Questions: IUPAC Nomenclature of Alkanes

Question 1

A compound has a 5-carbon parent chain with a methyl group on C2 and an ethyl group on C4. What is its correct IUPAC name?

• A. 2-methyl-4-ethylpentane
• B. 4-ethyl-2-methylpentane
• C. 2-ethyl-4-methylpentane
• D. 4-methyl-2-ethylpentane

Answer: (B)

Substituents are cited alphabetically, ignoring multiplying prefixes. 'Ethyl' (E) precedes 'methyl' (M) alphabetically, so ethyl is listed first: 4-ethyl-2-methylpentane. The locants 2 and 4 are correct because numbering from the end closer to the first substituent gives 2 (for the methyl end) vs. numbering from the other end, which would give a higher set. Option A lists substituents in reverse alphabetical order.

Question 2

In naming 2,5-dimethylheptane, the prefix 'di' is considered when alphabetizing substituents alongside other substituents.

• T. True
• F. False

Answer: False

Multiplying prefixes (di-, tri-, tetra-, etc.) are ignored when alphabetizing substituents in IUPAC nomenclature. If this compound also had an ethyl group, the order would be 'ethyl' before 'methyl' (E before M), regardless of the 'di' prefix on the methyls. The same rule applies to sec- and tert- prefixes, which are also ignored for alphabetizing, while 'iso-' and 'cyclo-' are included.

Question 3

A student draws a structural formula and, looking at the drawing, identifies the longest horizontal chain as 6 carbons, naming the compound 3-methylhexane. What error might the student be making, and how should they correct it?

Model answer: The student may be picking the longest horizontal path in the drawing rather than the longest actual carbon chain. Structural drawings are arbitrary in orientation — what appears as a branch might be part of a longer continuous chain. The student should trace every possible continuous carbon path and count carbons in each, choosing the longest regardless of drawing orientation.

IUPAC naming requires finding the longest continuous carbon chain in the molecule, not the longest horizontal segment in a particular drawing. A substituent drawn as a vertical branch might actually be connected to more carbons that, when followed through the drawing, form a longer chain than the horizontal one. Systematically tracing all paths is the only reliable method.