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

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Accepted Answer

> [!TLDR]
> The molecular formula $C_7H_8O$ with a phenolic –OH group means a benzene ring plus one methyl substituent and one –OH group — that is, a methylphenol (cresol). The three positional isomers are **2-methylphenol (o-cresol)**, **3-methylphenol (m-cresol)**, and **4-methylphenol (p-cresol)**.

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*. …

Position of –CH $_3$ relative to –OH	IUPAC name prefix
Adjacent (1,2)	ortho- (or 2-)
One carbon apart (1,3)	meta- (or 3-)
Opposite (1,4)	para- (or 4-)

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

Concept understanding — IUPAC Nomenclature

IUPAC Nomenclature (Organic Compounds)

The Recipe

Worked Example

Why this formula?

IUPAC Nomenclature: Why the Rules Work the Way They Do

1. The Core Principle: The Longest Carbon Chain

2. Numbering: Lowest Locants (The "First Point of Difference" Rule)

3. Alphabetical Order of Substituents

4. Multiple Bonds: The "Lowest Locant" Rule for Alkenes/Alkynes

5. The "Suffix" and "Prefix" System …

Method: Systematic Isomer Enumeration + IUPAC Nomenclature

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

Step 2: Enumerate positional isomers

Step 3: Draw structures and assign IUPAC names

1. 2-Methylphenol (ortho-cresol)

2. 3-Methylphenol (meta-cresol)

Step 1: Understand the molecular formula

Step 2: The correct structures and names

Common Mistakes & How to Avoid Them

✗ Mistake 1: Including benzyl alcohol as a monohydric phenol

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

✗ Mistake 3: Using common names instead of IUPAC names

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

Structure	IUPAC Name
2-methylphenol	2-methylphenol (or o-cresol)
3-methylphenol	3-methylphenol (or m-cresol)
4-methylphenol	4-methylphenol (or p-cresol)

Q.Give the structures and IUPAC names of monohydric phenols of molecular formula, C7H8OC_7H_8OC7​H8​O.

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

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