Question

Predict the products of the following reactions: (a) Toluene \(+\) phthalic anhydride \(+\mathrm{AICI}_{3} ;\) followed by \(\mathrm{H}_{2} \mathrm{O}\). (b) The product from (a) \(+\) Cone. \(\mathrm{H}_{2} \mathrm{SO}_{4}+\) heat.

Short answer

The products of the given reactions are as follows: (a) Toluene reacts with phthalic anhydride and AlCl3, followed by water to form p-methylbenzoic acid. (b) The product from (a), p-methylbenzoic acid, reacts with concentrated H2SO4 and heat to form methylphthalide.

Step-by-step solution

Predict the product of the first reaction: toluene + phthalic anhydride + AlCl3, followed by water.

In the first reaction, we have toluene, phthalic anhydride, and AlCl3 as reagents. Toluene is an aromatic compound, phthalic anhydride is an anhydride group, and AlCl3 is a Lewis acid catalyst. Toluene has an electron-rich aromatic ring containing a methyl group that is capable of Friedel-Crafts acylation. The reagent used for this acylation is phthalic anhydride, which can act as an acylating agent with the help of the Lewis acid catalyst AlCl3. The Friedel-Crafts acylation is a type of electrophilic aromatic substitution reaction. When toluene reacts with phthalic anhydride, toluene undergoes acylation at the ortho and para positions of the aromatic ring to give a mixture of isomeric products. Water will hydrolyze the acyl halide intermediate formed in the Friedel-Crafts acylation to give the product. The major product of the reaction is the para-isomer, p-methylbenzoic acid, which is formed due to lesser steric crowding.

Predict the product of the second reaction: product of step 1 + concentrated H2SO4 + heat.

In the second reaction, we use the product from the first reaction, p-methylbenzoic acid, along with concentrated H2SO4 and heat. The reaction that takes place in this step is an intramolecular dehydration reaction. Concentrated H2SO4 is a strong dehydrating agent and promotes the reaction. When p-methylbenzoic acid reacts with concentrated H2SO4, the -OH group in the carboxylic acid part of the product reacts with the adjacent -COOH carbonyl group to remove water, thereby forming an intramolecular cyclic ester, which is also known as a lactone. In our case, the lactone product is called methylphthalide. So, the final products of the given reactions are p-methylbenzoic acid (from the first reaction) and methylphthalide (after reacting p-methylbenzoic acid with concentrated H2SO4 and heat).

Key concepts

Electrophilic Aromatic Substitution

The concept of Electrophilic Aromatic Substitution (EAS) is pivotal to understanding how many reactions involving aromatic compounds, such as toluene, occur. This process allows an electrophile to replace a hydrogen atom on an aromatic ring. In this mechanism, the stability of the aromatic system plays a critical role as the ring momentarily loses its aromaticity during the reaction and subsequently regains it.

During EAS, the first step is the creation of a powerful electrophile that can attack the electron-rich aromatic ring. In our given reaction, phthalic anhydride itself is not a strong enough electrophile. This is where the Lewis acid catalyst, AlCl3, comes into play. It reacts with phthalic anhydride to form a highly reactive acylium ion that can act upon the toluene. In the textbook exercise, toluene is acylated, and through a series of steps—initial attack, formation of a sigma complex, and deprotonation—a new carbon-carbon bond forms. The result is one or more isomeric ketones, where the major product commonly has substituents in the para position due to steric and electronic factors.

Lewis Acid Catalyst

Without a Lewis Acid Catalyst, many EAS reactions, such as the Friedel-Crafts acylation, might be too slow or not proceed at all. Lewis acids, like AlCl3 used in our exercise, are critical because they accept electron pairs. In the context of the Friedel-Crafts acylation, AlCl3 accepts a pair of electrons from the carbonyl oxygen of the phthalic anhydride.

This action transforms the anhydride into a more reactive species, an acylium ion, by facilitating the loss of a leaving group. This positively charged ion is the true electrophile that the aromatic compound will attack. It's crucial for students to understand that without this catalyst, the electrophile's reactivity would be insufficient to engage the stable aromatic system, thus impeding the acylation reaction.

Intramolecular Dehydration Reaction

The second part of the exercise presents an intramolecular dehydration reaction, which is a type of reaction where a molecule loses a water molecule from two functional groups within the same molecule. This is present when we take the product from the first reaction, p-methylbenzoic acid, and subject it to concentrated H2SO4 and heat. The strong acidic conditions of concentrated H2SO4 can provoke the hydroxyl group (-OH) and the adjacent carbonyl group (-COOH) on the benzoic acid to interact, releasing a water molecule and forming a new cyclic structure.

This reaction is not only interesting because it produces water as a byproduct, but also because it creates a new ring system within the molecule, known as a lactone, in this case, methylphthalide. Understanding these concepts helps unravel the complexity of chemical reactions involving arenes and reminds students of the intricate dance between molecules leading to the formation and loss of water to create more complex structures.