Alkenes and Alkynes: Naming Unsaturated Hydrocarbons

Double and triple bonds

A ripening banana gives off a tiny amount of ethene gas (also called ethylene), a plant hormone that tells nearby fruit to ripen. Put a green banana in a paper bag with a ripe one and it ripens faster. Ethene is also one of the most-produced organic chemicals in the world, because it is the starting material for polyethylene plastic.

Ethene is an alkene: a hydrocarbon with a C=C double bond. Its cousin ethyne (acetylene), HC≡CH, is an alkyne, with a C≡C triple bond. It burns with oxygen in an extremely hot flame, hot enough to cut and weld steel.

Both families are unsaturated. Each double bond removes 2 H atoms compared with the alkane, and each triple bond removes 4:

• alkanes CₙH₂ₙ₊₂ (ethane C₂H₆)
• alkenes CₙH₂ₙ (ethene C₂H₄)
• alkynes CₙH₂ₙ₋₂ (ethyne C₂H₂)

Notice that alkenes and cycloalkanes share the formula CₙH₂ₙ. The formula alone does not tell you whether a molecule has a double bond or a ring.

Naming alkenes and alkynes

The rules are the alkane rules with three changes:

1. The parent chain is the longest chain that contains the double or triple bond.
2. Number the chain from the end nearer the multiple bond, even if a branch then gets a higher number. The multiple bond takes priority over branches.
3. Change the ending to -ene (double) or -yne (triple), and give the locant of the first carbon of the multiple bond.

Where does the number go? The current IUPAC style puts it just before the ending: but-2-ene, pent-1-yne, 2-methylbut-1-ene. Many textbooks and labels still use the older style with the number in front: 2-butene, 1-pentyne. Both mean the same molecule and both are accepted in this course.

• Two double bonds: -diene, with an "a" added to the root for easier saying: buta-1,3-diene (1,3-butadiene).
• Ethene, propene, ethyne and propyne need no number: there is only one place the multiple bond can be.
• Cycloalkenes: the double-bond carbons are always C1 and C2, so cyclohexene needs no number. Number through the double bond so that substituents get the lowest locants: 3-methylcyclohexene.

Cis and trans isomers

You saw in Lesson 1 that a double bond cannot rotate. That has a surprising result. In but-2-ene, each carbon of the C=C carries one H and one CH₃. The two CH₃ groups can be on the same side of the double bond (cis) or on opposite sides (trans). These are two different compounds with different boiling and melting points, and you cannot turn one into the other without breaking the π part of the double bond.

Molecules like these, with the same atoms joined in the same order but arranged differently in space, are stereoisomers. Cis and trans isomers are also called geometric isomers.

Test: an alkene has cis and trans forms only if each carbon of the double bond has two different groups attached.

• But-2-ene: C2 has H and CH₃; C3 has H and CH₃ → yes.
• But-1-ene: C1 has H and H (the same) → no.
• 2-methylbut-2-ene: C2 has CH₃ and CH₃ (the same) → no.

Put the prefix cis- or trans- in front of the name: cis-but-2-ene. In university you will also meet the more general E/Z system, which works for any groups.

Properties and uses

Alkenes and alkynes are non-polar, like alkanes, so their boiling points are similar to alkanes of the same size, and they do not dissolve in water. The big difference is reactivity. A double or triple bond is a region of concentrated electrons that other molecules can attack, so alkenes and alkynes undergo addition reactions easily (Lesson 6). That is why ethene and propene are used to make plastics, antifreeze and many other products, while alkanes are used mostly as fuels.

Many natural molecules are alkenes too: the long chains in vegetable oils contain cis C=C bonds, and β-carotene, the orange pigment in carrots, has 11 C=C double bonds in a row.