Alkenes

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  • Created by: Hindleyc
  • Created on: 17-05-18 17:44
What are alkenes?
Unsaturated hydrocarbons
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What do they contain?
Atleast one carbon-carbon double bond- a centre of high electron density
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What do this make alkenes in comparison to alkanes?
More reactive than alkanes as high electron density of c=c leads to attack on these molecules by electrophiles
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General formula?
CnH2n
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What is the arrangement of bonds around the C=C? Angle?
Planar, 120
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How do you test for alkenes?
Add bromine water and if double bond present de-colourises it
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How can it be used quantitatively?
To show presence of multiple double bonds in compounds eg polyunsaturated
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What does the double bond consist of?
One sigma bond and one Pi bond
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Which one is exposed?
Pi
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Which has high electron density?
Pi
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What does this mean?
Vulnerable to attack by electrophiles- species which like electrons
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Why are the electrons available to attack?
Further away
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Which bond is weaker and further away and therefore more susceptible to attack than sigma?
Pi bond
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Which bond is it that is broken when the double bond is broken?
Pi bond
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What does the presence of the pi bond do?
Prevent rotation around sigma bond
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What does this lack of rotation give rise to?
E-Z (geometrical) isomers
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What is this type of isomerism an example of?
Stereoisomerism
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What is a stereoisomer?
Have same structural formula but different spatial arrangement of atoms
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What can a single c-c bond do?
Easily rotate
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How do they arise?
Restricted rotation around c=c double bond. There are 2 different groups/atoms attached to both ends of the double bond
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How do you name an EZ isomer
Determine priority group on both sides of the double bond (atom with bigger Ar)
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How do split bond for EZ?
Vertically
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When is it labelled Z?
When the priority atom is on the same side of the double bond
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When is it labelled E?
If the priority atom is on the opposite side of the double bond
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What reactions do alkenes undergo?
Electrophilic addition
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What are the double bonds in alkenes?
Areas with high electron density
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What does this do?
Attracts electrophile and undergo addition reactions
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Whats an addition reaction?
Where two molecules react together to produce one
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Whats an electrophile?
Electron pair acceptor
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What are the electrophiles?
Bromine, hydrogenbromine and sulphuric acid
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What is the change is functional group with Bromine? what is the reagent? Conditions? Mechanism? Type of reagent?
Alkene to dihalogenoalkane. Br2. Room temp. Electrophilic addition. Electrophile- Br slightly positive
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How does it work?
As the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in Br-Br bond inducing a dipole so Br2 becomes polar and electrophilic
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Mechanism?
E- go from the double bond to the slightly +ve Br and form a bond, The e- in the Br2 molecule move to the slightly -ve Br atom forming a Br- and a carbocation. This negative ion is attracted to the positive charge on the carbocation and bonds
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What is a carbocation?
The intermediate formed which has a positive charge on the C atom
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Change is fg for Hydrogenbromide and alkenes? Reagent? Conditions? Mechanism? Type of reagent?
Alkene- halogenoalkane. HCl or HBr. Room temperature. Electrophilic addition. Electrophile- slightly positive H
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How does it work?
HBr is a polar molecule because Br is more electronegative than hydrogen . The slightly positive hydrogen is attracted to the electron rich pi bond.
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Mechanism?
E-s move from the double bond to the slightly +ve H atom and form a bond. The e-s then move from the h-br bond to the Br atom making it a negative ion and forming a carbocation. The br ion then bonds to the positive carbocation forming halogenalkane
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What can the reaction lead to? Why?
2 Products when the alkene is unsymmetrical
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Stage 1 Change is fg for Sulfuric acid and alkenes? Reagents? Conditions? Mechanism? Type of reagent?
Alkene to alkylhydrgoen sulphate. Concentrated H2SO4. Room temp. Electrophilic addition. Electrophile-H2SO4
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Overall role of sulphuric acid? how?
Catalyst as it is regenerated at the end
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Mechanism?
Electrons move from bond to slightly positive hydrogen, electrons in h-oso2oh bond move to oso2oh forming negative ion. Ion then bonds to carbocation forming CH3CH2SO4
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What happens in stage 2?
The alkyl hydrogen sulphate in converted into an alcohol
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What are the reagents?
Water
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Conditions?
Warm mixture
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Type of reaction?
Hydrolysis
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What is hydrolysis?
Reaction where the molecule is split by the addition of water
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What happens?
The OSO2OH turns into OH by the addition of water forming H2SO4 and alcohol
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What is Markownikoffs rule?
In most cases, the bromine will be added to the carbon which has the fewest hydrogens attaches to it
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If the alkene is unsymmetrical what can the addition of HBr lead to?
2 isomeric products
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Why?
Wants to form most stable carbocation. It is more stable if methyl groups are on either side of positive carbon as they are electron releasing so reduce charge on ion= stabilises it
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Order of stability for carbocation?
Tertiary>secondary>primary
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What do they then form?
Major and minor products
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What is the other reaction with alkenes?
Direct industrial hydration of alkenes to form alcohols
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How?
Reacted with water in presence of acid catalyst in one step
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Conditions? Temp? Pressure? Catalyst?
High- 300-600. High 70atm. Concentrated H3PO4
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Why can't it be done in a lab?
High pressure needed.
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Why is it preferred industrially?
No waste product and high atom economy. Separation of products easier and cheaper to carry out
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eqn?
CH2=CH2(g)+H2O(g)=CH3CH2OH(l)
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What are addition polymers with regards to society?
Commercially important and many uses in modern society
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How are they formed? what is the process called?
From alkenes and substituted alkenes. Addition polymerisation
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Reaction/eqn? what does n mean. What is only 1 repeating unit?
Alkene monomer eg ethene CH2=CH2 goes into a polymer [-CH2-CH2-]n poly(ethene). n=no. of repeating units. no n.
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When do you add n?
if writing an equation showing the reaction where ‘n’ monomers become ‘n’ repeating units
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What is poly(chloroethene)?
pvc- polymer that is waterproof- electrical insulator and doesn't react with acids
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What is it like in its pure form?
Rigid plastic from the strong IM forces so dent move over each other (upvc)
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What is upvc used in?
Windowframe coverings and gutterings
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What happens if plasticisers are added?
The IM forces are weakened- which allows chains to move more easily meaning more flexibility
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What is this used in?
insulation on electrical wires and waterproof clothing
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Why are poly(alkenes) like alkanes unreactive?
Due to strong c-c and c-h bonds
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What are addition polymer chains like? polarity? saturation- what does this mean?
HC chain long inc V.D.W and inc M.P and B.P. Non-polar so insoluble. Long and saturated- unreactive- can't react with electrophiles nucleophiles or addition reactions
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What is recycled?
Poly(propene) is recycled
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Other cards in this set

Card 2

Front

What do they contain?

Back

Atleast one carbon-carbon double bond- a centre of high electron density

Card 3

Front

What do this make alkenes in comparison to alkanes?

Back

Preview of the front of card 3

Card 4

Front

General formula?

Back

Preview of the front of card 4

Card 5

Front

What is the arrangement of bonds around the C=C? Angle?

Back

Preview of the front of card 5
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