Alkanes - Chemical Properties

Chemical properties

Reactions of the alkanes

• Alkanes do not undergo as many reactions as other hydrocarbons. This is due to the following key reasons:
• Alkanes only contain two elements – carbon and hydrogen.
• Alkanes only have non-polar bonds throughout the entire molecule.
• All bonds (carbon-carbon or carbon hydrogen) are single bonds. These are more stable than double or triple bonds and less prone to reactions.
• Perhaps the most significant reaction that alkanes undergo is combustion.
• Alkanes also undergo other types of reactions including substitution reactions and dehydrogenation reactions (these are a kind of elimination reaction), which will be discussed here.

Combustion reactions

As mentioned in the introduction, a common use for alkanes is to generate heat, light and energy through combustion. The general equation for the combustion reaction for alkanes is as follows:

C_nH_(2n+2) + (3n+1)/2 O₂→nCO₂ + (n+1)H₂O

For example the burning of ethane is:

C₂H₆ + 7/2 O₂→2CO₂ + 3H₂O

But remember we prefer to use whole number in the equation (multiply as required):

2C₂H₆ + 7O₂→4CO₂ + 6H₂O

The burning of alkanes produces a lot of energy, in the form of heat and light.
NOTE: The equation above is the ideal equation if the combustion of the alkane is complete. It is more common in real life to get a mixture of products, including carbon (which can be seen as soot), carbon monoxide, and other hydrocarbons. This you can demonstrate by holding a metal object (such as a spoon) over a candle flame.

Substitution Reactions

Substitution reactions are simply when one of the elements in a molecule is replaced by another. In the case of alkanes, the hydrogen atom is the easiest to replace. Important substitution reactions for alkanes include:

• Reaction with chlorine gas, which will be covered in more detail below.
• Amino group
• Sulphonation

An example of a substitution reaction is with chlorine gas to form chloroform and hydrogen chloride gas:
CH_{4 (g)} + Cl_{2 (g)} → CH₃Cl _(g) + HCl _(g)
* (in the presence of UV light or in the dark at 400℃)

The example reaction between methane and chlorine will not occur in the dark at room temperature. The reaction will proceed as soon as you add ultraviolet light to the reaction mixture. It is also possible to get the reaction to proceed in the dark, if the reaction is heated to 400°C. The mechanism for the reaction involves the formation of free radicals. This will be discussed further below. Mechanism for the free radical substitution in alkanes Free radicals are atoms or molecules which have a missing electron in an orbital of one of the atoms. Free radicals are very reactive. Once they collide with other molecules they can rip an electron from a bond in the molecule. A single atom of chlorine has 7 electrons in the outer shell. If you examine chlorine gas, the individual molecules are two chlorine atoms bonded together. It is not found as a single atom in nature because it has a single missing electron in an 3p orbital and it wants to gain another electron. This makes it very reactive. It is important to understand that free radicals exist for only a very short period of time. They are so reactive that almost as soon as they form they react.

The following steps on the detail the stages of the reaction:

1. Initiation
2. Propagation
3. Termination

Initiation

The reaction is started by the UV light hitting a chlorine molecule and splitting it. The electrons in the bond are split equally – the movement of the electrons in the bond is shown by the arrows. A splitting of a molecule which results in the bond electrons being shared equally is known as homolytic fission. The free radical products are drawn with a dot to show the unpaired electron.

Propogation

The reaction is able to continue if the free radicals can collide with other complete molecules, breaking them apart to make new products and other free radicals. This step is the principle part of the reaction and is what allows the reaction to be sustained. Here are two examples of how this can occur in this reaction:

Termination

If the reactive free radicals hit each other then they will form new molecules. These molecules do not take part in new reactions unless hit by free radicals. Since free radicals are crucial to continue the process, the below reactions lower the number of free radicals able to carry on the reaction.
If you analyse the products of the reaction, you will find a variety of compounds in the mix. The substitution is not limited to the replacement of just one hydrogen, so it is possible for the chloromethane to react with more chlorine radicals. The final mix can contain chloromethane, dichloromethane, trichloromethane and tetrachloromethane, as well as ethane, unreacted methane, hydrogen chloride and even chloroethanes. Note: There is a point of confusion in the above paragraph. A single substitution results in the product monochloromethane, but this is often called simply chloromethane. If we are talking about a mix of methane molecules with anywhere from 1 to 4 chlorine atoms in it then we call then collectively chloromethanes. Other substitution reactions: Other substitution reactions for alkanes happen in a similar manner to that described. The difference is in the group that is replaced.

Elimination reactions (dehydrogenation):

• The hydrogen atoms in an alkane can be removed (this is known as dehydrogenation).
• There are 3 main products of dehydrogenation depending up on the process:
  • Carbon
  • Alkenes
  • Alkynes
• Only the first product will be discussed briefly here the others will be discussed in later topics.
• When an alkane goes through an incomplete combustion it forms carbon (the black substance you see when holding a spoon over a candle).
• Alkanes can be processed in this way on an industrial scale to make carbon for use in rubber tyres and printing ink.

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