6B: Alkanes Flashcards
What is the general formula of alkanes?
C(n)H(2n+2)
Why are alkanes described as saturated?
All the carbon-carbon bonds are single bonds
All bonded to max number of hydrogens possible
What number of bonds can be found off each carbon?
4 single bonds
What is the general formula of a cycloalkane?
C(n)H(2n)
What is the process of breaking a covalent bond called?
Bond fission
What are the two types of bond fission?
Heterolytic fission - bond breaks unevenly, both electrons go to one atom
Homolytic fission - bond breaks evenly, each bonding atom receives one electron
What does homolytic fission produce?
Two free radicals
How do halogens react with alkanes to form halogenoalkanes?
Photochemical - requires UV light
Hydrogen substituted by a Cl/Br
Called radical substitution reaction
What are the three stages of radical substitution?
Initiation
Propagation
Termination
What occurs in initiation?
Homolytic fission of Cl2/Br2 with UV light
Produces 2 free radicals
What occurs in propagation?
1 Cl/Br free radical forms a bond with a hydrogen from an alkane, forms halogen halide and alkyl radical
Alkyl radical reacts with halogen to form halogenoalkane and another radical
What occurs in termination?
Alkyl radical reacts with halogen radical to form major product
Two alkyl radicals can react to form a minor product
As no radicals produced, ends chain reaction
What is the problem with radical substitution?
Produces a mix of products
Separation can be difficult
Can occur at different places on the carbon chains
What is crude oil/petroleum?
Mixture of hydrocarbons - mostly alkanes
Must be separated to be useful by fractional distillation
Describe the process of fractional distillation?
Crude oil is vaporised at about 350C
Vapor goes into fractionating column and rises
As vapour goes up column, gets cooler and fractions condense at different temps
After condensing they are drawn off the column
How is carbon monoxide a pollutant?
Produced when incomplete combustion occurs
Interferes with how red blood cells carry oxygen
Causes asphyxiation
How are particulates a pollutant?
Produced in incomplete combustion (soot)
Penetrates into lungs, increases risk of heart and lung disease
How are hydrocarbons a pollutant?
Unburnt engine fuel
Some are carcinogens
How are nitrogen oxides pollutants?
Nitrogen reacts with oxygen at high temp.
Creates acid rain which can kill aquatic life
Linked with asthma
How is sulphur dioxide a pollutant?
Caused by industry
Creates acid rain and breathing difficulties
What is a radical?
A species with an unpaired electron
Represented with a single dot in a mechanism
What is cracking?
Breaking long-chain alkanes in to smaller hydrocarbons
Thermal and catalytic
What does cracking produce?
Short alkanes and alkenes
Alkenes are useful for polymers etc.
What is a zeolite?
Hydrated aluminosilicate
Catalyst for cracking
Produces aromatic hydrocarbons and motor fuels
What is reforming?
The process of restructuring an alkane into cycloalkanes & aromatic hydrocarbons
Why is reforming done?
To stop the alkane “knocking”, whereby when compressed they explode
This is common in straight chain, not in cyclo
What does a catalytic converter do?
Platinum catalyst used
Convert pollutants into less harmful chemicals
What is the solubility of alkanes?
Insoluble in polar solvents - water
Soluble in non-polar solvents
Why are fossil fuels non-renewable?
Not sustainable, finite amount of them
What are biofuels?
Fuels made from living matter over a short period of time
What are some examples of biofuels?
Bioethanol - fermentation from sugar from crops
Biodiesel - refining renewable fats and oils
Biogas - breakdown of organic waste matter
Why are biofuels classed as carbon neutral?
Although produce CO2 when burnt, plants absorb it when growing
(doesn’t include transportation)
