AS 2 Chemistry mechanisations, definitions etc.
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- Created by: Anna McGuigan
- Created on: 16-03-24 18:34
What is the difference between E-Z isomers and Cis-Trans Isomers?
Cis-Trans isomers must have a H atom bonded to each of the atoms
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Factors Affecting Boiling Point of Alkanes
All to do with Van Der Waals
Bcos Alkanes only have VDW
With increasing RMM, BP incr, due to incr VDW
Bcos Alkanes only have VDW
With increasing RMM, BP incr, due to incr VDW
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A Second Factor Affecting BP of Alkanes
Branching
With higher branching, the lower the BP (less VDW)
With higher branching, the lower the BP (less VDW)
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Organic Compounds as Fuels
Organic compounds:
With a high C content (Alkanes/Alkenes) burn with a smoky orange flame - shows incomplete combustion
With a low C content (alcohols) burn with a non-sooty/ clean blue flame
With a high C content (Alkanes/Alkenes) burn with a smoky orange flame - shows incomplete combustion
With a low C content (alcohols) burn with a non-sooty/ clean blue flame
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Reactions of Alkanes
Complete combustion
Incomplete combustion
Free Radical Photochemical Substitution (monohalogenation)
Incomplete combustion
Free Radical Photochemical Substitution (monohalogenation)
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Steps in Free Radical Photochemical Substitution
(UV Light)
Initiation: m = r
Propagation: m + r = r + m
r + m = m + r
Termination: r + r = m
Initiation: m = r
Propagation: m + r = r + m
r + m = m + r
Termination: r + r = m
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What do mechanisms show?
Mechanisms show movement of electron and breaking and formation of covalent bonds
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What's a radical?
A particle with unpaired election
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What's homolytic fission?
Bond breaking in which one of the shared electrons goes to each atom
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Reaction of Alkenes
Complete combustion
Incomplete combustion
Bromination of ethene (HBr)
(Symmetrical and non-symmetrical)
Hydrogenation of Alkene (H2)
Hydration of Ethene (Steam)
Bromination with bromine (water) Br2
Addition polymerisation of alkenes
Incomplete combustion
Bromination of ethene (HBr)
(Symmetrical and non-symmetrical)
Hydrogenation of Alkene (H2)
Hydration of Ethene (Steam)
Bromination with bromine (water) Br2
Addition polymerisation of alkenes
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Bromination of Ethene (HBr)
Each Step
Mechanism
Each Step
Mechanism
Alkene = Carbocation = (Halogeno) Alkane
Electrophilic Addition
Electrophilic Addition
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Electrophile
An ion or molecule that attacks regions of high electron density
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Heterolytic Fission
Bond breaking in which both electron in the shared pair go to a single atom
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Non-symmetrical bromination of ethene
Works via the formation of a carbocation
Primary, secondary or tertiary
Tertiary is more stable than secondary which is more stable than primary
Primary, secondary or tertiary
Tertiary is more stable than secondary which is more stable than primary
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Hydrogenation of Alkene (H2)
Conditions
Product
How many products
Hydrogenation definition
Conditions
Product
How many products
Hydrogenation definition
Finely divided nickel catalyst, 180 C, 4 atm
Alkane (Only product)
The addition of a hydrogen molecule across a c=c bond
Also used to convert oils into solids fats
Alkane (Only product)
The addition of a hydrogen molecule across a c=c bond
Also used to convert oils into solids fats
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Hydration of ethene (Steam)
Conditions
Product
Conditions
Product
Concentrated Phosphoric Acid (H3PO4), 300 C, 60-70 atm
Ethanol
Ethanol
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Bromination with Bromine (water) Br2
Conditions
Product
Conditions
Product
Mix Bromine water with a liquid alkene or bubble a gaseous alkene through bromine water
dibromoalkane
dibromoalkane
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Test for a c=c bond
If yellow, brown bromine water changes to colourless in the presence of an organic compound, the compound contains a c=c bond
Chlorination of an alkene works in the same way however chlorine water is virtually colourless so its not a valid test for unsat
Chlorination of an alkene works in the same way however chlorine water is virtually colourless so its not a valid test for unsat
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Addition polymerisation of alkenes
The polymerisation of ethene produces polythene
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Polymerisation definition
The joining together of small molecules (monomers) to form a large molecule
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Monomers
Small molecules which join together to form a polymer
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Polymer
A large molecule formed when monomers join together
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How to name polymer
poly(alkene)
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What is a diene?
an alkene with 2 C=C bonds in main carbon chain
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E-Z isomers and dienes
A symmetrical diene with the ability to form E-Z Isomers at both C=C bonds will only form 3 isomers. E at both C=C, Z at both C=C and the E-Z isomer will be the same as the Z-E isomer.
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Trends in Boiling Point in Halogenoalkanes
Due to permanent dipole-dipole attractions
Higher BP due to breaking these bonds
BP will incr. as the mass of the halogen atom incr.
Bp will incr. as the carbon chain gets longer (caused by incr. in VDW)
Branching
Higher BP due to breaking these bonds
BP will incr. as the mass of the halogen atom incr.
Bp will incr. as the carbon chain gets longer (caused by incr. in VDW)
Branching
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Physical Properties
Sweet sickly smell
Immiscible with water
Immiscible with water
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Preparation of a halogenoalkane from an alcohol
Reacting an alcohol with a hydrogen halide
( the 8 step process)
( the 8 step process)
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Reactions that produce halogenoalkanes
1. From alkanes
2. From alkenes
3. From alcohols
2. From alkenes
3. From alcohols
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1. From alkanes
Equation
Conditions
Type of reaction
Mechanism
Equation
Conditions
Type of reaction
Mechanism
alkane + halogen = halogenoalkane + hydrogen halide
Ultraviolet light
Substitution
Free radical photochemical substitution
Ultraviolet light
Substitution
Free radical photochemical substitution
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2. From alkenes
Equation
Conditions
Type of reaction
Mechanism
Equation
Conditions
Type of reaction
Mechanism
alkene + halogen = halogenoalkane
none required
addition
electrophilic addition
none required
addition
electrophilic addition
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3. From alcohols
Equation
Conditions
Type of reaction
Equation
Conditions
Type of reaction
alcohol + halogen = halogenoalkane + water
HBr generated from NaBr + conc H2SO4 in situ, heat under reflux
substitution
HBr generated from NaBr + conc H2SO4 in situ, heat under reflux
substitution
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Reactions of halogenoalkanes
1. Reaction with alkali, OH-
2. Hydrolsis
3. Reaction with ammonia NH3
4. Reaction with cyanide ions, CN-
5. Reaction with potassium/ sodium hydroxide in ethanol
2. Hydrolsis
3. Reaction with ammonia NH3
4. Reaction with cyanide ions, CN-
5. Reaction with potassium/ sodium hydroxide in ethanol
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Reaction with alkali, OH-
Equation
Conditions
Type
Mechanism
Equation
Conditions
Type
Mechanism
halogenoalkane + OH- = alcohol + X-
heat under reflux with aqueous sodium hydroxide with ethanol for 1 hour
substitution / hydrolysis
nucleophilic substitution
heat under reflux with aqueous sodium hydroxide with ethanol for 1 hour
substitution / hydrolysis
nucleophilic substitution
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Hydrolsis
Molecules are broken up by reaction with water
One part of the molecule bonds with the H atom of water and the other part of the molecule bonds with OH
One part of the molecule bonds with the H atom of water and the other part of the molecule bonds with OH
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Procedure to test the ease of hydrolysis of halogenoalkanes depends on bond enthalpy of C-X bond
2 cm3 of ethanol in 3 test tubes with 1 cm3 of 1-chlorobutane in first, 1cm3 of 1-romobutane in second and 1 cm3 of 1-iodobutaane in 3rd.
1cm3 of silver nitrate added to each
then placed in beaker of hot water
1cm3 of silver nitrate added to each
then placed in beaker of hot water
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Explanation of this procedure
Reactivity of C-X bonds increases as go down group.
The longer the C-X bond, the lower the bond enthalpy and the more reactive
Iodine reacts first to form yellow ppt, then Bromine cream ppt and chlorine as white ppt
The longer the C-X bond, the lower the bond enthalpy and the more reactive
Iodine reacts first to form yellow ppt, then Bromine cream ppt and chlorine as white ppt
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Reaction with ammonia NH3
Equation
Conditions
Type
Mechanism
Equation
Conditions
Type
Mechanism
Halogenoalkane + ammonia = amine + HX
heated with ethanol ammonia in sealed tube
substitution
nucleophilic substitution
NH2 substitutes for halogen
heated with ethanol ammonia in sealed tube
substitution
nucleophilic substitution
NH2 substitutes for halogen
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Reaction with cyanide ions, CN-
Halogenoalkene + CN- = nitrile + X-
halogenoalkane dissolved in ethanol dropped into an aqueous solution of sodium or potassium cyanide. Heated under reflux
substitution
nucleophilic substitution
(lengthens carbon chain)
halogenoalkane dissolved in ethanol dropped into an aqueous solution of sodium or potassium cyanide. Heated under reflux
substitution
nucleophilic substitution
(lengthens carbon chain)
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Reaction with potassium/ sodium hydroxide in ethanol
Halogenoalkane + KOH = alkene + KX + H20
Halogenoalkane treated with hot ethanol potassium hydroxide soon.
Elimination
Halogenoalkane treated with hot ethanol potassium hydroxide soon.
Elimination
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Elimination from symmetrical and unsymmetrical halogenoalkanes
one product vs 2 products
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Nucleophile
An ion or molecule with a lone pair of electron that attacks regions of low electron density e.g. OH- or CN- or species with lone pair of electron NH3
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Hydrolysis of primary halogenoalkanes
SN2 mechanism
OH and X temporarily bonded to molecule at same time
OH bonds and X- leaves
OH and X temporarily bonded to molecule at same time
OH bonds and X- leaves
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Hydrolysis of tertiary halogenoalkanes
SN1 mechanism
C-X breaks and OH enters leaving X-
C-X breaks and OH enters leaving X-
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Preparation of Alcohols
Can be prepared from corresponding halogenoalkane by heating halogenoalkane under reflux with NaOH soln.
This is hydrolysis and occurs by a nucleophilic reaction
This is hydrolysis and occurs by a nucleophilic reaction
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Combustion of alcohols
Blue flame
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Reaction of Alcohols with PCL5
In fume cupboard add solid PCL5 to alcohol
Products - halogenoalkane, POCL3, HCL (g)
Products - halogenoalkane, POCL3, HCL (g)
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Reaction of alcohols with HBr
Heat under reflux with HBr
Products - halogenoalkane and water
Products - halogenoalkane and water
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Reaction of alcohols with Na
Small piece of Na added to alcohol
Sodium alkoxide and hydrogen
Removes H from OH and from ONa
Sodium alkoxide and hydrogen
Removes H from OH and from ONa
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Mild oxidation with acidified potassium dichromate (vi)
It maintains the carbon length - no c-c bonds broken. Sulphuric acid most commonly used to acidify the sole as sulphur in it is in its highest oxidation state
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Oxidation of primary alcohol
alcohol = aldehyde = carboxylic acid
Orange soon changes to green, change in smell
( If H removed at H20)
Orange soon changes to green, change in smell
( If H removed at H20)
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Oxidation of secondary alcohol
alcohol = ketone
orange soln. changes to green
( If H removed at H20)
orange soln. changes to green
( If H removed at H20)
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Other cards in this set
Card 2
Front
Factors Affecting Boiling Point of Alkanes
Back
All to do with Van Der Waals
Bcos Alkanes only have VDW
With increasing RMM, BP incr, due to incr VDW
Bcos Alkanes only have VDW
With increasing RMM, BP incr, due to incr VDW
Card 3
Front
A Second Factor Affecting BP of Alkanes
Back
Card 4
Front
Organic Compounds as Fuels
Back
Card 5
Front
Reactions of Alkanes
Back
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