Question
Name the following compound using IUPAC rules:
Also write the IUPAC name for: CH₃-C(CH₃)₂-CH₂-CH₃
Solution — Step by Step
Count all possible chains and pick the longest one. For the first compound, the longest chain has 5 carbons — that gives us the parent name pentane.
Don’t stop at the obvious horizontal chain. Branches are part of the molecule, and we must check if including them gives us a longer chain.
Number from the end that gives the first branch the smallest possible number. Here, numbering from the left gives us substituents at positions 2 and 4. Numbering from the right gives positions 2 and 4 as well — same set, so either end works. We then apply the first point of difference rule: go with the direction that gives the lower number at the first branch encountered.
For the second compound CH₃-C(CH₃)₂-CH₂-CH₃, the longest chain is 4 carbons (butane), and the two methyl groups sit on carbon 2.
- A
–CH₃group = methyl - A
–C₂H₅group = ethyl - Two identical substituents on the same carbon? Use the prefix di-, tri-, etc.
For compound 2: two methyl groups on C2 = 2,2-dimethyl
List substituents alphabetically (ignore di-, tri- when alphabetising). Then attach the parent chain name.
Compound 1: ethyl comes before methyl → 4-ethyl-2-methylpentane
Compound 2: two methyls on C2 → 2,2-dimethylbutane
Check: longest chain chosen ✓, lowest locant set ✓, substituents named correctly ✓, alphabetical order ✓.
Final answers: 4-ethyl-2-methylpentane and 2,2-dimethylbutane
Why This Works
IUPAC naming is essentially a coordinate system for molecules. Every rule exists so that exactly one name maps to exactly one structure — no ambiguity anywhere in the world.
The “lowest locant set” rule works like a tiebreaker in ranking. We compare the sets of numbers like we compare numbers digit by digit: {2,4} vs {2,4} is a tie, but {2,3} beats {3,4} because the first point of difference favours the former. This appeared as a direct one-mark question in CBSE 2024 Board Exam, so getting this logic right is scoring.
Alphabetical order of substituents is independent of their position numbers — we sort by the substituent name itself, not by the carbon number where it sits.
Alternative Method — Drawing the Structure First
When the compound is given as a condensed formula (like in board exams), draw the full structural formula before naming.
& \text{CH}_3 & & \text{C}_2\text{H}_5 \\ | & & & | \\ \text{CH}_3 & - & \text{CH} & -\text{CH}_2- & \text{CH} & -\text{CH}_3 \end{array}$$ Once you see it drawn out, the longest chain and branching points become obvious. Many students lose marks simply because they try to name from the condensed formula directly — the drawing step costs 30 seconds but saves errors. <Callout type="tip"> For JEE Main, condensed formulas are common in multiple-choice questions. Draw the structure immediately — it turns a confusing formula into a visual problem you can solve in under a minute. </Callout> --- ## Common Mistake <Callout type="mistake"> **Choosing a shorter chain because it "looks like the main chain."** Students often draw a horizontal line through the molecule and call that the parent chain — even when counting through a branch gives more carbons. Always count ALL possible continuous carbon paths. The longest one wins, regardless of how the structure is drawn on paper. In compound 1, if you miss the ethyl branch as part of a longer chain possibility, you'd write 2-methyl-4-something-butane — wrong parent chain, wrong name, zero marks for that part. </Callout> --- <Callout type="tip"> **PYQ Pattern:** CBSE boards consistently ask you to name one straight-chain and one branched compound in the same question (3 marks). JEE Main uses this as a trap in passage-based MCQs — they give you four compounds and ask which IUPAC name is correct. The trap is always either wrong numbering direction or wrong alphabetical order of substituents. </Callout>