Alkanes

Alkanes are a homologous series which have the general formula C n H 2n+2 The first four members of the homologous series are methane, ethane, propane and butane. They are good fuels because they burn in plentiful of oxygen to produce CO 2  and H 2 O. They react with halogens in UV light, but otherwise are quite unreactive.

As alkanes are non-polar or have very low polarity, they don’t dissolve in polar solvents such as water. However, they are good solvents since they can dissolve other non-polar compounds.
 * General properties of Alkanes **

Alkanes are very unreactive due to strong C-C and C-H bonds within them and also because the carbon atom cannot expand its octet.

Alkanes react with oxygen in air when sufficient activation energy is provided. The reaction is //exothermic.// The complete combustion of alkanes produces carbon dioxide and water: 2C 8 H 8  (l) + 25O 2 (g) ---> 16CO 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> + 18H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O (l)

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">If there is insufficient oxygen available then the alkanes undergo incomplete combustion. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The products of incomplete combustion are carbon monoxide, carbon, water and some carbon dioxide. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">C 8 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">H 18 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (l) + 9O 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (g) ---> C (s) + 5CO (g) + 9H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O (l) + 2CO 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(g)

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Alkanes react with halogens in the presence of ultraviolet light. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Eg: the reaction of methane with chlorine in ultraviolet light to produce chloromethane and hydrogen chloride: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">CH 4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (g) + Cl 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (g) ---> CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Cl (g) + HCl (g)

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">This reaction occurs in a series of steps and the reaction mechanism is what explains these steps. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The reaction mechanism of this reaction is known as a //free radical substitution//. This mechanism consists of an initiation step, a propagation step and a termination step. //(in order to understand the explanations below please look at the background info given at the bottom of the page.)//

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">In this step free radicals are formed. Ultraviolet light provides the energy required to break the Cl-Cl bond homolytically (see bottom of page for details on homolytic fission). <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The chlorine atoms formed are called free radicals because they contain an unpaired electron- they are a highly reactive and energetic species.
 * INITIATION STEPS **

Cl-Cl (g) ---> 2 Cl **. ** (g)

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">When a chlorine free radical collides with a methane molecule it reacts to form hydrogen chloride and the methyl radical (CH 3 . <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">). The methyl radical can react with another chlorine molecule to form chloromethane and another chlorine free radical. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">These are called propagation steps because in each step a radical reacts to generate new radicals.
 * PROPAGATION STEPS **

Cl **. ** (g) + CH 4 (g) ---> CH 3 **. ** (g) + HCl (g)

CH 3 **. ** (g) + Cl 2 (g) ---> CH 3 Cl (g) + Cl **. ** (g)

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">A termination step is where the radicals are consumed. In this step, radicals may collide with container walls or escape completely or free radicals may react with one another to produce a non-radical product.
 * TERMINATION STEPS **

Cl **. ** (g) + Cl **. ** (g) ---> Cl 2 (g)

Cl **. ** (g) + CH 3 **. ** (g) ---> CH 3 Cl (g)

CH 3 **. ** (g) + CH 3 **. ** (g) ---> C 2 H 6 (g)

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">If there is sufficient chlorine present in the reaction mixture, you get a variety of products ranging from chloromethane to tetrachloromethane. CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Cl or CH 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Cl 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> or CHCl 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> or CCl 4

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The overall reaction of ethane with bromine:

By the end of this lesson you should be able to: __**<span style="color: #0000ff; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">Next up- Functional groups **__
 * explain the low reactivity of alkanes in terms of bond enthalpy and bond polarity.
 * Use equations to show the complete and incomplete combustion of alkanes.
 * Use equations to show the equations of ethane and methane with chlorine and bromine.
 * Use the free radical mechanism to demonstrate the reactions of ethane and methane with chlorine and bromine.
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//Background information:// A covalent bond between two atoms X and Y involves the sharing of a pair of electrons. When the bond is broken, it can break homolytically or heterolytically. When the bond breaks such that one of the atoms keeps the shared electron pair, it is called heterolytic fission. X-Y ---> X + + Y - or alternatively X-Y ---> X - + Y +

When the bond breaks such that each atom keeps one of the electrons in the shared pair, it is called homolytic fission. X-Y ---> X**. ** + Y**. ** (where X** . ** and Y** . ** are called free radicals since they have an unpaired electron in their outershell so they are reactive and energetic).