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?

Step by step solution

01

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.

02

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.

03

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.

04

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.

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