Two things remain before the naming system covers everything you will meet in this course. One is what happens when the parent is a ring rather than a chain. The other is how to say where a double or triple bond sits. Neither needs a new principle. Both are the rules you already know, applied to a slightly different skeleton.
Rings: the parent gets cyclo-
Six carbons joined in a ring make cyclohexane. The name is the six-carbon root with cyclo- in front of it. Five carbons in a ring make cyclopentane, and three make cyclopropane.
A ring can also hang off something else as a substituent. It then takes the ending -yl, like any alkyl group: cyclohexyl, cyclopentyl, cyclopropyl.
A chain has two ends, and you number from one of them. A ring has no ends. You choose two things yourself: which ring carbon is C1, and which way round to count. The priorities you already use make that choice for you, as the next three examples show.
Start with one substituent. A methyl on cyclohexane is methylcyclohexane, with no number at all. Every carbon of the ring is equivalent, so there is nowhere else the methyl could be, and "1-methylcyclohexane" would add nothing.
Now take two substituents: an ethyl and a methyl with one ring carbon between them. Start at either substituted carbon and count the short way round to the other. Both choices put the substituents on C1 and C3, so the locant set is {1,3} either way and the numbers cannot decide. When the sets tie, the substituent that comes first in alphabetical order takes the lower number. Ethyl comes before methyl, so the name is 1-ethyl-3-methylcyclohexane, and the ethyl is the one that gets C1.
Three substituents show which rule comes first. Take a ring with a methyl, an ethyl on the next carbon, and a second methyl two carbons further round from the ethyl. If the ethyl takes C1, the lowest set it can reach is {1,2,5}. If the neighboring methyl takes C1 and the count runs through the ethyl, the set is {1,2,4}.
Compare the two sets term by term, at the first point of difference, exactly as you did for chains. The first terms tie at 1, the second terms tie at 2, and then 4 beats 5. The name is 2-ethyl-1,4-dimethylcyclohexane. Ethyl is first in the alphabet and still gets C2, because the sets are not tied. The alphabet never gets a say here.
A functional group on a ring follows the rule it follows on a chain. An OH on cyclohexane makes cyclohexanol, and a C=O in the ring makes cyclohexanone. The group is the principal characteristic group, so its carbon is C1. With nothing else on the ring, that 1 is not written.
Two groups on a ring can also point to the same face of the ring or to opposite faces. cis-1,2-Dimethylcyclohexane has both methyls on one face, and trans-1,2-dimethylcyclohexane has one on each face. They are different compounds. The drawing below uses the solid and dashed wedges from the Molecular geometry section of Foundations: a solid wedge comes toward you, and a dashed wedge goes away. Stereochemistry treats the cis and trans prefixes properly. For now, know that a ring name with two substituents is often incomplete without cis or trans.
Ring or chain: which one is the parent?
Start with methylcyclohexane again. It has a ring of six carbons and a chain of one. The ring is larger, so the ring is the parent and the methyl is a substituent on it.
Now bond the same ring to the end of an eight-carbon chain. The chain has eight carbons and the ring has six, so this time the chain is the parent. The ring becomes a cyclohexyl substituent on C1 of the chain: 1-cyclohexyloctane.
A five-carbon chain on a cyclopentane ring is a tie, five against five. A tie goes to the ring, so the name is pentylcyclopentane.
The rule behind all three: when a molecule has a ring and a chain, and neither one contains a principal characteristic group, the parent is whichever has more carbons. (The 2013 IUPAC recommendations go further and prefer the ring whatever the chain length, so newer sources call the second example octylcyclohexane; most course exams still count carbons.)
If a principal characteristic group is present, it overrides the carbon count, exactly as it did for chains. The parent must contain that group, whether the group sits on the ring or on the chain.
Double and triple bonds
You met -ene and its locant in The parent chain: but-2-ene has its double bond from C2 to C3, and the name cites only the lower number of the pair.
Move the double bond to the end of the chain, so that it joins C1 and C2, and you have a different compound, but-1-ene. The number is doing real work. To find it, number from the end nearer the double bond.
A triple bond works the same way, with the ending -yne in place of -ene. Two double bonds take -diene and two locants. In penta-1,3-diene one double bond joins C1 and C2 and the other joins C3 and C4. (The extra a in penta- is only there to make the name easier to say.)
The spelling rule of the previous section applies to -ene too. Its final e is dropped before a suffix that starts with a vowel, and also before -yne. It is kept before any other consonant. So you write hex-5-en-1-ol and pent-1-en-4-yne, but hex-5-enenitrile.
A multiple bond gets a lower number than a substituent
Take CH₃–CH=CH–CH(CH₃)–CH₃, a five-carbon chain with a double bond near one end and a methyl near the other. From the left, the double bond starts at C2 and the methyl sits on C4; from the right, the methyl sits on C2 and the double bond starts at C3.
The double bond claims the lower number, so you number from the left: 4-methylpent-2-ene. Numbering from the right to hand the methyl a 2 is the same slip as giving a methyl the low number over an OH.
A triple bond behaves the same way. In HC≡C–CH₂–CH(CH₃)–CH₃ the triple bond starts at C1 from the left and at C4 from the right, so the name is 4-methylpent-1-yne. Only the ending changes; the procedure does not. In both molecules the multiple bond takes the lowest locant it can, and the methyl gets what is left.
The parent chain must contain the double bond
Take a six-carbon chain with a =CH₂ on its third carbon. The longest chain has six carbons, but the double bond is not in it: it hangs off the side. The longest chain that runs through the double bond has five carbons, and it leaves two carbons over as an ethyl. That five-carbon chain is the parent, so the name is 2-ethylpent-1-ene, and not a methylidene-hexane (methylidene is the name for a =CH₂ used as a substituent).
This is the alcohol rule of the previous section again. The feature that sets the ending has to be on the parent. If the longest chain in the molecule does not contain the double bond, it is not the parent: take the longest chain that does. A triple bond is treated the same way.
The 2013 IUPAC recommendations dropped this rule and let chain length win, which gives 3-methylidenehexane. Textbooks and exams still apply it, and this course follows them.
A principal group outranks the multiple bond
Take a six-carbon chain with an OH on one end carbon and a double bond at the other end. Numbered from the OH end, the OH is on C1 and the double bond starts at C5, giving hex-5-en-1-ol; numbered from the other end, the double bond starts at C1 and the OH is on C6, giving hex-1-en-6-ol.
The alcohol is the principal characteristic group, and it outranks the double bond. So it claims the 1, and the first name is correct, even though the second gives the double bond the lower number.
In general, the principal characteristic group takes the lowest locant, and the multiple bond takes the lowest locant left after that. Both endings appear in the name: the -en- or -yn- comes first and the group's suffix comes last.
A double and a triple bond in one chain
CH₃–CH=CH–C≡CH has one double bond and one triple bond. From the left, the double bond starts at C2 and the triple bond at C4, so the set is {2, 4}. From the right, the triple bond starts at C1 and the double bond at C3, so the set is {1, 3}. Compare the sets at the first point of difference: 1 beats 2. The name is pent-3-en-1-yne, and the triple bond gets the 1.
CH₂=CH–CH₂–C≡CH gives {1, 4} from both ends. From the left the double bond is at 1; from the right the triple bond is at 1. The sets tie, so the double bond gets the lower number: pent-1-en-4-yne, not pent-4-en-1-yne.
So the lowest set of locants decides first, whichever bond it favors. Only a tie hands the 1 to the double bond.
The order of the endings in the name never changes: -en- comes before -yne, whatever the numbers turn out to be.
CH₂=CH–CH₂–CH(CH₃)–C≡CH gives {1, 5} for the multiple bonds from both ends. The sets tie, so the double bond takes the 1 and the chain is numbered from the CH₂= end. That puts the methyl on C4, not C3: 4-methylhex-1-en-5-yne. The double bond settles the tie before the methyl is considered at all.
Double bonds in rings
A cyclohexene ring carries a methyl group. The double bond joins two ring carbons, and the methyl sits on the carbon next to one of them.
Choose the parent. The ring, six carbons, containing the double bond: cyclohexene.
Number. The two carbons of a ring double bond get C1 and C2, so the count runs straight across the double bond. The double bond takes these lowest locants before the substituents are considered. That still leaves two directions, and you pick the one that gives the methyl the lower number.
Assemble. Going the way that reaches the methyl sooner puts it on C3: 3-methylcyclohex-1-ene. The 1 is often dropped in practice, as 3-methylcyclohexene.
A principal characteristic group still comes first in a ring. Put an OH on the carbon next to the double bond in cyclohexene. The OH carbon is C1, and you count toward the double bond, so the double bond joins C2 and C3: cyclohex-2-en-1-ol. Giving the double bond C1 and C2 would push the OH to C3, and that is wrong: the OH keeps C1, and the double bond takes the lowest locants left.
Cis, trans, E and Z — a forward reference
A double bond locks the two carbons it joins so they cannot rotate. That means a name like but-2-ene describes two different compounds: one with the two methyls on the same side of the double bond, and one with them on opposite sides. The name so far cannot tell them apart.
The prefixes cis- and trans- do it informally when each alkene carbon carries one hydrogen and one other group: cis for same side, trans for opposite. The general system, which works whatever the four groups are, is E and Z. It ranks the groups on each alkene carbon by a set of priority rules, taught in Stereochemistry.
The full system (the ranking, the traps and the worked assignments) is built in the Cis/trans and E/Z section of Stereochemistry. For now, know that an alkene name is often incomplete without one of these prefixes, and that a question asking for "the full name" of an alkene is usually asking for it.
Benzene rings with two substituents
Put two methyl groups on a benzene ring. They can sit on neighboring carbons, on carbons with one ring carbon between them, or directly across the ring. There is no fourth option. Number from one methyl and take the short way to the other, and the three give 1,2-, 1,3- and 1,4-dimethylbenzene.
Each of the three positions also has a name of its own, used constantly and interchangeably with the numbers:
The word is usually shortened to a single italic letter: o-xylene, m-xylene, p-xylene, where xylene is the retained name for dimethylbenzene in everyday use.
This vocabulary becomes essential in Aromatic Chemistry. There the whole question of directing effects is phrased in it: a substituent is called an ortho, para director or a meta director, and the words mean exactly what they mean here.
What carries forward
You can now name, or read the name of, essentially every compound in the remaining chapters. What is left is not more nomenclature but more chemistry, and from here the names are a tool rather than a topic. The forward references on this page are real, though: E/Z is finished in Stereochemistry, and ortho/meta/para becomes the language of Aromatic Chemistry. Both will assume you met the words here.