Nitrating Acetanilide and Methyl Benzoate: Electrophilic Aromatic Substitution

ABSTRACT:

The electrophilic aromatic substitution reaction is the attack of a benzene ring on an electrophilic species resulting in the substitution of a proton with a functional group. The electrophilic aromatic substitution reaction nitration is used to nitrate methyl benzoate and acetanilide with a nitronium ion. Crystallization was used to purify the product. The melting point was used to determine its purity and the regiochemistry of the products. The methyl benzoate reaction product, methyl nitrobenzoate, was determined to be meta-substituted and the acetanilide reaction product, nitroacetanilide, was determined to be para-substituted.

INTRODUCTION:

An electrophilic aromatic substitution reaction is the attack of an electrophile on an aromatic ring substituting for a proton. This reaction allows for the introduction of other functional groups onto the aromatic ring. The electrophile attacks the aromatic ring at the aliphatic position removing two electrons destabilizing the aromatic ring, but creating a resonance-stabilized carbocation called a sigma complex (arenium ion). Then the aliphatic proton is lost to give the substitution product.

The reaction rate with the aromatic ring depends on its substituents. Groups that increase the reaction rate with benzene are called activators, while ones that decrease the reaction rate are called deactivators. Activators stabilize the arenium ion by increasing the electron density on the aromatic ring by adding two electrons via resonance or through hyperconjugation, the overlap of neighboring sigma bond with the aromatic pi system. Deactivators decrease the electron density by removing two electrons via resonance or induction, resulting in the destabilization of the arenium ion (Rowland, et al, 224).

There are three positions of electrophilic substitution on the benzene ring based on the electronic nature of the substituents (activators and deactivators). These three positions are: ortho (1,2), meta (1,3) and para (1,4) (see Figure 1). The substituents can be ortho-para-directing activators, ortho-para-directing deactivators, or meta-directing deactivators. Activators of the benzene ring are ortho-para-directing creating a positive charge on the carbon that bears the substituent, which can be stabilized if the substituent is electron-donating. Ortho-para attack on a deactivated ring has a similar result. However, meta attack on a deactivated ring is favored because the meta-directing, electron-withdrawing groups avoid the positive charge on the carbon that carries the substituent. Halogens have both electron-withdrawing and electron-donating resonance effects making them ortho-para deactivators. Their high electronegativity deactivates the ring, but they stabilize it by sharing a lone pair electron (Rowland, et al, 224-225). The nitronium ion in the electrophilic aromatic substitution nitration is a meta-directing deactivator. Nitration is an important electrophilic aromatic substitution and it is used in this experiment. The nitronium ion is formed by the protonation and loss of water from nitric acid by a sulfuric the acid catalyst. A dehydrating reagent like CaCl3 can remove the hydronium ion created in this reaction. This prevents a reverse reaction.

Figure 1. Three positions of electrophilic substitution.

Methyl nitrobenzoate is the substitution product of the electrophilic aromatic substitution of methyl benzoate by nitric acid. According to the results the methyl benzoate was attacked at the meta position giving meta-methyl nitrobenzoate. Nitroacetanilide is the substitution product of the electrophilic aromatic substitution of acetanilide by nitric acid.

Figure 2. Reaction scheme of the electrophilic aromatic substitution of

methyl benzoate

Figure 3. Reaction scheme of the electrophilic aromatic substitution of

acetanilide.

A catalyst is a substance that induces a reaction but does not itself take part in the reaction. The sulfuric acid is used as a catalyst to allow the nitration of the aromatic ring to take place more rapidly and at lower temperatures. Without the sulfuric acid, the nitric acid has to be added at high temperatures and when added to any oxidizable material it might explode (Wade, 726).

Recrystallization is a purification process in which impurities are removed from solid products (Rowland, 29). In order to do this you have to have a solvent that only dissolves the wanted product at high temperatures, but dissolves impurities at high and low temperatures. The solvent is heated and then added to the solid. The solid is dissolved in the solvent. Then the pure solid is recrystallized, leaving the impurities behind in solution.

The melting point of a substance is the point at which a solid becomes a liquid. The boiling point can be used to determine the purity if a substance. If the melting point is found to be lower than it should be this indicates that it has an impurity. The boiling point can also be used to determine the regiochemistry of a substance. In aromatic substitution, there are ortho- and para- directing, as well as meta- directing groups that determine where the electrophile will add. These three positions change the melting point enough to be able to tell the substances regiochemistry. For example, methyl benzoate has the following melting points:

Ortho mp: -13C

Meta mp: 78-80C

Para mp: 94-96C

Determining the melting point therefore ultimately determines the regiochemistry of a product of electrophilic aromatic substitution.

The percent yield is a calculation of the amount of product obtained from an experiment compared to expected amount based on the mole ratio between the limiting reagent used with the product formed. The percent recovery is a calculation of the amount of pure product obtained as well as the percent of impurity removed from the pure product.

EXPERIMENTAL PROCEDURE:

An ice bath was prepared in a 400mL beaker so that it was 75% full. A 125mL Erlenmeyer flask was obtained and 30mL of distilled water was poured into. The water was placed in the ice bath to chill.

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