In this video, we will learn about the catalytic cracking of alkanes molecules, how this occurs in industry, and why it is so important. What does cracking mean in chemistry? In chemistry, cracking refers to the breaking or conversion of larger organic molecules into smaller molecules. We could class it as a decomposition reaction, since one reactant is breaking down to give smaller products. This is an endothermic process requiring the input of heat or thermal energy. And in some cases, a catalyst is used to speed up the rate of the reaction. We are going to focus on the cracking of the saturated hydrocarbons, the alkanes.
Crude oil contains many hydrocarbon compounds, especially saturated hydrocarbons. And many of these are alkanes. Typically, large alkanes are cracked or decomposed into smaller alkanes and alkenes. So, the process produces both saturated and unsaturated products. Some other products may be produced, such as pure carbon, hydrogen gas, and heteroatom containing molecules, heteroatoms referring to different elements such as nitrogen and sulfur, which are found in some organic compounds. The purpose of cracking is to produce smaller compounds which are often more useful than the original starting large alkanes. Cracking is used in oil refineries and produces useful byproducts like liquefied petroleum gas, diesel, and ethene.
Now let’s have a look at some of the chemistry that happens during cracking. There are three basic steps to a cracking reaction. In general, a carbon-carbon single bond in large alkane molecules breaks. A lot of energy is required to break a stable carbon-carbon single bond. In step two, a carbon-hydrogen bond breaks, and hydrogen atoms rearrange. And a new carbon-hydrogen bond is formed. Lastly, a new carbon-carbon double bond forms. A smaller alkane and an alkene result from this process. The large amounts of energy needed to initiate the first step are supplied using high temperature and sometimes high pressure through steam cracking or catalytic cracking.
Steam cracking tends to occur at very high temperatures, sometimes over 800 degrees Celsius, without the use of a catalyst, while catalytic cracking tends to be done at temperatures much lower than 800 degrees Celsius, normally about 500 to 700 degrees Celsius, with a catalyst. There are several methods of catalytic cracking, and some of these methods use zeolite catalysts. Zeolites are materials made of aluminum, silicon, and oxygen arranged in a complex lattice.
Now, let’s investigate a typical cracking reaction that might occur over a zeolite catalyst. We will use the three steps we saw earlier. In this example of catalytic cracking, we will use octane, which is a long saturated alkane molecule found in crude oil. This alkane is passed over a hot catalyst. A carbon-carbon single bond breaks. A carbon-hydrogen bond breaks. And a new carbon-hydrogen bond forms, as well as a carbon-carbon double bond, producing a shorter alkane, hexane, and an alkene, in this case ethene. Again, we have a larger alkane being converted or decomposed into a shorter alkane and an alkene.
In fact, octane can be cracked in various ways, not at just these two bonds. If these two bonds are cracked or broken, hexane and ethene are the products. What about the many other places the chain could break — for example, here and here or here — not to mention the carbon-hydrogen bonds, depending on where the chain breaks? Here are some of the alkanes and alkenes that can form. Pentane and propene can be produced, butane and but-1-ene is another option, or propane and pent-1-ene, or even ethane and hex-1-ene. The products of cracking can undergo further cracking to produce even smaller alkanes and alkenes.
So far, we’ve seen that long alkane molecules can be broken down into shorter alkanes and alkenes, which themselves can be cracked further. What is the importance of converting these long alkane molecules into smaller molecules? There are two main reasons. Firstly, cracking helps meet the demand for smaller hydrocarbons. Demand is the amount of a fraction from crude oil that a customer wants to purchase. Supply is the amount of a crude oil fraction that an oil refinery actually produces.
The problem that industry faces is this. Crude oil is a complex mixture of different hydrocarbons and other molecules. When it undergoes separation into fractions by fractional distillation, the products are predominantly the larger hydrocarbon molecules. A much smaller proportion of the shorter-chain hydrocarbons are produced. But there is a high demand for the shorter hydrocarbons in industry, for example, fuels such as gasoline or petrol. Cracking converts some of the less useful large hydrocarbons into the more useful small hydrocarbons, effectively increasing the supply of the desirable compounds. This in turn generates money.
A second important reason for cracking hydrocarbons is the production of unsaturated alkenes. Alkenes are important feedstocks in the petrochemical industry. The petrochemical industry is the industry that uses chemicals derived from petroleum or crude oil and natural gas to make polymers, plastics, detergents, synthetic rubber, and so on. Cracking products have many important applications. And this includes their use in food, water treatment, healthcare, household items and chemicals, vehicles, construction, electronics, and even agriculture.
Two specific examples of important alkenes that we get from cracking are ethene and propene. They can be used to make polymers such as polyethene or polyethylene and polypropene or polypropylene, which are highly useful and commonly used plastics. Now, it’s time to practice what we know about cracking.
Shown in the equation is one possible reaction in the cracking of heptane. Compound X is an unbranched hydrocarbon. What is the displayed formula of compound X?
The starting compound in the reaction scheme is the saturated hydrocarbon heptane with molecular formula C7H16. This is a relatively long alkane hydrocarbon. We are told that heptane is cracked and that this is only one possible reaction that occurs, which means there are other possible products. Two possible products are shown in this reaction scheme: an unsaturated alkene with a carbon-carbon double bond, in this case propene because there are three carbons in the chain, and an unknown compound X. But we are told that X is an unbranched hydrocarbon. Cracking is a type of decomposition reaction where larger, usually saturated organic molecules are broken down into smaller ones. These smaller molecules are usually more useful to industry.
The steps in a typical cracking reaction involve taking a large alkane molecule heating to high temperatures, sometimes with a catalyst. A carbon-carbon single bond breaks. A carbon-hydrogen bond breaks. Rearrangement occurs where a hydrogen atom bonds with a different carbon atom. This new carbon-hydrogen bond produces a shorter alkane. And in the other fragment of the original molecule, a double bond forms between two carbon atoms, forming an alkene.
The linear alkane given to us, heptane, was cracked or broken into the alkene propene and substance X. Therefore, X must be a shorter alkane and must contain the remaining carbon atoms. The alkene product propene has one, two, three of the carbon atoms from heptane. Therefore, X must contain four carbon atoms. A short alkane with four carbon atoms that is unbranched is butane. We were asked to give the displayed formula of compound X, and this is the displayed formula of butane.
Let’s review the key points of this video. Cracking is a reaction where larger, usually saturated hydrocarbon compounds are broken down or decomposed into smaller, usually more useful compounds. Typically, the products are a smaller alkane and an alkene, although other compounds can form too. Cracking is an endothermic process. Common types of cracking are steam cracking and catalytic cracking. Steam cracking generally uses very high temperatures, sometimes over 800 degrees Celsius, but does not use a catalyst, while catalytic cracking uses high temperatures in the range of 500 to 700 degrees Celsius, with a catalyst. Cracking can also employ high pressures.
Heavy crude oil fractions can be converted to lighter fractions, which are often more useful and more desirable to industry by cracking. There is a high demand for these light fractions in the petrochemical industry, which produces many economically important materials.