Preparation of Alkynes

Introduction

Alkynes are generally consists of triple bond between two Carbon atoms at a time. These triply bonded compounds are insoluble in water but slightly soluble in polar solvent. Ethyne is very common in Alkynes consist of triple bond between two Carbon atoms. Components like, Ethyne, Propyne, Butyne, and Pentyne are commonly in Alkyne series.

What are Alkynes?

Alkynes are mainly hydrocarbons with one or more triple bond. The triple bond is also called Acetylene bond. Alkynes exist with general formula of CnH2n-2. Alkynes exist in nature. The earliest member of this family is Ethyne C2H2, has a triple bond between two carbons atoms.

Figure 1 − C-C bonds in different Hydrocarbons

The reaction of double E2 commonly helps in the preparation of the Alkynes.

Functions of Alkynes

The functions of Alkynes are discussed below.

• Ethyne is utilised in welding and cutting purpose as oxyacetylene gas because it gives a hot flame at 3600K by burning in presence of Oxygen. Ethyne is used as raw material for polymer preparation as well as starting reactant in formation of most of the organic compounds like, Ethanoic acid, Ethanol, and Acrylic acid.

• Alkynes are used as fuel with the help of fractional oxidation of natural gases.

Alkynes: Properties

Unsaturated hydrocarbons are nonpolar. Alkynes have a higher boiling point than Alkenes. The boiling point of Ethane is −88.6°C and Ethene shows −103.7°C whereas, Ethyneshows boiling point −84°C which is higher than both Alkanes and Alkenes. . Three Alkynes are primarily gas and the other eight are liquids. As per molecular structure increases, melting point and boiling point also increases accordingly.

Figure 2 − Structure of Alkynes shows orbital overlapping

The three bonds between Carbon atoms makes the structure more electron rich hence, boiling point increases. Alkynes are hybridised, electronegative and acidic in nature. It’s pair of electrons are shared in covalent bond.

Preparation of Alkynes

The most commonly used method is the double $\mathrm{E^{2}}$ reaction in which vicinal or geminal halides are used as reactant molecules. In order to create two π bonds, E2 reaction takes place for two times. Hydroxide and Alkoxide are used to utilise their powerful basic nature.to carry out double $\mathrm{E^{2}}$ reaction. It helps in rearranging the position of triple bonds in Alkynes. Sodium Amide in Ammonia is also used as strong base. Alkynes are bonded in 180 degree angle and exist in sp hybridized bond. Antiparasite, antibacterial, and non-fungal properties of Alkynes make it useful.

Dehydrohalogenation process in Alkyl Dihalides

In this process, Alkynes are derived from the Halogen compounds. In the process of reaction, the Alkenes are added to halogens to form vicinal or germinal halide Alkane compound. Later, this Alkane Halides undergo elimination of hydro-halogen to produce Alkyne. Alcoholic KOH is used as suitable reagent to prepare Alkynes. Sodium Amide is also an effective reagent for Alkyne formation. The process of dehydrohalogenation is mainly eliminated halogen along with hydrogen to acquire Alkyne.

Making process of Calcium Carbide to Acetylene

Calcium Carbide $\mathrm{(CaC_{2})}$ reacts with water in huge scale preparation of Alkynes. Huge production of Alkyne result in by hitting quicklime in the presence of coke . The reaction of Calcium Carbide to produce Acetylene is as follows:

$$\mathrm{CaCO_{3}\:\rightarrow\:CaCO\:+\:CO_{2}}$$

$$\mathrm{CaO\:+\:3C\:\rightarrow\:CaC_{2}\:+\:CO}$$

$$\mathrm{CaC_{2}\:+\:2H_{2}O\:\rightarrow\:C_{2}H_{2}\:+\:Ca(OH)_{2}}$$

Preparation of Alkynes from Vicinal Dihalides

Alkynes are accruing from Vicinal Dihalides by dehalogenation process. Unsaturated halides are prepared in the first step. These halides are non-active in nature called Vinylic Halides. In the preparation of Alkynes, these halides particularly react with strong and powerful bases for further removal of hydro-halide group from unsaturated intermediate compound.

Homologous series of Alkynes

Similar chemical properties and successive members by adding−CH2 group in a organic compounds along with same functional group are called homologous series. In the group of compounds, many formulas are the same as each other in general. Alkanes formula in general $\mathrm{C_{n}H_{2n\:+\:2}}$ and alkynes formula in general $\mathrm{C_{n}H_{2n\:-\:2}}$. This is the most general homologous series of organic chemistry.Homologous series of alkynes are similar as Ketone, Hydroxyl, and Carbone group. The homologous series of Alkyne must contain Carbon triple bond in the compound.

Name	Molecular Name
Propyne	$\mathrm{C_{3}H_{4}}$
1-Butyne	$\mathrm{C_{4}H_{6}}$
1-Pentyne	$\mathrm{C_{5}H_{8}}$
1-Hexyne	$\mathrm{C_{6}H_{10}}$
1-Heptyne	$\mathrm{C_{7}H_{12}}$
1-Octyne	$\mathrm{C_{8}H_{14}}$
1-Nonyne	$\mathrm{C_{9}H_{16}}$
1-Decyne	$\mathrm{C_{10}H_{18}}$
Ethyne	$\mathrm{C_{2}H_{2}}$

Table 1 − Homologous series of Alkynes

Conclusion

In the above topic, we learn about Alkynes, different methods of preparation of lkynes and homologous series. Alkynes are hydrocarbons and used as fuel, gas produced when oxygen is not available, and they help to create natural compounds. Prepartion of Alkynes are, dehydrohalogenation of Alkyl Dihalides, process of Calcium Carbide to Acetylene, preparation of Alkynes from Vicinal Dihalides. Homologous is a series that general formulas are the same. The various types of Alkynes are presents like Eythene, Propyne, Butyne, and many more.

FAQs

1. What are the different types of names of Alkynes?

Ethyne, Propyne, Butyne, Pentyne etc. are known as Alkynes. It always ends with – yne suffix in nomenclature. Alkynes are unsaturated and hydrocarbon which has a triple carbon bond.

2. What is the process of preparing Alkynes?

There are many processes for preparation of Alkynes, like the dehalogenation process, where Alkynes react with halogens. Another process is Acetylene making from Calcium Carbide; helps to make a large number of alkynes. Another process is Vicinal Dihalides, where it has two steps by making small to large alkynes.

3. How to reduce Alkyne to Alkene?

Sodium dissolves in an Ammonia solvent to reduce alkyne from alkene. π bonds take an electron from a Na radical, triple bond between Carbon-Carbon. An anion that is formed in Ammonia solution, Hydrogen protonation can be done with it.