3.2 - Alkanes and Halogenoalkanes Flashcards
How is crude oil separated?
Crude oil is separated by fractional distillation
What are the steps in separated crude oil?
1) The crude oil is vaporised at around 620K
2) The vaporised crude oil travels up the fractionating column
3) Due to different sized alkanes having different boiling points, they condense at different temperatures in the column
4) The smallest hydrocarbons do not condense, and the largest do not vaporise (they are a thick liquid)
What is cracking?
Cracking is the process of breaking long-chain alkanes into smaller hydrocarbons (which can include alkenes)
What is thermal cracking?
Thermal cracking takes place at a high temperature (1250K) and a high pressure (up to 70 atm). Thermal cracking produces a lot of alkenes
What catalyst is used in catalytic cracking?
Catalytic cracking uses zeolite (hydrated aluminosilicate) as a catalyst
What is catalytic cracking?
A catalyst, slight pressure and a medium temperature (800K) are used to mainly produce aromatic hydrocarbons and the alkanes needed to produce motor fuels
What are the benefits of catalytic cracking?
Using a catalyst cuts costs and speeds up the rate of reaction
What are nitrous oxides?
Nitrogen oxides are a series of toxic and poisonous molecules that are produced in car engines
How is ozone formed in a car engine?
Unburnt hydrocarbons and nitrogen oxides react to form ozone, a major component in smog and can cause lung damage and aggravate asthma
What are catalytic converters?
Catalytic converters (made of precious metals like platinum, palladium and rhodium) remove pollutants like nitrogen oxides, unburnt hydrocarbons and carbon monoxide
How is sulphur dioxide removed from power stations?
It is reacted with calcium oxide or calcium carbonate to form calcium sulphate, a harmless salt
How does burning fossil fuels contribute to global warming?
The carbon dioxide released is a greenhouse gas, and therefore contributes to the enhanced greenhouse effect
What is formed when halogens react with alkanes in photochemical reactions?
They form a halogenoalkane. A hydrogen atom is substituted by a halogen; this is called free radical substitution
How are photochemical reactions started?
They are started by UV (ultraviolet) light
What is a free radical?
A free radical is a particle with an unpaired electron
How are free radicals formed?
Free radicals are formed when a covalent bond splits equally, giving one electron to each species.
How is a free radical shown?
A dot is put next to the atom
What are the three stages of free radical substitution?
Initiation, propagation and termination
What happens in the ‘Initiation stage?
Free radicals are produced under UV light e.g. Cl2 → 2Cl•
What happens during the ‘Propagation’ stage?
Free radicals are used up and created in a chain reaction. The first stage is when the free radical attack a molecule, forming another free radical and a separate compound. The second stage is when the free radical previously created attacks the halogen diatomic atom, creating the first free radical and a halogenoalkane
What are the two propagation stages in the formation of chloromethane?
First Stage: Cl• + CH4 → •CH3 + HCl
Second Stage: •CH3 + Cl2 → CH3Cl + Cl•
What happens in the reaction with chlorine and methane when the chlorine is in excess?
The products will go from chloromethane, dichloromethane, trichloromethane and tetrachloromethane
What happens during the ‘Termination’ stage?
Free radicals are used up; two free radicals join together. This ends the chain reaction
What are the three possible termination stages for the synthesis of chloromethane?
1) •CH3 + Cl• → CH3Cl
2) •CH3 + •CH3 → C2H6
3) Cl• + Cl• → Cl2
What are chlorofluorocarbons?
CFCs are halogenoalkane molecules where all of the hydrogen atoms have been replaced by chlorine or fluorine atoms
What is a halogenoalkane?
A halogenoalkane is an alkane with at least one halogen atom in place of a hydrogen atom
What are the polarities of halogenoalkanes?
Most carbon-halogen bonds are polar due to the fact that the halogen has a much higher electronegativity than carbon atoms. The slightly positive carbon means it can be attacked by a nucleophile
What is a nucleophile?
A nucleophile is an electron-pair donor
What happens in a nucleophilic substitution reaction?
A nucleophile attacks a polar molecule, kicks out a functional group and settles itself in its place. The lone pair of electrons on the nucleophile creates a new bond between the nucleophile and the carbon
What is produced when a halogenoalkane reacts with hydroxides and what conditions are needed?
An alcohol is formed. Warm aqueous sodium hydroxide or potassium hydroxide is needed
What is produced when a halogenoalkane reacts with ethanolic potassium cyanide?
A nitrile is formed - the solvent cannot be water as it could act as a competing nucleophile and some alcohol would be formed.
What is produced when a halogenoalkane reacts with ethanolic ammonia?
An amine is formed by a nucleophilic substitution reaction - the solvent cannot be water as it could act as a competing nucleophile and some alcohol would be formed
What is the trend in nucleophilic substitution reaction rates with different halogen polar bonds?
The stronger the bond, the slower the reaction takes place and therefore fluorine has the slowest reaction, followed by chlorine, bromine and iodine
What is an elimination reaction?
A small group of atoms break away from the larger molecule and are not replaced
What happens when you warm a halogenoalkane with hydroxide ions dissolved in ethanol (not water)?
An elimination reaction occurs and an alkene is formed. This is because the OH- ion acts as a base in an elimination reaction (attacking an H+ ion to form water) instead of acting like a nucleophile (attacking the delta positive carbon)
