- 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
Alkanes are hydrocarbons made up of only carbon and hydrogen atoms. They are saturated where 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 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) electron is called free radical. The energy required for this cleavage can be provided by light and heat.
Halogenation of alkanes
Alkanes reaction with halogens (F, Cl, Br, I) forms haloalkanes. The reaction is believed to follow free radical reaction mechanism in the presence of light or heat. However, different alkanes may exhibit slightly different reactivity depending upon the size of alkyl chain. Also same alkane may have different reactivity for different halogens. This is because of difference in bond dissociation energies of halogens. For example, methane reacts with fluorine vigorous and exothermic while its reaction with chlorine requires 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 specie or agent that triggers this formation is called 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 specie is called inhibitor.
The third step is called 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 example of simple alkane; methane in more details
Chlorination of methane
The initiation step is 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, chlorine atom must collide with some other molecule or atom. It may either collide with another chlorine molecule or methane molecule. It’s collision with chlorine molecule will not be productive as same chlorine atom will be generated. There is greater possibility of its collision with methane molecules and abstract its hydrogen atom to form hydrogen chloride and generate a methyl radical.
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 scheme above. This is propagation step goes on in a cycle until the system run out of starting material or an inhibitor is added into.
The reaction undergoes termination with the collision of a chlorine atom with another chlorine atom or methyl radical with another methyl radical to form ethane. In practical this means end of reactants because in chain propagation step, series (hundreds to thousands) of intermediate steps are taking place simultaneously.
Overall reaction converts methane into chloromethane and with continuous reaction, we get dichloromethane, trichloromethane (chloroform) and eventually tetrachloromethane (carbon tetrachloride). However under controlled conditions, 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 below scheme, reaction of methyl radical with chlorine is exothermic; means it is fast and releases energy. However, in the step B, 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, formation of methyl radical is the difficult step in the whole 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 the combination of radicals.
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.
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, enthalpy of 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).
Increasing number of carbon atoms in a molecule increases the heat of combustion. This is obviously 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 cleavage of long carbon chain molecules by heat. The reaction is carried out by passing alkanes through a chamber heated to 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.
Pyrolysis in 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.
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 petroleum industry to increase the yield of low boiling point fractions e.g. gasoline. Another important contribution of catalytic reforming is synthesis of aromatics. Aromatic molecules are better fuel and are precursors for various chemical industries.
Books for further study
- Morrison, R. T., and R. N. Boyd. "Organic chemistry 5th edition." (1987).
- Cary, A, F. Organic Chemistry, 3rd edition, (1996).
- Volhardt K, P, C. Organic Chemistry, (1987).
- Smith M, B and March, J. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 5th edition (2001).