Write structural formulas of the following compounds : (i) 3, 4, 4, 5–Tetramethylheptane (ii) 2,5-Dimethyhexane

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Accepted Answer

> [!TLDR]
> Draw the longest carbon chain first, number it correctly, then attach the methyl substituents at the positions specified in the IUPAC name. **(i)** has a 7-carbon chain with four methyls; **(ii)** has a 6-carbon chain with two methyls.

# Understanding Chain Isomerism and IUPAC Nomenclature

When you see a systematic IUPAC name, it's a precise instruction manual for building the molecule. The parent name (heptane, hexane) tells you the length of the longest continuous carbon chain. The numbers and prefixes (3,4,4,5-tetramethyl) tell you exactly where to attach the branches and how many there are.

The key insight: **always draw the parent chain first, number the carbons, then add substituents like decorating a Christmas tree.**

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## (i) 3,4,4,5–Tetramethylheptane

**Step-by-step construction:**

1. **Identify the parent chain.**  
   "Heptane" means a 7-carbon straight chain. Draw it:
   $$\ce{C-C-C-C-C-C-C}$$

2. **Number the chain.**  
   Number from left to right (or right to left—either works, but be consistent):
   $$\ce{C^1-C^2-C^3-C^4-C^5-C^6-C^7}$$

3. **Decode the substituents.**  
   "3,4,4,5-Tetramethyl" means four methyl groups ($\ce{-CH3}$) attached at positions 3, 4, 4, and 5. Notice position 4 appears twice—that carbon carries *two* methyl groups.

4. **Attach the methyls.**  
   - One $\ce{CH3}$ on carbon-3  
   - Two $\ce{CH3}$ groups on carbon-4  
   - One $\ce{CH3}$ on carbon-5

5. **Complete the structure with hydrogens.**  
   Each carbon forms four bonds total. Fill remaining valences with hydrogen.

**Structural formula:**

$$
\begin{array}{ccccccc}
& & \ce{CH3} & \ce{CH3} & \ce{CH3} & & \
& & | & | & | & & \
\ce{CH3} & - & \ce{CH2} & - & \ce{CH} & - & \ce{C} & - & \ce{CH} & - & \ce{CH2} & - & \ce{CH3} \
& & & & & & | & & | & & & \
& & & & & & \ce{CH3} & & \ce{CH3} & & &
\end{array}
$$

Or in condensed form:

$$\ce{CH3-CH2-CH(CH3)-C(CH3)2-CH(CH3)-CH2-CH3}$$

> [!WARNING]
> A common mistake is forgetting that carbon-4 has *t

Isomer Name	Common Name	Structure (Condensed)	Boiling Point (°C)
Pentane	n-Pentane	$CH_3(CH_2)_3CH_3$	36.1
2-Methylbutane	Isopentane	$(CH_3)_2CHCH_2CH_3$	27.7
2,2-Dimethylpropane	Neopentane	$C(CH_3)_4$	9.5

Q.Write structural formulas of the following compounds :

Concept understanding — Chain Isomerism

Chain Isomerism: The Skeleton of a Molecule

The Intuition

The Precise Statement

A Concrete Example: Pentane ( $C_5H_{12}$ )

When Does Chain Isomerism Occur?

Structural Formulas from IUPAC Names

Understanding Chain Isomerism and IUPAC Nomenclature

(i) 3,4,4,5–Tetramethylheptane

(ii) 2,5-Dimethylhexane

Q.Write structural formulas of the following compounds :

Understanding Chain Isomerism and IUPAC Nomenclature

(i) 3,4,4,5–Tetramethylheptane

(ii) 2,5-Dimethylhexane

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