Exp’t 440
2-(2,4-dinitrophenyl)pyridine: The Synthesis of a Photochromic
Compound
Adapted by R. Minard & Martin Schierhorn from E.D. Gilfillan and M.W. Pelter: “Dinitration of 2-Benzylpyridine: Microscale Synthesis of a Photochromic
Compound”, J. Chem. Educ. 1994, 71, A4 (rev 12/4/03)
Introduction
If you are tired of synthesizing either colorless liquids or white crystals, you are going to like the following experiment: The
synthesis of 2-(2,4dinitrophenyl)pyridine, 2, a photochromic compound (eq 1)!
NO2
70% HNO 3
N
2 benzylpyridine
(1)
N
18M H 2SO 4
(1)
NO2
2-[(2,4dinitrophenyl)]pyridine
(2)
(1)
This synthesis was first carried out by Tschitschibabin in 1925.1 The crystals of 2 are pale-brown in color when there is no light
present. However, when exposed to light, (try UV lamp), the color transforms to a deep-blue. This property is known as
photochromism, the ability of a compound to change color under exposure to light. In the case of 2, this process is indefinitely
reversible: the crystal return to its original color when stored in darkness for approximately a day. You should go to
http://JChemEd.chem.wisc.edu and find the November 2000 issue, pg 1386 for the article by Addison Ault that shows pictures of the
photochromic behavior in color.
An internal hydrogen transfer is responsible for the photochromic behavior (eq 2).
N
HH O
N
H
+
UV light
O
NO2
pale brown/yellow
H
N
O
N
darkness
+
O
(2)
NO2
dark blue
As you can see the tautomerization is favored because of the 6-membered transition state. Under light the nitro group’s oxygen
has enough energy to attract the methyl hydrogen and through an internal electron transfer, the dark blue form of 2 is formed.
Experiments regarding the photochromic behavior of the compound have been conducted with other substituents such as an alcohol
group instead of the pyridine. As you might expect, it has been concluded that the nitro group and a CH group have to be ortho to
each other in order for the isomerization to take place.1 This experiment can be done on either a microscale or macroscale level. Both
procedures are included.
Cautions
In this experiment you will be using concentrated sulfuric and nitric acid. Handle these chemicals with care. If you spill acid on
your skin, immediately wash the exposed area with cold water. 2-Benzyl pyridine is classified as a harmful irritant.2 Avoid inhaling
the chemical or exposing it to your skin. Work in the hood!
Macroscale procedure for making 2-(2,4-dinitrobenzyl)pyridine 2
Hazards The procedure calls for both concentrated sulfuric acid and concentrated nitric acid. These materials are corrosive and
should be handled with the care normally taken with concentrated acids. Keep 2-benzylpyridine off the skin.
Synthesis: Place 12 mL of concentrated sulfuric acid (0.22 mole) in a 50-mL Erlenmeyer flask. Cool the acid to 5 °C or below in an
ice bath. Support the flask so that it can be stirred or swirled in an ice bath and add gradually with good mixing 2.5 mL (2.6 grams;
0.016 mole) of 2-benzylpyridine. To this well-stirred mixture add drop by drop, over a period of about 3 minutes, 2.25 mL of
concentrated nitric acid (density 1.42 g/mL; 0.036 mole HNO3; (Note 1). The addition of the first few drops of the nitric acid will
cause a color change to deep brown, but the mixture will become lighter in color as the remainder of the acid is added. After all the
nitric acid has been added, heat the mixture for about 20 minutes in a hot water bath.
Isolation: At the end of the heating period, pour the mixture onto about 200 grams of ice in a 1-liter Erlenmeyer flask (Note 2).
Basify the solution to a pH of about 11 by adding almost all of a solution of 20 grams (0.5 mole) of sodium hydroxide in about 250
mL of water (Note 3). Toward the end of the addition of base the product separates to give a milky yellow suspension. Add about
200 mL of ether and stir the mixture for 10-15 minutes to extract the product into the ether layer. Separate the ethereal solution (Note
4), dry it over anhydrous sodium sulfate for a few minutes, filter, and remove the ether on a rotary evaporator. The product will
sometimes start to crystallize during the concentration of the solution. Complete the crystallization by cooling the mixture in an ice
bath. Collect the large sandy prisms by suction filtration, and wash them with a small amount of cold 95% ethanol. Yield: from 1 to 2
grams (from 25 to 50%). The product can be recrystallized from 95% ethanol with 90% recovery using about 10 mL of ethanol per
gram. While the mixture is heating add H2O until the hot solution just starts to get turbid or milky. Remove from the heat and let
stand undisturbed while cooling to room temperature.
Notes:
1. The original procedure called for red fuming nitric acid, which is produced only in China and costs more than $300 for 500 mL. We
discovered by accident during the summer of 1994 that an equivalent amount of concentrated nitric acid will work just as well; it costs
less than $20/lb).
2. A 1-liter beaker is almost as satisfactory, but it is easier to spill from a beaker than from an Erlenmeyer flask.
3. If you must make this solution from solid NaOH, put 20 grams of solid NaOH (0.5 mole NaOH) into a 500 mL Erlenmeyer flask or
beaker and add at first only about 20 mL of water. The mixture will get very hot and the sodium hydroxide will dissolve quite rapidly.
After almost all the solid has dissolved, add the remaining 225 mL of water. If you add all the water at once, dissolution takes much
longer! If you can make the sodium hydroxide solution from “50% NaOH”, add 26 mL of the “50% NaOH” (40 grams of 50% by
weight NaOH; 20 grams of NaOH; 0.5 mole of NaOH) to 230 mL of water.
4. This separation can be messy. First, the total volume will be about 500 mL, and separatory funnels large enough to hold the entire
mixture are not in your locker. You will need to borrow a 1000-mL separatory funned from the stockroom. Second, the layers
sometimes do not separate cleanly, and there is often some emulsion present at the interface. Try adding some saturated NaCl solution
to break the emulsion
Literature cited:
*”Cornell College Students Achieve the Highly Improbable” By Addison Ault, Cornell College, Mt. Vernon, IA 52314; [email protected] J. Chem. Educ. 2000,
77, 1386-1387.
1. Ault, A.; Kouba, C. "A Photochromic Compound", J. Chem. Educ. 1974, 51, 395.
2. Zaczek, N. M.; Levy, W. D.; Jordan, M. L.; Niemyer, J. A. "Synthesis of Photochromic 2-(2,4-Dinitrobenzyl)pyridine", J. Chem. Educ. 1982, 59, 705.
3. Gilfillan, E. D.; Pelter, M. W. "Dinitration of 2-Benzylpyridine: Microscale Synthesis of a Photochromic Compound", J. Chem. Educ. 1994, 71, A4.
Final Report
For your final report, obtain a UV/Vis spectrum. Record the melting point of your product and calculate the percent yield of your
reaction.
In addition, answer the following questions: (Include the answers in the Discussion section of your final report.)
1) What is the function of the H2SO4 in the reaction?
2) Why is the addition of NaOH necessary before extraction of the product into ether? Hint – Pyridine is an amine.
3) Why is it necessary for the nitro-group and the C-H group to be ortho to each other in order to ensure photochromic
behavior?
1)
2)
3)
4)
References
Gilfillan, E.D.; Pelter, M.W. J. Chem. Educ. 1994, 71, A4
Lancaster Fine Chemicals Catalogue; Lancaster Synthesis Inc., 1993-94, 133
Nunn, A.J.; Schofield, K. J. Chem. Soc. 1952, 583
March, J. Advanced Organic Chemistry, 4th ed, John Wiley and Sons Inc.,NY, 1992, pg 524