[Chemistry Class Notes] on Preparation of Alkynes Pdf for Exam

Alkynes are unsaturated hydrocarbons with the general formula (C_nH_n-2). Hydrocarbons are compounds containing C and H atoms. Unsaturated hydrocarbon means there is a presence of double or triple bonds between the atoms. Alkynes have a triple bond between C and H atoms. They are hard to find in their pure form, They are sp hybridised and the bond angle between them is 180°. Synthesis of alkynes is useful because of its antibacterial, antifungal, and antiparasitic properties. Here, we will discuss the various methods of preparation of alkynes. 

Methods of Preparation of Alkynes

Dehydrohalogenation

The loss of a hydrogen and halogen atom from adjacent alkane carbon atoms leads to the formation of an alkene. Further, the loss of additional hydrogen and halogen atoms from the double‐bonded carbon atoms leads to the formation of alkyne. The halogen atoms may be located on the same carbon or the adjacent carbon atoms.

During the second dehydrohalogenation process, in the presence of a strongly basic medium and high temperature, Vicinal tetra haloalkanes can be dehalogenation with zinc metal to form alkynes. This process is called dehydrohalogenation because hydrogen is eliminated along with a halogen in order to obtain an alkyne.

Preparation of Alkynes from Vicinal Dihalides

Alkynes are prepared from vicinal dihalides by the process of dehydrohalogenation. We know the group 17 elements are known as halogens. So, dehydrohalogenation means the removal of Hydrogen and Halogen atoms. The vicinal term is used when two similar atoms are attached at adjacent positions. Dihalides simply mean two halogen atoms. laboratory preparation of alkynes is done by this method.

The first step involves the preparation of unsaturated halides. These are vinylic halides and are not reactive in nature. These halides are reacted with a strong base which results in the formation of alkynes. By using Metal acetylides small alkynes are converted into large ones.

Preparation of Alkynes from Calcium Carbide

At the industrial level, the synthesis of alkynes is done using calcium carbide. Calcium Carbide is prepared by heating quicklime (CaO) in the presence of coke (C). When calcium carbide is made to react with water, It results in the formation of calcium hydroxide and acetylene.

CaCO₃  → CaO + CO₂

CaO + 3C → CaC₂ + CO

CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂

This method is now replaced by another method called pyrolysis of methane, In which methane is heated at a temperature of 1500^oC in an airless chamber. It forms the product within a fraction of a second with the liberation of hydrogen. (Air must be excluded from the reaction or oxidation process will occur).

The reaction is endothermic at ordinary temperatures and is thermodynamically favoured at high temperatures.