Cracking Of Alkanes Conditions

[Gaetane Eary]

Crackingis the name given to breaking up large hydrocarbon molecules into smaller and more useful bits. This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst.

The source of the large hydrocarbon molecules is often the naphtha fraction or the gas oil fraction from the fractional distillation of crude oil (petroleum). These fractions are obtained from the distillation process as liquids, but are re-vaporised before cracking.

There isn't any single unique reaction happening in the cracker. The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons, some of which have carbon-carbon double bonds. One possible reaction involving the hydrocarbon C15H32 might be:

This is only one way in which this particular molecule might break up. The ethene and propene are important materials for making plastics or producing other organic chemicals. The octane is one of the molecules found in petrol (gasoline).

Modern cracking uses zeolites as the catalyst. These are complex aluminosilicates, and are large lattices of aluminium, silicon and oxygen atoms carrying a negative charge. They are, of course, associated with positive ions such as sodium ions. You may have come across a zeolite if you know about ion exchange resins used in water softeners.

The zeolites used in catalytic cracking are chosen to give high percentages of hydrocarbons with between 5 and 10 carbon atoms - particularly useful for petrol (gasoline). It also produces high proportions of branched alkanes and aromatic hydrocarbons like benzene.

The zeolite catalyst has sites which can remove a hydrogen from an alkane together with the two electrons which bound it to the carbon. That leaves the carbon atom with a positive charge. Ions like this are called carbonium ions (or carbocations). Reorganisation of these leads to the various products of the reaction.

Note: If you are interested in other examples of catalysis in the petrochemical industry, you should follow this link. It will lead you to information on reforming and isomerisation (as well as a repeat of what you have just read about catalytic cracking).

In thermal cracking, high temperatures (typically in the range of 450C to 750C) and pressures (up to about 70 atmospheres) are used to break the large hydrocarbons into smaller ones. Thermal cracking gives mixtures of products containing high proportions of hydrocarbons with double bonds - alkenes.

Warning! This is a gross oversimplification, and is written to satisfy the needs of one of the UK A level Exam Boards (AQA). In fact, there are several versions of thermal cracking designed to produce different mixtures of products. These use completely different sets of conditions.

If you need to know about thermal cracking in detail, a Google search on thermal cracking will throw up lots of useful leads. Be careful to go to industry (or similarly reliable) sources.

Thermal cracking doesn't go via ionic intermediates like catalytic cracking. Instead, carbon-carbon bonds are broken so that each carbon atom ends up with a single electron. In other words, free radicals are formed.

Note: If you are interested, there is a lot of really useful information on cracking on this page from the Essential Chemical Industry. You will be reading that out of interest, not because you need to know it all!

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This experiment is intended as a demonstration, but could with the most competent students be a class practical. The main risk to be considered in making this choice is the handling of very hot glassware and manipulating the apparatus for the safe collection of the flammable gas mixture over water.

The demand for petrol is greater than the gasoline fraction obtained by distilling crude oil. Cracking larger hydrocarbons produces smaller alkanes that can be converted into petrol. It also produces small alkenes, which are used make many other useful organic chemicals (petrochemicals), especially plastics. This experiment models the industrial cracking process.

The gas mixture which collects has a characteristic smell, burns with a yellow flame, and decolourises bromine water and acidified potassium manganate(VII) solution. This shows the presence of unsaturated molecules.

This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.

CrackingclosecrackingThe breaking down of large hydrocarbon molecules into smaller, more useful hydrocarbon molecules by vaporising them and passing them over a hot catalyst. is a reaction in which larger saturated hydrocarbonclosehydrocarbonA compound that contains hydrogen and carbon only. moleculesclosemoleculeA collection of two or more atoms held together by chemical bonds. are broken down into smaller, more useful hydrocarbon molecules, some of which are unsaturated:

The starting compound will always fit the rule for an alkane, CnH2n+2. The first productcloseproductA substance formed in a chemical reaction. will also follow this rule. The second product will contain all the other C and H atoms. The second product is an alkene, so it will follow the rule CnH2n.

C16H34 is an alkane which can be used as the starting chemical in cracking. One of the products of cracking this compoundclosecompoundA substance formed by the chemical union of two or more elements. is an alkane which has 10 carbon atoms in it. Write a balanced symbol equation for this cracking reaction.

The supply is how much of a fraction an oil refinery produces. The demand is how much of a fraction customers want to buy. Very often, fractional distillationclosefractional distillationIn fractional distillation a mixture of several substances, such as crude oil, is distilled and the evaporated components are collected as they condense at different temperatures. of crude oilclosecrude oilMixture of hydrocarbons, mainly alkanes, formed over millions of years from the remains of ancient dead marine organisms. produces more of the larger hydrocarbons than can be sold, and less of the smaller hydrocarbons than customers want.

Smaller hydrocarbons are more useful as fuelsclosefuelMaterial that is used to produce heat, like coal, oil or gas. than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.

As a result, alkenes are more reactiveclosereactiveThe tendency of a substance to undergo a chemical reaction. than alkanes. Alkenes can take part in reactions that alkanes cannot. For example, ethene molecules can react together to form poly(ethene), a polymerclosepolymerA large molecule formed from many identical smaller molecules known as monomers..

Cycloalkanes are very important in components of food, pharmaceutical drugs, and much more. However, to use cycloalkanes in such applications, we must know the effects, functions, properties, and structures of cycloalkanes. Cycloalkanes are alkanes that are in the form of a ring; hence, the prefix cyclo- is used to name these alkanes. Stable cycloalkanes cannot be formed with carbon chains of just any length. Recall that in alkanes, carbon adopts the tetrahedral geometry in which the angles between bonds are 109.5.

For some cycloalkanes to form, the angle between bonds must deviate from this ideal angle, an effect known as angle strain. Additionally, some hydrogen atoms may come into closer proximity with each other than is desirable (become eclipsed), an effect called torsional strain. These destabilizing effects, angle strain and torsional strain are known together as ring strain. The smaller cycloalkanes, cyclopropane and cyclobutane, have particularly high ring strains because their bond angles deviate substantially from 109.5 and their hydrogens eclipse each other. Thus, both of these ring conformations are highly unfavorable and unstable. Cyclopentane is a more stable molecule with a small amount of ring strain, while cyclohexane is able to adopt the perfect geometry of a cycloalkane in which all angles are the ideal 109.5 and no hydrogens are eclipsed; it has no ring strain at all. Cycloalkanes larger than cyclohexane have ring strain and are not as commonly encountered in organic chemistry. Figure 7.6 provides examples of cycloalkane structures.

The number of carbon neighbors that a carbon atom has can help determine the reactivity of that carbon position. Thus, it is important to be able to recognize whether a carbon atom is primary, secondary, tertiary, or quaternary in its structure (Fig. 7.7).

Figure 7.7. Classification of carbon atoms as primary, secondary, tertiary, or quaternary. In the molecules above, the center carbon is evaluated for the number of carbon atoms that are bonded directly with the center carbon. A primary carbon is bonded to one carbon, a secondary carbon is bonded to two carbons, a tertiary carbon is bonded to three carbons, and a quaternary carbon is bonded to four carbons.

Regarding isomers, the more branched the chain, the lower the boiling point tends to be. London dispersion forces are smaller for shorter molecules and only operate over very short distances between one molecule and its neighbors. It is more difficult for short, bulky molecules (with substantial amounts of branching) to lie close together (compact) compared with long, thin molecules. Cycloalkanes are similar to alkanes in their general physical properties, but they have higher boiling points, melting points, and densities than alkanes. This is due to stronger London forces because the ring shape allows for a larger area of contact.

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