Chemistry Alkenes

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  • Created by: A.sx
  • Created on: 08-03-17 19:21
How does an alkene differ from an alkane?
Alkenes differ from alkanes in that they have a carbon to carbon double bond (C=C). This is a region of high electron density so they react with electrophiles
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What is an alkenes general formula?
The alkenes are hydrocarbons with general formula CnH2n.
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What is the structure and bonding within alkenes?
The bonding within alkenes contains a double bond which is made up of two distinct bonding regions the σ (sigma) bond and the π (pi) bond
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How is a σ-bond formed?
A σ-bond is formed when two orbitals overlap in a straight line which gives the highest possible electron density between the two nuclei. This is a single covalent bond.
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How is a π-bond formed?
A π-bond is formed when two p orbitals overlap sideways. It's got two parts to it- one 'above' and one 'below' the molecular axis. This is because the p orbitals which overlap are dumb-bell shaped
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Are π-bonds weaker than σ-bonds?
π-bonds are weaker than σ-bonds because the electron density is spread out above & below the nucleai. This means the electrostatic attraction between the nuclei & shared pair of electron is weaker, so less energy is needed to overcome these forces.
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What is the definition of structural isomerism?
Structural isomerism have the same molecular formula but have a different structural formula
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What are some examples of structural isomerism?
Some examples of structural isomerism are but-1-ene, but-2-ene, cyclobutane and methylpropene
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What is the definition of stereoisomers?
Stereoisomers have the same structural formula but they have a different arrangement of atoms in soace
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What is an example of stereoisomers?
An example of stereoisomers are E-but-2-ene (trans) and Z-but-2-ene (cis)
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When are cis/trans labels given?
Cis/trans labels are given when there are the same two groups on each carbon.
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How do give a cis/trans label if all the groups on the carbons are all different?
Decide which atoms has the highest priority on each carbon. The higher the atomic nmber the higher the priority grop. If the two highest groups are on the same side,the isomer is called Z. If they're on different sides the isomer is called E
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What if the atoms attached to the carbon atom double bond is the same when labelling the chain cis/trans?
You will need to find the first point of difference along the chain. The group which has the highest atomic number at the first point of difference is given the higher priority
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Explain why pent-1-ene does not exhibit stereoisomerism
Pent-1-ene does not show stereoisomerism as 2 of the groups on one of the c=c bonds are the same (H)
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Addition of hydrogen (Hydrogenation)
Reagents and conditions: Hydrpgen, Nickel catalyst, High temperature,and high pressure.
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What is the equation of hydrogenation with an ethene molecule and H2
C2H4 + H2 ------> C2H6
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Why is it important to hydrogenate vegetable oils?
The unsaturated alkene becomes saturated. It is used in the manufacture of magarine
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Addition of halogens (Halogenation)
Reagents and conditions: Cl2 or Br2 or I2 , room temperatre and a non-polar solvent e.g. CCl4
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What is the equation of the halogenation of ethene with Br2?
C2H4 + Br2 ----->C2H4Br
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What is the importance of the halogenation reaction?
The addition of bromine is laboratory test for the presence of double bonds. Also addition of iodine is used to test for unsaturation in vegetable oils
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What is the test for alkenes?
The test for alkenes is the decolourising of bromine water. First, you add the unsaturated compound to the bromine water, if the bromine water turns colourless there are double bonds present
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How do you synthesise haloalkanes?
Alkenes undergo electrophillic addition reactions with hydrogen halides to form haloalkanes
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What is the equation for the reaction between ethene and hydrogen bromide?
C2H4 + HBr -----> C2H5Br
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What is the trend for the reactivity of hydrogen halides?
The reactivity of the hydrogen halides increases from HF to HCl to HBr to HI, following the order of decreasing bond energy
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What do unsymmetrical alkenes produce?
Alkenes which are unsymmetrical will give a rise to two diferent products when reacting with hydrogen halides and with other non-symmetrical molecules
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When propene reacts which hydrogen bromide, what two isomeric products are formed?
1-bromopropane and 2-bromopropane
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Reaction with steam (hydration)
Reagents and conditions: Steam (H2O), 600K, 60 atm and a phosphoric acid (H3PO4) catalyst
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What is the equation of hydration reaction between ethene with H2O?
C2H4 + H2O -----> C2H4OH
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What is the importance of the reaction with steam (hydration)?
The addition of steam across a double bond is used in industrial manufactures of alcohols
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Why are symmetrical alkenes more reactive than alkanes?
Symmetrical alkenes are more reactive than alkanes as they contain a high electron density in the double bond. The elctrons in the double bond attract electrophiles
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What is the definition of electrophiles?
An electrophile is a lone electron pair acceptor
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What is a primary carbocation?
A primary carbocation has only one carbon bonded to carbocation
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What is a secondary carbocation?
A secondary carbocation has two carbons bonded to the carbocation
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What is a tertiaty carbocation?
A tertiary carbocation has three carbons bonded to the carbocation
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How can you tell the difference between a minor product and a major product?
The major product will be the one which results from the most stable carbocation
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Would a product formed with a primary carbocation be more or less stable than a product formed with a secondary carbocation?
The secondary carbocation is more stable than a primary carbocation. So the major product would be the one formed with a secondary carbocation and the minor product would be the one formed with a primary carbocation
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Would a product formed with a secondary carbocation be more or less stable than a product formed with a tertiary carbocation?
The tertiary carbocation is more stable than a secondary carbocation. So the major product would be the one formed with a tertiary carbocation and the minor product would be the one formed with a secondary carbocation
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What are polymers?
Polymers are long-chained molecules made by the joining of many small-chained molecules called monomers
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What is the definition of addition polymerisation?
A π bond is broken and two new σ bonds are made
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What do alkenes produce when they undergo addition polymerisation?
Alkenes (unsaturated) undergo addition polymerisation to produce saturated chains containing no double bonds.
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What are some problems with plastics?
They are chemically resistant products that do not break down naturally: they are non-biodegrable. As more non-biodegrable materials pile up,there is an increased threat to the environment; landfill is the most common way to dispose of waste products
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Do addition polymers break down?
Addition polymers do photodegrade (slowly break down in sunlight/ ultraviolet light
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What is the problem with recyling plastics?
It is time consuming as the plastics have be to sorted into different types by hand and then remelted and reformed
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What is an advantage of feedstock recyling (disposal/sustainability)
Polymers can be turned into chemicals which can be fed directly into the processes for manufactureing of useful products
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Alternative ways of disposing of polymers or of increasing sustainability
Incineration: when polymers are burnt they produce heat energy which could be used to generate electricity. Also, the development of degrable plastics from renewable materials such as starch,maizse,cellulose and lactic acid.
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Other cards in this set

Card 2

Front

What is an alkenes general formula?

Back

The alkenes are hydrocarbons with general formula CnH2n.

Card 3

Front

What is the structure and bonding within alkenes?

Back

Preview of the front of card 3

Card 4

Front

How is a σ-bond formed?

Back

Preview of the front of card 4

Card 5

Front

How is a π-bond formed?

Back

Preview of the front of card 5
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