Q.Explain the different types of polymerisation in ethyne.

CH2=CH2 + Br2 gives 1,2-dibromoethane (A); alcoholic KOH removes one HBr to give vinyl bromide (B); NaNH2 removes the second HBr to give acetylene (C).

Step 1 — Formation of A: Ethylene (CH2=CH2) reacts with bromine (Br2) in an electrophilic addition reaction — the pi bond of the alkene attacks Br2, and both bromine atoms add across the double bond:

CH2=CH2 + Br2 --> CH2Br-CH2Br (A = 1,2-dibromoethane, ethylene dibromide)

Step 2 — Formation of B: Treating the vicinal dibromide A with alcoholic KOH (a base, used specifically to favour ELIMINATION over substitution) removes one molecule of HBr (dehydrohalogenation), forming a carbon-carbon double bond again — this time with one bromine remaining:

CH2Br-CH2Br --(alc. KOH)--> CH2=CHBr (B = vinyl bromide, bromoethene)

Step 3 — Formation of C: Vinyl bromide B, when treated with sodamide (NaNH2, a very strong base), undergoes a second dehydrohalogenation — this time eliminating HBr from the vinylic system to introduce a second degree of unsaturation, converting the C=C double bond into a C-tripleBond-C triple bond:

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Ethyne undergoes two types of polymerisation: cyclic trimerisation (3 molecules -> benzene) and linear polymerisation (many molecules -> the polymer cuprene).

1. Cyclic polymerisation (trimerisation): When ethyne (acetylene) gas is passed through a red-hot iron or copper tube at about 873 K, three molecules of ethyne combine and cyclise to form one molecule of benzene:

3 HC-=CH --(red hot tube, 873 K)--> C6H6 (benzene)

This is historically one of the important laboratory routes for preparing benzene from a non-aromatic starting material, and it demonstrates that the six carbon atoms and six hydrogen atoms of three ethyne molecules can rearrange into the stable aromatic ring of benzene.

2. Linear polymerisation: When ethyne is passed over a copper catalyst (e.g., cuprous chloride/ammonium chloride solution, or activated copper) under different, milder conditions, many molecules of ethyne add on to each other one after another (end to end) rather than cyclising, giving a linear polymer chain:

n HC-=CH -> (-CH=CH-)n

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Ozonolysis of acetylene cleaves its triple bond, giving glyoxal (OHC-CHO) as the product after hydrolysis of the intermediate ozonide.

Acetylene (HC=CH, triple bond), like alkenes, undergoes an addition reaction with ozone (O3) to form an unstable cyclic intermediate called an ozonide. This ozonide is not isolated directly but is decomposed (hydrolysed) with water, typically in the presence of zinc dust (which acts as a mild reducing agent, preventing the further oxidation of the aldehyde product to a carboxylic acid). This hydrolysis cleaves the carbon-carbon triple bond completely, converting each of the two triply-bonded carbon atoms into a -CHO (aldehyde) group. Since both carbons of acetylene are equivalent (each bearing one hydrogen), the product formed is glyoxal (ethanedial), OHC-CHO: …