- Created by: geeraf97
- Created on: 28-11-14 17:23
Long chain alkanes heated up to be broken into smaller chain molecules. When the long chains are broken up they form alkenes and free radicals. (free radicals are highly reactive and bond quickly to form a variety of short chain molecules)
Conditions: High temperature of 700-1200K
High pressure of 7000KPa
This form of cracking mainly produces motor fuels. The products are mainly branched alkanes, cycloalkanes and aromatic compounds.
Conditions: Temperature of 720K
Low pressure but higher than usual atmosphere
Zeolite catalyst consisting of silicon dioxide and aliminium oxide
Zeolites have a honeycomb structure so large surface area
Test for Alkenes/Alkanes
To find out if a substance is an alkene or an alkane you must heat it with bromine solution.
If the solution goes colourless there are C=C bonds present.
Power stations produce sulfur dioxide by burning fossil fuels to produce energy. To absorb these harmful gases chimneys are coated with CaO or CaO3.
This is a neutralisation reaction to prevent the SO2 reactinh with water to become acid rain.
They are reagents that attack and form bonds with positively charged carbon atoms.
- A nucleophile is either a negatively charged ion or an atom with a temporary charge
- Has a lone pair of electrons
- Forms covalent bonds
- Lone pair situated on an electronegative atom
In nucleophilic substitution the nucleophile replaces a halogen. All the mechanisms are similar.
Nucleophilic Substitution (sodium hydroxide)
- Nucleophile is an OH- ion
- Reaction occurs slowly at room temperature
- warm temperature = good yeild
- Haloalkane dissolved in ethanol and aqueous sodium hydroxide
- Called a hydrosis reaction
- Alcohols are formed
Nucleophilic Substitution (cyanide ions)
Warmed with aqueous alcoholic solution of potassium cyanide. The nucleophile is the cyanide ion.
The product is a nitrile. It has 1 more carbon than the original haloalkane.
Nucleophilic Substitution (ammonia)
Nucleophile is the ammonia.
- Excess concentrated solution of ammonia in ethanol
- Carried out under pressure
- Produces a primary amine
Ammonia is the nucleophile because it has a lone pair of electrons that it can donate.
Elimination in Haloalkanes
A hydrogen halide is eliminated from the molecule, leaving a double bond behind. So alkenes are produced.
OH- acts as a base to remove a H+ ion from the haloalkane.
- Sodium or potassium hydrowide is dissolved in ethanol.
- This is then mixed with the haloalkane
- There is no water present
- Mixture is heated
The product is an alkene as there is a C=C bond formed.
Substitution or Elimination
Both use the OH- ion either as a base or a nucleophile. It's purpose and conditions decide on which reaction occurs.
Substution happens cold and in water. (producing alcohol)
Elimination happens hot and in ethanol. (producing alkenes)
Primary haloalkanes favour substitution, secondary will do both and tertiary favour elimination.
- breaking of a Cl-Cl bond to form 2 chlorines with UV radiation.
- Since both atoms are the same the bond breaks homolytically leaving each atom with one electron from the bond.
- These are called free radicals and they're highly reactive
- Chlorine free radical takes a hydrogen atom from methane to form hydrogen chloride, a stable compound. This leaves you with a methyl free radical.
- This free radical is also very reactive and so reacts with a chlorine molecule. This produces another chlorine free radical and a stable chloromethane molecule.
These 2 steps produce hydrogen chloride, chloromethane and a new Cl free radical. This step is a chain reaction that could occur many times over before the free radicals are destroyed in the next step.
- 2 free radicals react to produce a stable compound with no unpaired electrons
- Can happen in 3 ways; 2 Cl free radicals, 2 methyl free radicals and a Cl and methyl free radical.
- In any of the ways the free radicals are removed by termination.
- Double bond is in a different place.
- EXAMPLE between the middle carbons or 2 end carbons
- The longer the chain the more combinations
C-C=C-C OR C=C-C-C
- Form of stereoisomerism
- Same structural formula but bonds are differently arranged
- Double bond prevents the chain from twisting
- E isomers and Z isomers
Electrophilic Addition with Hydrogen Halides
Alkene reacts with HCl, HBr and HI
Alkane is formed
Electrophilic Addition with Halogens
React rapidly with Chlorine gas or solutions of bromine and iodine in organic solvents
The halogen acts as the electrophile
Electophilic Addition with Sulfuric Acid
- Concentrated sulfuric acid
- Room temperature
- Exothermic reaction
An alcohol is formed at the end of the reaction
Alkenes Reaction with Water
- Used industrially to make alcohols
- Carried out with steam (suitable temp and pressure)
- Catalyst of Phosphoric Acid
Polymerisation of Alkenes
Polymerise to form long chains with high molecular masses.
These are alkanes so fairly unreactive.
Called Addition Polymerisation.
- Seperate different types of plastics
- The plastics are then washed and sometimes ground into pellets
- These are then melted and reformed
- They are used to make fleece clothes among other things
- Plastics heated to a temperature that will break polymers into monomers
- Then used to make new plastics
- For some plastics you can only recycle it this way a certain number of times
- At each heating the bonds break and become shorter
From Crude Oil:
Ethene is created during cracking of ethane (natural gas). It is then hydrated to produce an alcohol- ethanol. This process is the reaction of alkenes with water. (as seen on a previous card)
Carbohydrates from plants are broken down and converted into ethanol from enzymes in yeast. The carbohydrates come from crops like sugar cane and sugar beet. The key step is anaerobic respiration.
Ethanol is useful as a motor fuel. Its production and use is often called carbon neutral. The CO2 taken in during photosynthesis is the same amount that is given out during its burning as a fuel, however transportation often adds to the CO2 release. The process is better than using fossil fuels but not entirely carbon neutral.
Oxidise Ethanol to Ethanal
- Dilute sulfuric acid
- Not quite enough potassium dichromate
- Heat gently to cause a reflux reaction
- The temperature should be around 21 degree celcius or 294 kelvin
Oxidise Ethanol to Ethanoic Acid
To change an alcohol into a carboxylic acid you need these conditions:
- Concentrated sulfuric acid
- An excess of potassium dichromate
- Begin a reflux addition and continue it for around 20 minutes
- Twice as much oxidising agent is needed
Tests for Aldehydes and Ketones
The Tollens Test (silver mirror test)
- Tollens reagent is a gentle oxidising agent
- Oxidises aldehydes but has no effect on ketones
- On warming the aldehyde causes a layer of silver on the edge of a test tube
The Fehling's Test
- A gentle oxidising agent
- Contain blue copper ions and during oxidation a brick red/orange precipitate forms
Dehydration of Alcohols
- Excess of hot concentrated sulfuric acid
- Passing their vapours over heated aliminium oxide
- An alkene is formed
- Phosphoric acid can also be used as an alternativedehydrating agent