Chemistry · Organic chemistry · Grade 10

Alkynes, benzene and styrene

What gas comes from calcium carbide and water, and why does it give a flame that melts metal?

The situation

In the workshop a welder opens two cylinders: oxygen in the blue one, acetylene in the white one. At the tip of the torch a tiny, dazzling sky-blue cone appears — next to it the temperature is about 3100 °C, and steel melts in an instant. Acetylene can be made in a school laboratory too: when water is dripped onto a lump of grey calcium carbide, gas “boils” out. And from acetylene, at high temperature, benzene can be made, and from benzene — styrene and polystyrene cups. What is the secret of the triple bond in the acetylene molecule and of the benzene ring?

Topic

Alkynes and aromatic hydrocarbons. Acetylene welding flame and the benzene ring: acetylene from carbide, benzene from acetylene, styrene from benzene

Key ideas

C≡C: σ + 2π, 180°
The benzene ring
Trimerisation, styrene

Formulas

C_{n} H_{2 n - 2}

Alkynes (one $C \equiv C$ : σ + 2π, 180°, sp); arenes — $C_{n} H_{2 n - 6}$ (the benzene ring).

CaC_{2} + 2 H_{2} O \overset{}{\to} C_{2} H_{2} ↑ + Ca (OH)_{2}

Acetylene in the laboratory: calcium carbide and water. 1 mol $CaC_{2}$ (64.1 g) → 22.4 L $C_{2} H_{2}$ (STP).

3 CH \equiv CH \overset{\overset{}{C_{act .}, 600^{\circ} C}}{\to} C_{6} H_{6}

Trimerisation (N.D. Zelinsky): benzene from three molecules of acetylene.

C_{6} H_{6} + Br_{2} \overset{\overset{}{FeBr_{3}}}{\to} C_{6} H_{5} Br + HBr

Substitution is typical of benzene: the ring is preserved, and Br replaces H.

C_{6} H_{5} - CH_{2} - CH_{3} \overset{\overset{}{Fe_{2} O_{3}, 600^{\circ} C}}{\to} C_{6} H_{5} - CH = CH_{2} + H_{2}

Styrene: dehydrogenation of ethylbenzene; styrene polymerises to give polystyrene.

Theory

Alkynes: structure, isomerism, naming (2.13)

Alkynes are hydrocarbons whose molecule contains one triple bond $C \equiv C$ , $C_{n} H_{2 n - 2}$ : ethyne (acetylene) $C_{2} H_{2}$ , propyne $C_{3} H_{4}$ , butyne $C_{4} H_{6}$ … The carbon atoms of the triple bond are sp-hybridised: two hybrid orbitals lie on one line (180°) and form σ-bonds, while the two unhybridised p-orbitals, perpendicular to each other, form two π-bonds — the π-cloud wraps around the bond axis like a cylinder. The length of $C \equiv C$ is 120 pm ( $C = C$ 134 pm, $C - C$ 154 pm). Naming: the “-ane” of the alkane name is replaced by “-yne”, and the position of the triple bond is given by a number: $CH \equiv C - CH_{2} - CH_{3}$ — but-1-yne, $CH_{3} - C \equiv C - CH_{3}$ — but-2-yne. Isomerism: of the carbon chain, of the position of the triple bond, and interclass — with alkadienes (but-1-yne, but-2-yne and buta-1,3-diene are all $C_{4} H_{6}$ ).

Acetylene: preparation, properties, uses (2.14)

Preparation: in the laboratory — by the action of water on calcium carbide (technical carbide has a foul smell, pure acetylene is almost odourless); in industry — by heating methane briefly to 1500 °C: $2 CH_{4} \overset{}{\to} C_{2} H_{2} + 3 H_{2}$ . Properties: it adds in two stages — hydrogen (Ni; first ethene, then ethane), bromine (bromine water is decolourised; 1 mol $C_{2} H_{2}$ takes up 2 mol $Br_{2}$ ), hydrogen chloride (vinyl chloride — the monomer of PVC), and water — the Kucherov reaction (1881, $Hg^{2 +}$ salts): ethanal (acetaldehyde) via unstable vinyl alcohol. It decolourises a $KMnO_{4}$ solution. The H atom at the end of a triple bond can be replaced by a metal: a precipitate of $AgC \equiv CAg$ forms from ammoniacal silver oxide solution — the qualitative test for a terminal triple bond. Over activated charcoal at 600 °C it trimerises to benzene (Zelinsky, 1927). In air it burns with a smoky flame, in oxygen with a flame of ≈ 3100 °C — used for welding and cutting metals. Safety: acetylene forms an explosive mixture with air from 2.5 % upwards — let the air escape from the apparatus before lighting; handle the carbide in a dry vessel and wear gloves; wear safety goggles.

The welding flame

In the torch, acetylene and oxygen mix. At about 1 volume of oxygen to 1 volume of acetylene the flame is neutral: a clear, bright inner cone, with a temperature of about 3100–3200 °C near its tip; the oxygen missing for complete combustion is taken from the air in the outer envelope ( $2 C_{2} H_{2} + 5 O_{2} \overset{}{\to} 4 CO_{2} + 2 H_{2} O$ , 1 mol of acetylene ≈ 1300 kJ). If there is excess acetylene the flame is carburising (a white “feather” around the cone, lower temperature, and the weld becomes brittle as it is saturated with carbon); if there is excess oxygen the flame is oxidising (a short, noisy cone, the hottest, but it oxidises the steel). Steel melts at about 1400–1500 °C, so the neutral flame is considerably hotter than that. By GOST standards, oxygen cylinders are painted blue and acetylene cylinders white.

Benzene and arenes: structure (2.15)

Aromatic hydrocarbons (arenes) contain the benzene ring; their homologous series is $C_{n} H_{2 n - 6}$ : benzene $C_{6} H_{6}$ , toluene (methylbenzene) $C_{7} H_{8}$ , ethylbenzene and the xylenes $C_{8} H_{10}$ . Benzene was isolated by M. Faraday in 1825 from the residue of illuminating gas, and the ring formula was proposed by A. Kekulé in 1865. In Kekulé's formula double and single bonds alternate, but in reality all six C–C bonds are the same — 139 pm (between a single and a double bond): the six carbon atoms are sp²-hybridised and the six p-orbitals form a single ring-shaped π-cloud (6 π-electrons). Arenes with two substituents show substituent position isomerism: ortho (1,2), meta (1,3), para (1,4) — for example o-, m- and p-xylene.

Properties of benzene; styrene (2.16–2.17)

The ring is stable: benzene does not decolourise bromine water or a $KMnO_{4}$ solution (if shaken with bromine water, the bromine passes into the benzene layer — it turns orange, but no reaction takes place). Substitution is typical of it: bromination ( $FeBr_{3}$ ), nitration ( $HNO_{3} + H_{2} SO_{4}$ → nitrobenzene), alkylation (ethene, $AlCl_{3}$ → ethylbenzene). Addition occurs only under severe conditions: hydrogenation (Ni, t, p → cyclohexane), chlorination in light ( $C_{6} H_{6} Cl_{6}$ ). Cyclohexene is hydrogenated with 120 kJ/mol of heat; a “Kekulé” ring with three double bonds should have given 360 kJ/mol, but benzene gives only 208 kJ/mol — the ring is more stable by ≈ 150 kJ/mol. In toluene the ring activates the methyl group: when heated, $KMnO_{4}$ oxidises it to benzoic acid. In industry benzene is obtained from petroleum reforming and from coal tar; it can also be made by trimerising acetylene. Styrene $C_{6} H_{5} - CH = CH_{2}$ is made by dehydrogenating ethylbenzene; because of the double bond in the side chain it decolourises bromine water and polymerises: polystyrene (disposable containers, foam plastic), and with butadiene — butadiene–styrene rubber. Safety: benzene is carcinogenic (Group 1 according to the International Agency for Research on Cancer — IARC): it is handled only in a fume cupboard and in very small amounts; in schools it is often replaced by toluene.

Problems and limits of the model (2.23)

Example: how many litres of acetylene can be obtained from 6.4 g of $CaC_{2}$ ? $n = 6.4 / 64 = 0.1$ mol, $V = 0.1 \cdot 22.4 = 2.24$ L (STP). How many litres of $O_{2}$ are used when 0.56 L of acetylene burns? $2 C_{2} H_{2} + 5 O_{2}$ : $0.025 \cdot 2.5 = 0.0625$ mol, 1.4 L. Model: the carbide is taken to be pure (technical carbide is not pure — it contains lime and other impurities, so it gives less gas); each drop (0.05 mL) of water reacts completely with the carbide, and the gas volume is shown in the gasometer corrected to standard conditions; in the test tubes bromine and permanganate are used up instantly and completely. The flame temperatures are approximate values from welding handbooks (sources differ by ±100 °C). The molecules are drawn with real bond lengths and angles, at a scale of 1 Å ≈ 4.5 cm; the reaction animations are slowed down and the mechanisms are simplified.

Did you know?

Benzene was isolated in 1825 by Michael Faraday in London from an oily residue that had collected in illuminating-gas cylinders, and he called it “bicarburet of hydrogen”. The ring formula was published by August Kekulé in 1865.
At the end of 2019, a complex producing 100 thousand tonnes of polyvinyl chloride (PVC) a year went into operation at the Navoiyazot plant in Navoi; it also includes production of caustic soda and acetylene.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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Alkenes and alkadienes — Grade 10
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Alcohols and ethers — Grade 10
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Acids, esters and soap — Grade 10
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