Question

Suggest the syntheses of (a) $\beta$ -naphthol from naphthalene (b) Orange II from $\beta$ -naphthol and sulfanilic acid.

Short answer

(a) To synthesize β-naphthol from naphthalene, perform a sulfonation reaction using fuming sulfuric acid, followed by treating the product with a strong base like sodium hydroxide, and then acidify the reaction mixture with hydrochloric acid, which results in the β-naphthol. (b) To synthesize Orange II from β-naphthol and sulfanilic acid, first, diazotize the sulfanilic acid using sodium nitrite in an acidic solution, and then perform an azo coupling reaction between the β-naphthol and the sulfanilic diazonium ion, yielding Orange II.

Step-by-step solution

Perform a sulfonation reaction

Sulfonate the naphthalene molecule by treating it with fuming sulfuric acid (H2SO4). The reaction follows electrophilic aromatic substitution, with the electrophile being sulfuric acid. The result is the formation of 1-naphthalenesulfonic acid. $\text{naphthalene}+H_{2}S{O}_4\to \text{1-naphthalenesulfonic acid}$

Transform sulfonic acid into phenol

Treat 1-naphthalenesulfonic acid with a strong base, such as sodium hydroxide (NaOH), in a heated solution. The strong base will deprotonate and eliminate the sulfonic acid group and create a phenoxide ion. $\text{1-naphthalenesulfonic acid}+NaOH\to \text{1-naphtholate}+NaHS{O}_4$

Protonate the phenoxide ion

Acidify the reaction mixture by adding a mineral acid, such as hydrochloric acid (HCl). Acidification will protonate the phenoxide ion and form the desired β-naphthol compound. $\text{1-naphtholate}+HCl\to \text{β-naphthol}+NaCl$ Now we have synthesized β-naphthol from naphthalene. #Part B: Synthesize Orange II from β-naphthol and sulfanilic acid#

Diazotize sulfanilic acid

In an acidic solution (e.g., using hydrochloric acid), add sodium nitrite (NaNO2) to sulfanilic acid. The reaction will generate a diazonium ion, an important intermediate for the synthesis of azo dyes. $\text{sulfanilic acid}+HCl+NaN{O}_2\to \text{sulfanilic diazonium ion}+NaCl+H_{2}O$

Perform an azo coupling reaction

Mix β-naphthol with the solution containing the sulfanilic diazonium ion. An azo coupling reaction will occur, which is an electrophilic aromatic substitution involving the diazonium ion as the electrophile and β-naphthol as the nucleophile. The resulting molecule is the azo dye Orange II. $\text{β-naphthol}+\text{sulfanilic diazonium ion}\to \text{Orange II}$ Now we have synthesized Orange II from β-naphthol and sulfanilic acid.

Key concepts

Electrophilic Aromatic Substitution

Understanding the electrophilic aromatic substitution (EAS) is fundamental in organic chemistry, especially when discussing the synthesis of naphthalene derivatives. In EAS, an electrophile attacks an aromatic ring, like benzene or naphthalene, and substitutes for one of the hydrogen atoms. The beauty of this mechanism lies in its ability to retain the aromaticity of the compound, a key factor for its stability and reactivity.

During this process, the aromatic compound undergoes a temporary loss of aromaticity to form a non-aromatic intermediate called a sigma complex or arenium ion. The re-gained aromaticity drives the reaction to completion, making it energetically favorable. This concept explains the first step in our original exercise, where we used fuming sulfuric acid, a strong electrophile, to add a sulfonic acid group to the naphthalene through EAS, creating 1-naphthalenesulfonic acid. The sulfonation step is reversible, and conditions are chosen to favor the desired sulfonation product.

Naphthalene Derivatives

Naphthalene derivatives are a group of compounds based on the naphthalene structure, which consists of two fused benzene rings. These derivatives serve as key starting materials in creating more complex molecules, including dyes, pharmaceutical compounds, and materials with particular electronic properties.

As shown in our exercise, transforming naphthalene into β-naphthol illustrates how base-induced cleavage of the sulfonic acid can introduce a hydroxyl group, a transformation known as a nucleophilic aromatic substitution. This change significantly alters the chemical behavior of the molecule, enhancing its reactivity towards other species. Such modifications bestow naphthalene derivatives with new properties, enabling their use in a wide array of chemical industries.

Azo Coupling Reaction

The azo coupling reaction is an organic reaction that forms an azo compound and is widely used to produce colorful azo dyes, which are characterized by their vivid colors and application in textile and printing industries. In this reaction, a diazonium ion, which is derived from an aromatic amine, couples with an electron-rich aromatic compound.

In our case, β-naphthol, an electron-rich aromatic compound, reacts with the sulfanilic diazonium ion to form Orange II, an azo dye. The unique aspect of the azo coupling reaction is its dependency on pH and temperature, determinants that influence the rate and outcome of the reaction. Azo dyes such as Orange II are used due to their strong bond to textile fibers and the ability to withstand washing and light exposure.