Question

Tropolone \(\left(\mathrm{I}, \mathrm{C}_{7} \mathrm{H}_{7} \mathrm{O}_{2}\right)\) has a flat molecule with all carbon-carbon bonds of the same length (1.40 A). The measured heat of combustion is \(20 \mathrm{kcal}\) lower than that calculated by the method of summing bond energies. Its dipole moment is \(3.71 \mathrm{D}\); that of 5-bromotropolone is \(2.07 \mathrm{D}\). Tropolone undergoes the Reimer-Tiemann reaction, couples with diazonium ions, and is nitrated by dilute nitric acid. It gives a green color with ferric chloride, and does not react with 2,4 -dinitrophenylhydrazine. Tropolone is both acidic \(\left(\mathrm{K}_{\mathrm{a}}=10^{-7}\right)\) and weakly basic, forming a hydrochloride in ether. (a) What class of compounds does tropolone resemble? Is it adequately represented by formula I? (b) Using both valence-bond and orbital structures, account for the properties of tropolone. (c) In what direction is the dipole moment of tropolone? Is this consistent with the structure you have proposed? (d) The infrared spectrum of tropolone shows a broad band at about \(3150 \mathrm{~cm}^{-1}\) that changes only slightly upon dilution. What does this tell you about the structure of tropolone?

Short answer

Tropolone resembles the class of phenols and can be represented by formula I. Its properties are due to its aromatic nature, resonance structure, and presence of a hydrogen bond within the molecule. The dipole moment is directed towards the oxygen atom of the carbonyl group, consistent with its structure. The infrared spectrum supports the formation of a hydrogen bond between the acidic hydrogen and the carbonyl oxygen.

Step-by-step solution

a) Class of compounds and formula representation

Tropolone resembles the class of phenols, as it shows acidic properties (with a Ka value of 10^-7) and gives a green color with ferric chloride, which is a typical test for phenols. It also undergoes reactions similar to phenols, such as Reimer-Tiemann reaction and coupling with diazonium ions. It does not react with 2,4-dinitrophenylhydrazine, indicating the absence of carbonyl groups. Therefore, tropolone can be considered a phenolic compound, and the formula I would represent it.

b) Valence-bond and orbital structures

Tropolone has a flat ring structure with all carbon-carbon bonds of the same length (1.40 Å). This indicates that the molecule is aromatic, exhibits resonance, and enjoys the stabilization of a conjugated system. The oxygen atom is sp^2-hybridized and is involved in the resonance, which allows the molecule to distribute negative charge and share the acidic proton (hydrogen), making the molecule both acidic and weakly basic. In resonance structures, the double bond of the carbonyl group can rotate within the ring, allowing the molecule to exhibit different characteristic reactions.

c) Dipole moment direction and consistency with the structure

The dipole moment of tropolone is 3.71 D, which indicates a significant charge separation in the molecule. Since tropolone is an aromatic and resonance structure (with keto and enol tautomers), the resulting dipole moment will be in the direction of the oxygen atom (along with the double bond) of the carbonyl group. The dipole moment of 5-bromotropolone is smaller (2.07 D) because the electron-withdrawing nature of the bromine reduces the charge separation in the molecule. The dipole moment's direction and magnitude are consistent with the proposed structure of tropolone.

d) Infrared spectrum analysis

The infrared spectrum of tropolone shows a broad band at around 3150 cm⁻¹, which does not change significantly upon dilution. This broad absorption can be attributed to the O-H stretching vibration, indicating a hydrogen bond within the molecule. The hydrogen bond can be formed between the acidic hydrogen and the oxygen atom of the carbonyl group in the enol tautomer. Thus, the infrared spectrum further supports the proposed structure of tropolone with an acidic hydrogen displaying a hydrogen bond with the carbonyl oxygen.

Key concepts

Phenolic Compounds

Phenolic compounds are characterized by having a hydroxyl group (-OH) attached to an aromatic hydrocarbon ring. Tropolone is classified as a phenolic compound due to its acidic nature. A common test for phenolic compounds is the reaction with ferric chloride, which produces a green color. Tropolone's ability to show this reaction confirms its phenolic character.
Furthermore, phenolic compounds can also undergo various chemical reactions much like tropolone, such as the Reimer-Tiemann reaction. This similarity in chemical behavior further supports the classification of tropolone as a phenolic compound.

Aromaticity

Aromaticity is a concept that describes the stability and unique electronic configuration of certain cyclic molecules. For a compound to be aromatic, it generally must have a continuous pi electron cloud above and below the plane of the molecule, contribute to resonance, and comply with Huckel's rule of having \(4n + 2\) pi electrons.
Tropolone displays aromaticity as it has a flat cyclic structure where electrons can delocalize across the ring. All carbon-carbon bond lengths in tropolone are identical, supporting the delocalization of electrons, a key feature of aromatic systems. This delocalization also accounts for the stability and reactivity of aromatic compounds like tropolone.

Resonance Structure

Resonance structures in chemistry depict the different ways electrons can be distributed within a molecule, illustrating delocalization. For tropolone, resonance structures show how electrons can move within the aromatic ring and across different functional groups.
In these structures, particularly involving tropolone’s keto and enol forms, the double bonds can shift, showing the multiple possible electron configurations. This shifting enhances the molecule’s stability due to the distribution of charge across the entire structure. Understanding these resonance forms is crucial to explaining tropolone's chemical properties, especially its ability to participate in reactions like nitration and coupling with diazonium ions.

Dipole Moment Analysis

Dipole moment is a measure of the polarity of a molecule, indicating how charges are separated within the molecule. Tropolone exhibits a significant dipole moment of 3.71 D, suggesting prominent charge separation. This separation occurs due to the molecule's inherent aromatic nature and the difference in electron affinity between atoms.
In tropolone, the dipole moment is directed towards the oxygen atom involved in resonance. The presence of keto and enol forms, where the oxygen participates in resonance, ensures that the dipole moment is consistent with the proposed structural theories. The smaller dipole moment of 5-bromotropolone shows how substituents can influence these properties.

Infrared Spectrum

The infrared spectrum provides insights into molecular bonds and functional groups through their absorption characteristics. For tropolone, a broad infrared absorption band at around 3150 cm⁻¹ reflects an O-H stretching vibration. Such broadness is indicative of strong hydrogen bonding.
This implies a hydrogen bond interaction between the enolic hydroxyl group and the carbonyl oxygen. The lack of significant change upon dilution suggests the hydrogen bond is intramolecular. Therefore, infrared spectral data align with the notion of internal hydrogen bonding within tropolone, supporting its enol form.

Reimer-Tiemann Reaction

The Reimer-Tiemann reaction is a chemical process used for formulating ortho-hydroxyaldehydes from phenols. Tropolone partakes in this reaction, further emphasizing its phenolic nature.
In this reaction, dichlorocarbene is generated and effectively introduces an aldehyde group at the ortho position of the aromatic ring. Tropolone's structural properties and resonance make it particularly suitable for such substitution reactions, thereby elucidating one of its key chemical reactivities. This chemistry not only highlights its behavior akin to other phenols but also expands its utility in aromatic compound manipulation.