Experiment 10

Electrophilic Aromatic Substitution: Nitration of Methyl Benzoate

Background

Benzene rings are components of many important natural products and other useful organic compounds. Therefore, the ability to put substituents on a benzene ring, at specific positions relative to each other, is a very important factor in synthesizing many organic compounds. The two main reaction types used for this are both substitutions: Electrophilic Aromatic Substitution (EAS) and Nucleophilic Aromatic Substitution (NAS). The benzene ring itself is electron-rich, which makes NAS difficult, unless there are a number of strongly electron-withdrawing substituents on the ring. EAS, on the other hand, is a very useful method for putting many different substituents on a benzene ring, even if there are other substituents already present. Chapter 12 in Organic Chemistry, by Carey, describes the factors involved in the regioselectivity for EAS reactions using benzene rings which already have substituents on them.

In this experiment you will put a nitro (—NO₂) group on a benzene ring which already has an ester group attached to it (methyl benzoate). The actual electrophile in the reaction is the nitronium ion (NO₂⁺), which is generated in situ ("in the reaction mixture") using concentrated nitric acid and concentrated sulfuric acid (see Carey and your lecture notes for the mechanism):

Product Name: Methyl m-nitrobenzoate (Information about naming esters is available online)

Note that only one product is isolated. Why is this the only product?  (You should draw resonance structures for the anticipated [meta-substitution] as well as ortho- and para-substituted products.)  Why is the ester group electron withdrawing?

Procedure

Safety: Concentrated nitric acid and concentrated sulfuric acid are both strong oxidizers, and strongly corrosive--wear gloves while handling them, and avoid breathing their vapors. Methyl benzoate and methyl m-nitrobenzoate are irritants -- wear gloves while handling them. Methanol is a flammable liquid, and is toxic -- no flames will be allowed in lab, wear gloves while handling it, and avoid breathing its vapors.

1. Add 3 mL of concentrated sulfuric acid to a 125-mL Erlenmeyer flask.  Let this flask, and its contents, cool in an ice bath (or ice-water mixture) for 5-10 minutes. (It must be close to 0 °C, but do not measure the temperature with a digital thermometer, since the probe will get corroded.)
2. Add 12 mmol (how many grams is this?) methyl benzoate (based on its molar mass) to the cold sulfuric acid in the flask.  (You can weigh out the methyl benzoate, just be ultra careful not to spill.)   
3. Let the sulfuric acid and methyl benzoate mixture sit on ice for an additional 5 minutes (do not worry about any color changes).
4. To prepare your H₂SO₄/HNO₃ mixture (nitration reagent), add 1 mL of concentrated sulfuric acid to 1 mL of concentracted. nitric acid in a small test tube.  Cool this acid mixture in an ice bath prior to using in the next step.
5. Using a Pasteur pipet, slowly add (drop-by-drop) the H₂SO₄/HNO₃ mixture to the H₂SO₄/methyl benzoate mixture (already in the flask). Swirl the mixture after each drop of your acid has been added. Keep the reaction flask in the ice bath during these additions.
6. When the addition of the concentrated H₂SO₄/HNO₃ mixture has been completed, allow the entire reaction mixture to warm to room temperature.  Allow the reaction mixture stand for an additional 15 minutes to allow reaction to proceed to completion.
7. Pour the entire reaction mixture onto about 10 g of crushed ice
   • You do no need to weigh the ice, so just add enough ice to come up to about the 20-mL mark on a 50-mL beaker
   • Too much ice will not be a problem, but all the ice must be melted prior to doing your vacuum filtration
8. Isolate the solid product by suction filtration using a small Büchner funnel..
   • Save a small amount of this crude product in a labelled test tube until the next lab period for a melt point determination (do not seal the tube, since this will prevent evaporation of solvent)
9. Recrystallize your crude product (collected via vacuum filtration) using methanol as your recrystallization solvent.
   • Use a minimal amount of solvent for this recrystallization, following the recrystallization protocols from previous experiments (if you think you are not getting a good yield, evaporate some of the methanol)
10. Record your product yield (crude and recrystallized product), melt points, and IR (if instructed on how to do an IR of a solid sample) of the purified product.  
11. Show your sample to your instructor.

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Copyright © Donald L. Robertson (Modified: 09/18/2006)