Questions: IUPAC Nomenclature of Alcohols, Ethers, and Thiols

Question 1

A molecule contains both a hydroxyl group (–OH) and an ether linkage (–O–). How should it be named under IUPAC rules?

• A. As an ether with a hydroxy substituent, because the ether oxygen bridges two carbons
• B. As an alcohol with an alkoxy substituent, because the hydroxyl group outranks the ether in the functional group hierarchy
• C. The choice depends on which functional group is attached to the longer carbon chain
• D. As an alkoxyalcohol, treating both groups as co-principal functional groups

Answer: (B)

IUPAC rules rank functional groups in a strict priority hierarchy: alcohols outrank ethers. Only the highest-priority group earns the suffix; everything else becomes a prefix or substituent. So the molecule is named as an alcohol (suffix -ol) with the ether oxygen treated as an alkoxy- substituent. Option A reverses the hierarchy — ethers are always substituents when a higher-priority group is present.

Question 2

When naming the compound CH₃–CH(OH)–CH₂–CH₃, why is the carbon bearing the –OH numbered as carbon 2 rather than carbon 3?

• A. IUPAC rules require numbering from the end closest to the most branches
• B. IUPAC rules require the principal functional group to receive the lowest possible locant
• C. The numbering starts from the end with the most substituents, regardless of functional group
• D. Both numbering directions give the same name for this molecule

Answer: (B)

For alcohols (and all oxygenated compounds), the chain is numbered so that the functional group carbon gets the lowest possible locant. Numbering from the left gives –OH on C2 (butan-2-ol); numbering from the right gives –OH on C3 (butan-3-ol). IUPAC chooses C2 because 2 < 3. This is a direct extension of the lowest-locant rule you already know for substituents on alkanes, now applied to the principal functional group.

Question 3

Thiols are named using the same rules as alcohols, substituting the suffix -thiol for -ol, and the sulfur-bearing carbon must be included in and numbered within the parent chain.

• T. True
• F. False

Answer: True

Correct. Thiols (R–SH) follow the same naming logic as alcohols (R–OH): the -e of the parent alkane is replaced by -thiol, the SH carbon must be part of the main chain count, and the chain is numbered to give the SH carbon the lowest possible locant. For example, CH₃CH₂SH is ethanethiol, parallel to ethanol.

Question 4

In IUPAC nomenclature, an ether can serve as the principal functional group (receiving the -oxy suffix) even when a hydroxyl group is present in the same molecule.

• T. True
• F. False

Answer: False

Ethers rank below alcohols, aldehydes, ketones, and carboxylic acids in IUPAC's functional group priority hierarchy. When a higher-priority group is present, the ether oxygen is always treated as an alkoxy substituent (prefix), not as the principal group. The suffix is reserved for the highest-priority functional group only.

Question 5

Why does the IUPAC functional group priority hierarchy require that only one group receive the suffix when multiple oxygenated groups are present?

Model answer: Because giving multiple groups suffix status would create ambiguous or contradictory names — the suffix encodes the primary chemical reactivity and class of the compound. The hierarchy ensures a unique, unambiguous name: the most reactive or highest-priority group determines the compound's class (suffix), while all others are described as substituents (prefixes). This lets a chemist immediately identify the compound's principal reactivity from the name alone.

The suffix communicates the compound's functional class and guides chemical reactivity predictions. If both –OH and an ether could each claim the suffix, two different valid names would exist for the same compound, violating IUPAC's goal of one compound → one name. The strict hierarchy resolves every tie by rank, producing a unique name regardless of molecular complexity.