Alkenes: structure, preparation and properties: WASSCE Chemistry

Put an unripe banana in a closed bag with a ripe one and it ripens faster. The ripe fruit gives off ethene, the simplest alkene. Alkenes contain a carbon–carbon double bond, which makes them far more reactive than alkanes and makes them the starting point for plastics, ethanol and many other products.

Learning objectives

By the end of this lesson you should be able to:

state the general formula of the alkenes and name the first members
describe the structure and shape of the C=C double bond
draw and name the isomers of butene, including cis–trans isomers
describe the laboratory preparation of ethene
state the sources of alkenes and the physical properties of ethene

Structure and names

Definition

Alkenes are unsaturated hydrocarbons that contain one carbon–carbon double bond (C=C). General formula: CₙH₂ₙ.

Name	Molecular formula	Condensed formula	Boiling point
Ethene	C₂H₄	CH₂=CH₂	−104 °C
Propene	C₃H₆	CH₃CH=CH₂	−48 °C
But-1-ene	C₄H₈	CH₂=CHCH₂CH₃	−6 °C
But-2-ene	C₄H₈	CH₃CH=CHCH₃	1–4 °C (cis and trans)
Pent-1-ene	C₅H₁₀	CH₂=CH(CH₂)₂CH₃	30 °C

There is no "methene": a double bond needs at least two carbon atoms.

The C=C bond. The double bond is made of one strong σ (sigma) bond and one weaker π (pi) bond. The π bond breaks easily, which is why alkenes undergo addition reactions.

Shape. The two carbons of the double bond and the four atoms attached to them all lie in one plane, with bond angles of about 120° (sp² hybridisation). The π bond prevents rotation about the C=C bond.

Isomerism in alkenes

C₄H₈ has these alkene isomers:

Structure	Name	Type of isomerism
CH₂=CHCH₂CH₃	But-1-ene	Position isomer of but-2-ene
CH₃CH=CHCH₃ (both CH₃ on the same side)	cis-But-2-ene	Geometric (cis)
CH₃CH=CHCH₃ (CH₃ groups on opposite sides)	trans-But-2-ene	Geometric (trans)
CH₂=C(CH₃)CH₃	2-Methylpropene	Chain isomer

Cis–trans isomerism needs two different groups on each carbon of the double bond. Propene (CH₃CH=CH₂) has two H atoms on one carbon, so it has no cis–trans isomers.

Laboratory preparation of ethene

Ethanol and excess concentrated sulphuric acid heated in a flask at 170 °C; the gas passes through sodium hydroxide solution and is collected over water

Figure 1. Preparing ethene by dehydrating ethanol. WASSCE Tutor diagram.

Heat ethanol with excess concentrated sulphuric acid at about 170 °C. The acid acts as a dehydrating agent, removing water from ethanol:

\text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{conc. H}_2\text{SO}_4,\ 170\,^\circ\text{C}} \text{C}_2\text{H}_4 + \text{H}_2\text{O}

The gas is passed through sodium hydroxide solution to remove sulphur(IV) oxide and carbon(IV) oxide, formed when the acid oxidises some ethanol.
Ethene is collected over water, as it is only slightly soluble.
Broken porcelain (or sand) is added so that the mixture boils smoothly.
If the temperature is only about 140 °C, the main product is ethoxyethane (an ether), not ethene.

Alternative method: pass ethanol vapour over heated aluminium oxide (or broken porcelain) at about 350 °C:

\text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{Al}_2\text{O}_3,\ 350\,^\circ\text{C}} \text{C}_2\text{H}_4 + \text{H}_2\text{O}

Industrial source: almost all ethene and propene are made by cracking naphtha or ethane from crude oil and natural gas (see the Petroleum topic).

Physical properties of ethene

Colourless gas with a faint sweet smell
Slightly less dense than air
Almost insoluble in water; soluble in organic solvents
Burns with a luminous, slightly smoky flame, because it has a higher proportion of carbon than ethane

\text{C}_2\text{H}_4 + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 2\text{H}_2\text{O}

\begin{gathered}\text{C}_2\text{H}_4 + 3\text{O}_2 \\[6pt] \rightarrow 2\text{CO}_2 + 2\text{H}_2\text{O}\end{gathered}

Worked example

Percentage of carbon. Compare ethane and ethene (C = 12, H = 1).

Ethane, C₂H₆ (Mᵣ = 30): % C $= 24 \div 30 \times 100 = 80\%$
Ethene, C₂H₄ (Mᵣ = 28): % C $= 24 \div 28 \times 100 = 85.7\%$

The higher carbon content of ethene explains its smokier flame.

In Sierra Leone

Traders at markets such as Dove Cot or Bo's main market sometimes ripen bananas, plantains and mangoes by wrapping them with a ripe fruit or storing them in closed sacks. The ripe fruit releases ethene, a plant hormone, which speeds up ripening of the rest. Commercial ripening rooms use controlled amounts of ethene gas.

Common mistake

The temperature matters in the ethanol and sulphuric acid reaction. At 170 °C (excess acid) ethene is formed; at 140 °C (excess ethanol) ethoxyethane is formed instead.

Key points

Alkenes, CₙH₂ₙ, contain one C=C (a σ and a π bond); they are unsaturated.
Atoms around the C=C are planar with angles of about 120°; there is no rotation about the double bond, which allows cis–trans isomerism.
Ethene is prepared by dehydrating ethanol with excess conc. H₂SO₄ at 170 °C (or over hot Al₂O₃); impurities are removed with NaOH(aq); collected over water.
Industrially, alkenes come from cracking.
Ethene burns with a smokier flame than ethane (higher % carbon).

WASSCE examination focus

WAEC tests this

Expect to describe the laboratory preparation of ethene (reagents, conditions, purification, collection and equation), to name and draw alkenes and their isomers, and to explain why the alkenes are unsaturated. The role of the concentrated sulphuric acid (dehydrating agent) and of the NaOH wash are frequently asked.

Quick check

Write the general formula of the alkenes.
What is the role of concentrated sulphuric acid in the preparation of ethene?
Why is the gas passed through sodium hydroxide solution?
Why does propene not show cis–trans isomerism?

Answers:

CₙH₂ₙ.
It is a dehydrating agent: it removes water from ethanol.
To remove the acidic impurities sulphur(IV) oxide and carbon(IV) oxide.
One carbon of its double bond carries two identical groups (two hydrogen atoms).

Alkenes: structure, preparation and properties