Unit 3: section by 2 - Alkanes and halogenoalkanes MDY * Flashcards
Alkanes and petroleum alkanes as fuels chloroalkanes and CFCs halogenoalkanes
what is the general formula for alkanes?
CnH2n+2
what do alkanes contain?
only carbon and hydrogen - they’re hydrocarbons
how many bonds does a carbon have in an alkane?
every carbon has 4 single bonds with other atoms, carbons can only have 4 bonds so alkanes are saturated
what are cycloalkanes?
they have a ring of carbon atoms with 2 hydrogens attached to each carbon
it is also saturated
what is the general formula for cycloalkanes?
CnH2n
assuming they have only 1 ring
what is petroleum?
crude oil, extracted from the ground with oil wells
what does crude oil contain?
a mixture of hydrocarbons, mostly alkanes
how is crude oil separated?
by fractional distillation at an oil refinery
how is fractional distillation carried out?
crude oil vaporised at about 350*C
vapour is passed into a tower which is hot at the bottom and cooler at the top (temperature gradient)
as vapour goes up fractioning column it cools
molecules condense at different heights in the tower as they have different boiling points
how are fractions collected?
large hydrocarbons don’t vaporise at all, they collect at the bottom as a gooey residue
alkane molecules have different chain lengths, so they have different boiling points, so each fraction condenses at a different temperature
hydrocarbons with the lowest boiling points don’t condense, they’re drawn off as gas at the top of the column
what sized hydrocarbons does each fraction collect?
20*C = 1-4 = fuel gas 40*C = 5-12 = gasoline 110*C = 7-14 = naphtha 180*C = 11-15 = kerosene 250*C = 15-19 = diesel 340*C = 20-30 = lubricating oil 30 - 50+ = residue
what are the uses of fuel gas?
liquefied petroleum gas (LPG)
camping gas
what are the uses of gasoline?
petrol as fuel for cars
what are the uses of naptha?
its processed to make petrochemicals
what are the uses of kerosene?
jet fuel
petrochemicals
central heating fuel
what are the uses of diesel?
diesel fuel
central heating fuel
what are the uses of lubricating oil?
lubricating oil
what does residue contain?
30-40 = fuel oil 40-50 = wax, grease 50+ = roofing, = bitumen
what are the uses of fuel oil?
ships, power stations
what are the uses of wax, grease?
candles, lubrication
what are the uses of bitumen?
roofing, road surfacing
what is the supply and demand like for hydrocarbons?
there is high supply of large hydrocarbons and low supply of short hydrocarbons
there is high demand for small hydrocarbons and low demand for large hydrocarbons
what is cracking?
breaking long-chain alkanes into smaller hydrocarbons. it involves breaking the C-C bonds
what are the 2 types of cracking?
thermal cracking
catalytic cracking
what are the conditions for thermal cracking?
high temperatures (up to 1000*C) high pressure (up to 70 atm) this is because bonds in hydrocarbons are so strong
what are the conditions for catalytic cracking?
high temperature (about 450*C) slight pressure zeolite catalysts (hydrated aluminosilicate)
what does thermal cracking produce?
lots of alkenes - these are used to make lots of valuable products like polymers e.g. poly(ethene)
and straight chain alkanes
what does catalytic cracking produce?
branched hydrocarbons
cyclic hydrocarbons
aromatic hydrocarbons like benzene
these are useful as fuels
what are the advantages of using zeolite in catalytic cracking?
zeolites have microscopic pores and channels so they have large surface area
it cuts costs, because the reaction can be done at a low temperature and pressure.
catalyst also speeds up reaction, saving time
what determines where in the fractioning column hydrocarbons condense?
the larger the hydrocarbon the higher the boiling point because it has more electrons so has stronger Van der Waals forces between
how is crude oil formed?
sea plants and animals died and were buried on the ocean floor
overtime they were covered by sand and silt
over millions of years enormous heat and pressure turn them into oil and gas
how is crude oil obtained?
we drill through layers of sand, silt and rock to reach the rock formations that contain oil and gas deposits
why do branched chain alkanes have lower boiling points than their straight chain isomers?
because branches mean alkanes can’t lie flat so there are weaker van der Waals forces between hydrocarbons
what is a complete combustion reaction?
if you burn (oxidise) alkanes (and other hydrocarbons) with plenty of oxygen, you get carbon dioxide and water
e.g.
C3H8 + 5O2 –> 3CO2 +4H2O
what are the only products of complete combustion?
water and carbon dioxide are the only products
how do alkanes make great fuels?
burning just a small amount releases a large amount of energy
it burns very exothermically with oxygen from the air
where are alkanes burnt?
in power stations, central heating systems and to power car engines
what is the downside of burning alkanes?
it produces lots of pollutants
what happens if there’s not enough oxygen for hydrocarbons to burn in?
incomplete combustion.
this produces carbon monoxide, carbon and unburnt hydrocarbons
as well as the original products
what is the disadvantage of carbon monoxide as a product of incomplete combustion?
carbon monoxide gas is poisonous
and colourless and odourless
what does carbon monoxide do?
it binds irreversibly to haemoglobin so oxygen can’t be carried around the body
results in suffocation
what is the problem of soot as a product of incomplete combustion?
pass deep into lungs and aren’t removed so cause breathing problems
build up in engines
contribute to global dimming
how does burning fossil fuels contribute to global warming?
burning fossil fuels produces carbon dioxide, which is a greenhouse gas
what do greenhouse gases do?
greenhouse gases in our atmosphere are really good at absorbing infrared energy (heat). they emit some of the energy they absorb back towards the earth. this is called the greenhouse effect
what is global warming?
by increasing the amount of carbon dioxide in our atmosphere, we are making the Earth warmer
how are oxides of nitrogen (NOx) produced?
when the high pressure and temperature in a car engine and sparks cause the nitrogen and oxygen atoms from the air to react
what is smog?
hydrocarbons and nitrogen oxides react in the presence of sunlight to form ground-level ozone (O3), which is a major component of smog
what negative impacts does ground-level ozone have?
it irritates people’s eyes
aggravates respiratory problems
causes lung damage
how can harmful products be removed from the products of engines?
carbon monoxide, unburnt hydrocarbons and oxides of nitrogen can be removed from the exhaust by catalytic converters
how can sulfur dioxide be produced from burning fuel?
some fossil fuels contain sulfur, which, when burnt, reacts to form sulfur dioxide gas (SO2)
why is sulfur dioxide bad?
it dissolves in the moisture of the atmosphere and is converted into sulfuric acid.
this causes acid rain
what harm does acid rain do?
it destroys trees and vegetation, as well as corroding buildings and statues and killing fish in lakes
how can sulfur dioxide be removed from power station flue gases before it gets into the atmosphere?
flue gas desulphurisation
what is flue gas desulfurization?
waste gases pass through a scrubber
this is calcium oxide or calcium carbonate
this is an acid-base reaction
the reaction forms gypsum which can make plaster board
what happens when soot blocks up engine parts?
less energy given out by the fuel
engine is less powerful
engine needs to burn more fuel to get the same energy
increased costs due to need to use more fuel
what problems do unburnt hydrocarbons cause?
this wastes fuel
harmful because they’re carcinogenic and air pollutants
hydrocarbons act as greenhouse gases
how can unburnt hydrocarbons be removed before being released into the atmosphere?
careful mixing of air/fuel reduces unburnt hydrocarbons
can be removed in catalytic convertors
what are catalytic convertors made of?
they have a ceramic honeycomb coated with a thin layer of catalyst metals (Pt, Pd, Rh)
this is cheaper and gives a larger surface area
how do catalytic convertors remove CO, NOx and unburned hydrocarbons?
2CO +2NO –> 2CO2 +N2
C3H8 +10NO –> 3CO2 +5N2 +4H2O
what reactions take place in flue gas desulfurization?
CaO +SO2 –> CaSO3
CaCO3 +SO2 –> CaSO3 + CO2
what is a free radical?
a particle with an unpaired electron
this makes them very reactive
a free radical can be shown as a dot e.g. Cl•
how do free radicals form?
when a covalent bond splits equally, giving an electron to each atom
what is a photochemical reaction?
when halogens react with alkanes that are started by ultraviolet light
a hydrogen atom is substituted by chlorine or bromine this is a free-radical substitution reaction
what are the steps of a free-radical substitution reaction?
initiation - free radicals are produced
propagation - free radicals used up and created in a chain reaction
termination - free radicals are gotten rid of
what happens in initiation in the reaction between chlorine and methane?
sunlight provides enough energy to break Cl-Cl bond, this is photodissociation
the bonds split equally and each atom becomes a highly reactive free radical
Cl2 —UV —> 2Cl•
what happens in the propagation step of the reaction between chlorine and methane?
Cl• reacts with a methane molecule
Cl• +CH4 –> CH3• + HCl
new methyl free radical can react with another Cl2 molecule
CH3• + Cl2 –> CH3Cl + Cl•
these reactions can then happen on repeat
what happens in the termination step of the reaction between chlorine and methane?
if 2 free radicals join together, they make a stable molecule. the 2 unpaired electrons form a covalent bond
Cl• + CH3• –> CH3Cl
CH3• + CH3• –> C2H6
what happens after a free-radical substitution reaction between chlorine and methane?
if chlorine’s in excess, Cl• free radicals start attacking chloromethane, to produce dichloromethane, trichloromethane and tetrachloromethane
if methane is in excess, the products will mostly be chloromethane
what are chlorofluorocarbons?
CFCs are halogenoalkane molecules where all of the hydrogen atoms have been replaced by chlorine and fluorine
what does ozone do in the upper atmosphere?
it absorbs a lot of ultraviolet radiation from the sun, stopping it from reaching us
UV radiation can cause sunburn and even skin cancer
how is ozone formed?
it forms naturally when an oxygen molecule is broken down into 2 free radicals by ultraviolet radiation, the free radicals attack other molecules forming ozone
O2 —-> O• + O•
O2 + •O• –> O3
how are chlorine free radicals formed naturally?
formed in the upper atmosphere when C-Cl bonds in CFCs are broken down by ultraviolet radiation.
these free radicals are catalysts
how do chlorine free radicals react with ozone?
Cl• + O3 –> O2 +ClO•
ClO• +O3 –> 2O2 + Cl•
so overall 2O3 –> 3O2 and Cl• is a ctalyst
what are the properties of CFCs?
they are pretty unreactive, non-flammable and non-toxic
they used to be used as coolant in fridges
why were CFCs banned?
in the 1970s research by several different scientific groups demonstrated that CFCs were causing damage to the ozone layer (ozone hole).
the advantages were outweighed by the environmental problems so they were banned
how have CFCs been replaced?
chemists have developed safer alternatives which contain no chlorine. like HFCs (hydrofluorocarbons) and hydrocarbons
what does infrared radiation do?
excites the C=O C-H and O-H bonds in carbon dioxide, methane and water vapour.
this causes bonds to vibrate more vigorously
molecules then re-emit energy in all directions as heat
what are some examples of greenhouse gases?
carbon dioxide hydrocarbons water vapour methane nitrous oxide
what does the greenhouse effect of a given gas depend on?
its atmospheric concentration - greater concentration = more molecules that can absorb IR
its ability to absorb infrared radiation - some re-emit more strongly than others
what is the evidence for global warming?
the carbon dioxide and methane concentrations in the atmosphere are increasing and they have been increasing rapidly since the 1800s
global average temperature has also been increasing rapidly proportionally to greenhouse gas levels
what is a halogenoalkane?
an alkane with at least 1 halogen atom in place of a hydrogen atom
how are carbon-halogen bonds polar?
halogens are much more electronegative than carbon, so the bonds are polar
the 𝛿+ charge on the carbon atom makes it prone to attacks from nucleophiles
what is a nucleophile?
an electron-pair donor
it donates an electron pair to somewhere without enough electrons
e.g. negative ion, 𝛿-, lone pair
what are some examples of nucleophiles that can react with halogenoalkanes?
OH-, CN- and NH3
what is a nucleophilic substitution reaction?
a nucleophile can react with a polar molecule by kicking out the functional group and taking its place
how can the movement of electrons been shown in an equation?
curly arrows show the movement of electron pairs
how can a halogenoalkane become an alcohol?
by reacting with hydroxides
the 𝛿+ carbon attracts a lone pair of electrons from an OH- ion, breaking the C-halo bond
halogen leaves taking electron pair with it, a new bond forms between the carbon and OH-
how are nitriles formed?
if you warm a halogenoalkane with ethanolic potassium cyanide (potassium cyanide dissolved in ethanol)
you get a nitrile.
its a nucleophilic substitution reaction - the cyanide ion is the nucleophile
how to form amines?
if you warm a halogenoalkane with excess ethanolic ammonia, the ammonia swaps places with the halogen - its a nucleophilic substitution reaction
ammonia molecule removes a hydrogen from NH3 group forming amine and an ammonium ion (NH4 +)
the ammonium ion can react with the halogen to form ammonium halide
e.g.
CH3CH2Br + 2NH3 -ethanol-> CH3CH2NH2 +NH4Br
which halogenoalkanes react fastest?
iodoalkanes react fastest
fluoroalkanes react slowest
the carbon-halogen bond strength (enthalpy) decides reactivity. this is because that bond needs to break for any reaction to occur
why is the C-F bond strongest?
it has the highest bond enthalpy. so fluoroalkanes undergo nucleophilic substitution reactions more slowly than other halogenoalkanes
why is the C-I bond weakest?
it has the lowest enthalpy, so its easier to break. this means that iodoalkanes are substituted more quickly
what is the bond enthalpy of each halogenoalkane?
C-F = 467 kJ/mol
C-Cl = 346 kJ/mol
C-Br =290 kJ/mol
C-I 228 kJ/mol
how do halogenoalkanes undergo elimination reactions?
if you warm a halogenoalkane with hydroxide ions dissolved in ethanol instead of water, an elimination reaction happens and you end up with an alkene. it must be heated under reflux so you don’t lose volatile substances
how does the halogenoalkane elimination reaction work?
OH- acts as a base and takes a proton H+ from the carbon on the left. this makes water
the left carbon now has a spare electron, so it forms a double bond with the middle carbon
to form the double bond, the middle carbon has to let go of the halogen, which drops off as a halide
what is a halogenoalkane elimination reaction?
a small group of atoms break away from a molecule. this group is not replaced by anything else
what happens when halogenoalkanes react with hydroxides?
they can undergo either nucleophilic substitution or elimination. the 2 reactions are said to be competing
if hydroxide is dissolved in water OH- acts as nucleophile in substitution
if hydroxide is dissolved in ethanol that OH- acts as base in elimination
what does a mixture of water and ethanol do to a halogenoalkane reacting with hydroxides?
both substitution and elimination happen, and you’ll get a mixture of 2 products
what 2 ways can a covalent bond be broken?
using energy - endothermic
heterolytically - bond splits unevenly and 1 atom takes both electrons
homolytically - bond splits evenly (using UV)
what is a reaction mechanism?
a series of steps that shows what happens in a chemical reaction
arrows are used to show the movement of electrons
what is a substitution reaction?
a reaction where an atom/ group replaces another atom/ group
what is the Montreal protocol?
worldwide legislation in 1987 to ban CFC production in 1989
what is the general formula for a halogenoalkane?
Cn H2n+1 X
shortened to R-X
how are halogenoalkanes named?
prefixes are used to show which halogen is present (fluoro, chloro, bromo, iodo)
if multiple are present, they’re listed in alphabetical order
numbers show which carbon the halogen is attached to
prefixes are used to say how many atoms of each halogen are present (di, tri, tetra)
what are primary, secondary and tertiary halogenoalkanes?
primary - carbon it’s attached to is only bonded to 1 other carbon
secondary - carbon it’s attached to is bonded to 2 other carbons
tertiary - carbon it’s attached to is bonded to 3 other carbons
what is hydrolysis?
splitting a molecule by using water
how does polarity make hydrocarbons more reactive?
the more electronegative halogens remove charge from carbon its bonded to
this makes electron-deficient carbon
its vulnerable to attack by electron-rich nucleophiles
how does enthalpy make hydrocarbons more reactive?
its the amount of energy required to break bonds, so the lower the bond enthalpy the easier the bonds break for reactions to occur
does bond polarity or enthalpy determine how reactive halogenoalkanes are?
if bond polarity is important for the reaction then R-F will be most reactive
if bond enthalpy is important for the reaction then R-I will be most reactive
what are the steps of nucleophilic substitution?
lone pair of electrons on nucleophile are attracted to electron deficient carbon
nucleophile causes electrons in C-X bond to be repelled towards more electronegative halogen, forming halide ion
what conditions are needed for nucleophilic substitution with :OH-?
warm, aqueous
what conditions are needed for nucleophilic substitution with :CN-?
ethanolic, warm
what conditions are needed for nucleophilic substitution with :NH3?
excess concentrated ammonia dissolved in ethanol at pressure in a sealed container
why is ethanol needed in nucleophilic substitutions?
if not with an :OH- nucleophile, OH in water will react instead of intended nucleophile
what conditions are needed for a nucleophilic elimination reaction?
heat
ethanol as a solvent (no water present)
concentrated potassium hydroxide
what type of halogenoalkanes are more likely to undergo substitution or elimination?
primary - substitution
tertiary - elimination
