I can't find the rule allowing the name "ethenyl group" in IUPAC Blue Book

Question

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1. What is the question?

A student is trying to name the substituent –CH=CH₂ (the “ethylene‑derived” group) using the IUPAC Blue Book rules.

They think the name must contain the locant 1 (‑ethen‑1‑yl) because
P‑32.1.1 says that all locants for the free valences of an acyclic parent must be cited.
Yet P‑14.3.4.2(d) (the “omit locant 1” rule) appears to allow the name ethenyl with the locant omitted.

The student therefore sees a contradiction: must the “1” be written or not?

The task is to resolve the apparent conflict and show, step‑by‑step, why the correct substituent name is ethenyl (or the accepted trivial name vinyl) and why the locant “1” is not required.

2. Systematic walk‑through of the relevant rules

Below the exact wording of the Blue Book rules (2013 edition, “Nomenclature of Organic Chemistry – IUPAC Recommendations”) is reproduced, followed by an explanation of how they interact.

Rule	Text (re‑phrased)	What it means for our case
P‑20	Acyclic parent hydrides are always saturated.	Ethene (CH₂=CH₂) is not a parent hydride; it is a parent unsaturated compound (see P‑31).
P‑31.1.1.1	The parent of a simple open‑chain alkene is named ‑ene (e.g., ethene).	The parent we start from is ethene.
P‑32	Names of substituents derived from unsaturated compounds are formed by adding the suffixes –yl, –ylidene, –ylidyne …	The substituent derived from ethene will end in ‑yl (or ‑ylidene etc.).
P‑32.1.1	For acyclic parent structures, all locants for the free valences, including “1”, must be cited.	At first sight this forces us to write ethen‑1‑yl.
P‑32.2.1 (exception)	The locant “1” may be omitted in the name of a substituent derived from a parent consisting of only two identical atoms (i.e., a dinuclear alkene or alkyne).	Ethene has exactly two carbon atoms, both identical, so the “1” can be left out.
P‑14.3.4.2(d)	The locant “1” is omitted in any dinuclear, unsubstituted, trinuclear alkenes and alkynes.	This is the same special case; it reinforces the exception in P‑32.2.1.
P‑29.2 (formation of the substituent prefix)	(2) The locants for the atoms of free valences are as low as is consistent with any established numbering of the parent hydride and, except for mononuclear parent hydrides or the suffix “‑ylidyne”, the locant “1” must be cited.	This is the general rule for substituent prefixes. The “except for …” clause does not apply here, but the higher‑order exception in P‑32.2.1 (and the reinforcing P‑14.3.4.2) overrides it.

2.1 Hierarchy of the rules

The Blue Book is explicit that specific rules (or exceptions) trump more general ones. The sequence is:

General requirement – P‑32.1.1 (cite every locant).
Specific exception – P‑32.2.1 (omit “1” for dinuclear alkenes/alkynes).
Reinforcing statement – P‑14.3.4.2(d) (same omission rule, placed in the numbering‑section part of the book).

Thus, when a specific exception exists, the “must‑cite‑locant” rule does not apply.

2.2 Applying the rules to the –CH=CH₂ group

Step	Action	Reason
1	Identify the parent unsaturated compound.	Ethene (CH₂=CH₂) – named by P‑31.1.1.1.
2	Decide which suffix to use for a substituent that retains a single free valence.	The substituent retains one free valence → suffix ‑yl (P‑32).
3	Write the base name of the parent without the “‑e” that indicates the hydride.	Remove the terminal “e” of ethene → ethen‑ (P‑32.0).
4	Check whether a locant is required.	General rule would demand “1” (P‑32.1.1).
5	Look for an exception.	Ethene is a dinuclear (two‑carbon) alkene, therefore the exception in P‑32.2.1 (and reinforced by P‑14.3.4.2(d)) allows omission of “1”.
6	Assemble the final substituent name.	ethen‑yl → after omitting the hyphen (as per P‑29.2) → ethenyl.

Result: the substituent is named ethenyl (or, by the widely‑accepted trivial name, vinyl).

3. Final answer

The correct IUPAC substituent name for –CH=CH₂ is “ethenyl”.
The locant “1” is not required because ethene is a dinuclear (two‑carbon) alkene, and the Blue Book contains an explicit exception (P‑32.2.1, reinforced by P‑14.3.4.2(d)) that permits the omission of the “1” locant for substituents derived from such parents.

4. Common mistakes for this type of problem

Mistake	Why it happens	How to avoid it
Forgetting the exception in P‑32.2.1 and applying the “always cite locants” rule verbatim.	The “all locants must be cited” rule looks absolute and is easy to overlook.	When you encounter a rule that seems to conflict, search the same chapter for “exceptions” or “omission of locant 1”.
Citing the locant “1” anyway (writing ethen‑1‑yl).	Habit from naming saturated substituents (e.g., ethyl → ethan‑1‑yl).	Remember that the “1” omission exception applies only to substituents derived from dinuclear alkenes/alkynes, not to longer chains.
Confusing the “parent hydride” vs. “parent unsaturated compound” distinction.	P‑20 is sometimes mis‑read as forbidding any unsaturated parent.	Keep in mind that parent hydrides (the basis for the “‑ane” series) must be saturated, but unsaturated parents (‑ene, ‑yne) are treated separately in P‑31.
*Assuming P‑14.3.4.2 applies only to numbering of the parent* and not to substituents**.	The rule is placed in the numbering‑section, so it can be missed when naming substituents.	Treat P‑14.3.4.2 as a global exception that also influences the way substituent prefixes are written.
Using the trivial name “vinyl” without checking its IUPAC status.	“Vinyl” is widely used, but the exam or formal report may require the systematic “ethenyl”.	Verify the required naming style: if the IUPAC systematic name is requested, use ethenyl; otherwise, “vinyl” is acceptable as a retained IUPAC name (RIN).

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