Q.Structures and IUPAC names of some hydrocarbons are given below. Explain why the names given in the parentheses are incorrect. (Examples include: 2,5,6-Trimethyloctane not 3,4,7-trimethyloctane; 3-Ethyl-5-methylheptane not 5-ethyl-3-methylheptane; 3-Ethyl-1,1-dimethylcyclohexane not 1-ethyl-3,3-dimethylcyclohexane; 5-(2-Ethylbutyl)-3,3-dimethyldecane not 5-(2,2-dimethylbutyl)-3-ethyldecane.)

Concept: IUPAC Naming Rules — Lowest Locant Set & Alphabetical Order

The names in parentheses are incorrect because they violate two core IUPAC rules:

1. Lowest locant rule: When numbering the parent chain, the first point of difference must give the smallest possible number to the substituent.
2. Alphabetical order: Substituents are listed alphabetically (ignoring prefixes like di-, tri-), and the numbering must reflect this order when there is a tie.

Step-by-step reasoning for each example:

• 2,5,6-Trimethyloctane (not 3,4,7-trimethyloctane): Comparing the two numbering directions locant by locant: 2,5,6 vs 3,4,7. At the first point of difference 2 < 3, so 2,5,6 is the lower set — the sum of the locants is not the criterion.

• 3-Ethyl-5-methylheptane (not 5-ethyl-3-methylheptane): Both numbering directions give the same first locant (3). Alphabetical order requires ethyl before methyl, so the ethyl gets the lower number (3), not the methyl. …

The Concept: Why Numbering Matters

IUPAC naming is a precise language. The parent chain (or ring) is numbered so that substituents get the smallest possible numbers when compared locant by locant — not by summing them or picking the smallest first number alone. This is called the lowest set of locants rule.

When two numbering directions are possible, you compare the two sets of locants in increasing order. The set that has the smaller number at the first point of difference wins. This is the single most common source of error in naming.

Let’s apply this to each example.

1. 2,5,6-Trimethyloctane (not 3,4,7-trimethyloctane)

Step 1: Identify the parent chain.

The compound is an octane — an 8-carbon straight chain.

Step 2: Number from the end that gives the lowest locants.

Numbering from left to right gives methyl groups at positions 2, 5, and 6.

Numbering from right to left gives positions 3, 4, and 7.

Step 3: Apply the lowest set rule.

Compare the two sets in increasing order:

• Set A (correct): 2, 5, 6
• Set B (incorrect): 3, 4, 7

At the first position, 2 < 3, so Set A wins. The name 2,5,6-trimethyloctane is correct.

2. 3-Ethyl-5-methylheptane (not 5-ethyl-3-methylheptane)

Step 1: Identify the parent chain.

Heptane — 7 carbons.

Step 2: Number from the end that gives the lowest locants.

Numbering left to right: ethyl at 3, methyl at 5 → set {3,5}.

Numbering right to left: ethyl at 5, methyl at 3 → set {3,5} as well (just swapped).

Step 3: When locant sets are identical, alphabetical order decides the name.

Ethyl (e) comes before methyl (m). So the correct name lists ethyl first: 3-ethyl-5-methylheptane. The incorrect name lists methyl first, which violates alphabetical order.

3. 3-Ethyl-1,1-dimethylcyclohexane (not 1-ethyl-3,3-dimethylcyclohexane)

Step 1: Identify the parent ring.

Cyclohexane — a 6-carbon ring.

Step 2: Number the ring to give the lowest set of locants.

For a ring, you can start at any carbon and go clockwise or anticlockwise. The goal is to minimise locants.

• Correct numbering: start at the carbon bearing the ethyl group (position 1), then go anticlockwise: methyls at 1 and 3 → set {1,1,3}.
• Incorrect numbering: start at a methyl-bearing carbon (position 1), then ethyl at 3, other methyl at 3 → set {1,3,3}.

Step 3: Compare sets.

Set A (correct): 1,1,3

Set B (incorrect): 1,3,3

At the first position, both are 1. At the second position, 1 < 3, so Set A wins. The correct name is 3-ethyl-1,1-dimethylcyclohexane.

4. 5-(2-Ethylbutyl)-3,3-dimethyldecane (not 5-(2,2-dimethylbutyl)-3-ethyldecane) …