What Is Tropolone?
Tropolone is an aromatic organic compound where the 2-position of tropone is substituted by a hydroxy group.
Tropone, also known as 2,4,6-cycloheptatrien-1-one, is a nonbenzenoid aromatic ring comprising seven carbon atoms featuring three conjugated alkenes and a carbonyl group.
Tropolone, alternatively called hydroxytropolone or 2-hydroxy-2,4,6-cycloheptatrien-1-one, possesses a hydroxy group adjacent to the carbonyl group.
Uses of Tropolone
Hinokitiol, a derivative of tropolone, occurs naturally in the essential oils of cypress trees like Formosan red cypress and Aomori cypress. Exhibiting antibacterial properties against various bacteria including Escherichia coli and Staphylococcus aureus, hinokitiol finds application as an antiseptic in skincare and cleansing products.
Moreover, hinokitiol has been reported to inhibit the growth of Malassezia, the bacterium responsible for dandruff, making it a common ingredient in anti-dandruff shampoos and scalp care products.
Properties of Tropolone
Tropolone has a melting point ranging from 50-52°C, a boiling point of 290°C, and a flash point of 112°C. It appears as a pale yellow solid at room temperature.
It is soluble in various organic solvents such as ether, ethanol, and benzene. With two oxygen atoms, tropolone readily forms chelates upon reacting with different metals.
The hydroxyl group in tropolone exhibits weak acidic properties akin to phenol, with an acid dissociation constant (pKa) of 6.89. It produces a dark green color reaction when treated with iron chloride.
Structure of Tropolone
The chemical formula of tropolone is C7H6O2, with a molar mass of 122.12 g/mol and a density of 1.1483 g/mL.
Featuring a seven-membered ring structure, tropolone is aromatic. The ring constitutes a 6π-electron system, with the negative charge being biased towards the oxygen atom.
The hydroxy and carbonyl groups in tropolone are indistinguishable, and the tropolone nucleus is stabilized by a hybridization of resonance structures. With six possible limiting structural formulas, the properties of the hydroxy and carbonyl groups are nearly identical.
Natural compounds with tropolone as a backbone include hinokitiol, colchicine, and pulprogarin, with stipitatinic acid demonstrating specific antimicrobial properties.
Other Information on Tropolone
1. Synthesis of Tropolone
Tropolone can be synthesized through various methods. One approach involves bromination of 1,2-cycloheptanedione using N-bromosuccinimide, followed by dehalogenation at high temperatures.
Alternatively, it can be synthesized via the acyloin condensation of diethyl pimelate, followed by the oxidation of the resulting acyloin with bromine.
Furthermore, [2+2] cycloaddition of cyclopentadiene with ketene yields bicyclo[3.2.0]heptyl, which upon hydrolysis forms a monocyclic ring to yield tropolone.
2. Reactions of Tropolone
Tropolone readily undergoes O-alkylation to produce cycloheptatrienyl derivatives, serving as a versatile synthetic intermediate.
When treated with sulfamic acid, nitric acid, or bromine, tropolone undergoes electrophilic substitution to yield sulfonic acids, nitro compounds, and bromo compounds, respectively. It also undergoes diazo coupling with diazonium salts.
Deprotonation by metal cations results in the formation of bidentate ligands, as seen in the Cu(O2C7H5)2 complex.