Chemical Reaction of Alkanes – Notes for Class 11 Chemistry

Chemical Reaction of Alkanes Notes for Class 11 Chemistry

Chemical Reaction of Alkanes Notes for Class 11 Chemistry

Alkanes are generally less reactive, as their C-C bonds are stable and are not easily broken. They are inert against ionic or other polar substances hence they are also called “paraffin”(Latin “para + affinis” = “lacking affinity”).

Gaseous alkanes are highly explosive in air, the liquid alkanes are highly flammable. The most common reactions occurring with alkanes are reactions are combustion and free radical halogenation reaction.

  • Reactions with Oxygen/ Combustion Reaction

Alkanes are called saturated hydrocarbons (having single bonds between carbon atoms). They burn with a non-luminous flame insufficient supply of air to release energy; hence they are used as fuels. Complete combustion of alkanes produces carbon dioxide and water vapor.

Alkanes react with oxygen in a combustion reaction. The general equation for complete combustion is:

2 CnH2n+2 + (3n+1) O2 —–> 2(n+1) H2O + 2n CO2

In the absence of sufficient oxygen, incomplete combustion takes place, hence leading to the formation of carbon monoxide and/or soot can be formed, as shown here for methane:

2 CH4 + 3 O2 —–> 2 CO + 4 H2O

CH4 + O2 —–> C + 2 H2O

The standard enthalpy changes of combustion, for alkanes increases by about 650 kJ/mol per CH2 group.

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Oxidation of Cycloalkanes

They are oxidized by alkaline potassium permanganate to dicarboxylic acids involving ring fusion.

Notes for Chemical Reaction of Alkanes, Class 11 Chemistry

  • Reactions with halogens / Halogenation of Alkanes

The replacement of one or more hydrogen atoms in alkane by a halogen (fluorine, chlorine, bromine or iodine) is called as halogenation reaction. The halogenation reactions of alkanes vary, depending on the halogen involved in the reaction. Fluorine reacts explosively with alkanes and can hardly be controlled; chlorine and bromine react satisfactorily. Bromine reacts much slower than chlorine, and iodine is unreactive. The calculated heats of reaction (kcal/mol)for the halogenation reaction with different halogen groups are:

Fluorine:  -116

chlorine:  -27

bromine:  -10

iodine:   +13

The halogenation of an alkane is a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed. Free halogen radicals are very reactive species. For chlorine and bromine, the free radicals have to be created by light and UV radiation, respectively.  The fluorination is difficult to control.

The chlorination of alkane is a three-step process which leads to a mixture of products. Chlorination of methane is shown here as an example:

1. Initiation: During this stage, the splitting of the chlorine molecule into two chlorine atoms with unpaired electrons (free radical) occurs. This step is initiated by ultraviolet radiation. Thus, chlorination of alkanes does not occur in the absence of light.

Cl2 —->  2 Cl·

2. Propagation: A hydrogen atom is drawn off from methane resulting in a methyl radical. The methyl radical pulls a chlorine atom from the Cl2 molecule, leaving another chlorine radical.

  1. CH4 + Cl —–>· CH3· + HClCH3· + Cl2 —– > CH3Cl + Cl·Now, the chlorinated product is formed. This radical so formed, will then go on to take part in another propagation reaction causing a chain reaction.

3. Termination: The chain reaction stops if two free radicals recombine:

  1. Cl· + Cl —–> ·Cl2
  2. CH3· + Cl ——> CH3Cl
  3. CH3· + CH3·  ——> C2H6
  4. Methane and ethane produce randomly distributed products as all hydrogen atoms are equivalent (having an equal chance of being replaced). In higher alkanes, the hydrogen atoms of CH2 or CH groups are preferentially substituted.

 

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Halogenation of Cycloalkanes 

The reactions of the cycloalkanes are same as the alkanes, with the exception of the very small cycloalkanes, i.e. cyclopropane. Cyclopropane and cyclobutane are comparatively less stable due to more strain in the ring. They tend to break up and open the ring to act like a double bond species to give addition reactions. Cyclopentane and onwards, the cycloalkane shows remarkable similarities with alkanes due to their stability. The stability of higher cycloalkanes is due to the less strained ring as well as each carbon atom being sp3 hybridized like alkanes. Higher cycloalkanes do not react by acids, alkalies and common oxidizing under normal conditions.Notes for Chemical Reaction of Alkanes, Class 11 Chemistry

In the presence of UV light/ sunlight, cyclopropane undergoes substitution reactions with chlorine or bromine in the same way as a non-cyclic alkane. However, cyclopropane can also react in the dark. in the absence of UV light. In such reaction conditions, it undergoes addition reactions in which the ring is broken.

Notes for Chemical Reaction of Alkanes, Class 11 Chemistry

Addition reaction of cyclopropane in dark

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