Organic Chemistry 1: Chemical properties of alkynes

3.0 Organic Chemistry 1

3.3.5 Chemical properties of alkynes

Observe the video demonstration of properties of alkynes

Figure 3.3.5: Some chemical properties of ethyne (CHCH)

Questions 3.3.5

1. Like other hydrocarbons, ethyne burns in oxygen gas. Write an equation for the reaction.
2. Like ethene, ethyne undergoes addition reaction with halogens. Complete the equations
   CHCH(g) + Br₂ (g) (assume two moles of bromine gas are used)
   CHCH(g) + I₂(g) (assume only one mole of iodine vapour is used)
   How can you obtain iodine vapour (I₂)?
3. Reaction of ethyne with chlorine gas is explosive and produces carbon (C), steam (H₂O) and carbon (II) oxide (CO). Explain this.
4. Like alkenes, ethyne reacts with hydrogen halides (HCl, HBr, HF). Write the corresponding equations for complete reactions (with only single bonds remaining between carbon atoms).
5. Like alkenes, ethyne undergoes addition reaction with hydrogen gas (hydrogenation). Write an equation for complete hydrogenation of ethyne.
6. Explain why ethyne shares many similarities with alkenes.
7. 0.26 g of ethyne was completely burnt in air. Determine the mass of oxygen used in the reaction (C = 12; H = 1; O = 16).

Reactions of ethyne are similar to those of ethene. They do not require presence of ultraviolet light to react with halogens. They decolourize chlorine water, bromine water, and acidified potassium manganate (VII) solution. But unlike alkenes, ethyne forms a yellow precipitate with ammoniacal silver nitrate solution.