Alkenes: Structure and Names

25 min

The gas that ripens bananas

Traders who want green bananas to ripen quickly close them up with a few ripe ones, in a sack or a closed room. The ripe fruit gives off a gas that tells the others to ripen. That gas is ethene, a plant hormone and the simplest member of the alkenes. Ripening rooms in fruit warehouses add measured amounts of ethene for the same reason.

Ripe yellow bananas for sale at Kimironko Market in Kigali. Photo: François Terrier, CC BY 2.0, via Wikimedia Commons.

Definition

Alkene: a hydrocarbon that contains one carbon–carbon double bond, C=C. Alkenes with one double bond have the general formula $\mathrm{C_nH_{2n}}$ , where n is 2 or more. Example: ethene, $\mathrm{C_2H_4}$ .

Unsaturated hydrocarbon: a hydrocarbon with at least one C=C or C≡C bond, so more atoms can be added to it. Example: propene, $\mathrm{C_3H_6}$ .

Functional group of the alkenes: the C=C double bond, the part of the molecule where its reactions happen.

The double bond and the shape it gives

In a C=C double bond the two carbon atoms share two pairs of electrons. Each carbon of the double bond is joined to only three other atoms, so it has two fewer hydrogen atoms than the alkane with the same number of carbons: ethane is $\mathrm{C_2H_6}$ , ethene is $\mathrm{C_2H_4}$ . The three groups around each of these carbons spread out in a flat triangle, about 120° apart, and the whole ethene molecule is flat. The double bond cannot twist, unlike a single bond.

Naming alkenes

Find the longest carbon chain that contains the double bond. Its stem gives the name, with the ending -ene.
Number the chain from the end nearer the double bond.
Put the number of the first carbon of the double bond in front of -ene: $\mathrm{CH_2{=}CHCH_2CH_3}$ is but-1-ene.
Name branches as for alkanes, after the double bond has its lowest number: $\mathrm{CH_2{=}C(CH_3)CH_2CH_3}$ is 2-methylbut-1-ene.

Ethene and propene need no number: in a chain of two or three carbons the double bond can only be between carbons 1 and 2. The alkene series starts at two carbons, because a double bond needs two carbon atoms.

Table 1.1: The first alkenes, with the double bond at carbon 1 (C = 12, H = 1)

n	Name	Molecular formula	Condensed formula	$M_{r}$	Boiling point / °C	State at 25 °C
2	ethene	$\mathrm{C_2H_4}$	$\mathrm{CH_2{=}CH_2}$	28	−104	gas
3	propene	$\mathrm{C_3H_6}$	$\mathrm{CH_2{=}CHCH_3}$	42	−48	gas
4	but-1-ene	$\mathrm{C_4H_8}$	$\mathrm{CH_2{=}CHCH_2CH_3}$	56	−6	gas
5	pent-1-ene	$\mathrm{C_5H_{10}}$	$\mathrm{CH_2{=}CH(CH_2)_2CH_3}$	70	30	liquid
6	hex-1-ene	$\mathrm{C_6H_{12}}$	$\mathrm{CH_2{=}CH(CH_2)_3CH_3}$	84	63	liquid

As with alkanes, the boiling point rises along the series: C₂ to C₄ alkenes are gases and pent-1-ene and above are liquids at 25 °C.

Worked example

Problem. Name: (a) $\mathrm{CH_3CH{=}CH_2}$ ; (b) $\mathrm{CH_3CH_2CH{=}CH_2}$ ; (c) $\mathrm{CH_3CH{=}CHCH_2CH_3}$ ; (d) $\mathrm{CH_2{=}C(CH_3)CH_3}$ .

Solution. (a) Three carbons with a double bond: propene. (b) Four carbons. From the left the double bond starts at carbon 3; from the right, at carbon 1. Use the lower number: but-1-ene. (c) Five carbons; from the left the double bond starts at carbon 2, from the right at carbon 3: pent-2-ene. (d) The chain with the double bond has three carbons, and a $\mathrm{CH_3}$ branch sits on carbon 2: 2-methylpropene.

Worked example

Problem. (a) An alkene has $M_{r} = 84$ . Find its molecular formula. (b) Write the molecular formula of the alkene with 9 carbon atoms. (C = 12, H = 1)

Solution. (a) For $\mathrm{C_nH_{2n}}$ : $M_{r} = 12 n + 2 n = 14 n$ . So $14 n = 84$ , $n = 6$ : $\mathrm{C_6H_{12}}$ , a hexene. (b) $n = 9$ , hydrogens $= 2 \times 9 = 18$ : $\mathrm{C_9H_{18}}$ , a nonene.

Warning

Common mistakes. (1) Number the chain from the end nearer the double bond, even if a branch is nearer the other end. (2) The position number is the first carbon of the double bond: $\mathrm{CH_3CH{=}CHCH_3}$ is but-2-ene, not but-2,3-ene. (3) Each carbon of a C=C has only two other bonds left: drawing two hydrogens and a $\mathrm{CH_3}$ on one of them gives that carbon five bonds.

Tip

$\mathrm{M_r = 14n}$ for alkenes. The alkene has two hydrogens fewer than the alkane, so its mass is 2 less: ethane 30, ethene 28.

FuseSchool: Alkanes and Alkenes. Watch how the double bond changes the formula and the reactivity, and what alkanes and alkenes have in common.

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Practice

Define an alkene and state its general formula.
Explain why there is no alkene called "methene".
Draw the displayed formula of propene and label the functional group.
Name: (a) $\mathrm{CH_2{=}CHCH_2CH_2CH_3}$ ; (b) $\mathrm{CH_3CH_2CH{=}CHCH_3}$ ; (c) $\mathrm{CH_3CH{=}C(CH_3)CH_3}$ .
Write condensed formulas for (a) but-2-ene; (b) hex-1-ene; (c) 3-methylbut-1-ene.
An alkene has a relative molecular mass of 56. Find its molecular formula. (C = 12, H = 1)
Describe the shape of an ethene molecule and give the bond angle.
A student names $\mathrm{CH_3CH_2CH_2CH{=}CH_2}$ "pent-4-ene". Correct the name and explain.

Show answersHide answers

A hydrocarbon with a C=C double bond; $\mathrm{C_nH_{2n}}$ .
A double bond needs two carbon atoms, so the series starts at ethene (two carbons).
$\mathrm{CH_2{=}CH{-}CH_3}$ drawn with all bonds; the C=C between carbons 1 and 2 is the functional group.
(a) pent-1-ene; (b) pent-2-ene (number from the right); (c) 2-methylbut-2-ene.
(a) $\mathrm{CH_3CH{=}CHCH_3}$ ; (b) $\mathrm{CH_2{=}CH(CH_2)_3CH_3}$ ; (c) $\mathrm{CH_2{=}CHCH(CH_3)CH_3}$ .
$14 n = 56$ , $n = 4$ : $\mathrm{C_4H_8}$ , a butene.
Flat (planar): all six atoms lie in one plane, with angles of about 120° around each carbon.
Pent-1-ene: the chain must be numbered from the end nearer the double bond, which gives 1, not 4.

Sources and credits

Photo: Bananas @Kimironko Market, François Terrier, via Wikimedia Commons, CC BY 2.0, used unchanged.
Video: Alkanes and alkenes, FuseSchool, embedded from YouTube.
Written by Ganzaa. The diagrams, table, worked examples and Practice items were drawn and written by Ganzaa; the boiling points are standard reference values, rounded.

1The gas that ripens bananas

2The double bond and the shape it gives

3Naming alkenes

The gas that ripens bananas

The double bond and the shape it gives

Naming alkenes