Alkanes- As level notes edexcel igcse notes

Associated with these alkanes we have alkyl radicals of
general formula CnH2n+1 (symbol R)

e.g. CH3- methyl,
C2H5- ethyl,
C3H7- propyl
This family of alkanes forms an homologous series.

Combustion in a limited and plentiful supply of air.

Combustion

The alkanes are used as fuels and burn in excess air or oxygen producing carbon dioxide and water.
2C2H6 + 7O2 ---- 4CO2 + 6H2O
C3H8 + 5O2 ---- 3CO2 + 4H2O

In general
CnH2n+2 + (3n+1) O2 -nCO2 + (n+1) H2O

In a limited air supply carbon monoxide is produced
2C2H6 + 5O2--- 4CO + 6H2O


Reaction with chlorine or bromine , monohalogenation only (except for methane and chlorine.


Alkanes are generally unreactive. They are saturated and react by substitution.
Halogenation
Chlorine and bromine react with alkanes in the presence of strong sunlight or u.v. light giving a series of products formed by successive replacement of a hydrogen atom by a halogen atom.
R-H + X2 R-X + HX

Mechanism of the photochemical reaction between chlorine and methane viewed as a free radical substitution.

Chlorine reacts explosively with methane in the presence of strong sunlight or u.v. light to give a mixture of products.
CH4 + Cl2 --- CH3Cl + HCl

This is photochemical chlorination.

Mechanism
The mechanism of a reaction is the course believed to be followed by the reactants in combining together and the various stages inv olved in reaching the final products.This is a free radical chain reaction.
Initiation
Cl2 ---2Cl. (a few)

Propogation
Cl. + CH4 CH3. + HCl
CH3. + Cl2 CH3Cl + Cl.
CH3Cl + Cl. CH2Cl. + HCl
CH2Cl. + Cl2 CH2Cl2 + Cl.

Termination
2Cl. Cl2
CH3. + Cl. CH3Cl
2CH3. C2H6


Isomerism in Organic Compounds

Structural isomerism for aliphatic compounds containing up to six carbon atoms, to include branched structures. (Cyclic structures excluded)

Isomerism

Strutural isomerism occurs when 2 or more compounds have the same molecular formula but different structural formula. (ie same number of atoms but bonded together differently).

Draw and name all the structural isomers of C4H10, C5H12 and C6H14.

Environmental problems associated with spillage and combustion of alkane fuels.

Unreactive nature of alkanes towards electrophiles and nucleophiles.