Nomenclature and Isomerism

9.2.1 Nomenclature and Isomerism

The Foundation: Why Alkanes Have Isomers

The first three alkanes — methane ($CH_4$), ethane ($C_2H_6$), and propane ($C_3H_8$) — each exist as only one structure. There is exactly one way to arrange the carbon atoms in each case. But starting with butane ($C_4H_{10}$), multiple arrangements become possible.

Four carbon atoms can be joined in two fundamentally different ways: as a continuous, unbranched chain, or as a chain with a branch. These two arrangements give two distinct compounds with the same molecular formula $C_4H_{10}$:

Structure I (continuous chain):

Butane (n-butane), boiling point 273 K

Structure II (branched chain):

2-Methylpropane (isobutane), boiling point 261 K

These two compounds have the same molecular formula but different structures — and therefore different properties. They are structural isomers of each other.

Chain Isomerism

When structural isomers differ only in the arrangement of the carbon skeleton — whether the chain is straight or branched — they are called chain isomers.

For $C_5H_{12}$, five carbon atoms can be arranged in three distinct ways:

Structure III (continuous chain):

Pentane (n-pentane), b.p. 309 K

Structure IV (one branch):

2-Methylbutane (isopentane), b.p. 301 K

Structure V (two branches):

2,2-Dimethylpropane (neopentane), b.p. 282.5 K

Notice that structures III, IV, and V all have the molecular formula $C_5H_{12}$ but differ in their carbon skeletons. Structures I and III have continuous chains; structures II, IV, and V have branched chains.

The Number of Isomers Grows Rapidly

As the number of carbon atoms increases, the number of possible chain isomers grows dramatically:

Problem 9.1: Chain Isomers of $C_6H_{14}$

Write structures and IUPAC names for all chain isomers of hexane.

Solution:

1. $CH_3-CH_2-CH_2-CH_2-CH_2-CH_3$ — n-Hexane
2. $CH_3-CH(CH_3)-CH_2-CH_2-CH_3$ — 2-Methylpentane
3. $CH_3-CH_2-CH(CH_3)-CH_2-CH_3$ — 3-Methylpentane
4. $CH_3-CH(CH_3)-CH(CH_3)-CH_3$ — 2,3-Dimethylbutane
5. $CH_3-C(CH_3)_2-CH_2-CH_3$ — 2,2-Dimethylbutane

Classification of Carbon Atoms

Based on the number of other carbon atoms directly attached to a given carbon atom, we classify carbon atoms as:

• Primary ($1^\circ$) carbon: attached to no other carbon atom (as in methane) or to only one other carbon atom (as in ethane). Terminal carbon atoms are always primary.
• Secondary ($2^\circ$) carbon: attached to two other carbon atoms.
• Tertiary ($3^\circ$) carbon: attached to three other carbon atoms.
• Quaternary ($4^\circ$) or neo carbon: attached to four other carbon atoms.

Alkyl Groups

When one hydrogen atom is removed from an alkane, the resulting group is called an alkyl group. The general formula for an alkyl group is $C_nH_{2n+1}$, and it is commonly represented as $R-$.

Common alkyl groups include:

• Methyl: $-CH_3$ (from methane)
• Ethyl: $-C_2H_5$ (from ethane)
• Propyl: $-C_3H_7$ (from propane)

Problem 9.2: Isomeric Alkyl Groups $C_5H_{11}$ and Their Alcohols

Write structures of different isomeric alkyl groups corresponding to $C_5H_{11}$. Write IUPAC names of alcohols obtained by attachment of $-OH$ group at different carbons of the chain.

Solution:

Nomenclature Rules in Practice

The general rules for IUPAC nomenclature (covered in Unit 8) apply to alkanes. Key points to remember:

1. Identify the longest continuous carbon chain — this gives the parent alkane name.
2. Number the chain from the end nearest a substituent.
3. Name substituents as alkyl groups, with their position numbers.
4. Use prefixes (di-, tri-, tetra-) for multiple identical substituents.
5. List substituents in alphabetical order (ignoring prefixes like di-, tri-).

Table 9.1: Nomenclature of Selected Organic Compounds

Writing Structures from IUPAC Names

It is equally important to be able to write the correct structure from a given IUPAC name. Follow these steps:

Step 1: Draw the longest chain of carbon atoms corresponding to the parent alkane.

Step 2: Number the carbon atoms.

Step 3: Attach the substituents to the correct carbon atoms.

Step 4: Satisfy the valence of each carbon atom by putting the correct number of hydrogen atoms.

Example: 3-Ethyl-2,2-dimethylpentane

Step 1: The parent is pentane — a chain of five carbons:

Step 2: Number the chain:

Step 3: Attach an ethyl group at carbon 3 and two methyl groups at carbon 2:

with $C_2$ having two $-CH_3$ groups and $C_3$ having one $-C_2H_5$ group.

Step 4: Add hydrogen atoms to satisfy tetravalency:

Problem 9.3: IUPAC Names from Structures

Write IUPAC names of the following compounds:

(i) $(CH_3)_3C-CH_2-C(CH_3)_3$

Solution: The longest chain has 5 carbons (pentane). There are four methyl groups at positions 2 and 4. Name: 2,2,4,4-Tetramethylpentane

(ii) $(CH_3)_2C(C_2H_5)_2$ …