4c: Alkanes

alkane

saturated hydrocarbons

saturated - definition

only single carbon-carbon bonds

general formula for alkanes

**C**n**H**2n+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

**CH**4

ethane - molecular formula & structural formula

**C**2**H**6

propane - molecular formula & structural formula

**C**3**H**8

butane - molecular formula & structural formula

**C**4**H**10

pentane - molecular formula & structural formula

**C**5**H**12

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)

**CH**4 + **Br**2 → **CH**3**Br** + **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)

**CH**4 + **Cl**2 → **CH**3**Cl** + **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 **CH**2

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