Alkenes and Alkynes

Lessons

Senior 3 · Chemistry

Alkenes and Alkynes

5 lessons

Lesson 5 of 5

Properties and Uses of Alkynes

30 min

A flame that cuts steel

A metal workshop can cut a thick steel plate in a few seconds with a torch that burns ethyne in pure oxygen. The flame is hotter than 3000 °C, hot enough to melt steel, and a jet of extra oxygen then burns the hot iron away along the cut. No other common fuel gas burns as hot. The reasons lie in the triple bond, which also makes alkynes even more reactive in addition reactions than alkenes.

Physical properties of ethyne

A colourless gas. Pure ethyne has only a faint smell; the garlic smell of laboratory ethyne comes from impurities.
Slightly soluble in water, but it can still be collected over water.
Slightly less dense than air ( $M_{r} = 26$ , compared with about 29 for air).
Compressed ethyne can explode on its own, so gas cylinders hold it dissolved in propanone (acetone) soaked into a porous filling.

The higher alkynes follow the usual pattern: insoluble in water, less dense than water, and boiling points rising with chain length.

Combustion

In air, ethyne burns with a very bright, very smoky flame, because 92.3% of its mass is carbon ( $\frac{24}{26} \times 100$ ), more than in ethene (85.7%) or ethane (80.0%). Burned with enough pure oxygen, it burns completely and gives the hottest flame of any common fuel:

\mathrm{2C_2H_2(g) + 5O_2(g) \rightarrow 4CO_2(g) + 2H_2O(l)}

Addition reactions, in two steps

A triple bond can open twice, so an alkyne adds two molecules of a reagent. The first addition leaves a C=C double bond, which can then add a second molecule.

Hydrogen (nickel catalyst, heat)

\mathrm{C_2H_2(g) + H_2(g) \rightarrow C_2H_4(g)}

\mathrm{C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)}

Bromine

\mathrm{C_2H_2(g) + Br_2(aq) \rightarrow CHBr{=}CHBr(l)}

\mathrm{CHBr{=}CHBr(l) + Br_2(aq) \rightarrow CHBr_2CHBr_2(l)}

Bromine water is decolourised, just as with an alkene. Acidified potassium manganate(VII) is also decolourised.

Hydrogen chloride

\mathrm{C_2H_2(g) + HCl(g) \rightarrow CH_2{=}CHCl(g)}

The product, chloroethene, is the monomer of PVC. The reaction needs a catalyst.

Definition

Terminal alkyne: an alkyne whose triple bond is at the end of the chain, so that a hydrogen atom is bonded to a triple-bonded carbon. Example: ethyne, propyne and but-1-yne; but-2-yne is not terminal.

Ammoniacal copper(I) chloride: copper(I) chloride dissolved in aqueous ammonia, a reagent that gives a red-brown precipitate with terminal alkynes. Example: ethyne bubbled into it.

Telling an alkyne from an alkene

Bromine water cannot tell ethene from ethyne: both decolourise it. The hydrogen atoms on a triple-bonded carbon, however, can be replaced by metal atoms. Bubble the gas into ammoniacal copper(I) chloride: ethyne gives a red-brown precipitate of copper(I) dicarbide. With ammoniacal silver nitrate it gives a white precipitate. Ethene and the alkanes give no precipitate.

Table 5.1: Alkanes, alkenes and alkynes compared

	Alkanes	Alkenes	Alkynes
general formula	CₙH₂ₙ₊₂	CₙH₂ₙ	CₙH₂ₙ₋₂
carbon–carbon bonds	single only	one C=C	one C≡C
shape at that bond	tetrahedral, 109.5°	flat, 120°	straight line, 180°
saturated?	yes	no	no
typical reaction	substitution (needs light)	addition	addition, in two steps
bromine water	stays orange	decolourised quickly	decolourised quickly
acidified KMnO₄	stays purple	decolourised	decolourised
ammoniacal copper(I) chloride	no change	no change	red-brown precipitate (ethyne and other alkynes with C≡C at the chain end)
flame in air	blue, little soot	yellow, some soot	very bright and very sooty

Warning

Safety. The copper and silver precipitates formed with ethyne explode when they are dry and are struck or heated. Make only a test-tube amount, keep it wet, and destroy it with dilute acid as your teacher directs. Never use copper pipes or brass fittings with high-pressure ethyne.

Worked example

Problem. Three gas jars hold methane, ethene and ethyne, but the labels have fallen off. Describe how to identify each gas with two tests.

Solution. (1) Shake a sample of each with a few drops of bromine water. The gas that leaves it orange is methane (saturated). (2) Bubble each of the other two into ammoniacal copper(I) chloride. The gas that gives a red-brown precipitate is ethyne; the one that gives no precipitate is ethene.

Worked example

Problem. (a) What volume of hydrogen is needed to turn 50 cm³ of ethyne completely into ethane, all volumes measured at the same temperature and pressure? (b) What mass of bromine adds to 1.3 g of ethyne to form 1,1,2,2-tetrabromoethane? (C = 12, H = 1, Br = 80)

Solution. (a) Overall $\mathrm{C_2H_2 + 2H_2 \rightarrow C_2H_6}$ . Equal volumes of gases contain equal numbers of moles, so the volume of hydrogen is $2 \times 50 = 100$ cm³.

(b) $n(\mathrm{C_2H_2}) = \frac{1.3}{26} = 0.050$ mol. Each mole adds 2 mol of $\mathrm{Br_2}$ : 0.10 mol. Mass $= 0.10 \times 160 = 16$ g.

Uses of ethyne

Table 5.2: Uses of ethyne

Use	Property used
oxy-acetylene welding and cutting of metals	burns in pure oxygen with a flame of over 3000 °C
making chloroethene, then PVC pipes and cable coating	adds hydrogen chloride across the triple bond
making other chemicals, such as ethanal and ethanoic acid	the reactive triple bond
carbide lamps, formerly used by miners and cavers	made easily from calcium carbide and water; bright flame

Warning

Common mistakes. (1) An alkyne adds two molecules of hydrogen or bromine for complete saturation, not one. (2) The copper(I) test works only for terminal alkynes: but-2-yne gives no precipitate, although it is an alkyne. (3) Ethyne is not the "most stable" fuel: its cylinders are special because compressed ethyne can explode.

Tip

Three tests, three answers. Bromine water separates saturated from unsaturated. Ammoniacal copper(I) chloride then separates terminal alkynes from alkenes. The sootiness of the flame gives a rough check: alkane least, alkyne most.

Anne Schmidt: Calcium Carbide and Water, and Combustion of Acetylene. Watch how quickly the gas forms when the carbide meets water, and how the gas burns.

✦

Practice

Give three physical properties of ethyne.
Write balanced equations, with state symbols, for (a) the complete combustion of ethyne; (b) the two-step addition of hydrogen to ethyne.
Explain why ethyne burns in air with a sootier flame than ethane. Use percentages by mass of carbon.
Name the products of the two-step reaction of ethyne with bromine.
Write the equation for the addition of hydrogen chloride to ethyne and give one use of the product.
Describe a chemical test that gives a positive result with ethyne but not with ethene, and give the observation.
Calculate the volume of hydrogen needed to convert 30 cm³ of propyne completely into propane (same temperature and pressure).
A student claims that bromine water can be used to tell ethene from ethyne. Evaluate the claim and suggest a better test.

Show answersHide answers

Colourless gas; slightly soluble in water; slightly less dense than air; nearly odourless when pure.
(a) $\mathrm{2C_2H_2(g) + 5O_2(g) \rightarrow 4CO_2(g) + 2H_2O(l)}$ ; (b) $\mathrm{C_2H_2(g) + H_2(g) \rightarrow C_2H_4(g)}$ , then $\mathrm{C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)}$ , with a nickel catalyst.
Ethyne is 92.3% carbon, ethane 80.0%. In air there is not enough oxygen to burn all of that carbon, so some leaves the flame as soot.
1,2-dibromoethene, then 1,1,2,2-tetrabromoethane.
$\mathrm{C_2H_2(g) + HCl(g) \rightarrow CH_2{=}CHCl(g)}$ ; chloroethene is polymerised to PVC for water pipes.
Bubble the gas into ammoniacal copper(I) chloride: ethyne gives a red-brown precipitate; ethene gives none. (Or ammoniacal silver nitrate: a white precipitate.)
$\mathrm{C_3H_4 + 2H_2 \rightarrow C_3H_8}$ : $2 \times 30 = 60$ cm³.
The claim is wrong: both are unsaturated and both decolourise bromine water quickly. Use ammoniacal copper(I) chloride: only ethyne gives a red-brown precipitate.

1A flame that cuts steel

2Physical properties of ethyne

3Combustion

4Addition reactions, in two steps