CCEA ADVANCED SUBSIDIARY

CHEMISTRY

MODULE 2

2.3 Alkanes

ALKANES	General formula CnH2n+2		Functional group (C-H)
FORMULA	NAME	melt. pt.	boil. pt.	density
CH4	methane	-183	-162	0.42
C2H6	ethane	-183	-89	0.55
C3H8	propane	-188	-42	0.58
C4H10	butane	-138	-1	0.58
C5H12	pentane	-130	36	0.63
C6H14	hexane	-95	69	0.66

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

C_nH_2n+2 + (3n+1) O₂ nCO₂ + (n+1) H₂O

2

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

CH₂Cl₂

CHCl₃

CCl₄

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

2.3.1 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.