Video Transcript
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.
