Problem 29 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of isomers possible for \(\mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O}\) are (a) 3 (b) 4 (c) 5 (d) 6

Step-by-Step Solution

Answer

The correct answer is (c) 5.

Step 1: Analyze the Molecular Formula

The molecular formula given is \( \mathrm{C}_7 \mathrm{H}_8 \mathrm{O} \), which suggests that it is an aromatic compound as it fits the formula \( \mathrm{C}_n \mathrm{H}_{2n-6} \mathrm{O} \) typical for benzene derivatives. Considering this, the core structure is likely a benzene ring (\( \mathrm{C}_6 \mathrm{H}_5 \)) with additional groups.

Step 2: Determine the Possible Structural Groups

Given the formula, the extra carbon and the oxygen can form groups like a methyl group (\( \mathrm{CH}_3 \)), hydroxyl group (\( \mathrm{OH} \)), or a methoxy group (\( \mathrm{OCH}_3 \)). These groups can be arranged in different ways around a benzene ring.

Step 3: List Potential Structures

1. **Cresols** - These are isomers where a hydroxyl group and a methyl group are attached to the benzene ring. There are three positions possible: ortho, meta, and para, leading to 3 cresol isomers (\( \mathrm{o{-}C}_6 \mathrm{H}_4 \mathrm{(CH}_3\mathrm{)OH} \), \( \mathrm{m{-}C}_6 \mathrm{H}_4 \mathrm{(CH}_3\mathrm{)OH} \), \( \mathrm{p{-}C}_6 \mathrm{H}_4 \mathrm{(CH}_3\mathrm{)OH} \)).2. **Benzyl Alcohol** - Here, the \( \mathrm{OH} \) group is attached to a \( \mathrm{CH}_2 \) group which is connected to the benzene ring (\( \mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{OH} \)).3. **Methoxybenzene (Anisole)** - The \( \mathrm{OCH}_3 \) group is attached to the benzene ring (\( \mathrm{C}_6 \mathrm{H}_5 \mathrm{OCH}_3 \)).

Step 4: Count the Total Isomers

Calculate all unique isomers by considering different arrangements: three cresols, one benzyl alcohol, and one methoxybenzene. This makes for a total of five isomers.

Step 5: Choose the Correct Answer

From the analysis and counting above, there are exactly 5 isomers possible for \( \mathrm{C}_7 \mathrm{H}_8 \mathrm{O} \). Therefore, the correct option is (c) 5.

Key Concepts

Aromatic Compounds

Aromatic compounds are a special class of organic molecules known for their stability and unique chemical properties. At the heart of these compounds is the benzene ring, a ring consisting of six carbon atoms connected in a hexagonal arrangement. Each carbon atom is bonded to one hydrogen atom, resulting in the molecular formula \( \text{C}_6\text{H}_6 \).
These compounds exhibit a distinct feature called aromaticity, characterized by a delocalized cloud of electrons above and below the plane of the ring. This electron delocalization provides aromatic compounds with extra stability compared to other cyclic compounds.
Some key points about aromatic compounds include:

• They have a notable scent or aroma, hence the name "aromatic."
• They are found in many natural and synthetic substances, including essential oils and pharmaceuticals.
• The classic example of an aromatic compound is benzene, from which many derivatives are formed.

Understanding aromatic compounds is essential when studying organic chemistry, especially when dealing with derivatives like benzene derivatives and various functional groups.

Benzene Derivatives

Benzene derivatives are compounds formed by substituting one or more hydrogen atoms in the benzene ring with other atoms or groups of atoms. These compounds retain the aromatic nature of benzene and often exhibit interesting chemical properties due to the presence of additional functional groups.
Common benzene derivatives include:

• Toluene: A benzene ring with a methyl group (\( \text{CH}_3 \)) attached, widely used as an industrial solvent.
• Phenol: A benzene ring with a hydroxyl group (\( \text{OH} \)) attached, known for its antiseptic properties.
• Aniline: A benzene ring with an amino group (\( \text{NH}_2 \)), used in making dyes.

When studying isomerism in benzene derivatives, it’s important to note that substituents can occupy different positions on the benzene ring. These positions are known as ortho, meta, and para positions, influencing the chemical properties and reactivity of the compound.
Benzene derivatives like cresols, benzyl alcohol, and methoxybenzene play significant roles in organic chemistry, illustrating the diversity and complexity of aromatic compounds.

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical reactions and properties of those molecules. In aromatic compounds, functional groups can be attached to the benzene ring affecting the compound's overall reactivity and interactions.
Some important functional groups in the context of \( \text{C}_7 \text{H}_8 \text{O} \) include:

• Methyl Group (\( \text{CH}_3 \)): Often found in toluene, it is a simple alkyl group that can slightly increase the molecule’s overall size and change its solubility.
• Hydroxyl Group (\( \text{OH} \)): This group imparts alcohol-like properties, increasing solubility in water and typically making the compound act as a weak acid.
• Methoxy Group (\( \text{OCH}_3 \)): Found in anisole or methoxybenzene, this group influences the compound's polarity and reactivity.

Understanding these functional groups allows us to predict and explain the behavior of organic molecules during chemical reactions. When attached to a benzene ring, these groups not only contribute to the stability and uniqueness of the compound but also offer insight into potential chemical transformations or reactions the molecule can undergo.