Oxidation, Polymerisation and Uses of Alkenes – Alkenes and Alkynes (Senior 5 Chemistry) | Ganzaa

Alkenes and Alkynes

Lessons

Senior 5 · Chemistry

Alkenes and Alkynes

5 lessons

Lesson 4 of 5

Oxidation, Polymerisation and Uses of Alkenes

25 min

A country without plastic bags

Rwanda banned polythene carry bags in 2008, and Law N° 17 of 2019 extended the ban to most single-use plastic items, enforced by the Rwanda Environment Management Authority (REMA). The bags were banned because poly(ethene) is so unreactive that it lasts for centuries in soil and water. Yet it is made from ethene, one of the most reactive hydrocarbons in industry. Polymerisation is one of several ways the C=C bond is used up; this lesson covers the others: oxidation, cleavage and hydrogenation.

Definition

Oxidative cleavage: a reaction that breaks a C=C bond completely, turning each of its carbons into a C=O group.

Ozonolysis: oxidative cleavage by ozone, $\mathrm{O_3}$ , followed by zinc and water; it gives aldehydes and ketones.

Addition polymerisation: the joining of many unsaturated monomer molecules, through their C=C bonds, into one long saturated chain, with no other product. Monomer: the small starting molecule. Repeat unit: the part of the chain that repeats.

Oxidation by potassium manganate(VII)

Cold, dilute manganate(VII) adds an –OH group to each carbon of the C=C, giving a diol. In acid the purple colour disappears; in alkaline solution a brown precipitate of manganese(IV) oxide forms. This is a second test for C=C (but other easily oxidised compounds, such as aldehydes, also decolourise it):
$\mathrm{CH_2{=}CH_2 + [O] + H_2O \rightarrow HOCH_2CH_2OH}$ (ethane-1,2-diol). Here [O] stands for oxygen supplied by the oxidising agent.
Hot, concentrated, acidified manganate(VII) breaks the C=C completely. A $\mathrm{{=}CR_2}$ end becomes a ketone; a $\mathrm{{=}CHR}$ end becomes a carboxylic acid (the aldehyde first formed is oxidised further); a $\mathrm{{=}CH_2}$ end becomes $\mathrm{CO_2}$ and water.

Potassium manganate(VII) solutions of two concentrations in volumetric flasks. Shaken with an alkene, the purple colour fades. Photo: Pradana Aumars, CC0, via Wikimedia Commons

Ozonolysis: finding the double bond

Ozone adds across the C=C to form an unstable ozonide; zinc and water then break it into two carbonyl compounds. Because the aldehyde is not oxidised further, the products show exactly what was on each side of the double bond.

Figure 4.1: Breaking a C=C bond. Each carbon of the double bond becomes a C=O group, so the products show where the double bond was.

Other oxidations used in industry: ethene and oxygen over a silver catalyst at about 250 °C give epoxyethane, a three-membered ring with an oxygen atom, which reacts with water to give ethane-1,2-diol for antifreeze and for making polyester (PET) bottles.

Hydrogenation

Hydrogen adds across C=C over nickel (Unit 2). The food industry hardens liquid vegetable oils this way: removing some C=C bonds raises the melting point and gives spreadable margarine. Partial hydrogenation also turns some cis C=C bonds into trans ones, and these "trans fats" raise the risk of heart disease; the World Health Organization has called for industrially produced trans fats to be eliminated from food.

Addition polymerisation

Figure 4.2: n monomer molecules join into one long chain. The repeat unit is drawn in brackets with "extension" bonds through them.

The reaction is started by a small amount of an initiator that forms free radicals, and it needs heat and, for poly(ethene), high pressure or a special catalyst. The polymer is saturated, so it has the low reactivity of an alkane: this makes it durable and safe for food and water, and it is also why it does not biodegrade. Burning PVC releases hydrogen chloride and toxic dioxins, so polymers should be reused or recycled, not burned in the open.

Table 4.1: Common addition polymers

Monomer	Polymer	Repeat unit	Typical uses
ethene, $\mathrm{CH_2{=}CH_2}$	poly(ethene), polythene	$\mathrm{{-}CH_2{-}CH_2{-}}$	bags and film (low density); bottles, crates, pipes (high density)
propene, $\mathrm{CH_2{=}CHCH_3}$	poly(propene)	$\mathrm{{-}CH_2{-}CH(CH_3){-}}$	ropes, woven sacks, food tubs, carpets
chloroethene, $\mathrm{CH_2{=}CHCl}$	poly(chloroethene), PVC	$\mathrm{{-}CH_2{-}CHCl{-}}$	water and drain pipes, cable insulation, window frames
phenylethene, $\mathrm{CH_2{=}CHC_6H_5}$	poly(phenylethene), polystyrene	$\mathrm{{-}CH_2{-}CH(C_6H_5){-}}$	packaging foam, disposable cups
tetrafluoroethene, $\mathrm{CF_2{=}CF_2}$	PTFE	$\mathrm{{-}CF_2{-}CF_2{-}}$	non-stick pan coatings, plumber's tape

Worked example

Problem. An alkene X, $\mathrm{C_5H_{10}}$ , undergoes ozonolysis (then Zn and water) to give ethanal, $\mathrm{CH_3CHO}$ , and propanone, $\mathrm{CH_3COCH_3}$ . Identify X, and predict the products if X were instead heated with concentrated acidified manganate(VII).

Solution. Remove the two C=O oxygen atoms and join the two carbonyl carbons with a double bond: $\mathrm{CH_3CH{=}C(CH_3)_2}$ , 2-methylbut-2-ene (check: 5 C, 10 H). With hot manganate(VII), the $\mathrm{{=}C(CH_3)_2}$ end still gives propanone, but the $\mathrm{{=}CHCH_3}$ end is oxidised past ethanal to ethanoic acid.

Worked example

Problem. (a) Write the repeat unit of poly(propene). (b) A poly(propene) chain has a relative molecular mass of 42 000. How many monomer units does it contain?

Solution. (a) Open the C=C of $\mathrm{CH_2{=}CHCH_3}$ and join the units: $\mathrm{{-}[CH_2{-}CH(CH_3)]_n{-}}$ . The methyl group is a side branch, not part of the main chain.

(b) $M_r(\mathrm{C_3H_6}) = 3 \times 12.0 + 6 \times 1.0 = 42.0$ ; $n = \frac{42\,000}{42.0} = 1000$ units.

Warning

The repeat unit has NO double bond, and only the two C=C carbons go into the main chain. A common error is writing poly(propene) as $\mathrm{{-}CH_2CH_2CH_2{-}}$ , which puts the methyl carbon in the chain. Keep every other group on the side.

Tip

Ozonolysis backwards: write the two products with their C=O groups facing each other, cross out both O atoms, and join the carbons with "=". That is the original alkene.

Professor Dave Explains: Ozonolysis (3 min). Watch how ozone breaks the C=C bond and how the carbonyl products reveal where the double bond was in the starting alkene.

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Practice

Name the product when propene reacts with cold, dilute, acidified potassium manganate(VII), and describe the colour change.
Name the products of ozonolysis (then Zn, $\mathrm{H_2O}$ ) of (a) but-2-ene; (b) 2,3-dimethylbut-2-ene; (c) but-1-ene.
An alkene $\mathrm{C_6H_{12}}$ gives only propanone on ozonolysis. Identify it.
What products form when but-1-ene is heated with concentrated acidified manganate(VII)?
Draw the repeat unit of PTFE and name its monomer.
Explain why poly(ethene) is unreactive although ethene is reactive.
Why is hydrogenation used to make margarine from vegetable oil, and what is the health concern?

Show answersHide answers

Propane-1,2-diol, $\mathrm{CH_3CH(OH)CH_2OH}$ ; purple to colourless.
(a) Ethanal only (two molecules). (b) Propanone only. (c) Propanal and methanal.
2,3-dimethylbut-2-ene, $\mathrm{(CH_3)_2C{=}C(CH_3)_2}$ .
Propanoic acid, carbon dioxide and water (the $\mathrm{{=}CH_2}$ end becomes $\mathrm{CO_2}$ ).
$\mathrm{{-}[CF_2{-}CF_2]_n{-}}$ ; tetrafluoroethene, $\mathrm{CF_2{=}CF_2}$ .
Polymerisation uses up every C=C bond; the polymer is a long saturated chain with only strong, non-polar C–C and C–H bonds, like an alkane.
Adding hydrogen removes some C=C bonds and raises the melting point, so the oil becomes a soft solid. Partial hydrogenation also forms trans fats, which raise the risk of heart disease.

Sources and credits

Written by Ganzaa.
Rwanda's bans: Rwanda Environment Management Authority (REMA), which enforces the 2008 ban on polythene bags and Law N° 17 of 2019 on plastic carry bags and single-use plastic items.
Trans fats: Trans fat fact sheet, World Health Organization.
Image: Two solutions of potassium permanganate with different concentrations.jpg by Pradana Aumars, via Wikimedia Commons, dedicated to the public domain under CC0 1.0.
Image: Bundled PVC pipes for drainage in Awka.jpg by Johnnybam, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: Ozonolysis, Professor Dave Explains, embedded from YouTube.
The text, diagrams, table, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A country without plastic bags

2Oxidation by potassium manganate(VII)

3Ozonolysis: finding the double bond

4Hydrogenation

5Addition polymerisation

A country without plastic bags

Oxidation by potassium manganate(VII)

Ozonolysis: finding the double bond

Hydrogenation

Addition polymerisation