Question

Give the structures and IUPAC names of monohydric phenols of molecular formula, \(\mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O}\).

Short answer

The IUPAC names are 2-methylphenol, 3-methylphenol, and 4-methylphenol.

Step-by-step solution

Understanding the Components of Monohydric Phenols

A monohydric phenol contains a phenol group (an aromatic ring bonded to an -OH group) and one additional group attached to the benzene ring. For the molecular formula \( C_7H_8O \), it must accommodate a phenol group (C6H5OH).

Analyzing the Base Structure

Since the molecular formula is \( C_7H_8O \), start with a phenol (\( C_6H_5OH \)) and add one methyl group (CH3) to achieve the required molecular formula. The methyl group is the only group that can be added to reach the molecular formula without violating the phenol structure.

Identifying Isomers

The phenol (C6H5OH) can have a methyl group substituted at different positions in the aromatic ring. Possible positions are ortho, meta, and para relative to the hydroxyl group.

Naming the Ortho Isomer

When the methyl group is at the ortho position (adjacent to the OH group), the structure is named 2-methylphenol. The structure is: 

Naming the Meta Isomer

When the methyl group is at the meta position (one carbon away from the OH group), the structure is named 3-methylphenol. The structure is: 

Naming the Para Isomer

When the methyl group is at the para position (opposite the OH group), the structure is named 4-methylphenol. The structure is: 

Key concepts

Phenol Structure

The structure of phenol is one of the simplest and most studied in organic chemistry. At its core, phenol is a combination of an aromatic ring, benzene, and a hydroxyl group (-OH) attached to it. This combination makes phenol unique due to its aromatic nature, lending it special properties in terms of reactivity and chemical behavior.
In the case of monohydric phenols with the molecular formula \( \mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O} \), the phenol base (\( \text{C}_6\text{H}_5\text{OH} \)) is modified by adding one additional group. This group often alters how the phenol interacts with other chemicals, impacts its solubility, and changes its boiling and melting points.

• The phenol group is characterized by the aromatic ring, which gives stability due to resonance.
• The hydroxyl (-OH) group can act as a weak acid, donating a hydrogen to form a phenoxide ion under certain conditions.
• Phenolic structures can further be modified by adding other groups such as methyl, whose position on the ring leads to different isomers.

Understanding the phenol structure is key to predicting and explaining chemical reactions and behavior in these compounds.

IUPAC Naming

The International Union of Pure and Applied Chemistry (IUPAC) system is a universal standard for naming chemical compounds. For monohydric phenols, IUPAC naming involves identifying the base structure (phenol) and the position and type of any additional groups.
For example, with the molecular formula \( \mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O} \), the additional group is a methyl group that can attach at different positions on the aromatic ring. The simplest way to name these structures is to number the carbon atoms in the benzene ring starting from the carbon bearing the hydroxyl group.

• Ortho isomer: The methyl group is adjacent to the hydroxyl (OH) group, and it is named as 2-methylphenol.
• Meta isomer: The methyl group is one carbon away from the hydroxyl group, and it is named as 3-methylphenol.
• Para isomer: The methyl group is opposite the hydroxyl group, named as 4-methylphenol.

Numbering and naming according to IUPAC helps in methodically identifying the structure of any phenolic compound, making communication about these compounds clear and consistent worldwide.

Isomer Identification

Isomers are compounds that have the same molecular formula but different structures or arrangements of atoms in space. In the case of monohydric phenols, specifically with the formula \( \mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O} \), isomers arise from the different possible positions of the methyl group on the benzene ring.
These position variations lead to distinct compounds, each with unique physical and chemical properties. The three isomers of phenol with an added methyl group are:

• Ortho Isomer (2-methylphenol): The methyl group is directly adjacent to the -OH group.
• Meta Isomer (3-methylphenol): The methyl group is one carbon away from the -OH group.
• Para Isomer (4-methylphenol): The methyl group is positioned opposite the -OH group.

Each of these isomers interacts differently with other substances because of their unique spatial arrangements, which is crucial for chemists when predicting reactions and outcomes of chemical processes.

Aromatic Compounds

Aromatic compounds are a special class of organic compounds that contain a benzene ring—a structure consisting of six carbon atoms with alternating double bonds. These compounds are notable for their strong, distinct odors and special stability, often exhibited in resonance.
Phenol, which is firmly categorized as an aromatic compound, benefits from this characteristic stability due to its benzene ring. This stability is due to the delocalization of electrons across the carbon atoms in the ring, making these compounds less reactive than typical alkenes. However, the presence of the hydroxyl group in phenol can enhance its reactivity, allowing it to participate in certain types of reactions that are not typical for benzene alone.

• Aromatic compounds are prevalent in various natural and synthetic materials, many of which have antibacterial and preservative properties.
• They provide the backbone for many pharmaceuticals, dyes, and plastics.
• The aromaticity in phenols plays a critical role in these applications, affecting how materials interact and perform under different conditions.

Understanding aromatic compounds like phenol is essential for many fields of chemistry and industry, offering insights into their uses and the mechanisms behind their effects.