Hydrocarbons Flashcards
(26 cards)
Alkanes
Very unreactive
Combust completely and incompletely
Free radical substitution reaction by chlorine or bromine
Free radical substitution
Three stages
Initiation
Propagation
Termination
Initiation
Homolytic fission of breaking of Cl-Cl covalent bond to form two Cl radicals
Propogation
Free Cl radicals attack alkanes to produce methyl radical as C-H bond breaks homolytically
Methyl radical can attack chlorine molecule forming chloromethane and regenerating Cl radical
These two propogation steps can go on and on as long as methyl and Cl still present
Termination
Eventually two free radicals collide with each other and a single molecule is formed
As no free radicals are made from this the chain reaction stops
Aliphatic hydrocarbons
Organised into chain
Aromatic hydrocarbons
Organised into benzene ring
Cracking
Used to obtain more useful alkanes and alkenes of lower Mr from larger hydrocarbon molecules
Long chain alkane—>shorter alkanes+alkenes
Alkenes
Much more reactive than alkanes
Unsaturated
Alkene plus Hydrogen
Alkane
Using nickel catalyst
Electrophilic addition reaction
Alkene plus steam
Alcohol
Using sulfuric or phosphoric acid catalyst
Electrophilic addition reaction
Alkene plus hydrogen halides
Haloalkane
Electrophilic addition reaction
Major product
Halogen atom bonded to the carbon with the least number of H attached
Minor product
Halogen atom bonded to the carbon with the most number of H attached
Alkene plus halogen
Dihaloalkane
Electrophilic addition reaction
Alkene plus cold, dilute, acidified manganate ions
Diol
Oxidation reaction
[O]
Alkene plus hot, concentrated, acidified manganate ions
Products depend on what is bonded to the carbon atoms involved in the C=C bonds
C=C breaks completely
The OH group in the diol formed initially are further oxidised to ketones, aldehydes, carboxylic acids or carbon dioxide
Alkene with only hydrogens=CO2
Alkene with one alkyl group and a hydrogen=aldehyde which can then be further oxidised to a carboxylic acid
Alkene with two different alkyl groups=ketone
Addition polymerisation
Alkene undergoes addition reaction with itself to form a longer polymer
Electrophilic addition
Br approaches ethene and π bond repels electrons in Br-Br bond forcing an induced dipole
Eventually Br-Br bond breaks forming bonds between the carbon and bromine
Polyalkenes
Chemically inert and biodegrade very slowly so can cause environmental problems
Can be burnt but need very high temperature to avoid poisonous combustion products being formed
CO
Product of incomplete combustion of alkanes
Will bind to Hb so poses threat to humans
Nitrogen in car engines
Will react with oxygen to produce oxides of nitrogen
These react with water in atmosphere to produce nitric acid which is a main cause of acid rain
Unburnt hydrocarbons
Lead to serious health problems
Carcinogens
Catalytic converters
Catalyse reactions in car engines using platinum
CO+NO—>CO2+1/2N2
CO+1/2O2—>CO2
Unburnt hydrocarbons+O2—>CO2+H2O
Even though catalytic converters remove poisonous pollutants, they can’t do anything for the greenhouse gas carbon dioxide being produced
