IChO 1969 · Theory problem 4

Products obtainable from calcium carbide and water

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Calcium carbide and water are the basic raw materials in the production of:

a) ethanol

b) acetic acid

c) ethylene and polyethylene

d) vinyl chloride

e) benzene

Problem:

Give basic chemical equations for each reaction by which the above mentioned compounds can be obtained.

Official English text of the problem.

Answers

All routes start with the formation of acetylene (ethyne), C#C:

\[ \ce{CaC2 + 2H2O -> C2H2 + Ca(OH)2} \]

  • a) Ethanol, CCO: hydrate acetylene to ethanal, then hydrogenate. \[ \ce{C2H2 + H2O -> CH3CHO} \] \[ \ce{CH3CHO + H2 -> CH3CH2OH} \]

  • b) Acetic acid, CC(=O)O: hydrate acetylene to ethanal, then oxidize. \[ \ce{C2H2 + H2O -> CH3CHO} \] \[ \ce{2CH3CHO + O2 -> 2CH3COOH} \]

  • c) Ethylene (ethene), C=C, and polyethylene, repeat-unit SMILES [*]CC[*]: partially hydrogenate acetylene, then polymerize. \[ \ce{C2H2 + H2 -> CH2=CH2} \] \[ \ce{n CH2=CH2 -> [-CH2-CH2-]_{n}} \] The stars in the repeat-unit SMILES mark connections to the continuing chain; a complete molecular SMILES would require a specified chain length and end groups.

  • d) Vinyl chloride (chloroethene), C=CCl: add hydrogen chloride to acetylene. \[ \ce{C2H2 + HCl -> CH2=CHCl} \]

  • e) Benzene, c1ccccc1: cyclotrimerize acetylene. \[ \ce{3C2H2 -> C6H6} \]

Solution

The common starting reaction is the hydrolysis of calcium carbide. Water protonates its acetylide component, releasing acetylene and leaving calcium hydroxide. The substances are calcium carbide, [Ca+2].[C-]#[C-]; water, O; and calcium hydroxide, [Ca+2].[OH-].[OH-]. The disconnected salt SMILES represent ionic formula units, not discrete molecules.

Hydrogen, [H][H], and oxygen, O=O, are auxiliary reagents in the routes below. They can also be obtained from water by electrolysis:

\[ \ce{2H2O -> 2H2 + O2} \]

a) Ethanol

Hydration of acetylene in acidic aqueous solution, catalysed by mercury(II) ions, [Hg+2], gives ethanal (acetaldehyde), CC=O. The initially formed enol is vinyl alcohol (ethenol), C=CO, which rearranges by keto–enol tautomerism:

\[ \ce{CH2=CHOH -> CH3CHO} \]

Thus the net hydration equation is \(\ce{C2H2 + H2O -> CH3CHO}\).

Catalytic hydrogenation of ethanal, for example over nickel, reduces its carbonyl group to a primary alcohol:

\[ \ce{CH3CHO + H2 -> CH3CH2OH} \]

The two-carbon skeleton is retained, giving ethanol.

b) Acetic acid

First prepare ethanal by the same hydration reaction as in (a). Oxidation converts its aldehyde group into a carboxyl group:

\[ \ce{2CH3CHO + O2 -> 2CH3COOH} \]

Each ethanal molecule gains one oxygen atom, so one molecule of oxygen oxidizes two molecules of ethanal.

c) Ethylene and polyethylene

Selective catalytic hydrogenation adds one molecule of hydrogen across part of acetylene’s triple bond:

\[ \ce{C2H2 + H2 -> CH2=CH2} \]

The catalyst and reaction conditions must favour stopping at ethylene.

Ethylene then undergoes addition polymerization, for example using high temperature and pressure with radical initiation:

\[ \ce{n CH2=CH2 -> [-CH2-CH2-]_{n}} \]

The double bonds become single bonds as the monomers join into a chain. Every monomer contributes one \(\ce{-CH2-CH2-}\) repeat unit, without elimination of a small molecule.

d) Vinyl chloride

Hydrogen chloride, Cl, adds across acetylene’s triple bond. A conventional catalyst is mercury(II) chloride, Cl[Hg]Cl:

\[ \ce{C2H2 + HCl -> CH2=CHCl} \]

Addition of one equivalent gives a double bond bearing one chlorine substituent: vinyl chloride. Hydrogen chloride supplies the chlorine, which is absent from the two basic raw materials.

e) Benzene

On heating over a suitable catalyst, such as activated carbon, three acetylene molecules undergo cyclotrimerization:

\[ \ce{3C2H2 -> C6H6} \]

Their six carbon atoms form the aromatic six-membered ring of benzene. All six hydrogen atoms are retained, and no coproduct is required.

The solution was checked against the official solution.