Alkenes and Alkynes

Lessons

Senior 5 · Chemistry

Alkenes and Alkynes

5 lessons

Lesson 5 of 5

Alkynes: Structure, Preparation and Reactions

25 min

A gas that is stored dissolved

Ethyne, $\mathrm{C_2H_2}$ , is the fuel of the welding torch you met in Senior 3, and it is also burned in the flame of laboratory instruments that measure metals in water and soil. It is never sold as a compressed gas on its own: under pressure it can decompose explosively into carbon and hydrogen. Instead a cylinder of ethyne is packed with a porous solid soaked in propanone (acetone), and the ethyne is dissolved in the liquid. Both the stored energy and the reactivity come from the triple bond.

Definition

Alkyne: an unsaturated hydrocarbon containing a C≡C triple bond; general formula $\mathrm{C_nH_{2n-2}}$ . Example: propyne, $\mathrm{CH_3C{\equiv}CH}$ .

Terminal alkyne: one with the triple bond at the end of the chain, so that it carries a hydrogen atom, $\mathrm{R{-}C{\equiv}C{-}H}$ . Internal alkyne: one with the triple bond inside the chain, $\mathrm{R{-}C{\equiv}C{-}R'}$ .

Tautomers: structural isomers that interconvert rapidly by the movement of a hydrogen atom and a double bond. Example: ethenol and ethanal (keto–enol tautomerism).

Structure, names and isomers

Each carbon of C≡C is $s p$ hybridised, so the two carbons and the atoms joined to them lie in a straight line (180°). The triple bond is one σ bond and two π bonds at right angles to each other, giving a cylinder of electron density round the axis. It is short (120 pm) and strong (838 kJ mol⁻¹), but the π bonds still break more easily than σ bonds.

Names use "-yne" with the locant of the first triple-bond carbon: but-1-yne, $\mathrm{HC{\equiv}CCH_2CH_3}$ ; but-2-yne, $\mathrm{CH_3C{\equiv}CCH_3}$ ; 3-methylbut-1-yne, $\mathrm{HC{\equiv}CCH(CH_3)_2}$ . A molecule with both bonds is an "enyne": $\mathrm{CH_2{=}CHC{\equiv}CH}$ is but-1-en-3-yne. Alkynes show chain and position isomerism, and are functional-group isomers of dienes and cycloalkenes ( $\mathrm{C_4H_6}$ : but-1-yne, but-2-yne, buta-1,3-diene, cyclobutene…). Because the C≡C unit is linear, alkynes have no E/Z isomers about the triple bond.

Preparation

Calcium carbide and water (Senior 3): $\mathrm{CaC_2(s) + 2H_2O(l) \rightarrow C_2H_2(g) + Ca(OH)_2(aq)}$ .
Double elimination: a 1,2-dihalogenoalkane heated with excess KOH in ethanol (or with sodium amide, $\mathrm{NaNH_2}$ ) loses two molecules of HX: $\mathrm{CH_2BrCH_2Br + 2KOH \xrightarrow{\text{ethanol, heat}} HC{\equiv}CH + 2KBr + 2H_2O}$ . Since a 1,2-dibromide is made by adding $\mathrm{Br_2}$ to an alkene, this turns an alkene into an alkyne.
Industry: methane is heated very briefly to about 1500 °C and quenched: $\mathrm{2CH_4(g) \rightarrow C_2H_2(g) + 3H_2(g)}$ .

Physical properties

The small alkynes are colourless gases (ethyne, propyne, but-1-yne); their boiling points are a little higher than those of the alkenes with the same number of carbons, because the linear molecules pack closely. They are almost insoluble in water but dissolve well in organic solvents such as propanone.

Reactions

Figure 5.1: The main reactions of a terminal alkyne, using propyne as the example.

Hydrogenation. Over nickel, two molecules of $\mathrm{H_2}$ add and an alkane forms. Over a Lindlar catalyst (palladium partly "poisoned" with lead compounds), the reaction stops at the alkene, and both H atoms add from the same side, giving the Z (cis) alkene: but-2-yne gives (Z)-but-2-ene.
Halogens and hydrogen halides add in two steps. Bromine gives 1,2-dibromoalkenes, then 1,1,2,2-tetrabromoalkanes. HBr follows Markovnikov's rule both times, so both Br atoms end up on the same carbon: $\mathrm{CH_3C{\equiv}CH + 2HBr \rightarrow CH_3CBr_2CH_3}$ (2,2-dibromopropane).
Hydration. With dilute sulfuric acid and a mercury(II) sulfate catalyst, water adds once. The product, an enol, rearranges at once to a carbonyl compound (Figure 5.2). Ethyne gives ethanal; every other alkyne gives a ketone (propyne gives propanone, by Markovnikov addition).
Acidity of terminal alkynes. An $s p$ orbital has 50% s character, so its electrons are held close to the carbon nucleus; the C–H bond of a terminal alkyne can lose $\mathrm{H^+}$ to a strong base, leaving a stable carbanion ( $\mathrm{p}K_a \approx 25$ , far more acidic than alkanes and alkenes, though far less than water). Sodium reacts: $\mathrm{2HC{\equiv}CH + 2Na \rightarrow 2HC{\equiv}C^-Na^+ + H_2}$ . Ammoniacal silver nitrate gives a white precipitate, and ammoniacal copper(I) chloride a red-brown one:
$\mathrm{HC{\equiv}CH(g) + 2[Ag(NH_3)_2]^+(aq) \rightarrow AgC{\equiv}CAg(s) + 2NH_4^+(aq) + 2NH_3(aq)}$
Internal alkynes have no such H and give no precipitate.
Polymerisation. Passed through a red-hot iron tube, three ethyne molecules join into a ring: $\mathrm{3C_2H_2 \rightarrow C_6H_6}$ (benzene, Senior 6).

Figure 5.2: The enol formed first is unstable and tautomerises to the carbonyl compound. The equilibrium lies almost entirely on the side of ethanal.

Table 5.1: Telling hydrocarbons apart

Test	alkane (hexane)	alkene (hex-1-ene)	terminal alkyne (hex-1-yne)	internal alkyne (hex-3-yne)
bromine water	stays orange	decolourised	decolourised	decolourised
cold acidified $\mathrm{KMnO_4}$	stays purple	decolourised	decolourised	decolourised
ammoniacal silver nitrate	no change	no change	white precipitate	no change
ammoniacal copper(I) chloride	no change	no change	red-brown precipitate	no change

Uses

Oxy-ethyne welding and cutting; fuel for atomic absorption spectrometers; making chloroethene (for PVC) by adding HCl, and ethanal and ethanoic acid by hydration; synthetic rubbers. Propyne mixed with propadiene is a welding gas too.

Worked example

Problem. Three unlabelled bottles hold but-1-yne, but-2-yne and but-1-ene. Describe tests to identify each.

Solution. Add bromine water to a sample of each: all three decolourise it, so this test alone cannot separate them. Add ammoniacal silver nitrate: only but-1-yne, a terminal alkyne, gives a white precipitate. Of the other two, take a known mass and measure the hydrogen or bromine taken up: but-2-yne adds two moles of $\mathrm{Br_2}$ per mole, but-1-ene only one. (Ozonolysis also works: but-1-ene gives propanal and methanal.)

Worked example

Problem. Give the main organic product when propyne reacts with: (a) excess HBr; (b) water, sulfuric acid and mercury(II) sulfate; (c) hydrogen over a Lindlar catalyst. In (b), explain why the product has no –OH group.

Solution. (a) Markovnikov twice: 2,2-dibromopropane, $\mathrm{CH_3CBr_2CH_3}$ . (b) Water adds by Markovnikov's rule, putting –OH on C2: the enol $\mathrm{CH_3C(OH){=}CH_2}$ . An enol is unstable: the H on oxygen moves to C1 and the double bond moves to C=O, giving propanone, $\mathrm{CH_3COCH_3}$ . (c) One molecule of $\mathrm{H_2}$ adds: propene (propene has no E/Z isomers, so here the cis addition makes no difference).

Warning

Safety. Ethyne is extremely flammable over a very wide range of mixtures with air. Dry silver and copper(I) ethynides explode when struck or heated: in the test, keep the precipitate wet and destroy it straight away with dilute nitric acid. Mercury(II) salts are toxic and must be collected for safe disposal, never poured down the sink.

Tip

Only a C≡C–H end gives a precipitate with ammoniacal $\mathrm{Ag^+}$ or $\mathrm{Cu^+}$ . Bromine water and manganate(VII) cannot tell an alkene from an alkyne.

Organic Chemistry with Victor: Hydration of Alkynes = Markovnikov Addition of Water | Keto-Enol Tautomerism (9 min). Watch how the enol formed by Markovnikov addition rearranges into a ketone.

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Practice

Name: (a) $\mathrm{CH_3C{\equiv}CCH_2CH_3}$ ; (b) $\mathrm{HC{\equiv}CCH(CH_3)_2}$ ; (c) $\mathrm{CH_2{=}CHC{\equiv}CH}$ .
How many alkynes have the formula $\mathrm{C_5H_8}$ ? Name them.
Write an equation for making propyne from 1,2-dibromopropane, with conditions.
Name the product of but-2-yne with hydrogen over a Lindlar catalyst, including its stereochemistry.
Name the product of hydrating but-1-yne with dilute sulfuric acid and mercury(II) sulfate.
Describe a test that distinguishes pent-1-yne from pent-2-yne, with the result for each.
Calculate the volume of ethyne, at room temperature and pressure, made from 3.20 g of calcium carbide ( $M = 64.1\ \mathrm{g\,mol^{-1}}$ ); molar volume $24.0\ \mathrm{dm^3\,mol^{-1}}$ .

Show answersHide answers

(a) pent-2-yne; (b) 3-methylbut-1-yne; (c) but-1-en-3-yne.
Three: pent-1-yne, pent-2-yne and 3-methylbut-1-yne.
$\mathrm{CH_3CHBrCH_2Br + 2KOH \rightarrow CH_3C{\equiv}CH + 2KBr + 2H_2O}$ ; excess KOH in ethanol, heated (or sodium amide).
(Z)-but-2-ene (cis-but-2-ene).
Butanone, $\mathrm{CH_3COCH_2CH_3}$ (Markovnikov addition gives the enol with –OH on C2, which tautomerises).
Add ammoniacal silver nitrate: pent-1-yne gives a white precipitate (it has a C≡C–H end); pent-2-yne gives none.
$n = \frac{3.20}{64.1} = 0.0499$ mol; $V = 0.0499 \times 24.0 = 1.20\ \mathrm{dm^3}$ .

1A gas that is stored dissolved

2Structure, names and isomers

3Preparation

4Physical properties

5Reactions

6Uses