Upon direct photolysis or heating at \(220^{\circ} \mathrm{C}\), the dimer of 2,5 -dimethyl-3,4-diphenyl-2,4-cyclopentadien1-one (A) loses carbon monoxide and yields compound B, mp \(166-168^{\circ} \mathrm{C}\). Spectral data for compound \(\mathbf{B}\) are summarized below. Deduce the structure of compound \(\mathbf{B}\) and assign the peaks. Compound B Mass spectrum: \(\mathrm{m} / \mathrm{z}=492(\mathrm{M})\) IR \(\left(\mathrm{CCl}_{4}\right): 1704 \mathrm{~cm}^{-1}\) \({ }^{1} \mathrm{H}\) NMR \(\left(\mathrm{CDCl}_{3}\right): \delta 0.73(\mathrm{~s}, 3 \mathrm{H})\) $$ \begin{aligned} &0.73(s, 3 H) \\ &0.92(s, 3 H) \\ &1.51(s, 3 H) \\ &1.88(s, 3 H) \\ &6.6-7.5(\mathrm{~m}, 20 \mathrm{H}) \end{aligned} $$

Step by step solution

01

- Analyze the direct photolysis or heating

Given that compound A undergoes direct photolysis or heating at 220°C and loses carbon monoxide to yield compound B, the first step is to understand this transformation. Compound A loses CO, simplifying and reducing its molecular weight.

02

- Calculate the molecular formula of Compound B

Determine the change in molecular formula by the loss of carbon monoxide (CO) from Compound A: The molecular weight of CO is 28 amu. Subtracting this from the initial molecular weight of Compound A will help deduce the molecular weight of compound B.

03

- Analyze the Mass Spectrum

The mass spectrum shows that compound B has an m/z value of 492, which indicates that the molecular weight of compound B is 492 amu. This helps confirm the formula of compound B.

04

- Interpret the IR Spectrum

The IR spectrum shows a key absorption at 1704 cm⁻¹, which typically indicates the presence of a carbonyl (C=O) group in compound B.

05

- Analyze the ¹H NMR Spectrum

The ¹H NMR spectrum provides information on the hydrogen environment in compound B: - δ 0.73 (s, 3H) - δ 0.92 (s, 3H) - δ 1.51 (s, 3H) - δ 1.88 (s, 3H) - δ 6.6-7.5 (m, 20H) These peaks indicate the presence of four different environments of methyl protons and aromatic protons.

06

- Deduce the Structure

Combining all the data: - Indicating a carbonyl group from IR - Methyl group environments from ¹H NMR - Aromatic protons indicating phenyl rings The structure of compound B is deduced to be a derivative of biphenyl with carbonyl and methyl substituents. The exact structure is 4,4'-Dimethyl-2,2'-benzil.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Photolysis

Photolysis is a process wherein chemical bonds are broken down by photons, specifically using light. In our problem, this change happens upon direct photolysis or when heating compound A to 220°C. This process results in the dimer of 2,5-dimethyl-3,4-diphenyl-2,4-cyclopentadien-1-one losing a molecule of carbon monoxide (CO).When the energy from light hits the compound, it weakens and breaks specific chemical bonds. Given the heat condition, it further supports the degradation of the compound. Knowing this is crucial for understanding how compound A transforms into compound B.

Mass Spectrum Analysis

Mass spectrometry helps us determine the molecular weight of a compound and some structural details. By analyzing the mass-to-charge ratio (m/z) of ionized particles, we get critical information about the compound's makeup. For compound B, the mass spectrum shows an m/z value of 492. Since mass spectrometry shows the molecular weight of ionized fragments, the molecular weight of compound B is confirmed to be 492 amu. This information helps solidify the deduced molecular formula of compound B.

IR Spectrum Interpretation

Infrared (IR) spectroscopy allows us to identify certain functional groups in a compound by observing absorption bands at specific wavelengths. In the case of compound B, the key absorption at 1704 cm⁻¹ indicates the presence of a carbonyl group (C=O). Carbonyl compounds typically show strong absorption around this region because the carbon-oxygen double bond stretches and absorbs IR radiation. This piece of data confirms that a C=O group is present in compound B, guiding us towards its final structure.

NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy provides detailed information about the environment of hydrogen atoms in a molecule. The ¹H NMR spectrum of compound B presents various peaks that lead us to identify different types of protons in the structure:

• δ 0.73 (s, 3H)
• δ 0.92 (s, 3H)
• δ 1.51 (s, 3H)
• δ 1.88 (s, 3H)
• δ 6.6-7.5 (m, 20H)

The singlets at lower ppm values indicate the presence of methyl groups, while the multiplets at higher ppm values signal aromatic protons, likely from phenyl groups. All these details converge towards the structure of compound B as a substituted biphenyl.

Molecular Formula Calculation

To deduce the molecular formula of compound B, we must calculate the shift resulting from the loss of CO during the transformation from compound A. By determining the initial molecular weight of compound A, we can subtract the molecular weight of CO (28 amu) to find the molecular weight of compound B. Given the mass spectrum data showing a molecular weight of 492 amu, we derive the molecular formula consistent with this weight. We then use complementary data from IR and NMR spectra to confirm the molecular structure, leading us to identify compound B as 4,4'-Dimethyl-2,2'-benzil.