4c: Alkanes

alkane

alkane

saturated hydrocarbons

saturated - definition

only single carbon-carbon bonds

general formula for alkanes

CnH2n+2

properties of alkanes

• colourless compounds

• gradual change in physical properties as carbon atoms increase

• generally unreactive

• undergo combustion

• can be cracked into smaller molecules

• can react with halogens in the presence of light

first 5 members of the alkane homologous series

1. methane

2. ethane

3. propane

4. butane

5. pentane

uses - methane, ethane, propane

• methane: major component of natural gas

• ethane, propane: present in natural gas and LPG

LPG

liquefied petroleum gas

methane - molecular formula & structural formula

CH4

ethane - molecular formula & structural formula

C2H6

propane - molecular formula & structural formula

C3H8

butane - molecular formula & structural formula

C4H10

pentane - molecular formula & structural formula

C5H12

what happens in a substition reaction

a halogen atom takes place of a hydrogen atom

what has to be present for alkanes to undergo a substitution reaction with halogens

UV radiation

methane and bromine - substitution reaction in the presence of UV radiation (symbol equation)

CH4 + Br2 → CH3Br + HBr

methane and bromine - substitution reaction in the presence of UV radiation (word equation)

methane + bromine → bromomethane + hydrogen bromide

methane and chlorine - substitution reaction in the presence of UV radiation (symbol equation)

CH4 + Cl2 → CH3Cl + HCl

methane and chlorine - substitution reaction in the presence of UV radiation (word equation)

methane + chlorine → chloromethane + hydrogen chloride

which family do the products of the substitution reactions in the presence of UV radiation belong to

halogenoalkanes / haloalkanes

CFC

chlorofluorocarbon - a class of halogenoalkanes

isomers

compounds with the same molecular formula but different structural formulae

properties of a homologous series

• same functional group

• similar chemical properties

• trend in physical properties

• described by same general formula

• differ from the next by CH2

free radical

the atoms formed when substitution reactions occur due to the presence of unpaired electrons

mono-substitution

when only one hydrogen atom in the alkane is replaced by a halogen atom