Alkenes and organic chemistry revision
Why does a major product form instead of a minor?
"This product is produced by the route via the more stable carbocation"
Ways in which polymer waste can be put to other uses?
(Reduce) Reuse, recycle, incinerate, use as a chemical feedstock
Give the name for the product of the reaction between ethene and bromine, Br2
1,2 dibromoethane
Comment on the atom economy of the formation of polyethene from ethene
100% because it is an addition reaction where all the reactants form one product
HBr plus propene makes 1-bromopropane and 2-bromopropane. Which is the major product?
2 - bromopropane because the mechanism proceeds via the more stable secondary carbocation
How many compounds have the formula C5H12?
3
How many alkyl groups does a tertiary carbocation have?
3 (and no hydrogens)
How many structural isomers does the alkane C6H14?
5
What is the propagation step of halogenation?
A Cl free radical replaces takes a H from an alkane to make H-Cl and a alkane radical. Then another Cl2 molecule reacts with the alkane free radical to form a halogenoalkane and a Cl free radical. This continues...
Definition of a saturated compound
A compound in which all the covalent bonds are single bonds
Definition of a hydrocarbon
A compound of hydrogen and carbon only
Definition of a homologous series
A family of compounds which all contain the same functional group and each member of the series contains one -CH2- unit more than the previous member. (Also have similar chemical properties and show a steady gradation in physical properties)
Definition of a general formula
A formula that represents all members of a homologous series
What is a free radical?
A neutral species, i.e. An atom or a group of atoms, which possesses an unpaired electron
Alkene plus H2 type of reaction
Addition and reduction
What is a free radical?
An atom or covalently bonded group of atoms with an unpaired electron
Suggest two reasons why cracking of larger alkane molecules is important in industry
As a source of H2/smaller alkanes burn more efficiently/less pollution/shorter chains are more useful products.
How does the boiling points of alkanes change as you increase the number of carbon atoms per molecule ?
Because the number of electrons increases, the strength of the London dispersion forces increase and so the boiling point increases.
What is the definition of heterolysis?
Both electrons in a bond go to one atom (double headed curly arrow) when this takes place, ions are formed
Bromine water is what?
Br2 (aq) - water molecules are more abundant than Br- ions
The main product of alkene with bromine water is what
Bromoalcohol... there is the addition of OH and Br and there is a H+ ion left over
What two ways can you form alkenes?
By dehydrating alcohol or by dehyrohalgenation of halogenoalkanes... also cracking can do some
Why should PTFE (polytetrafluoroethene) be particularly resistant to chemical attack?
C-F bond is really strong and F is very close to each other so slippy and barricaded surface. (Due to helix structure of polymer)
When methane reacts with Cl2, a mixture of products forms. Which product provides the strongest evidence for a free radical mechanism?
C2H6
What is a phenyl group?
C6H5 or in skeletal hexagon with a circle inside
State and explain main environmental problem from complete combustion of alkane fuels
CO2 conc rise causes global warming from it being a greenhouse gas. Traps the infrared radiation re-radiating from the earth.
Environmental problems of polymer production?
CO2 released from burning fossil fuels to provide energy for production/transport and incineration of polymers. Atmospheric pollution by incinerating. Landfill. Plastics pollution - waste materials impact flora, fauna, fish...
State 2 problems that result from incomplete combustion of alkane fuels
Carbon monoxide is toxic, sooty deposits (in atmosphere global dimming), hydrocarbon deposits are toxic, insufficient energy produced.
What is the initiation step of halogenation?
Cl2 molecule has UV light shone at it and splits the molecule into two free radicals
Cyclic alkene has the general formula
CnH2n-2
Define Stereoisomerism
Compounds with the same molecular and structural formula but different arrangements of atoms.
Definition of structural isomerism
Compounds with the same molecular formula but different structural formulae.
What condition do you need to dehydrate an alcohol?
Conc. sulphuric acid at 170degrees C
What are electrophiles?
Electrophiles are species which are electron deficient or have a positive charge and so can accept a pair of electrons from an electron-rich site forming a new covalent bond.
Which of the following can be determined, for an unknown alkene, using only percentage composition by mass data?(Molecular, empirical or structural formula)
Empirical (simplest) formula
True or false. Large hydrocarbon molecules ignite easier than small molecules?
False. Small molecules are easily ignited, flow easily, very volatile and low bp.
Methane with bromine in sunlight. How is this reaction best described?
Free radical substitution
What type of reaction is halogenation of an alkane? (Alkane plus Cl2 gases)
Free radical substitution
Alkenes can be produced by cracking when they have what conditions?
High temp and aluminium/silicon oxide
What makes a flame sootier?
Higher percentage of carbon in the hydrocarbon
State the type of bond fission involved in the initiation step of the reaction of butane with chlorine
Homolytic fission
What makes a polymer stronger?
If the polymer has more Carbons then it has more electrons and so the London dispersion forces are stronger and chains can't slide over each other due to branches.
State the type of reaction occurring in step 1 where chlorine forms two free radicals.
Initiation
What is bad about using sulphuric acid as a dehydrating agent ?
It is also an oxidising agent so causes some of the alcohol to oxidise to CO2 and leaves acidic impurities, also acid is reduced to sulphur dioxide.
What does the halogenoalkane lose when it makes an alkene?
It loses the halogen atom and a hydrogen atom (from a carbon atom next to the carbon atom with the halogen attached).
Definition of a polymer
Large (long chain) molecule made up of lots of small molecules called monomers
What makes polymers so strong/resistant?
Large size of molecules and strength of C=C and C=halogen bonds. Also, because they are not naturally occurring substances, no enzymes exist that can break them down.
Advantages to PVC aka polychloroethene?
Long life & not easily hydralised or biodegraded
Life cycle analysis/assessment
Main requirements for energy input. Environmental impact and sustainability of making materials from natural resources. Environmental impact of making and using product. Environmental impact of disposing of product by incineration, landfill or recycling.
How would u expect 2-methylpent-2-ene to react with HBr?
Mechanism proceeds via the more stable tertiary carbocation therefore 2bromo-2-methylpentane is the major product.
Suggest a reason why oil companies reform alkanes such as heptane into cyclic alkanes?
More efficient combustion/easier to burn/combusts more easily
What charge does the 2nd Br in the reaction between ethene and bromine have?
Negative (and 2 electron dots)
Would you expect alkanes to dissolve well in water ?
No (ie oil sits on top)
Are alkanes polar or non-polar?
Non polar
What is a primary carbocation?
One alkyl group and 2 hydrogens attached to the positively charged carbon atom.
Colour change of alkene plus bromine water?
Orange to colourless
What are biodegradable polymers?
Polymers that break down by microbes in the environment. Also they are often made from plant material.
Colour change of alkene plus potassium manganate?
Purple to pale pink (or purple colour completely disappears if alkene is in excess)
Poly...ene has no C=C double bonds so is...
Saturated and therefore does not decolorise bromine water
Catalyst and conditions for cracking
Silica (silicon oxide) or alumina (aluminium oxide) and 400-500degrees C
Definition of a monomer
Simple compound that can be joined together to make a polymer
What condition is needed for halogenation of an alkane? (Alkane plus Cl2 gases)
Sunlight. (If chlorine is in excess then the main product is CCl4 and if methane is in excess the main product I'd CH3Cl but there is always a mixture of products... the products then need separating by fractional distillation)
Disadvantages to PVC aka polychloroethene?
Takes up space in landfill because it doesn't biodegrade and if it is incinerated it gives off toxic gases (e.g. HCl)
Which carbon on the c=c bond does the hydrogen add onto?
The Carbon with the most hydrogen atoms already
Suggest why a small amount of UV light can result in the formation of a large amount of product
The chlorine radical is regenerated in the propagation step (chain reaction)
Why are the melting points of even numbered carbon chains higher than the odd numbered carbon chains?
The even numbered chains pack together more efficiently in the solid than odd numbered chains
Definition of a functional group
The group of atoms which gives an organic compound its characteristic properties and reactions
How come polymers have the same empirical formula as their monomers?
The pi bond of a C=C bond opens up and the alkene molecules join on to each other. The polymer therefore has the same percentage composition and same empirical formula but a higher relative molecular mass as the monomer.
What is the definition of a hazard?
The potential to do harm
Define risk
The probability that a particular hazard will actually cause harm
Definition/purpose of reforming hydrocarbons
The processing of straight-chain hydrocarbons into branched-chain alkanes and cyclic hydrocarbons for efficient combustion.
What is reforming
The processing of straight-chain hydrocarbons into branched-chain alkanes and cyclic hydrocarbons for efficient combustion.
Why do branched alkanes have lower bp. than there straight-chain isomers?
There are fewer points of contact between adjacent molecules. (i.e. Submarines don't stack well)
Disadvantages to biopolymers
They have a short life. They can not be recycled, incinerated or used as a chemical feedstock.
What is the termination step of halogenation?
Two free radicals join together to form a neutral molecule (it doesn't produce another free radical). E.g. Two alkane free radicals to make a longer chain alkane, or an alkane free radical and a Cl free radical to make just a halogenoalkane.
Use the fact C-H bond ethalpy is 435 to suggest why no H2 is produced (energy of UV light is 400)
UV light does not have enough energy to break C-H bond therefore no H free radicals are formed so u cannot get H2
What is meant by the term unsaturated as applied to alkenes?
Unsaturated means that a molecule contains a double bond
What is cracking
When an alkane is heated to a high temp in the absence of air it breaks down to give an alkene and hydrogen or an alkene and another alkane.
When does an addition reaction occur?
When two substances react together to form a single substance.
How do u produce alkenes from halogenoalkanes ?
When u treat the halogenoalkane with hot conc. solution of potassium hydroxide in alcohol it undergoes an elimination reaction giving an alkene
Name 3 ways in which u can reduce risk?
Work on a smaller scale, take specific precautions & carry out the reaction using an alternative method that involves less hazardous substances
Would you expect alkanes to dissolve well in non-polar solvents ?
Yes (like dissolve like)
