest point. Use the short wave ultraviolet lamp in the hood (always point this lamp
away from you) to outline the developed spots with pencil. Use the centers of these spots
to calculate the Rf values (Fig. 5.55) for each component of the mixture. Since
a compound will have about the same Rf value whether it is run by itself or in
a mixture, one of the spots from the mother liquor should coincide with the spot for the
pure solid product. While it is under the lamp, make a visual estimate of the
relative amounts of the two spots from the mother liquor (e.g. 1 part a : 3 parts
b).
Spotting and developing the plates takes time and practice; be prepared to repeat this
process many times until you get satisfactory results. Save the best plate in a sealed
plastic bag and make a careful, scaled drawing of the spots on your plate in your
notebook. Place the "best" chromatogram in a sealed plastic
bag in your notebook.
Discussion: Usually the compound in a product mixture with the higher melting point
will form crystals first. Was this the case in your product mixture? Identify the
compounds you formed and provide a rationale for your answer.
With TLC, the stationary phase, silica, is more polar than the solvent, hexane-chloroform;
therefore, the less polar compounds will associate more with the mobile phase and thus
travel farther. Based on drawings of the nitrated products, decide which isomer will have
a larger Rf value. Is this the case? Explain completely.
Look up in the Handbook the melting points, molar masses and densities of the mono-nitro
derivatives you produced. Create a Reaction Table based on a balanced equation for the
nitration (combine all products as if they were one product for this calculation) and
tabulate under each compound the molar mass, density, and mass used as necessary to
calculate the number of moles used of each reactant. Based on the amount of crude yield
(crystals plus mother liquor) collected, calculate the percent yield overall for the
nitration reaction.
Now setting the total crude yield as 100%, calculate the amounts of the individual
products; one of the isomers will be all the crystals plus a fraction of the mother liquor
based on the visual inspection of your TLC. Show your calculations and reasoning.
Compare this percent you obtained with the data in McMurry Table 16.1.
Conclusions: Identify the principal products of the reaction, their percent
yields and melting points. Comment on the relationship between each compound's structure
and its mobility in TLC. What information did the PMR spectrum provide? Should you
be able to distinguish between the isomers using PMR? Why or why not? What
other technique could you use to differentiate between the isomers and specifically what
results would you anticipate for each?
References:
"Mayo et al.": Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991
"McMurry": McMurry, John, Organic Chemistry, 4th ed., Brooks/Cole Publishing Co.: Pacific Grove, CA, 1995.
The "Handbook": any of the recent editions of Weast, R.D. Handbook of
Chemistry and Physics; The Chemical Rubber Co.: Cleveland, 1960-present.
Revised by SLP on 3/22/99.