Alaknes And Haloalkanes Flashcards
What is an alkane and what is it’s formula?
Hydrocarbons
Saturated and only contain C-C single bonds
CnH2n+2
Can be straight chained, branched or cyclic
What is the bonding and shape of an alkane?
Each carbon atom can make 4 bonds (sigma bonds)
And so every carbon atom has a tetrahedral shape
What are the intermolecular forces found in alkanes?
Non polar molecules
And so have weak intermolecular forces of…
Van der waals
Temporary induced dipole-dipole forces
What is the trend in boiling point of alkanes?
Longer the hydrocarbon chain, the higher the boiling point
Since molecules have a large surface area and therefore a greater degree of contact between them allowing for an increased abundance of van der waals forces. Therefore more energy is required to overcome the forces and the boiling point increases
How does branching affect the boiling point of alkanes?
More branching reduces the surface area of the molecule for intermolecular forces to act between.
Therefore branched alkanes have fewer van der waals forces and hence have lower boiling points
What is crude oil?
Fossil fuel
Non renewable energy source
Mixture of hydrocarbons (mainly alkanes)
Why are longer chained hydrocarbons converted into shorter chain hydrocarbons in industry?
Global demand for energy is rising
Shorter chain hydrocarbons are in higher demand (eg for petrol)
Longer chained hydrocarbons can be converted into the more useful and valuable shorter chained hydrocarbons.
How is crude oil processed?
Fractional distillation
Cracking
Reforming
What is fractional distillation?
Process used to separate a liquid mixture into fractions based on their boiling points
How is fractional distillation done?
- Crude oil is heated in a furnace to vaporise it and the vapours pass into the bottom of a fractionating column.
- Temperature gradient is achieved in the column (cooler at the top)
- Vapours pass up the column and condense at different points into trays (so they can be removed)
- Depending on boiling points, different fractions condense at different temperatures
Shorter hydrocarbons have lower boiling points and condense nearer the top of the column.
What is reforming?
Where straight chain hydrocarbons are converted into branched or cyclic hydrocarbons as they can burn more efficiently
What is cracking and what are the two methods?
Where longer chain hydrocarbons are broken down into shorter chain hydrocarbons
Catalytic cracking
Thermal cracking
What is catalytic cracking and it’s conditions?
Using a zeolite catalyst
Uses lower temperature and pressures
Larger molecules are broken into smaller molecules
Uses less energy
Usually safer
Produces a high proportion of
Branched alkanes
Cycloalkanes
Aromatic compounds
What is thermal cracking and it’s conditions?
Alkanes are heated to a high temperature and at a high pressure
700-1200K
7000KPa
Produces at least 1 alkene
Thermal cracking produces a high proportion of alkenes
What is a haloalkane?
A hydrocarbon which has at least 1 halogen attached to it.
Fluorine / bromine / chlorine / iodine
What are primary secondary and tertiary haloalkanes?
Based on number of alkyl groups
Primary haloalkane:
1 alkyl group
Secondary haloalkane:
2 alkyl groups
Tertiary haloalkane:
3 alkyl groups
What is the binding like in a haloalkane?
Due to the electronegativity of the halogen, the C-X bond is polar.
Meaning that the carbon is slightly positively charged
Why are alkanes not very reactive?
The C-H bond is not polar and these bonds have large bond enthalpy so lots of energy is required to break them.
What happens when alkanes are combusted completely?
Burnt in excess air with a clean flame
Produces water and carbon dioxide (greenhouse gas may contribute to global warming)
What happens when alkanes are combusted incompletely?
Burnt in limited air supply with a dirty yellow flame
Produces carbon (spot) or carbon monoxide (binds to haemoglobin and can kill)
What are the impure products of combustion and what do they cause?
Sulphur oxides
Nitrogen oxides
Leads to the production of acid rain
What are catalytic converters and what metals catalysts are found in them?
Fitted to exhausts to road vehicles in order to convert pollutant gases into less harmful ones
Usually use platinum, rhodium or palladium catalysts
How does a catalytic converted work and give equations to show how the pollutant gases are made less harmful.
Gases are passed over the catalyst
2CO +O2 ——> 2CO2
2NO + 2CO2 ——> N2 + CO2
What happens when haloalkanes are mixed with water?
Undergo a substitution reaction (hydrolysis) where the oxygen atom is attracted to the carbon in the haloalkane and an OH group is substituted for the halogen.
Forms an alcohol
How can you measure the rate of hydrolysis of a haloalkane?
Add silver nitrate as silver ions will react with halide ions and form a silver mirror
Measure the time taken for a precipitate to form = rate of hydrolysis
Conditions:
Ethanolic solvent
Controlled temperature
Controlled concentration
What is a nucleophile?
Electron pair donor
A species attracted to an area of electron deficiency.
What happens when aqueous alkali (eg. KOH) is mixed with a haloalkane?
Nucleophilic substitution reaction occurs
Nucleophile: :OH- ion
Forms an alcohol
(Faster than using water)
What happens when a haloalkane is mixed with potassium cyanide?
State the conditions and product formed
Nucleophilic substitution reaction
Nucleophile is the CN ion
Conditions:
Ethanol
Heat under reflux
Forms a nitrile
What happens when a haloalkane is mixed with ammonia? And then with excess ammonia?
State condition and product formed?
Nucleophilic substitution reaction
Nucleophile is ammonia
Conditions;
Heat
Ethanol
Sealed tube (prevent ammonia escaping)
Forms a salt
With excess ammonia forms a primary amine
How are alkenes formed from a haloalkane?
State how the nucleophile acts
State the conditions
State the product formed
Using an elimination reaction mechanism
Using a nucleophile to act as a base
Removes H+ from haloalkane
(Does not act as a nucleophile)
Conditions:
Ethanolic solvent (sodium or potsssium hydroxide)
Anhydrous conditions (no water present)
Heat
Forms an alkene
How do you produce an alcohol or an alkene from a haloalkane?
React a haloalkane with hydroxide ions
Substitution forms an alcohol
Elimination forms an alkene
How can you favour a substitution or elimination reaction?
Substitution:
Aqueous conditions (with water)
Room temperature
Use primary haloalkanes
Elimination:
Ethanolic solvent
High temperature
Use tertiary haloalkanes
Secondary haloalkanes go either way
What are some uses of haloalkanes?
Used as
refrigerants
Pharmaceuticals
Solvents
Fire fighting equipment
Restricted due to the effect of CFC’s on the atmosphere
What are chlorofluorocarbons?
Organic compounds containing chlorine and fluorine
They are inert and non toxic
Used as refrigerants
How did CFC’s affect the atmosphere?
CFC’s could diffuse through the layers of atmosphere and become exposed to UV radiation causing the C-Cl bonds to undergo homolytic fission and form free radicals.
How do free radicals affect the Ozone layer?
Ozone layer protects us from UV radiation.
Chlorine free radicals catalyse the decomposition of the O3 molecules
- Cl +O3 ——> •ClO + O2
- ClO +O3 ——> •Cl + 2O2
Overall:
2O3 ——> 3O2
