Determining the Molecular Symmetry Group of Product 3 in a Three-Step Reaction Sequence

An In-Depth Analysis of Reaction Mechanisms and Symmetry Considerations

Key Takeaways

• Toluene undergoes nitration to form para-nitrotoluene as Product 1.
• Oxidation of para-nitrotoluene yields para-nitrobenzoic acid as Product 2.
• Aldol condensation of para-nitrobenzoic acid with acetone produces Product 3, whose molecular symmetry group is C_2h.

Introduction

Understanding the molecular symmetry of chemical compounds is essential in fields such as spectroscopy, quantum chemistry, and crystallography. In this analysis, we will delve into a three-step reaction sequence starting from toluene to determine the molecular symmetry group of the final product, referred to as Product 3. The reaction sequence involves nitration, oxidation, and condensation reactions, leading to a compound whose symmetry elements define its molecular symmetry group. We will explore each reaction step in detail, examine the structural features of the final product, and identify its symmetry elements to determine its molecular symmetry group from the options provided: ['c2h', 'cs', 'd2h', 'c3'].

Step 1: Nitration of Toluene to Form Product 1

Reaction Mechanism and Product Formation

Toluene, also known as methylbenzene, is an aromatic hydrocarbon with the chemical formula C₆H₅CH₃. When toluene is treated with a mixture of concentrated nitric acid (HNO₃) and sulfuric acid (H₂SO₄), it undergoes an electrophilic aromatic substitution reaction known as nitration.

The overall reaction is:

\[ \text{C}_{6}\text{H}_{5}\text{CH}_{3} + \text{HNO}_{3} \rightarrow \text{C}_{6}\text{H}_{4}(\text{CH}_{3})(\text{NO}_{2}) + \text{H}_{2}\text{O} \]

The methyl group is an electron-donating, ortho/para-directing substituent, which activates the benzene ring towards electrophilic attack and directs the incoming nitro group (–NO₂) to the ortho and para positions relative to the methyl group. Under controlled conditions, the major product is para-nitrotoluene due to steric hindrance at the ortho positions.

Detailed Mechanism of Nitration

The nitration of toluene involves the generation of the nitronium ion (NO₂⁺), the active electrophile in this reaction:

\[ \text{HNO}_{3} + \text{H}_{2}\text{SO}_{4} \rightarrow \text{NO}_{2}^{+} + \text{HSO}_{4}^{-} + \text{H}_{2}\text{O} \]

Stepwise mechanism:

1. Formation of the σ-complex: The benzene ring donates a pair of electrons to the nitronium ion, forming a carbocation intermediate (σ-complex).
2. Deprotonation: A base (HSO₄⁻) removes a proton from the intermediate, restoring aromaticity and yielding para-nitrotoluene.

Product 1: Para-nitrotoluene (p-nitrotoluene)

Structure of Product 1

Para-nitrotoluene consists of a benzene ring with a methyl group (–CH₃) at position 1 and a nitro group (–NO₂) at position 4, opposite to each other:

• Position 1: –CH₃
• Position 4: –NO₂

This arrangement imparts certain symmetry elements to the molecule, significant for later analysis.

Step 2: Oxidation of Product 1 to Form Product 2

Reaction Mechanism and Product Formation

Product 1, para-nitrotoluene, is treated with manganese dioxide (MnO₂) in the presence of sulfuric acid (H₂SO₄). Manganese dioxide is a strong oxidizing agent that selectively oxidizes benzylic methyl groups to carboxylic acids under acidic conditions.

The overall reaction is:

\[ \text{C}_{6}\text{H}_{4}(\text{CH}_{3})(\text{NO}_{2}) + \text{[O]} \rightarrow \text{C}_{6}\text{H}_{4}(\text{COOH})(\text{NO}_{2}) \]

Product 2: Para-nitrobenzoic acid (p-nitrobenzoic acid)

Detailed Mechanism of Oxidation

The oxidation involves converting the methyl group (–CH₃) to a carboxyl group (–COOH). The mechanism proceeds through the formation of a benzylic radical or cation intermediate, which is further oxidized:

1. Formation of benzylic cation: Removal of a hydrogen atom from the methyl group.
2. Oxidation: Addition of oxygen to form a benzyl alcohol intermediate.
3. Further oxidation: Conversion of the alcohol to a carboxylic acid.

Structure of Product 2

Para-nitrobenzoic acid has:

• Position 1: –COOH
• Position 4: –NO₂

The molecule remains planar, and the substituents are directly opposite each other on the benzene ring.

Step 3: Condensation of Product 2 with Acetone to Form Product 3

Reaction Mechanism and Product Formation

Product 2 reacts with acetone in the presence of aqueous sodium hydroxide (NaOH). The reaction likely involves a base-catalyzed condensation process. One plausible pathway is the decarboxylation of para-nitrobenzoic acid under basic conditions to form para-nitrophenolate ion, which can undergo condensation with acetone.

The overall reaction can be summarized as:

\[ \text{C}_{6}\text{H}_{4}(\text{COOH})(\text{NO}_{2}) + \text{CH}_{3}\text{COCH}_{3} \rightarrow \text{C}_{6}\text{H}_{4}(\text{COCH}_{3})(\text{NO}_{2}) + \text{CO}_{2} \]

Product 3: Para-nitroacetophenone (p-nitroacetophenone)

Detailed Mechanism of Condensation with Acetone

The mechanism involves:

1. Decarboxylation: The carboxylate ion loses CO₂, forming a phenoxide ion.
2. Nucleophilic attack: The phenoxide ion attacks the carbonyl carbon of acetone.
3. Elimination: Removal of a hydroxide ion reforms the carbonyl group, yielding para-nitroacetophenone.

Structure of Product 3

Para-nitroacetophenone has:

• Position 1: –COCH₃
• Position 4: –NO₂

The molecule is planar with substituents opposite each other on the benzene ring.

Summary of Reaction Steps

Step	Reactants	Conditions	Products
1	Toluene + HNO3	Conc. H2SO4	Para-nitrotoluene (Product 1)
2	Product 1 + MnO2	H2SO4	Para-nitrobenzoic acid (Product 2)
3	Product 2 + Acetone	Aqueous NaOH	Para-nitroacetophenone (Product 3)

Symmetry Analysis of Product 3

Identifying Symmetry Elements

To determine the molecular symmetry group of Product 3, we identify its symmetry elements:

• Principal axis of rotation (C₂): A twofold rotation axis perpendicular to the plane of the benzene ring, passing through its center.
• Horizontal mirror plane (σ_h): The plane of the molecule itself is a mirror plane, reflecting one half of the molecule onto the other.
• Center of inversion (i): At the center of the benzene ring, inversion transforms each atom into an equivalent atom on the opposite side.

Determining the Point Group

The presence of these symmetry elements places Product 3 in the C_2h point group, which includes:

• A C₂ rotational axis.
• A horizontal mirror plane (σ_h).
• A center of inversion (i).

Comparison with Other Point Groups

Considering the other options:

• Cs: Contains only a single mirror plane. Product 3 has additional symmetry elements.
• D_2h: Requires three C₂ axes and multiple mirror planes. Product 3 does not possess this level of symmetry.
• C₃: Contains a threefold rotational axis. Product 3 lacks this feature.

Thus, C_2h is the correct molecular symmetry group for Product 3.

Conclusion

Through a detailed analysis of the reaction sequence and the structural features of the final product, we have determined that Product 3 is para-nitroacetophenone. By identifying its symmetry elements—namely, a C₂ axis, a horizontal mirror plane, and a center of inversion—we have assigned it to the C_2h point group. Therefore, among the given options, the molecular symmetry group of Product 3 is c2h.

References