Naming and drawing alkenes and alkynes
How -ene and -yne names place C=C and C≡C bonds, how chains with multiple bonds are numbered, and how dienes and substituted alkenes are drawn.
Alkenes contain at least one carbon–carbon double bond and alkynes at least one triple bond. Their names keep the alkane stem and change the ending:-ane becomes -ene for a C=C and -yne for a C≡C. The only new piece of information is where the multiple bond is, given by a locant placed directly in front of the ending.
Reading the locant
The locant of a multiple bond is the lower-numbered of its two carbons. "but-2-ene" means a double bond between C2 and C3.
The chain is numbered so that the multiple bond gets the lowest possible locant. There is no "but-3-ene": numbered from the other end, it is but-1-ene. Remember to draw the two carbons of a triple bond and their neighbours in a straight line.
Substituents and several multiple bonds
Substituents are added as prefixes exactly as for alkanes, but the multiple bond takes priority when numbering. In2-methylbut-2-ene the double bond gets locant 2 whichever end of the four-carbon chain you number from, so the substituent decides: numbering from the end nearer the methyl puts it on C2, not C3.
Two double bonds give a diene, three a triene. Each needs a locant, and an extra "a" is added to the stem for pronunciation:buta-1,3-diene, not "but-1,3-diene".
When a chain has both a double and a triple bond, the name ends -en…-yne, and the numbering gives the lowest locants to the multiple bonds taken together. If there is still a choice, the double bond gets the lower number:
Rings
In a cycloalkene the double bond is always between C1 and C2, so its locant is omitted: cyclohexene. Numbering continues through the double bond in the direction that gives substituents the lowest locants, so a methyl on one of the double-bond carbons is at C1.
Cis/trans (E/Z) isomers
The two carbons of a C=C cannot rotate relative to each other. When each of them carries two different groups, as in but-2-ene, there are two distinct compounds: one with the methyl groups on the same side of the double bond and one with them on opposite sides. Their full names include a stereodescriptor, (Z)- or (E)-. Exercises in this topic use compounds where that question does not arise (for example but-1-ene, where C1 carries two hydrogens); double-bond geometry is practised in the stereochemistry topic.
Conventions differ for chain selection. Older IUPAC rules, still taught in many courses, require the parent chain to contain as many multiple bonds as possible. The 2013 IUPAC recommendations instead choose the longest chain first. The two rules can give different names for the same compound: CH2=C(CH2CH2CH3)2 is "2-propylpent-1-ene" under the older rule and "4-methylideneheptane" under the 2013 rule.
The exercises here avoid compounds where the two conventions disagree.
Common mistakes
- Off-by-one locants: pent-2-ene has its double bond between C2 and C3, not between C1 and C2.
- Too many hydrogens on alkene carbons: a carbon in C=C with two other carbon neighbours has no hydrogens.
- Bent triple bonds: the checker accepts them, but your examiner may not; draw C–C≡C–C in a straight line.
- Leaving out the extra "a": it is buta-1,3-diene and hexa-1,4-diene.