Preparation of Hydrocarbons – Alkynes

Hydrocarbons are organic compounds made up of essential atoms called hydrogen and carbons. Generally, these are colourless gases with nearly no odour. Based on their types, hydrocarbons may possess simple or complex structures. These are mainly classified into four major parts – alkenes, alkanes, alkynes, and aromatics. 

What are Hydrocarbons?

Hydrocarbons can be described as organic compounds comprising two atoms – carbon and hydrogen. Hydrocarbons vary based on their structures which can be either simple or complex. Further, they can be divided into six major parts: Saturated hydrocarbons, Unsaturated hydrocarbons, Cycloalkanes, Aromatic hydrocarbons, Aliphatic hydrocarbons, and Alicyclic hydrocarbons.

What are Alkynes? 

Belonging to the family of organic chemistry, alkyne or alkynes are unsaturated hydrocarbons that contain at least one carbon-carbon triple bond. The formula for alkynes is CnH2n-2. The triple bond is also referred to as an acetylenic bond. In the alkyne family, ethyne is the first member. Its chemical formula is C2H2. 

Properties of Alkynes 

• Alkynes are unique because of hybridization. The non-polar bonding strength of alkyne, its acidity, and its linearity is because of the triple bonds featured in these compounds. 

• When in water, these compounds are insoluble, whereas, in polar solvents, these are slightly soluble. Alkynes possess the capability to dissolve in organic solvents because the solution density is less. 

• Alkynes have a higher boiling point than alkanes and alkenes. For example, Ethane’s boiling point is -88.6°C, ethene’s boiling point is -103.7°C, and the boiling point of ethyne is -84°C. As carbon atoms increase, the boiling point of these compounds also increases. 

• The acidity of alkynes is much higher than that of alkanes and alkenes. Ethane’s pKa value is 50, making it less acidic, while the pKa value of ethene is 44. On the other hand, ethyne is the most acidic, whose pKa value is 25. 

Preparation of Alkynes

There are several methods considered to prepare alkynes. Because of its antibacterial, antifungal and antiparasitic properties, the synthesis of Alkynes is extremely useful. Here are some of the methods used for preparing alkynes. 

Dehydrohalogenation of alkyl dihalides

The first method is the dehydrohalogenation of alkyl dihalides, where alkenes react with the halogen. Because of this reaction, another alkane is achieved called the “substituted alkane.” The formed alkynes are further processed through the alcoholic KOH for the formation of the substituted alkenes. Later, it is reacted with the sodium amide to form alkynes. Since hydrogen is not present along with the halogen, the process is referred to as dehydrohalogenation. 

Here is the dehydrohalogenation of alkyl dihalides – 

Preparation from calcium carbide 

Experts prefer this method when the alkynes are produced on a larger scale. The chemical formula of calcium carbide is CaC2. Quicklime is heated in the presence of coke to prepare Calcium Carbide. After that, calcium carbide is made to react with water to get acetylene. Here are the reactions for preparing acetylene from the calcium carbide – 

CaCO3 → CaO + CO2

CaO + 3C → CaC2 + CO

CaC2 + 2H2O → Ca(OH)2 + C2H2

Preparation from Vicinal dihalides

During this process, alkenes react with a halogen. The process of dehydrohalogenation is performed to obtain alkynes, and it is carried out in two major steps. 

The first step in the preparation of alkynes involves preparing the unsaturated halides. The formed halides possess the halogen, which is further attached to the double-bonded carbon. Such halides are referred to as vinylic halides. Note that these halides are not reactive in nature, but they are made to react with a strong base that forms alkynes. In order to convert the small alkynes into larger ones, Metal acetylides are used in the second step. 

Here is the preparation of alkynes from vicinal dihalides –

Difference between Alkenes and Alkanes  

Alkenes come from the family of hydrocarbons. Alkenes at least have one double bond between the two adjacent carbon atoms. The chemical formula for alkenes is CnH2n. Alkanes can be described as an organic compound that possesses single-bonded hydrogen and carbon compounds. CnH2n+2 is the formula of Alkanes. It is further divided into three major groups – cycloalkanes, branched alkanes, and chain alkanes.  

What is Clemmensen’s Reduction

Clemmensen’s Reduction is the reaction used to reduce ketones or aldehydes to the alkanes by using hydrochloric acid and zinc amalgam. This reaction has been named after Erik Christian Clemmensen, a Danish chemist. 

Conclusion 

Hydrocarbons are organic compounds made up of two atoms – hydrogen and carbon. Alkynes are unsaturated hydrocarbons which contain at least one carbon-carbon triple bond. There are several methods to prepare alkynes. Some of those methods include dehydrohalogenation of alkyl dihalides, preparation of alkynes from calcium carbides, and preparation of alkynes from Vicinal dihalides. The formula for alkynes is CnH2n-2.