Q.Give the structures and IUPAC names of monohydric phenols of molecular formula, $C_7H_8O$.

Concept: IUPAC Nomenclature of Monohydric Phenols

A monohydric phenol has one –OH group directly attached to a benzene ring. For $C_7H_8O$, the benzene ring ($C_6H_5$) accounts for 6 carbons and 5 hydrogens, leaving one extra carbon and three hydrogens — that extra carbon must be a methyl substituent on the ring.

Reasoning steps:

1. The –OH group is the principal functional group (suffix: -ol). The benzene ring is the parent chain (phenol).
2. The methyl group can be placed at three positions relative to –OH: ortho (1,2), meta (1,3), or para (1,4).
3. Name each isomer using locants, with –OH at position 1 (implied in "phenol").

Structures and IUPAC names: …

The key to solving this is to first recognise what “monohydric phenol” means. A phenol has a hydroxyl group (–OH) directly attached to a benzene ring. “Monohydric” tells us there is exactly one such –OH group. So the core is a benzene ring ($C_6H_5$–) with one –OH, which accounts for $C_6H_5O$ (that’s 6 carbons, 5 hydrogens, 1 oxygen). The molecular formula given is $C_7H_8O$. Subtract the phenol core: $C_7H_8O - C_6H_5O = CH_3$. That leftover is exactly one methyl group ($-CH_3$). So the molecule is a benzene ring with an –OH and a –CH₃ attached — a methylphenol, commonly called a cresol.

Now, the methyl group and the hydroxyl group can be placed in three different relative positions on the benzene ring. These are the ortho, meta, and para isomers. In IUPAC nomenclature, we number the ring so that the –OH gets the lowest possible number (since –OH is the principal functional group for phenols). The methyl group then takes the position number accordingly.

Let’s go through each isomer step by step.

1. Ortho isomer — The methyl group is on carbon 2, adjacent to the –OH (which is on carbon 1).

   Structure: a benzene ring with –OH at position 1 and –CH₃ at position 2.

   IUPAC name: 2-methylphenol. Common name: o-cresol.

2. Meta isomer — The methyl group is on carbon 3, one carbon away from the –OH.

   Structure: –OH at position 1, –CH₃ at position 3.

   IUPAC name: 3-methylphenol. Common name: m-cresol.

3. Para isomer — The methyl group is on carbon 4, directly opposite the –OH.

   Structure: –OH at position 1, –CH₃ at position 4.

   IUPAC name: 4-methylphenol. Common name: p-cresol. …

Method: Systematic Isomer Enumeration + IUPAC Nomenclature

This method works by first listing all possible structural isomers (different carbon skeletons and positions of the –OH group), then naming each according to IUPAC rules.

Step 1: Determine the molecular formula and identify the functional group

• Formula: $C_7H_8O$
• “Monohydric phenol” means one –OH group directly attached to a benzene ring.
• So the benzene ring contributes $C_6H_5$ — the remaining part is $CH_3$ (a methyl group).

Thus, all isomers are methylphenols (cresols) — one methyl group and one hydroxyl group on the benzene ring.

Step 2: Enumerate positional isomers

The –OH and –CH$_3$ groups can be placed in three relative positions on the benzene ring:

Step 3: Draw structures and assign IUPAC names

1. 2-Methylphenol (ortho-cresol)

      OH
       |
    — C₆H₄ — CH₃  (at position 2)

• IUPAC: 2-methylphenol
• Common name: o-cresol

2. 3-Methylphenol (meta-cresol)

      OH
       |
    — C₆H₄ — CH₃  (at position 3)

• IUPAC: 3-methylphenol
• Common name: m-cresol …

Step 1: Understand the molecular formula

The formula is $C_7H_8O$.

For a monohydric phenol, we have:

• A benzene ring ($C_6H_5$–)
• One –OH group (phenolic)
• Remaining: $C_7H_8O - C_6H_5OH = CH_3$

So the extra carbon is a methyl group attached to the ring.

Step 2: The correct structures and names

There are three positional isomers:

All are monohydric phenols with formula $C_7H_8O$.

Common Mistakes & How to Avoid Them

✗ Mistake 1: Including benzyl alcohol as a monohydric phenol

Why it's wrong:

Benzyl alcohol ($C_6H_5CH_2OH$) has the –OH on a side chain, not directly on the benzene ring. That makes it an alcohol, not a phenol.

How to avoid:

Remember: phenol = –OH directly attached to an aromatic ring. If the –OH is on a carbon chain attached to the ring, it's an alcohol.

✗ Mistake 2: Forgetting the methyl group can be at positions 2, 3, or 4

Why it's wrong:

Some students only write o-cresol (2-methylphenol) and p-cresol (4-methylphenol), missing the m-cresol (3-methylphenol).

How to avoid:

Always systematically check all possible positions on the ring: 2, 3, and 4 (since position 1 is taken by –OH).

✗ Mistake 3: Using common names instead of IUPAC names

Why it's wrong:

"Cresol" is a common name. In exams, IUPAC names like 2-methylphenol are required.

How to avoid:

Learn the IUPAC naming rules:

• Phenol is the parent.
• Substituents are named as prefixes with locants.
• Number the ring so that –OH gets position 1.

✗ Mistake 4: Incorrect numbering (giving –OH a number > 1)

Why it's wrong:

In phenols, the –OH group always gets position 1 by priority. Some students number from the methyl group.

How to avoid: …