Alkynes: Structure, Isomers and the Preparation of Ethyne – Alkenes and Alkynes (Senior 3 Chemistry) | Ganzaa

Alkenes and Alkynes

Lessons

Senior 3 · Chemistry

Alkenes and Alkynes

5 lessons

Lesson 4 of 5

Alkynes: Structure, Isomers and the Preparation of Ethyne

35 min

A lamp that runs on water and stone

Before electric head torches, miners and cave explorers carried brass lamps with two chambers. Water from the top chamber dripped slowly onto grey lumps of calcium carbide in the bottom one. The gas that formed hissed out of a small jet in front of a polished reflector and burned with a bright white flame. That gas is ethyne, also called acetylene, the first member of the alkynes.

A lit brass carbide lamp made by Justrite. The flame burns at a small jet in the centre of a round polished reflector; the water and carbide chambers form the body behind it. Photo: Scott Ehardt, public domain, via Wikimedia Commons.

Definition

Alkyne: a hydrocarbon that contains one carbon–carbon triple bond, C≡C. Alkynes with one triple bond have the general formula $\mathrm{C_nH_{2n-2}}$ , where n is 2 or more. Example: ethyne, $\mathrm{C_2H_2}$ .

Triple bond: a covalent bond in which two atoms share three pairs of electrons. Example: the C≡C bond in ethyne.

Structure and shape

Each carbon of a triple bond has used three of its four bonds on the other carbon, so it can hold only one more atom. An alkyne therefore has four hydrogen atoms fewer than the alkane with the same number of carbons: ethane $\mathrm{C_2H_6}$ , ethene $\mathrm{C_2H_4}$ , ethyne $\mathrm{C_2H_2}$ . The two bonds around each triple-bonded carbon point in opposite directions, so the atoms H–C≡C–H lie in a straight line, with bond angles of 180°.

Naming alkynes

Use the same rules as for alkenes, with the ending -yne: find the longest chain containing the triple bond, number it from the end nearer the triple bond, and put the number of the first triple-bonded carbon before -yne. Ethyne and propyne need no number.

Table 4.1: The first alkynes, with the triple bond at carbon 1 (C = 12, H = 1)

n	Name	Molecular formula	Condensed formula	$M_{r}$	Boiling point / °C
2	ethyne	$\mathrm{C_2H_2}$	$\mathrm{HC{\equiv}CH}$	26	−84 (sublimes)
3	propyne	$\mathrm{C_3H_4}$	$\mathrm{HC{\equiv}CCH_3}$	40	−23
4	but-1-yne	$\mathrm{C_4H_6}$	$\mathrm{HC{\equiv}CCH_2CH_3}$	54	8
5	pent-1-yne	$\mathrm{C_5H_8}$	$\mathrm{HC{\equiv}C(CH_2)_2CH_3}$	68	40
6	hex-1-yne	$\mathrm{C_6H_{10}}$	$\mathrm{HC{\equiv}C(CH_2)_3CH_3}$	82	71

As in the other series, the boiling point rises with chain length; ethyne turns straight from solid to gas at −84 °C.

Worked example

Problem. Name: (a) $\mathrm{HC{\equiv}CCH_2CH_3}$ ; (b) $\mathrm{CH_3C{\equiv}CCH_3}$ ; (c) $\mathrm{CH_3C{\equiv}CCH_2CH_3}$ ; (d) $\mathrm{HC{\equiv}CCH(CH_3)CH_3}$ .

Solution. (a) Four carbons, triple bond from carbon 1: but-1-yne. (b) Four carbons, triple bond between carbons 2 and 3: but-2-yne. (c) Five carbons; from the left the triple bond starts at carbon 2, from the right at carbon 3: pent-2-yne. (d) Four-carbon chain, triple bond at carbon 1 and a $\mathrm{CH_3}$ branch on carbon 3: 3-methylbut-1-yne.

Structural isomerism in alkynes

Alkynes show the same two kinds of structural isomerism as alkenes. $\mathrm{C_4H_6}$ has two alkyne isomers, but-1-yne and but-2-yne: position isomers. $\mathrm{C_5H_8}$ has three: pent-1-yne and pent-2-yne (position isomers) and 3-methylbut-1-yne (a chain isomer of both). There is no "2-methylbut-1-yne": carbon 2 already has three bonds to carbon 1 and one to carbon 3, so it has no room for a branch.

Because the triple bond holds its neighbours in a straight line, alkynes do not have cis and trans forms.

Making ethyne in the laboratory

Ethyne is made by adding water to calcium carbide:

\mathrm{CaC_2(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + C_2H_2(g)}

Spread a layer of sand in a conical flask and put a few small lumps of calcium carbide on it. The reaction gives out heat, and the sand protects the glass.
Let water drip slowly from a dropping funnel. Controlling the drops controls the rate of gas production.
Collect the gas over water (Figure 4.3) and let the first jar escape, as it is mostly air.
Commercial carbide contains traces of compounds that give phosphine and hydrogen sulfide, which cause a garlic-like smell. Bubbling the gas through acidified copper(II) sulfate solution removes them.

The cloudy liquid left in the flask is calcium hydroxide. Industry makes calcium carbide by heating calcium oxide with coke at about 2000 °C in an electric furnace:

\mathrm{CaO(s) + 3C(s) \rightarrow CaC_2(s) + CO(g)}

Calcium carbide, CaC₂, as a pale powder on a watch glass. Technical calcium carbide is usually sold as grey lumps. Photo: Ondřej Mangl, public domain, via Wikimedia Commons.

Warning

Safety. Mixtures of ethyne and air explode when lit over a very wide range of proportions: no flames near the apparatus, and test the gas only in a small tube held at arm's length. Store calcium carbide in a sealed, dry tin, because moist air releases ethyne. Do not let ethyne come into contact with copper or silver salts except in the small test-tube tests of the next lesson.

Worked example

Problem. What volume of ethyne at room temperature and pressure is made from 3.2 g of calcium carbide, and what mass of water reacts? (Ca = 40, C = 12, H = 1, O = 16; 24 dm³ per mole at r.t.p.)

Solution. $M_r(\mathrm{CaC_2}) = 40 + 24 = 64$ , so $n = \frac{3.2}{64} = 0.050$ mol. One mole of $\mathrm{CaC_2}$ gives one mole of $\mathrm{C_2H_2}$ : volume $= 0.050 \times 24 = 1.2$ dm³. It uses two moles of water: $0.10 \times 18 = 1.8$ g.

Warning

Common mistakes. (1) The general formula is $\mathrm{C_nH_{2n-2}}$ : the alkyne with 5 carbons is $\mathrm{C_5H_8}$ , not $\mathrm{C_5H_{10}}$ . (2) The equation for calcium carbide needs two water molecules. (3) A carbon in a triple bond can carry only one more atom: never draw a branch or two hydrogens on it.

Tip

Ane, ene, yne. Each step from single to double to triple bond removes two hydrogen atoms and lowers the relative molecular mass by 2: ethane 30, ethene 28, ethyne 26.

The Organic Chemistry Tutor: Naming Alkynes, IUPAC Nomenclature and Common Names. Watch how the chain is numbered from the end nearer the triple bond; the later examples with several branches go beyond this unit.

✦

Practice

Define an alkyne and give its general formula.
Write the molecular formula and draw the displayed formula of propyne. Describe the shape of the molecule around the triple bond.
Name: (a) $\mathrm{CH_3CH_2C{\equiv}CH}$ ; (b) $\mathrm{CH_3CH_2C{\equiv}CCH_3}$ ; (c) $\mathrm{CH_3C{\equiv}CCH_2CH_2CH_3}$ .
Draw and name the three alkyne isomers of $\mathrm{C_5H_8}$ . Which pairs are position isomers?
Explain why "2-methylbut-2-yne" cannot exist.
Write the equation, with state symbols, for the preparation of ethyne. Give two reasons why the water is added drop by drop.
Calculate the mass of calcium carbide needed to make 2.4 dm³ of ethyne at r.t.p. (Ca = 40, C = 12; 24 dm³ per mole)
An alkyne has $M_{r} = 82$ . Find its molecular formula. (C = 12, H = 1)

Show answersHide answers

A hydrocarbon with a C≡C triple bond; $\mathrm{C_nH_{2n-2}}$ .
$\mathrm{C_3H_4}$ ; $\mathrm{H{-}C{\equiv}C{-}CH_3}$ with every bond drawn. H, the two triple-bonded carbons and carbon 3 lie in a straight line (180°).
(a) but-1-yne; (b) pent-2-yne; (c) hex-2-yne.
Pent-1-yne, pent-2-yne, 3-methylbut-1-yne (Figure 4.2). Pent-1-yne and pent-2-yne are position isomers.
Carbon 2 would have three bonds to carbon 3, one to carbon 1 and one to the methyl group: five bonds. Carbon forms only four.
$\mathrm{CaC_2(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + C_2H_2(g)}$ . To control the rate of gas production, and to stop the flask overheating, since the reaction is exothermic.
$n = \frac{2.4}{24} = 0.10$ mol; $0.10 \times 64 = 6.4$ g.
$M_{r} = 12 n + 2 n - 2 = 14 n - 2 = 82$ , so $n = 6$ : $\mathrm{C_6H_{10}}$ , a hexyne.

Sources and credits

Photo: Carbide lamp lit, Scott Ehardt, via Wikimedia Commons, public domain (released by the author).
Photo: Karbid vápenatý, Ondřej Mangl, via Wikimedia Commons, public domain (released by the author).
Video: Naming Alkynes, IUPAC Nomenclature and Common Names, The Organic Chemistry Tutor, 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.

1A lamp that runs on water and stone

2Structure and shape

3Naming alkynes

4Structural isomerism in alkynes

5Making ethyne in the laboratory

A lamp that runs on water and stone

Structure and shape

Naming alkynes

Structural isomerism in alkynes

Making ethyne in the laboratory