Phenolphthalein synthesis


The purpose of the experiment is to become familiar with the methods of synthesis of dyes of the phthaleins type. The synthesis is carried out by condensation of phthalic anhydride with phenol, and involves an (acid-catalyzed) reaction of electrophilic aromatic substitution SEAr.

phenolphthalein synthesis
Phenolphthalein synthesis

The phenolphthalein is purified by recrystallization of the final product. Once recrystallized, the behavior of phenolphthalein, as an indicator, will be observed in acidic and basic media.


Phenolphthalein (C20H14O4) is a pH indicator. In acidic solutions it remains colorless, but in the presence of basic solutions it takes a pinkish color with a turning point between pH = 8.0 (colorless) to pH = 9.8 (magenta or pinkish).

However, at extreme pH (very acidic or basic) it shows other changes in coloration: at strongly basic pH it becomes colorless, while at strongly acidic pH it changes to orange.

This color change is due to the conjugation of the π electrons of the phenolphthalein derivative which at different pH absorbs at visible light wavelengths. It is related to the different open and closed structure of the lactonic ring present in these compounds.

The different forms of phenolphthalein are summarized in the following figures:

phenolphthalein synthesis - Cationic form of phenolphthalein
Cationic form of phenolphthalein

Synthesis of Phenolphthalein 3

Phenolphthalein is insoluble in water and is usually dissolved in alcohols for use. It is a weak acid that can lose H protons in solution. Therefore, the main applications of phenolphthalein derivatives are as dyes and pH indicators.

Phthalein derivatives can be synthesized from the reaction between two moles of the phenolic derivative with one mole of the anhydride derivative in the presence of different catalysts, among which we can mention: AlCl3, SnCl4, ZnCl2 and H2SO4, among others.

Reaction mechanism

The reaction is catalyzed by acid (methanesulfonic acid, MeSO4H). The reaction mechanism starts, in a first step, with the protonation of the phthalic anhydride as shown in figure, giving rise to the corresponding carbocation which is in a tautomeric equilibrium.

Synthesis of Phenolphthalein 4

In a second step, a π-bond of the phenol attacks the carbocation that has formed. The corresponding Wheland intermediate of reaction SEAr is produced. Next, the unshared electron pair of an oxygen attacks the hydrogen in the para position of the initial phenol. Subsequently, a water molecule is released, which generates another carbocation. The reaction continues, with another SEAr attack of another phenol molecule, and ends with the loss of a proton to give phenolphthalein.

Experimental procedure

0.1552 g (1 mmol) of phthalic anhydride, 0.1881 g (2 mmol) of phenol and methanesulfonic acid, MeSO4H, 3 ml (0.046 mmol) are placed in a 100 ml round bottom flask. A reflux cooler, with magnetic stirring, is attached and the flask is placed in an oil bath. The bath is heated for 2 hours at 90.5 °C, and the reaction is monitored by TLC every 30 minutes.

Once the process is complete, the reaction is terminated by adding 2 mL of methanol, MeOH, to the round bottom flask and cooling slightly, before allowing it to cool, subsequently, further in an ice water bath for 5 minutes, to allow crystallization of the phenolphthalein.

Once the precipitate is formed, it is filtered to vacuum and the phenolphthalein crystals are collected. The product is recrystallized from methanol and tan-brown crystals are obtained. The yield of the dry product is calculated. The estimated yield is 73-94 %, and the melting point 259-261 ºC.

Phenolphthalein identification reaction

Dissolve some crystals of phenolphthalein in 1 mL of ethanol, add 1 mL of distilled water, shake and add a few drops of soda solution. Note your observations.

Physico-chemical properties

This table collects data for the molecular weight (Mw), melting point (M.p.) boiling point (B.p.) and density of the reactives and compounds used in this laboratory experiment.

Name Mw (g/mol) M.p. (ºC) B.p. (ºC) Density (g/ml)
HCl 36.46-30>1001.200
Methanesulfonic acid96.1017-191691.480
Phthalic anhydride148.10131.62951.530

GHS pictograms

Hazard pictograms form part of the international Globally Harmonized System of Classification and Labelling of Chemicals (GHS) and are collected in the followinf Table for the chemical compounds used in this experiment.

Name GHS
HCl ghs05  Danger Warning Corrosive cat. 1 pictogram ghs07  Warning Toxic cat. 4 Irritant cat. 2 or 3 Lower systemic health hazards pictogram
Methanesulfonic acidghs05  Danger Warning Corrosive cat. 1 pictogram
Phthalic anhydrideghs05  Danger Warning Corrosive cat. 1 pictogram ghs07  Warning Toxic cat. 4 Irritant cat. 2 or 3 Lower systemic health hazards pictogram ghs08  Danger Warning Systemic health hazards pictogram
EtOH ghs02  Danger Warning Flammable pictogram
Phenol ghs06  Danger Toxic cat. 1–3 pictogram ghs08  Danger Warning Systemic health hazards pictogram ghs05  Danger Warning Corrosive cat. 1 pictogram
Phenolphthaleinghs08  Danger Warning Systemic health hazards pictogram
NaOH ghs05  Danger Warning Corrosive cat. 1 pictogram
MeOH ghs02  Danger Warning Flammable pictogram ghs06  Danger Toxic cat. 1–3 pictogram ghs08  Danger Warning Systemic health hazards pictogram

International Chemical Identifier

The IUPAC InChI key identifiers for the main compounds used in this experiment are provided to facilitate the nomenclature and formulation of chemical compounds and the search for information on the Internet for these compounds.


Video on phenolphthalein synthesis


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