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Reactions of Alkanes

Summary

  • Alkanes are saturated hydrocarbons.
  • Methane is a simple example of alkanes
  • They show low chemical reactivity due to saturation compared to alkenes or alkynes.
  • The halogenation of alkanes forms haloalkanes via free radical reaction mechanism.
  • Halogenation reaction involves three steps, initiation, propagation & termination.
  • Different halogens exhibit different activity for alkanes due to their difference in their bond dissociation energies.
  • Alkanes undergo combustion generating heat. They are consumed as source of energy for domestic and industrial.
  • Keep reading to learn more about the reactions of alkanes.

Alkanes

Alkanes are hydrocarbons made up of only carbon and hydrogen atoms. They are saturated where the central carbon atom is attached to four other atoms or groups. In terms of hybridization, Alkanes are Sp3 hybridization where p orbitals of carbons are overlapping the s orbitals of four hydrogen atoms. This saturation is responsible for a relatively low reactivity of alkanes.  Traditionally in old times, saturated hydrocarbons were known as paraffins meaning “little affinity”. However, alkanes undergo some important chemical reactions and act as precursors for series of other chemical products. Due to its saturation, reactions of alkanes are mostly targeting carbon – carbon or carbon – hydrogen bonds. The cleavage of these bonds generates some activated species that act as intermediates in chemical reactions. Breaking a bond is known as Bond Dissociation and the energy required to break a bond is called Bond dissociation energy (ΔHo)

The bond-breaking where two bonding electrons are equally divided between the two participating atoms is called hemolytic cleavage. Alkanes mostly undergo this type of bond breakage and as a result of this, generate species containing unpaired electrons called free radicals. An atom of groups of atoms possessing odd (unpaired) electrons is called a free radical. The energy required for this cleavage can be provided by light and heat. 

A diagram of simple alkanes in order to understand reactions of alkanes
Figure 1: Chemical structure of simple alkanes. 

Learn more about Alkanes

Halogenation of alkanes

Alkanes' reaction with halogens (F, Cl, Br, I) forms haloalkanes. The reaction is believed to follow a free radical reaction mechanism in the presence of light or heat. However, different alkanes may exhibit slightly different reactivity depending upon the size of the alkyl chain. Also the same alkane may have different reactivity for different halogens. This is because of the difference in bond dissociation energies of halogens. For example, methane reacts with fluorine vigorous and exothermic while its reaction with chlorine requires the presence of light or heat.

Free radical reaction mechanism of halogenation

The reaction mechanism can be described in three steps. The first step is initiation generating halogen atoms. Any species or agent that triggers this formation is called the initiator.

In the second step, highly reactive halogen atoms react with alkane molecules to trigger the chain propagation reaction. The step goes on continuously generating a series of more free radicals until a foreign species is added to disrupt the chain sequence. Such species is called the inhibitor

The third step is called the termination step where reacting species are completely consumed and no more free radicals are generated. 

To understand more about the halogenation of alkanes, let’s examine an example of simple alkane; methane in more detail:

Chlorination of methane

Equation 1: Chlorination of methane to offer mono-chloromethane, dichloromethane, trichloromethane and tetrachloromethane. 

The initiation step is the cleavage of Cl – Cl bonds. The bond dissociation energy for chlorine is 58 kcal/mol. This much energy is provided by light or heat. In the following step, the chlorine atom must collide with some other molecule or atom. It may either collide with another chlorine molecule or methane molecule. Its collision with the chlorine molecule will not be productive as the same chlorine atom will be generated. There is a greater possibility of its collision with methane molecules and abstract its hydrogen atom to form hydrogen chloride and generate a methyl radical.  

Scheme 1: Free radical reaction mechanism of chlorination of methane.

Once this methyl radical is formed, the reaction enters into a cycle. Methyl radical collides with chlorine molecule (reactant) to form chloromethane and another chlorine atom as shown in the scheme above. This is propagation step goes on in a cycle until the system runs out of starting material or an inhibitor is added into it. 

The reaction undergoes termination with the collision of a chlorine atom with another chlorine atom or a methyl radical with another methyl radical to form ethane. In practice, this means the end of reactants because, in the chain propagation step, series (hundreds to thousands) of intermediate steps are taking place simultaneously. 

The overall reaction converts methane into chloromethane and with continuous reaction, we get dichloromethane, trichloromethane (chloroform), and eventually tetrachloromethane (carbon tetrachloride). However, under controlled conditions, the reaction can be stopped at any stage to obtain a particular desired chloromethane. 

Potential energy diagram of methane chlorination

Comparing the heat changes during the reaction, the first step (A) of Cl – Cl bond cleavage is endothermic and requires energy to proceed. The third step (C) in the below scheme, the reaction of methyl radical with chlorine is exothermic; means it is fast and releases energy. However, in step B, the formation of methyl radical is slightly endothermic. The amount of energy required for the formation of methyl radical can be described in terms of enthalpy change of step C where methyl radical is consumed very quickly. Therefore, the formation of methyl radicals is the difficult step in the whole chlorination of methane. 

Scheme 2: Enthalpy changes in the chlorination of methane

In summary, halogens (except iodine) react with alkanes in a free radical mechanism. The reaction is capable of maintaining the free radical chain reaction by (1) hydrogen abstraction of methane to give methyl radical and (2) reaction of methyl radical with chlorine (Cl2) to give chlorine atom. The reaction terminates with a combination of radicals. 

Combustion

Combustion is a chemical reaction with oxygen in which alkane is converted into carbon dioxide and water with the release of heat energy. In case of combustion, we need slight ignition or electric spark to provide activation energy for this reaction. The heat released on complete combustion of one mole of a substance is called heat of combustion and the reaction is called Exothermic.

Equation 2: Combustion of methane.

ΔHo = Hoproducts - Horeactants

Here H is the heat content or enthalpy in its standard state (at room temperature and at atmospheric pressure). In an exothermic reaction, the enthalpy of the product is less than starting reactants and thus reaction generates heat energy and ΔHo is negative. SI unit for heat energy & enthalpy is Kilojoule/mole (kJ/mol). 

Equation 3: Combustion of butane. 

An increasing number of carbon atoms in a molecule increases the heat of combustion. This is obvious as more carbon atoms are available for burning and more bonds undergoing changes. Equation 1 & 2 demonstrates this where methane, a single carbon molecule generates less heat energy compared to butane with four carbon atoms generating more heat energy.

Pyrolysis & Cracking

The cleavage of C - C or C – H bonds under the influence of heat is called pyrolysis. In simple words, it is the cleavage of long carbon chain molecules by heat. The reaction is carried out by passing alkanes through a chamber heated to a high temperature (400-450 oC) in the presence of a catalyst. The heat energy provided breaks the carbon-carbon or carbon-hydrogen bonds generating highly reactive free radicals. These free radicals subsequently combine with other free radicals to form series of new hydrocarbons. Pyrolysis reaction is very complex and requires special conditions to control and proceed to a particular desired product. 

Equation 4: Pyrolysis of n-hexane.
Equation 5: Reforming of various carbon-centered radicals formed as a result of pyrolysis.

Pyrolysis in the petroleum industry is known as cracking. This process is important in modifying the carbon chain length of higher hydrocarbons obtained by the distillation of crude oil.

Catalytic reforming

The conversion of long-chain aliphatic hydrocarbons into aromatic hydrocarbons with the same number of carbon atoms is called reforming. The chemical reaction is carried out under high pressure and temperature in the presence of a catalyst. Reforming is utilized in the petroleum industry to increase the yield of low boiling point fractions e.g. gasoline. Another important contribution of catalytic reforming is the synthesis of aromatics. Aromatic molecules are better fuels and are precursors for various chemical industries. 

Equation 6: Catalytic reforming of n-heptane into toluene.

Books for further study

  1. Morrison, R. T., and R. N. Boyd. "Organic chemistry 5th edition." (1987).
  2. Cary, A, F. Organic Chemistry, 3rd edition, (1996). 
  3. Volhardt K, P, C. Organic Chemistry, (1987).
  4. Smith M, B and March, J. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 5th edition (2001).

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