Question

Suggest a mechanism for the steps in the synthesis of phenolphthalein.

Short answer

The synthesis of phenolphthalein involves a series of reactions, beginning with the formation of phenoxide ion by reacting phenol with a strong base (NaOH or KOH). The phenoxide ion, as a strong nucleophile, attacks the electrophilic carbon atom of one carbonyl group in the phthalic anhydride molecule to form an ester linkage and generate an intermediate molecule. A second alkoxide ion is formed by introducing another molecule of phenol and reacting it with a strong base. This second alkoxide ion then attacks the remaining carbonyl group in the intermediate molecule, forming another ester linkage and producing the phenolphthalein molecule.

Step-by-step solution

Starting materials

Phenolphthalein can be synthesized from the reaction of phenol and phthalic anhydride in the presence of a strong base (usually sodium hydroxide or potassium hydroxide). Step 2: Formation of alkoxide ion

Formation of alkoxide ion

The phenol reacts with the strong base (NaOH or KOH) to form a phenoxide ion. The base abstracts the acidic hydrogen from phenol, resulting in the phenoxide ion and a water molecule. Phenol acts as an acid and the strong base as the base. The reaction is as follows: \[ Phenol + NaOH \rightarrow Phenoxide\, ion + H_2O \] Step 3: Nucleophilic attack

Nucleophilic attack

The phenoxide ion, being a strong nucleophile, attacks the electrophilic carbon atom of one carbonyl group in the phthalic anhydride molecule. This step forms the ester linkage between the phenol and phthalic anhydride and generates an intermediate molecule. \[ Phthalic\, anhydride + Phenoxide\, ion \rightarrow Intermediate \] Step 4: Formation of second alkoxide ion

Formation of second alkoxide ion

We need to introduce another molecule of phenol and convert it into a phenoxide ion in a similar manner as in Step 2. This is done by reacting phenol with another equivalent of strong base (NaOH or KOH). Step 5: Nucleophilic attack by the second alkoxide ion

Nucleophilic attack by the second alkoxide ion

The second phenoxide ion (generated in step 4) reacts with the intermediate molecule from step 3 and attacks the remaining carbonyl group. This step forms another ester linkage, similar to the one formed in step 3, and produces the phenolphthalein molecule. \[ Intermediate + Phenoxide\, ion \rightarrow Phenolphthalein \] In conclusion, phenolphthalein is synthesized through a series of reactions including the formation of phenoxide ions and nucleophilic attacks on the phthalic anhydride molecule by these ions.

Key concepts

Nucleophilic Attack

Nucleophilic attack is a crucial step in organic synthesis, especially in the creation of phenolphthalein. This step involves the phenoxide ion, which is a strong nucleophile. A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond.
The phenoxide ion, equipped with extra electrons, is capable of attacking the carbon atom of a carbonyl group in the phthalic anhydride. Phthalic anhydride has two electrophilic carbonyl groups susceptible to nucleophilic attack.

• The result is the formation of a new covalent bond.
• This bond links the phenoxide ion with the carbon atom of the anhydride.

It is important to note that each nucleophilic attack creates an ester link. This link is essential for building the structure of phenolphthalein. The entire process should be executed twice to yield phenolphthalein from the phenol and phthalic anhydride.

Phenol Reaction

In the synthesis of phenolphthalein, phenol plays a fundamental role in the formation of phenoxide ions. Initially, phenol reacts with a strong base like sodium hydroxide (NaOH) or potassium hydroxide (KOH).
This reaction leads to the formation of a phenoxide ion through the removal of a hydrogen ion from the phenol. In turn, this transforms the phenol into an even stronger reacting species.

• Phenol essentially acts as an acid, and the applied base removes its acidic hydrogen.
• The generated water results from the deprotonation process.

Once the phenoxide ion is formed, it is ready to react further by undergoing the nucleophilic attack described earlier. Bringing about this transformation empowers phenol for further involvement with other reactants like phthalic anhydride.

Phthalic Anhydride

Phthalic anhydride serves as a key reactant for synthesizing phenolphthalein. It is an organic compound characterized by its two carbonyl groups, which are highly reactive.
These reactive carbonyl groups are electrophiles, capable of undergoing reaction with nucleophiles. During the synthesis of phenolphthalein, phthalic anhydride behaves as follows:

• Each carbonyl group is sequentially attacked by a phenoxide ion in two steps.
• This results in the formation of ester linkages that connect phenol molecules to the phthalic acid framework.

The role of phthalic anhydride extends beyond being a mere reactant; it provides the backbone for phenolphthalein's molecular structure. The process effectively incorporates multiple phenol units into a more complex and functional molecule.