Alkanes and Halogenoalkanes Flashcards
Alkanes are ________ hydrocarbons
Saturated
Every carbon atom in an alkane has four single bonds with other atoms. It is impossible for carbon to make any more bonds so alkanes are saturated.
What is the general formula of an alkane?
CnH2n+2
You can get ____________ too. They have a ring of carbon atoms with two hydrogens attached to each carbon.
What is different about the general formula?
cycloalkanes
CnH2n (1 ring) - still saturated
Crude oil is mainly alkanes but it isn’t very useful - describe fractional distillation
1) The crude oil is vaporised at about 350 degrees Celsius
2) The vaporised crude oil goes into a fractionating column and rises up through the trays. The largest hydrocarbons don’t vaporise at all, because their boiling point is too high - they just run to the bottom to form a gooey residue (used for road surfacing etc)
3) As the crude oil vapour goes up the column, it gets cooler. Because the 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.
4) The hydrocarbons with the lowest bp don’t condense. They are drawn off as gases at the top of the column.
Heavy fraction can be ________ to make smaller molecules
cracked
Give an example of how decane could be cracked
C10H22 –> C2H4 + C8H18
involves breaking the c-c bonds
Describe the properties of thermal cracking
high temp (up to 1000 degrees Celsius)
high pressure (up to 70 atm)
produces a lot of alkenes
these alkenes are used to make a lot of valuable products, like polymers(plastics). a good example is poly(ethene) which is made from ethene.
Describe the properties of catalytic cracking
uses a zeolite catalyst
slight pressure and high temp (around 450 degrees celsius)
produces mostly aromatic hydrocarbons and motor fuels
using a catalyst cuts costs because the reaction can be done at low temp and pressure and catalyst speeds up reaction saving time and money
Alkanes are useful _____
fuels
burning a small amount releases a lot of energy
If you burn(oxidise) alkanes with plenty of oxygen, you get carbon dioxide and water - its a complete __________ reaction
combustion
they are burned in power stations, central heating systems and power car engines.
however they do produce pollutants
Incomplete combustion happens when there’s not enough _______
oxygen
What happens when incomplete combustion takes place?
You get carbon monoxide gas instead of carbon dioxide - it is poisonous and binds to same sites of haemoglobin molecules as the oxygen so oxygen cannot be carried around the body.
Carbon monoxide can be removed from exhaust gases by catalytic convertors on cars
Carbon particulates can be formed also
Soot causes breathing problems and also can build up in engines causing them to not work correctly
Unburnt ___________ and oxides of _______ can cause smog
hydrocarbons
nitrogen
How are oxides of nitrogen (NOx) produced?
the high pressure and temperature in a car engine cause the nitrogen and oxygen atoms from the air react together
When do hydrocarbons and nitrogen oxides react? And what can they cause?
react in the presence of sunlight to form ground level ozone which is a major component of smog
ground level ozone irritates people eyes, aggravates respiratory problems and can cause lung damage.
catalytic converters on cars remove unburnt hydrocarbons ad oxides of nitrogen from the exhaust.
Acid rain is caused by _____ _______
sulfur dioxide
some fossil fuels contain sulfur, when burned sulfur reacts to form sulfur dioxide gas
it dissolves moisture in and is converted into sulfuric acid
acid rain destroys trees and vegetation, as well as corroding buildings and statues and killing fish in lakes.
it can be removed from power station flue gases before getting into atmosphere by an alkaline slurry
A free radical is a particle with an _________ _________
unpaired electron
How do free radicals form?
when a covalent bond splits equally giving one electron to each atom
the unpaired electron makes them very reactive
page 146 for free radical equations
Chlorofluorocarbons are halogenoalkanes molecules where all of the _____ atoms have been replaced with chlorine and fluorine atoms
hydrogen
page 147 for CFC and ozone layer
Why are CFCs now banned?
they are unreactive, non flammable and non toxic and used to be used as coolant gas in fridges however it was found that they were causing damage to the ozone layer
chemists have now developed safer alternatives which contain no chlorine such as HFCs and hydrocarbons.
Halogenoalkanes are alkanes with _______ atoms
give an example
halogen
trichloromethane
Halogens are much more electronegative than carbon, so carbon-halogen bonds are _____
polar
the delta positive charge on the carbon makes it prone to attacks from nucleophiles
define nucleophile
an electron pair donor
page 148-149 to see the substitution reactions
Halogenoalkanes react with _________ to form alcohols
hydroxides
Nitriles are formed by reacting halogenoalkanes with _______
cyanide
Reacting halogenoalkanes with ammonia forms ______
amines
What decides the reactivity of the halogenoalkanes?
Carbon-halogen bons strength
C-F bond is the strongest as it has the highest bond enthalpy so fluoroalkanes undergo nucleophilic substitution slower than other halogenoalkanes
C-I has lowest bond enthalpy so it is easier to break so iodoalkanes are substituted more quickly.
Halogenoalkanes also undergo _________ reactions
elimination
page 149 for the mechanism
What decides on whether a halogenoalkane undergoes nucleophilic substitution or elimination ?
Aqueous conditions - nucleophilic
Anhydrous conditions - elimination
