Question

Indanthrene Brilliant Orange can be synthesized by heating naphthalene-1, \(4,5,8\) -tetracarboxylic acid with 2 moles of o-phenylenediamine. The product is a scarlet mixture (which itself is useful as a vat dye) of Indanthrene Brilliant Orange and an isomer called Indenthrene Bordeaux (red-purple). Formulate a reasonable mechanism for the formation of Indanthrene Brilliant Orange from the reaction of the tetracarboxylic acid and the diamine with attention to the possible formation of isomeric products.

Short answer

In summary, the formation of Indanthrene Brilliant Orange from the reaction between naphthalene-1,4,5,8-tetracarboxylic acid and o-phenylenediamine involves a series of nucleophilic attacks, formation of amide linkages, and intramolecular ring formation. The isomeric product, Indenthrene Bordeaux, can be formed through similar interactions between the reactants but in a different order or fashion, possibly due to steric hindrance or differences in the order of amine-carbonyl interactions.

Step-by-step solution

Understand the structure of Reagents

First, we need to draw the structure of Naphthalene-1,4,5,8-tetracarboxylic acid and o-phenylenediamine. Naphthalene-1,4,5,8-tetracarboxylic acid: This is a naphthalene molecule with carboxylic acid (-COOH) groups at positions 1, 4, 5, and 8. \( \chemfig{*6(=-=(-(-[:60](-[:0])=[:-60]O))=(-(-[:60]O=[:-60](-[:0])))-=)}\) o-phenylenediamine: This is a benzene molecule with amine (-NH2) groups at the 1 and 2 positions (ortho position). \( \chemfig{*6(=-=(-(NH_{2}))-(-(-(NH_{2}))-)=(-)=)}\) Now that we have the structures of the reactants, we can investigate the reaction mechanism to form Indanthrene Brilliant Orange dye and the isomeric Indenthrene Bordeaux dye.

Suggest a mechanism for the formation of Indanthrene Brilliant Orange

Here are the possible steps for the formation of Indanthrene Brilliant Orange: 1. Nucleophilic attack by one of the amine groups of o-phenylenediamine on a carboxyl group of the naphthalene-1,4,5,8-tetracarboxylic acid. The carbonyl oxygen will get a negative charge. 2. The electrons on the carbonyl oxygen will form a π-bond, expelling the -OH group as a leaving group. 3. The amide linkages will form between the aromatic ring of the o-phenylenediamine and the naphthalene-1,4,5,8-tetracarboxylic acid. 4. A second amine group from the o-phenylenediamine will react with another carboxyl group of the naphthalene-1,4,5,8-tetracarboxylic acid through the same mechanism, forming a bis-amide. 5. An intramolecular nucleophilic attack of the amine nitrogen on one of the carbonyl carbons of the other amide will lead to ring formation. 6. Oxidation of the formed intermediate (using an external oxidizing agent) will give the final Indanthrene Brilliant Orange dye.

Investigate the formation of the isomeric product - Indenthrene Bordeaux

Since both Indanthrene Brilliant Orange and Indenthrene Bordeaux are isomers, the difference in their structures can be based on the positions of the amide linkages formed during the reaction between o-phenylenediamine and naphthalene-1,4,5,8-tetracarboxylic acid. This implies that the isomeric product might be formed via a slightly different interaction between o-phenylenediamine and naphthalene-1,4,5,8-tetracarboxylic acid. To ensure the formation of the isomeric Indenthrene Bordeaux, some steric hindrance or change in the order of amine-carbonyl interactions can lead to the alternative product. In conclusion, the mechanism for the formation of Indanthrene Brilliant Orange from the reaction of the tetracarboxylic acid and the diamine involves amide formation and intramolecular ring formation. The isomeric product (Indenthrene Bordeaux) can possibly form through similar interactions between the reactants but in a different order or fashion.

Key concepts

Nucleophilic Attack

At the core of many organic synthesis processes, including the synthesis of Indanthrene Brilliant Orange, is the nucleophilic attack. In this reaction, the nucleophile, o-phenylenediamine, with its lone pair of electrons on the nitrogen atom, targets an electrophilic center. Specifically, the nucleophile attacks the carbon atom of the carboxyl group on the naphthalene-1,4,5,8-tetracarboxylic acid, which is electrophilic due to its double bond with oxygen.

In this case, the amine's nitrogen donates a pair of electrons to form a new bond with the carbon, displacing the double bond electrons onto the oxygen, resulting in a negatively charged intermediate. Understanding this nucleophilic attack is fundamental because it initiates the series of steps leading to the synthesis of vat dyes like Indanthrene Brilliant Orange.

Amide Formation

Following the nucleophilic attack is the formation of an amide bond, a key reaction in the synthesis of Indanthrene Brilliant Orange. Amide formation occurs when the intermediate formed from the nucleophilic attack loses a water molecule. The nitrogen and carbon, previously involved in the nucleophilic attack, now share a bond, and a -CONH- linkage is formed.

This step is repeated as the o-phenylenediamine possesses two amine groups, and each can form an amide linkage with the acid groups present on the naphthalene compound. The development of two amide bonds is pivotal as it stabilizes the structure and sets the stage for further intramolecular reactions that lead to the final dye structure.

Intramolecular Ring Formation

In the progression towards the Indanthrene Brilliant Orange dye, intramolecular ring formation is a critical step. This event transpires after the bis-amide has been formed through repeated amide bond formation. One of the amine groups then initiates another nucleophilic attack, this time on the electrophilic carbonyl carbon of the other amide group within the same molecule.

Such intramolecular reactions are favored under the right conditions because they enhance molecular stability by reducing strain and potential steric hindrance. The ring closure helps to create the complex polycyclic structure characteristic of many vat dyes, including Indanthrene Brilliant Orange.

Vat Dye Chemistry

Vat dyes, including Indanthrene Brilliant Orange, are a class of dyes renowned for their colorfastness and widespread use in textile dyeing. The chemistry of vat dyes involves creating an insoluble dye that can be applied to fabric by first rendering it soluble. This is accomplished by reducing the dye to form a leuco compound which can be dissolved in a vat, hence the name 'vat dye'.

In the synthesis of vat dyes like Indanthrene Brilliant Orange, an important consideration is the use of an oxidizing agent to convert the intermediate into the final dye form. The brightly colored end product is then reintroduced to oxygen to regain its insoluble state, allowing it to permanently adhere to the fibers of the fabric.

Isomeric Products

The synthesis of Indanthrene Brilliant Orange can yield isomeric products due to the multiple reactive sites present in the starting materials. An isomer, in this context, refers to a compound that shares the same molecular formula as another compound but has a different arrangement of atoms. Indenthrene Bordeaux is one such isomeric product which can occur if the order of the nucleophilic attacks and the subsequent amide formation differs from the pathway leading to Indanthrene Brilliant Orange.

Isomers can display varying properties despite their similar compositions. The orientation of the bonds and the three-dimensional arrangement of the atoms can influence the color and other characteristics of the dye, demonstrating the subtle but crucial interplay between molecular structure and chemical properties in synthetic chemistry.