Unit 2- Alkenes

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  • Created by: Heather
  • Created on: 28-03-15 16:16
What is the definition of an alkene?
An unsaturated hydrocarbon with C=C double bonds,
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What is their general formula?
CnH2n
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Why can alkenes be grouped by this general formula?
As they are a homologous series of alkenes,
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Why can carbon-carbon double bonds not easily rotate?
Due to the electron clouds present above and below the plane of the bond
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When an alkene has two different groups at each end of the double bond, what two different isomers can form?
E-Z stereoisomers
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What is the definition of stereoisomers?
Compounds which have the same structural formula but the bonds are arranged differently in space,
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Why type category of isomerisms do E-Z stereoisomers come under?
Geometrical isomers,
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What does E mean in E-Z stereoisomerism mean?
E means that two groups, which may be identical, are on opposite sides of the double bond,
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What does Z in E-Z stereoisomerism mean?
Z means that two groups, which may be identical, are on the same side,
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Give the names of the two E-Z stereoisomers of but-2-ene?
1) Z- but-2-ene 2) E- but-2-ene
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Why is methylpropene not an E-Z stereoisomer?
AS there are two identical groups joined to the same carbon in a double bond-It has two methyl groups on one carbon and two hydrogen atoms on the other,
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Why type os isomer is methylpropene to but-2-ene then?
A structual isomer
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Which E-isomers or Z-isomers have a higher boiling point and why?
Z-isomers have a higher boiling point as they have some polarity, whereas E-isomers are less polar,
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Which E-isomers or Z-isomers have a higher melting point and why?
E-isomers as they can pack closer together better,
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For alkenes in general, what are the only intermolecular forces present?
Van der Waal forces,
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Describe the melting and boiling points of alkenes?
Very similar to alkanes- They increase with the number of carbon atoms present,
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Are alkenes soluble in water?
No
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Describe the bond ethalpies between alkenes and alkanes and the predicted reactivities for alkenes?
The bond enthalpy is lower for alkanes than alkenes, therefore you would predict that alkenes would be less reactive,
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Why are alkenes actually more reactive than alkanes?
As the C=C forms an electron rich area in the molecule, which can easily be attacked by positively charged reagents,
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What are the poitively charged reagents and their definition?
Electrophilies, -Electron pair acceptors,
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Give two examples of elecrophilies?
-Hydrogen Bromide, -Bromine, -Sulfuric acid,
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The reaction of alkenes with small molecules occurs by what mechanism?
Electrophilic addition,
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What is the definition of addition?
When two smaller molecules join together to form a larger one or when the electrophile joins on to the alkene to form one new molecule,
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What states does hydrogen bromide need to be in to react with alkenes in electrophilic addition?
-Gas, -Concentrated aqueous solution,
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Give the equation for the reaction of ethene with HBr?
H2C=CH2 + HBr -> CH2BrCH3
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Why can the hydrogen act as the electrophile in this reaction?
As Bromine is more electronegative than hydrogen, so the HBr molcules is polar. The electron deficient or delta positive hydrogen actom therefore acts as the electrophile,
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When the hydrogen forms a bond with the carbon atom, one of the carbon atoms is electron deficient. What is the name of this compound?
A carbocation
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What does the bromide ion act as by using a lone pair of electrons to form a new bond with the carbocation?
A nucleophile,
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If the reaction is in aqueous solution, what acts as the nucleophile?
Hydrogen + (aq) ions,
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What conditions of the sulfuric acid are needed for alkenes to react with sulfuric acid in electrophilic addition?
Cold, concentrated sulfuric acid,
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What if the end product?
Alkyl hydrogensulfates
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Give the overall reaction for the electrophilic addition of ethene with sulfuric acid?
H2C=CH2 + H2SO4 -> CH3CH2(OSO2OH)
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What is the name of the product CH3CH2(OSO2OH)
Ethyl hydrogensulfate
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if water is added to the product, what is formed?
An alcohol, ethanol, and sulfuric acid,
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If the alkene is unsymmetrical, how many possible products can be formed?
2
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What property will make one of the products most likely to form?
The most stable carbocation,
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What is the order of the stability of carbon cations?
Tertiary > secondary > primary
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What is the difference between a tertiary carbocation and a primary carbocation?
A tertiary carbocation has three alkyl groups attached to C+ whereas a primary carbocation has one alkyl group attached to C+
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What is the simplest way to work out the most stable product with hydrogen halides?
When hydrogen halides add on to alkenes, the hydrogen adds on to the carbon atom which already has the most hydrogens,
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What does the direct hydration of an alkene form?
An alcohol
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What forms is the water used to react with the alkene?
Steam
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What catalyst is used?
Phosphoric acid, H3PO4
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What conditions are need to form ethanol from ethene?
Temp: 300C Pressure: 6500KPa
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What is the definition of a polymer?
A long chain molecule made from lots of small molecules, monomers, joining together,
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What is the definition of a monomer?
Smaller molecules that join together to make polymers
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What is the definition of addition polymerisation?
The formation of long chain molcules from lots of small molecules joining together with no other products,
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In addition polymers, why are the end of the chains reactive?
As it is a radical which is formed at the beginning of the reaction when the double carbon bonds break to single,
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What type of catalyst is used?
Organic peroxides,
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What is the peroxides role in the formation of addition polymers?
The peroxide molecules readily split into radicals to initiate the chain growth and a radical is reformed at the end of the chain after each addition of a monomer,
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Give the equation for the addition polymerisation of ethene?
nH2C=CH2 -> -(-CH2CH2-)- n
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Why are polyalkenes unreactive?
As, like alkanes, are saturated so have strong covalent bonds between atoms and a lack of bond polarity,
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Give an example of a polymer which can be recycled and how?
-Poly(propene) -It is collected, cleaned and cut into small pieces. It is then melted down and remoulded into a new object,
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What is poly(ethene) used for?
Packaging e.g. plastic bags,
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What is poly(propene) used for?
Rigid containers and objects such as car bumbers, -Fibres used as the backing for carpets and in thermal clothing,
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Card 2

Front

What is their general formula?

Back

CnH2n

Card 3

Front

Why can alkenes be grouped by this general formula?

Back

Preview of the front of card 3

Card 4

Front

Why can carbon-carbon double bonds not easily rotate?

Back

Preview of the front of card 4

Card 5

Front

When an alkene has two different groups at each end of the double bond, what two different isomers can form?

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Preview of the front of card 5
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