Cracking is done in the presence of heat or catalysts. So cracking of alkynes is explained as the breaking of long hydrocarbon chains of alkynes in presence of heat or catalyst. This is done by using lower pressure or temperature and the presence of a catalyst or by using high pressure or temperature in absence of the catalyst. Let us discuss cracking and different types of cracking in detail. 

Cracking –

Cracking is defined as the process of breaking down anything into simpler parts. Like in chemistry, cracking is the breaking of complex organic compounds into simpler molecules by breaking carbon-carbon bonds. The temperature and the catalyst present or not are directly correlated by the rate of cracking and the end products of the reaction. A large amount of heat is required in this process. For example- By the process of fluid catalytic cracking,  petrol and LPG of high yield can be produced through this process. 

Types of Cracking

Two different types of cracking are:-

1. Thermal Cracking

2. Catalytic Cracking

Now, we will study in detail about them. 

Thermal Cracking

In thermal cracking, high temperature and pressure are used to break long chains of hydrocarbons. For which, the temperature should be in the range of 450 degrees Celsius to 750 degrees Celsius. And pressure should be up to 70 atm pressure.

With the double bond compounds which are alkenes, thermal cracking would result in a mixture of products that contain high proportions of hydrocarbons. This example is very simple. But, thermal cracking gives different mixtures of products, but with different sets of situations. 

Thermal cracking is based on the formation of free radicals, unlike catalytic cracking which is based on the formation of intermediates. As, in thermal cracking, carbon-carbon bonds break, that’s why free radicals are formed instead of the formation of ionic intermediates.

Propyne can also be prepared by the thermal cracking of hydrocarbons. And, it is also industrially used widely.

Catalytic Cracking

Nowadays in catalytic cracking, Zeolites are used as catalysts. 

[Zeolites are basically very complex aluminosilicates, and they have large lattices of aluminum, silicon, and oxygen atoms that carry a negative charge. And, these are even positive ions of sodium. ]

If you know about ion exchange resins used in water softeners, you have come across a zeolite. 

The zeolites that are used in catalytic cracking are chosen to give high percentages of long-chain hydrocarbons of 5 to 20 carbon atoms. These are particularly used for petrol also known as gasoline. 

Catalytic cracking also produces high proportions of branched alkanes and aromatic hydrocarbons like benzene. The zeolite catalyst has some special sites which can remove hydrogen from an alkene together with the two electrons which bind it to the carbon atom. And it leads to a positive charge on carbon atoms. These ions are called Carbonium ions or Carbocations. Similarly, we can produce many products of the reaction by reorganizing it.

FLUID CATALYTIC CRACKING –

Let us discuss fluid catalytic cracking in detail. Under this category, the process includes solid catalysts like silica-alumina. As the catalyst is used to promote the formation of carbocations. This process is widely used in the refinery because of the high demand for gasoline. Here, the alkenes formed to cause the instability of hydrocarbon fuels. 

Hydrocracking- 

It is also a catalytic cracking process that involves the presence of added hydrogen gas. It uses hydrogen in order to break C-C bonds. The product which is obtained are the saturated hydrocarbons. The obtained products through the process of hydrocracking are jet fuel and diesel. 

Natural Gas

As we know, the most common alkyne is Acetylene. It is used as a fuel and a precursor to other compounds, such as acrylates. 

By partial oxidation of natural gases, hundreds of millions of alkynes are produced annually.

Reaction to this is,   

                                2 CH4 + 3/2 O2 → HC≡CH + 3 H2O

Acrylates

Salts, esters, and conjugate bases of acrylic acids are known as acrylates. 

CH2=CHCOO–

The above-written anion is known as an acrylate ion.

Acrylates belong to the group of vinyl polymers that are made up of acrylate monomers. The most common acrylate member of this family is methacrylate. 

It is made up of monomers of esters that contain vinylic groups. Hence, acrylates are bifunctional. 

Conclusion

We have read about the cracking of alkynes. The cracking of alkynes is the breaking down of complex hydrocarbons into smaller ones. And cracking is of two types i.e., thermal cracking which requires high temperature and high pressure and another one is catalytic cracking which does not require high temperature it works at low temperature but with a catalyst.

Also, the two main products of cracking are alkanes and alkenes. It is mainly used in petroleum which is the gasoline and diesel industries.