Alkanes and Halogenalkanes lol Flashcards
What do alkenes contain?
Only hydrogen and carbon do hydrocarbons
How many bonds does every carbon atom in an alkane make?
Four single bonds with other atoms so impossible for carbon to make more than four bonds so alkenes are saturated (only contain single bonds)
Alkanes first four
Methane
Ethane
Propane
Butane
What else can you get like alkanes?
Cycloalkanes but have different general formula
CnH2n assuming only one ring but still saturated
Petroleum what is it?
Crude oil
Mixture of hydrocarbons (mainly alkanes ranging from small alkanes like pentane to massive alkanes of more than 50 carbons.
Is crude oil useful?
Not as it is it needs to undergo fractional distillation
Fractional distillation step 1
1) first the crude oil is vaporised at about 350 degrees
Fractional distillation step 2
Vaporised crude oil goes into a fractional column and rises up through the trays. The largest hydrocarbons don’t vaporise at all, because their boiling points are too high- they just run to the bottom and form a gooey residue
Fractional distillation step 3
As crude oil vapour goes up the fractionating column it gets cooler because alkane molecules have different chain lengths they have different boiling points so each fraction condenses at a different temperature. The fractions are drawn off at different levels in the column. They’re drawn off as gases at the top of the column
Gases
1-4 carbons
Liquefied petroleum gas (LPG) camping gas
40 degrees
Petrol
5-12 carbons
Petrol
110 degrees
Naphtha
7-14 carbons
Processed to make petrochemicals
180 degrees
Kerosene (paraffin)
11-15 carbons
Jet fuels. Petrochemicals, central heating
250 degrees
Gas oil (diesel)
15-19 carbons
Diesel fuel, central heating fuel
340 degrees
Mineral oil (lubricating)
20-30 carbons
Lubricating oil
Residue
Fuel oil, 30-40 carbons, ships, power stations
wax, grease, 40-50 candles lubrication
bitumen
50+ roofing, road surfacing
What’s most in demand from crude oil?
Lighter fractions of crude oil like petrol and naphtha
Heavier stuff like bitumen
So more expensive to get stuff in demand
How is this demand met?
Less popular heavier fractions are cracked. Cracking is breaking long-carbon chain alkanes into smaller hydrocarbons (including alkenes) involved breaking c-c bonds
Can you choose a certain length to crack to?
No where the chain breaks is random so you’ll get a different mixture of products everything you crack a hydrocarbon
General formula for alkane a?
CnH2n+2
What do alkenes contain?
Only hydrogen and carbon do hydrocarbons
How many bonds does every carbon atom in an alkane make?
Four single bonds with other atoms so impossible for carbon to make more than four bonds so alkenes are saturated (only contain single bonds)
Alkanes first four
Methane
Ethane
Propane
Butane
What else can you get like alkanes?
Cycloalkanes but have different general formula
CnH2n assuming only one ring but still saturated
Petroleum what is it?
Crude oil
Mixture of hydrocarbons (mainly alkanes ranging from small alkanes like pentane to massive alkanes of more than 50 carbons.
Is crude oil useful?
Not as it is it needs to undergo fractional distillation
Fractional distillation step 1
1) first the crude oil is vaporised at about 350 degrees
Fractional distillation step 2
Vaporised crude oil goes into a fractional column and rises up through the trays. The largest hydrocarbons don’t vaporise at all, because their boiling points are too high- they just run to the bottom and form a gooey residue
Fractional distillation step 3
As crude oil vapour goes up the fractionating column it gets cooler because alkane molecules have different chain lengths they have different boiling points so each fraction condenses at a different temperature. The fractions are drawn off at different levels in the column. They’re drawn off as gases at the top of the column
Gases
1-4 carbons
Liquefied petroleum gas (LPG) camping gas
40 degrees
Petrol
5-12 carbons
Petrol
110 degrees
Naphtha
7-14 carbons
Processed to make petrochemicals
180 degrees
Kerosene (paraffin)
11-15 carbons
Jet fuels. Petrochemicals, central heating
250 degrees
Gas oil (diesel)
15-19 carbons
Diesel fuel, central heating fuel
340 degrees
Mineral oil (lubricating)
20-30 carbons
Lubricating oil
Residue
Fuel oil, 30-40 carbons, ships, power stations
wax, grease, 40-50 candles lubrication
bitumen
50+ roofing, road surfacing
What’s most in demand from crude oil?
Lighter fractions of crude oil like petrol and naphtha
Heavier stuff like bitumen
So more expensive to get stuff in demand
How is this demand met?
Less popular heavier fractions are cracked. Cracking is breaking long-carbon chain alkanes into smaller hydrocarbons (including alkenes) involved breaking c-c bonds
Can you choose a certain length to crack to?
No where the chain breaks is random so you’ll get a different mixture of products everything you crack a hydrocarbon
General formula for alkane a?
CnH2n+2
Two types of cracking?
Thermal cracking
Catalyst cracking
What conditions does thermal cracking need?
A high temperature( up to 1000oC) High pressure (up to 70 atomspheres)
What does thermal cracking produce?
A lot of alkenes
Alkenes are used to make heaps of valuable products like polymers (plastic). A good example is poly(ethene) which is made from ethene.
What conditions does catalytic cracking need?
A zeolite catalyst (hydrated aluminosilicate)
Slight pressure
High pressure (about 450oC)
What does catalytic cracking produce?
Aromatic hydrocarbons and motor fuels
Describe aromatic compounds
Contain benzene rings
Benzene rings have six carbons with three double bonds
Pretty stable because electrons are delocalised around the carbon ring
Why is catalytic cracking cheaper than thermal cracking?
Using a catalyst cuts costs because the reaction can be done at a low pressure and a lower temperature
The catalyst also speeds up the reaction saving time and time is money
What happens if you burn (oxidise) alkanes and other hydrocarbons with plenty of oxygen?
Carbon dioxide and water
Combustion reaction
Complete combustion
What do alkanes make?
Great fuels
Burning just a small amount releases a humongous amount of energy
Where are alkanes used?
Burnt in power stations, central heating systems and to power a car
Downside of alkanes?
Produce a lot of pollutants
What happens if there’s not enough oxygen around?
Hydrocarbons combust incompletely to produce either carbon monoxide or carbon particles
When a hydrocarbon undergoes incomplete combustion?
You get carbon monoxide gas instead or as well as carbon dioxide
Why is carbon monoxide bad news?
It’s poisonous
Binds to the same sites on haemoglobin molecules in red blood cells as oxygen molecules so oxides can’t be carried around the body.
Luckily, carbon monoxide can be removed from exhaust gases by catalytic converters on cars
Soot can be formed by?
Incomplete combustion
Why is soot bad news?
Thought to cause breathing problems
Can build up in engines meaning they don’t work properly
Burning fossil girls produces what?
Carbon dioxide
What is carbon dioxide?
Greenhouse gas
Greenhouse effect?
Greenhouse gases in our atomspheres are really good at absorbed infrared energy (heat). Emit some of the energy they absorb back towards the Earth keeping it warm.
What do most scientists agree?
Increasing the amount of carbon dioxide in our atomsphere we are making the Earth warmer
It’s called global warming
Just learn this rubbish
What don’t engines do?
Burn all fuel molecules some will come out as unburnt hydrocarbons
When are oxides of nitrogen (NOx) produced?
When high pressure and temperature in a car engine cause the nitrogen and oxygen atoms from the air to react together
When do hydrocarbons and nitrogen oxides react together?
In the presence of sunlight to form ground-level ozone (O3) which is a major component of smog.
What does ground-level ozone do?
Irritate people’s eyes
Aggravates respiratory problems
Causes lung damage (ozone isn’t nice stuff unless it’s high up in the atomsphere as part of the doubt layer)
What do catalyst converters do?
Remove unburnt hydrocarbons and oxides of nitrogen from the exhaust
What do some fossil fuels contain?
Sulfur
When burnt sulfur reacts to form sulfur dioxide gas (SO2)
Sulfur dioxide is it good?
NO
If it gets into the atomsphere it dissolves into the moisture and is converted into sulfuric acid which causes acid rain.
What else causes acid rain?
When nitrogen oxides escape into the atomsphere- nitric acid is produced
Why does acid rain matter?
Destroys trees and vegetation
Corroding building and statues
Kills fish in lakes
Sulfur dioxide can be removed how from power stations flue gases before it gets into the atomsphere?
Powdered calcium carbonate (limestone) or calcium oxide mixed with water to make an alkaline slurry. When flue gases mix with alkaline slurry, the acidic sulfur dioxide gas reacts with calcium compounds to form harmless calcium sulfate
What’s a free radical?
A particle with an unpaired electron
Unpaired electron makes them very reactive
Represented by •
When do free radicals form?
When a covalent bond splits equally giving one electron to each atom
When do halogen react with alkanes?
In photochemical reactions-reactions started by UV light
Hydrogen atom substituted by chlorine or bromine. Free radical substitution reaction
Chlorine and methane goes to?
UV
CH4+Cl2=> CH3Cl+HCl
Three stages of free-radical substitution reaction?
Initiation reaction
Propagation reaction
Termination
Initation reaction
Free radicals produced
Sunlight provides enough energy to break Cl-Cl bond (photodissociation)
Cl2=UV=> 2Cl•
Bond splits equally and each atom gets to keep one electron. Atom becomes highly reactive free radical Cl• because of unpaired electron
Proposition reaction
Free radical used up and created in chain reaction
Cl• +CH3-> CH3• +HCl
CH3• +Cl2=> CH3+Cl•
New Cl• can attack another CH4 molecule and do on until all Cl2 or CH4 molecules are used up
Termination reaction
Free radicals mopped up
If two free radicals join together they form a stable molecule
The two unpaired electrons form covalent bond
Heaps of possible termination reactions
Cl• +CH3• => CH3Cl
CH3• +CH3•=> C2H6
What happens after termination
If chlorine in excess Cl• free radicals will start attacking chloromethane producing CH2Cl2 CHCl3 CCl4
But if methane in excess product will mostly be chloromethane
What a are CFC’s
Chlorofluorocarbons
Halogenalkane molecules where all of the hydrogen atoms have been replaced by chlorine and fluorine
What does ozone in the upper atomsphere do?
Acts as a chemical sunscreen
Absorbs lots of UV radiation from the Sun stopping it from reaching us
UV radiation can cause sunburn or even skin cancer
How is ozone formed naturally?
When an oxygen molecule is broken down into two free radicals by UV radiation
Free radicals attack other oxygen molecules forming ozone
O2=UV=> O•+O•
O2+O•=>O3
How are CFC’s causing holes in the ozone?
Chlorine free radicals are formed in the upper atmosphere when C-Cl bonds in CFCs are broken down by UV radiation
CCl3F=UV=> CCl2F+Cl•
Free radicals are catalysts
React with ozone to form intermediate ClO• and an oxygen molecule
Cl• +O3=> O2+ ClO•
ClO•+O3=> 2O2+Cl•
Overall equation
2O3=> 3O2
Cl• is the catalyst
Chlorine free radical regenerated?
Can go straight on to attack another ozone molecule so only takes one chlorine free radical to destroy loads of ozone molecules
CFC’s were used because?
Pretty unreactive, non-flammable and non-toxic
Used to be used as coolant gas in fridges as solvents and as propellants in aerosols
Why did they stop using them?
1970’s research by several different scientific groups demonstrated that CFCs were causing damage to the ozone. The advantages of CFCs couldn’t outright the environmental problems they were causing so they were banned.
How come we still can have fridges and stuff now CFCs are banned?
Chemists have developed safer alternatives to CFCs which contain no chlorine such as HFC hydrofluorocarbons and hydrocarbons
What’s a halogenoalkane?
An alkane with at least one halogen atom in place of a hydrogen atom
Halogens are much more?
Electronegative than carbon so carbon-halogen bonds are polar
δ+ charge on the carbon does what?
Makes it prone to attacks from nucleophiles
What’s a nucleophile?
An electron-pair donor
Donates an electron pair to somewhere without electrons
OH-, CN-, NH3 are all nucleophiles that can react with halogenoalkanes
What can a nucleophile react with?
With a polar molecule by kicking out the functional group and taking its place (nucleophilic substitution reaction go learn it)
The carbon- halogen strength decides what?
Reactivity for any reaction to occur the carbon-halogen bond needs to break
What bond is the strongest?
C-F
It has the highest bond enthalpy
So fluoroslksnrd undergo nucleophilic substitution reactions slower the other halogenoalkanes
Lowest bond enthalpy?
C-I bond
So easier to break
Iodoalkanes substituted quicker
Explain elimination reaction theory
Warm halogenoalkane with HO- ions dissolved in ethanol instead of water an elimination reaction happens and you end up with an alkene
Anhydrous
Under reflux or you’ll lose volatile stuff
Learn the mechanism
What happens when halogenalkanes react with hydroxide?
Either they will undergo nucleophilic substitution or elimination depending on the conditions and are said to be competing
What conditions favour nucleophilic substitution?
OH-/ water
Reflux
Aqueous conditions
OH- acts as a nucleophile
What conditions favour elimination?
Anhydrous conditions
OH- acts as a base
OH-/ ethanol
Reflux
What would happen if you used a mixture of water and ethanol as a solvent?
Both reactions will happen and you’ll get a mixture of two products
