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CSEC Chemistry: Reactions of Alkanes and Alkenes

By now, you have already become familiar with the general formulae of alkanes (CnH2n+2) and alkenes (CnH2n). Alkanes are saturated hydrocarbons, that is, they are composed of only single carbon bonds, and thus contain the maximum number of hydrogen atoms in their structure. Alkenes are unsaturated because they contain carbon double bonds in their structures, and so, they do not contain the maximum number of hydrogen atoms in their structure.


Chemical Reactions of Alkanes

Since they belong to the same homologous series, alkanes have similar chemical properties and behave similarly in reactions.


Combustion

Alkanes will burn in an excess of air (or oxygen) when ignited to produce carbon dioxide, water and heat. This is known as a complete combustion reaction.

For example, the complete combustion of pentane is shown as:

C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O + heat

So, the reactants (an alkane and oxygen) will burn under the condition of some heat of ignition to form the products carbon dioxide, water and heat.

Because heat is released in this equation, this reaction is exothermic. (Read our previous post on energetics if you are unfamiliar with the concept)


NB: Sometimes, the supply of oxygen in the reaction is insufficient, so the alkane is not completely oxidised to carbon dioxide and water. In these incomplete combustion reactions, products such as carbon, carbon monoxide and hydrogen may be formed instead.


Halogenation (Reactions with Halogens)

In the presence of bright light, alkanes will undergo substitution reactions with the halogens. In substitution reactions, one or more hydrogen atoms in a saturated compound are replaced by another atom or group of atoms- in this case, halogens.

The reaction will usually yield a mixture of organic products and a corresponding hydrogen halide. In halogenation reactions involving chlorine, this hydrogen halide will be hydrogen chloride gas. For example:

CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

The organic product there is called monochloromethane.

If this product is reacted with more chlorine gas, there can be further substitution:

CH₃Cl(g) + Cl₂(g) → CH₂Cl₂(g) + HCl(g)

(dichloromethane)

CH₂Cl₂(g) + Cl₂(g) → CHCl₃(l) + HCl(g)

(trichloromethane)

CHCl₃(l) + Cl₂(g) → CCl₄(l) + HCl(g)

(tetrachloromethane)


How do we test to determine if substitution is occurring? Well, we simply have to test for the release of hydrogen chloride gas by reacting what is released in the reaction with concentrated ammonia. If hydrogen chloride is being released, then white clouds of ammonium chloride will be produced:

HCl(g) + NH3(g) → NH4Cl(s)

The products of these halogenation reactions are known as halogenoalkanes.

If you are wondering why the condition of bright light is needed for halogenation reactions of alkanes, it is because the energy of light (or similar radiation) will break the single bond of the halogen molecule to produce reactive halogen atoms or free radicals (atoms containing an unpaired electron). So, in the case of chlorine:

Cl₂ → 2Cl

chlorine in bright light breaks down into two free radical chlorine atoms

These atoms can then react with the alkanes to form chloroalkanes and hydrogen chloride gas.


Uses of Alkanes

Alkanes are often used as fuels because they produce a lot of heat when they burn. This heat, known as their heat of combustion is said to be very large. They are also suitable fuels because they ignite easily but not spontaneously, they burn well and not explosively, they have low smoke and ash release during burning, they are inexpensive, safe to use and are easy to store and transport.

Halogenoalkanes are also widely used:

Some are solvents (eg tetrachloromethane), while some are used as anaesthetics (eg halothane and chloromethane). Halogenoalkanes are also used in pesticides. Apart form these beneficial uses, chlorofluorocarbons (a type of halogenated carbon) are harmful to the environment.


Chemical Reactions of Alkenes


Combustion

Similarly to alkanes, alkenes will burn in excess oxygen to produce carbon dioxide, water and heat. This reaction is also very exothermic.

C2H4(g) + 3O2(g) → 2CO2(g) + 2H2o(g) + heat

It is possible to use combustion reactions to determine the molecular formula of the hydrocarbon used. (The following question is taken from CSEC Chemistry Paper 2 2007)


X and Y are two hydrocarbons. When X is completely burnt in air, 0.50 moles of X produce 60 dm3 of C02 and 3 moles of water at r.t.p. Deduce the molecular formula of X. (1 mole of gas at r.t.p. has a volume of 24 dm3)


Step 1:

The question states that X was completely burn in air, so complete combustion took place. That means that all the carbon in X has been converted into CO2, and so, the amount of carbon in X and in 60 dm^3 of CO2 are the same.


So, 60 dm^3 of CO2 is (60/24) mol = 2.5 mol


0.5 mol of X produced 2.5 mol of CO2

So 1 mol of X will produce (2.5/0.5) = 5 mol CO2


5 mol CO2 contains 5 mol of Carbon, so there are (5×12)g= 60g of carbon in 1 mol of X

Now we know that there are 60g of carbon in 1 mol of X.


Step 2:

To find the molar mass of the hydrogen in X, we must first realize that the amount of hydrogen in the water produced and the amount of hydrogen in the hydrocarbon X are the same. Then, 0.5 mol of X produces 3 mol of water.

1 mol of X will produce 3/0.5= 6 mol of water.

6 mol of water contains 12 mol of hydrogen. So, there are 12 × 1g of hydrogen in 1 mol of X.

Combining these two values, 60g (5 mol) of carbon and 12g (12 mol) of hydrogen, we get the molecular formula C5H12 (pentane).



Addition Reactions

Typically, alkenes will undergo addition reactions, where they react with a given reagent to form a saturated compound as the only product.


Reaction with Hydrogen (Hydrogenation)

In the presence of finely divided nickel (the catalyst of the reaction), hydrogen adds to alkenes to form alkanes.


As you can see, one carbon atom is added to each of the carbon atoms in the double bond.


Hydration of Alkanes to Produce Alkanols

Before, we looked at hydrogenation, the addition of a hydrogen atom across a double bond. Hydration is the addition of a water molecule across a double bond to form an alkanol.

As you can see, the water molecule is added in two separate parts: a hydrogen atom and a hydroxyl group, creating an alkanol.

This hydration shown above is direct, and there exists a way to do it indirectly using sulphuric acid to form an intermediate.



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