4.1.4 · D5General Organic Chemistry (GOC)
Question bank — IUPAC nomenclature — alkanes, alkenes, alkynes, aromatics, alcohols, ethers, aldehydes, ketones, acids, esters, amines,
The three words this whole page hangs on
Before any trap, pin down the vocabulary. If you already know these cold, skim — but most "traps" are really just one of these three words being misremembered.
True or false — justify
Every item: decide true/false, THEN give the one-line reason. A bare verdict scores zero — the reasoning is the point. (Answers are the text after the ::: — the question is before it.)
T/F: The longest carbon chain is always chosen as the parent chain.
False — the parent chain must first contain the principal characteristic group; only among chains that qualify do you then pick the longest. A longer chain that misses the boss group loses.
T/F: When two chains are tied for length, you pick either one at random.
False — ties break by the chain with more substituents, then by lower locants; nothing in IUPAC is ever "random", the algorithm is fully deterministic.
T/F: A double bond always gets the lowest possible locant.
False — only when no senior suffix group is present. If –OH, =O, or –COOH is in the chain, that boss group claims the low numbers and the C=C takes what's left (e.g. but-3-en-1-ol).
T/F: Ethers and halides can occasionally be a suffix if they are the only group present.
False — they are structurally incapable of being a suffix and are always prefixes (alkoxy-, halo-); if they're all you have, the compound is named as a substituted alkane.
T/F: "Di", "tri", "tetra" are ignored when alphabetizing substituents.
True — multiplying prefixes don't count, so "diethyl" alphabetizes at e; but iso- and cyclo- are part of the substituent name and DO count.
T/F: In an aldehyde and a carboxylic acid, the functional-group carbon is always locant 1.
True — –CHO and –COOH can only sit at a chain end, so their carbon is necessarily the lowest locant, C1; the locant is usually omitted because it can't be anything else.
T/F: A ketone carbon can be locant 1 of the chain.
False — a ketone needs carbons on both sides of the C=O, so it is never terminal; the smallest ketone is propan-2-one (acetone), never a "C1" ketone.
T/F: Phenol and aniline are systematic IUPAC names built from the algorithm.
False — they are retained names on IUPAC's approved historic list; the algorithm would give hydroxybenzene / aminobenzene, but the official retained list overrides that.
T/F: The E/Z labels on a double bond can simply be replaced by cis/trans in every case.
False — cis/trans only works cleanly when each double-bond carbon has one H; when all four substituents differ you must use E/Z, which ranks groups by CIP priority (see Priority and Seniority Rules (CIP vs IUPAC)).
Stereochemistry traps (E/Z, cis/trans, R/S)
T/F: "cis" always means the same as "Z".
False — they often agree, but cis compares the two like/main groups while Z compares the two highest CIP-priority groups; when the priority group is not the "main-chain" group, cis and Z can point at opposite arrangements.
Spot the trap: a student labels a stereocentre R without turning the lowest group to the back.
You must view with the lowest-priority group pointing away from you first; reading the clockwise/anticlockwise sense from any other viewpoint can flip R and S, giving the wrong descriptor.
Why does E/Z exist when cis/trans already did?
Because cis/trans becomes ambiguous once a double-bond carbon carries two different non-H groups — there is no unique "cis" side. E/Z uses CIP priority to give a single unambiguous answer every time.
Edge case: is a carbon with two identical groups (say two methyls) ever a stereocentre?
No — a stereocentre needs four different groups; two identical substituents make the mirror images superimposable, so no R/S label applies. This links to Isomerism (stereoisomers require genuine handedness).
Spot the error
Each name below is wrong. Say WHAT rule was broken.
Error in "but-1-en-4-ol" for .
The –OH is senior to the C=C, so numbering must start from the OH end giving it locant 1; the correct name is but-3-en-1-ol. The namer wrongly minimized the double-bond locant.
Error in "2-methylbutan-4-ol".
Locants must be as low as possible for the boss group first — numbering from the other end gives the OH the lower number, yielding 3-methylbutan-1-ol.
Error in "4-oxobutanal" for .
The principal characteristic group is the carboxylic acid, not the aldehyde; –COOH must be the suffix (-oic acid) and –CHO becomes the prefix oxo-, giving 4-oxobutanoic acid.
Error in "ethylmethyl ether" written as an IUPAC name.
That's a common (functional-class) name; systematic IUPAC treats the smaller group as an alkoxy prefix, giving methoxyethane.
Error in "3-chloro-2-methylbutane" numbered to give methyl the low number.
With only substituents (no suffix group), you compare the whole set of locants; when the set is a tie, the group first in alphabetical order gets the lower locant — chloro (c) beats methyl (m): 2-chloro-3-methylbutane.
Error in naming as "but-3-ene".
The double bond is between C2 and C3; you cite the lower locant, and numbering from either symmetric end gives 2, so it is but-2-ene — never the higher number.
Error in mixing conventions: writing "2-methyl-o-xylene".
You may name this ring either systematically (1,2-dimethylbenzene, "di" ignored so it alphabetizes at m) or by the retained name (o-xylene) — but never blend the two conventions in one name.
Why questions
Why must exactly ONE functional group become the suffix, not several?
A single suffix guarantees one deterministic name per structure; if two groups could both be suffixes, two competing names would exist and the "one name ↔ one structure" contract would break. (This is the whole reason Isomerism cases still get unique names.)
Why does the boss group get the lowest locant rather than a high one?
It is a fixed rule chosen to remove ambiguity: giving the most important group the smallest number means two chemists numbering independently always land on the same name.
Why are ethers and halides forbidden from ever being a suffix?
A suffix must attach through a carbon that "owns" the group's descriptor carbon; –O– and –X hang off the chain without such a carbon, so IUPAC only allows them as substituent prefixes. See Functional Groups Overview for the full prefix/suffix table.
Why do we cite the lower locant of a double bond and not both carbons?
The bond spans two adjacent carbons, so one number plus the "-ene" suffix unambiguously fixes it; citing both would be redundant. Each such bond also raises the count in Degree of Unsaturation (rings + multiple bonds), which is why locating them matters.
Why does the acid carbon count as part of the parent chain?
The –COOH carbon is a real chain carbon bearing the functional group; excluding it would shorten the root and mis-locate the group, so it is always counted and always locant 1.
Why keep any retained names (phenol, aniline, benzoic acid) at all?
They are so universally entrenched that abandoning them would hurt communication more than help; IUPAC preserves a short curated list — see Aromatic Compounds — Benzene Derivatives.
Edge cases
What if a molecule has TWO identical senior groups (e.g. two –COOH)?
Both are expressed in the suffix with a multiplying prefix: is propanedioic acid — the chain must be numbered to include and lowest-locate both.
What is the name of where a C=C and C≡C compete for low locants?
With no other suffix, you give the lowest locant set to the unsaturation as a whole; here numbering yields pent-1-en-4-yne (double before triple at an equal-first tie, "-ene" cited before "-yne").
What if the longest chain and a shorter chain both contain the senior group but the shorter one has more branches?
The chain containing the senior group and being longest wins; branch count only breaks length ties, never overrides a genuinely longer qualifying chain.
How do you name a compound whose only "feature" is a halogen, with no functional group at all?
You name it as a substituted alkane with the halogen as a prefix — e.g. is 2-chloropropane; there is simply no suffix slot to fill.
What happens to numbering when both ends give the boss group the same locant (a symmetric molecule)?
The tie is resolved by the next rule — lowest locants to the remaining substituents, then first alphabetical point of difference — so symmetry never leaves the name undecided.
If a nitrile –C≡N and an aldehyde –CHO are both present, which is the suffix?
The nitrile is senior (rank 4 vs 5 on the seniority ladder), so it becomes -nitrile and the aldehyde is demoted to the prefix oxo-; consult the ladder, never intuition about reactivity.
Active recall
Recall Q: The one-sentence rule that resolves 90% of "which end do I number from?" traps.
A: Number so the principal characteristic group (the boss) gets the lowest locant; only if there's no suffix group do double/triple bonds, then substituents, decide direction.
Recall Q: Why can "but-1-en-4-ol" never be correct?
A: Because –OH outranks a C=C, so OH must be locant 1; the correct name is but-3-en-1-ol.
Recall Q: Two prefixes that DO count for alphabetizing (unlike di/tri).
A: iso- and cyclo- are counted; the multiplying prefixes di/tri/tetra are not.
Recall Q: When must you use E/Z instead of cis/trans?
A: When a double-bond carbon carries two different non-H groups, cis/trans is ambiguous, so E/Z (ranked by CIP priority) is required.