lab organic chemistry

NITRATION OF METHYL BENZOATE

OBJECTIVES: To perform an electrophilic aromatic substitution using methyl benzoate as a substrate. To characterize the product by NMR and mp

Background

Electrophilic aromatic substitution.

Benzene does not react with most HX reagents like alkenes and dienes, due to its stability that would require a very high activation energy for the reaction to happen. However, it reacts with electrophiles in “electrophilic aromatic substitutions”. The first part is an addition reaction that generates a resonance stabilized intermediate, followed by a deprotonation steps, during which the aromaticity is regenerated. Nitration is an example of an electrophilic aromatic substitution.

In the case of monosubstituted aromatic rings, there are three possible products that can obtained: substitution can occur in -meta, -ortho, or -para position. The site of substitution of the second substituent is determined by the first substituent. These substituents may be divided into two groups. One directs the second substituent into the ortho or para positions. These ortho, para directors include donating groups such as alkyl groups, hydroxyl groups, and halogens. The second group directs the second substituents into the meta position and includes electron withdrawing substituents such as esters, carboxylic acid, and nitro groups. Your starting material is depicted below

The nitration of methyl benzoate is a typical electrophilic aromatic substitution reaction. In the first step of the

reaction a nitronium ion (NO2+) is formed by the reaction of nitric acid and sulfuric acid. The nitronium ion acts as the electrophile and reacts with the aromatic ring at the position where the electron density is the greatest. The non-aromatic carbocation intermediate is stabilized by resonance structures. The reaction is completed by proton loss from the nitro-bearing carbon, regenerating the aromatic ring and giving the substitution product.

Experimental

1. In a small round-bottom flask, with a stir bar, cool 0.6 mL of sulfuric acid to 0°C. (measure out using syringe provided)

2. Add 0.30 mL (density = 1.088 g/mL) of methyl benzoate while stirring and cooling with an ice bath.

3. In a small test tube, measure 0.25 mL sulfuric acid and 0.25 mL nitric acids. *It is important to mix the solution. (Use a small pipet bulb and “push” air into test tube)

4. Add the acid mixture drop wise to the methyl benzoate solution in the small round-bottom flask. (***Do not add acid drops until the temperature is below ~15 ºC AND mix the acid solution before each addition-take your time here!)

5. Warm to room temperature, while stirring with the stir bar, for 25 min.

6. Pipet the mixture onto 2.5 g ice in a small beaker.

7. Filter the product using suction filtration through the plastic Hirsch funnel.

8. Wash the product with water several times, then with ~0.2 mL of ice-cold methanol.

9. Weigh the product and crystallize in a small test tube from an equal weight of methanol (with cooling).

About 50% yield of product with a mp of 78°C is expected. If significantly lower yield is obtained, decrease the volume of the crystallization solution and try again. If still too low, see your TA for an alternative method.

10. Take a 1H-NMR in CDCl3 and a melting point of the product.

11. Assign signals to the structure on the NMR spectra.

Safety Notes: Nitric and sulfuric acids are strong oxidizers and can damage skin and clothing.

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POST LAB ASSIGNMENT

1) Write the introduction for the current experiment in not more than 5 sentences

2) Write the abstract using not more than 4 sentences

3) Write a short discussion (include the mechanism of the reaction)

4) Write the experimental section

5) Write the conclusions section