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Free Radical Substitution
Homolytic Fission
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Substitution Rxn(free radical substitution)
• Is a chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms
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Mechanism of reaction
• Is the detailed step by step description of how the overall reaction occurs
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Cl
Hydrogen Chloride
Methane
+ =
H
Cl
Cl
Cl
H
Chlorine
Chloromethane
+
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ClHydrogen Chloride
Methane H
ClChlorine
Chloromethane
Hydrogen and Chlorine have swapped places
Substitution
Simple mechanism
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Stage 1
InitiationGetting Started
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Chlorine molecule
Cl2
Ultra violet light breaks the bond
2 Chlorine radicals each
with an unpaired electronBoth species are the same
Called Homolytic Fission
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Stage 2
Keeping it going
Propagation
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ClH
Methane
Lets put in the 2 electrons in this bond
Chlorine radical
Methyl radical
Hydrogen chloride
The methyl radical is now free to react with a chlorine molecule
The chlorine radical pulls the hydrogen and one electron across to it.
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Cl
Methylradical
Cl
Chlorine
Chlorineradical
Chloromethane
Chlorine radical can now go and react with a methane molecule
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Stage 3
Grinding to a halt
Termination
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• Three different ways this can happen
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Chlorine radical
Cl
Chlorine radical
Cl
Chlorine molecule
Reaction stops
No free radicals to keep it going
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Methylradical
Cl
Chlorine radicalChloromethane forms
Reaction stops
Because there are no free radicals to keep it going
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Methylradical
Methylradical
Reaction stops because no free radicals produced to keep it going
The formation of ethane proves that this is the mechanism
Reaction speeded up by sources of free radicals such as tetramethyl lead.
Ethane
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Proof of mechanism
•Small amount of ethane detected
•Not initiated (start) in dark needs UV light
•Tetra methyl lead decomposes to form methyl free radicals, if Tetra methyl lead added it increases rate of reaction
Pb (CH3)4 = Pb + 4 CH3º
THERFORE Methyl radicals are used in reaction
•Halogenated alkanes are used as flame retardants
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Step1: Initiation UV light stimulates rxn. Cl-Cl molecule splits equally (homolytic fission)
Step 2. Propagation
Free Cl° atoms (RADICAL) attacks methane and forms HCL
Step 3. Propagation
Methyl free radical attacks a Cl-Cl molecule and forms chloromethane. Chain rxn continues.
Step 4: Termination
Cl°+ Cl° =Cl-ClCl°+ °CH3= CH3Cl
CH3+ CH3= C2H6
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Tetramethyl lead is added to speed up the rxn
• It supplies the solution with methl free radicals.
• Evidence for free radicals comes from small amounts of ethane being found in the solution
• Halogenation of alknaes makes them more flame resistant
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Addition Reaction (pg. 367)
• When two substances react together to form a single substance
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Addition Reaction Mechanism
and evidence for
Heterolytic Fission
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Step 1Polarising of the bond
in bromine
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Ethene
δ+ δ-
Concentration of negative chargeBecause 4 electrons in this area
Bromine Br2
Moves in this direction
At this point the negative charge of the double bond in ethene forces the electrons to the right Br and it becomes δ− and other one becomes δ+
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Step 2Heterolytic Fission
Occurs
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δ+ δ-
At this point the negative charge of the double bond in ethene forces the electrons to the right Br and it becomes δ− and other one becomes δ+
Br+ Br -Br -
The two electrons of the bond have been forced across to the right Br making it Br- while the other is Br+
The Br2 has been split into 2 different species i.e. Br+ and Br- this is called Heterolytic Fission
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Step 3Formation of the Carbonium Ion
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Br+ Br -Br -
The two electrons of the bond have been forced across to the right Br making it Br- while the other is Br+
The Br2 has been split into 2 different species i.e. Br+ and Br- this is called Heterolytic Fission
+
Carbonium ion
Cyclic bromium ion
BrLet us put in the two electrons of this bond
At this point a lot of things happen at the same time
• The two electrons are pulled to the Br+
• A bond is formed
• one of the bonds between the two carbons disappears
• The lower carbon becomes +ve because it has lost an electron
• The two hydrogens on the upper carbon move to make way for the Br
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Step 4Attack on
carbonium ion
by Br-
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Carbonium ion
+Br
-Br -Br -Br -
Br
The negative bromide ion is attracted by the positive carbonium ion
The two electrons of the bromide ion are used to form the bond
The two hydrogen atoms move round to allow the Br in
The negative and positive cancel each other out
Br
1,2 dibromoethane
Called Ionic Addition because the species are ions when they add on
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Step 5Proof of
mechanism
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+Br
-Br -Br -Cl -
Br
Proof of the mechanism is that if there are Cl- in the environment then some 1-bromo, 2-chloroethane will be formed.
This can be identified by its different Relative Molecular Mass
Cl
Br -Br -Br -Br -
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Step1:
σ+Br-σ-Br-
Carbon double bond is region of high e-
density. Br2
becomes polar as comes close
Step 2. Ionic addition
Br2 splits into
ions.heterolytic fission because Br+ and Br- created
Step 3. Ionic addition
Br+ molecule attacks double bond and forms cyclic bromonium ion/ carbonium ion
Step 4: Termination Br- now attacks the carbonium ion
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Hydrogenation
• Adding of hydrogen's into a molecule (addition)
• Occurs in manufacture of margarine
• Add hydrogen into double bonds causes oils to become solid
• Unsaturated fats are better for you that saturated fats
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Evidence for the carbonium ion
• When bromine and chlorine ions present
• Ethene forms 1-bromo-2-chloroethane as well as 1, 2 dibromoethane
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Polymerisation rxns
• Molecules that contain double bonds undergo addition to become less unsaturated (addition polymers such as polythene and polypropene)
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Polymerisation Reactions
• Example of an addition reaction
• Ethene molecules add together
• Polymers are long chain molecules made by joining together many small molecules
+ =
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Polymers
• Commonly reffered to as plastics
• Polyethene used for plastic bags, bowls, lunch boxes, bottles etc
• Polypropene is used in toys, jugs, chairs etc
• Crude oil is raw material for their manufacture
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Elimination reactions
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Elimination reactions
• When a small molecule is removed from a larger molecule to leave a double bond in the larger molecule
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Elimination rxns• a compound breaks down into 2 or more simpler
substances
• Double bond created
• only one reactant
AB A + B
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Elimination reactions
• Ethene is made from ethanol from removing water using AlO as catalyst
• Elimination reaction is one in which a small molecule is removed from a larger molecule to leave a double bond in the larger molecules
• Dehydration reaction
• Only need to know dehydration of alcohol
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Elimination reaction
• Dehydration of an alcohol is an example of an elimination reaction
• In this reaction, a larger alcohol molecule reacts to form a smaller alkene molecule and an even smaller water molecule
• The change in structure is from tetrahedral to planar
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Dehydration of ethanol
• Ethanol is dehydrated to ethene
• This reaction is used in the preparation of ethene
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Dehydration of ethanol to ethene
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Reaction conditions
• Heat
• Aluminium oxide catalyst
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Preparation of ethene
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Elimination rxn
• Is when a small molecule is removed from a larger molecule to leave a double bond in the larger molecule
• Alcohol =water + alkene• Dehydration reaction since water is removed• Ethanol=ethene + water• 2 methanol +sulphuric acid =methoxymethane ether +water
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C. Decomposition
2 H2O(l) 2 H2(g) + O2(g)
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Redox reactions
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Redox reactions
• These reactions involves oxidation and reduction reactions
• The removal or addition of lectrons from the molecule
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-3 -2 -1 0 1 2 3
Reduction Oxidation
Receives electrons
Looses electrons
Reducing agents give electrons
Oxidation agents take electrons
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Redox reactions of primary alcohols
• Primary alcohols react with oxidising agents such as potassium manganate(VII) or sodium dichromate(VI), forming the corresponding aldehyde
• For example, ethanol reacts forming ethanal
• Ethanal is also formed in the metabolism of ethanol in the human body
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Redox reaction
• Primary alcohol oxidised to an aldehyde
• Oxidising agent: sodium dichromate or potassium permanganate
• The oxidising agent must be limited to prevent the aldehyde from being further oxidised to an carboxylic acid
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Reaction of ethanol with sodium dichromate(VI)
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Reaction of ethanol with sodium dichromate(VI)
• This reaction is used in the preparation of ethanal
• Reaction conditions: heat, excess ethanol, acidified sodium dichromate(VI) solution
• The aldehyde is distilled off as it is formed in order to prevent further oxidation to ethanoic acid
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Preparation of ethanal
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Oxidation of primary alcohols
• Primary alcohols such as ethanol are oxidised to the corresponding aldehydes, which can be further oxidised to the corresponding carboxylic acids.
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Oxidation of ethanol
CC
OH
H
H
H
H
H
C C
O
H
H
H
H
CC
O
OH
H
H
HN a 2 C r 2 O 7 / H + N a 2 C r 2 O 7 / H +
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Reaction of ethanol with sodium dichromate(VI)
• This reaction is used in the preparation of ethanoic acid
• Reaction conditions: heat, excess acidified sodium dichromate(VI) solution
• The reaction mixture is refluxed in order to bring about oxidation to ethanoic acid
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Preparation of ethanoic acid
Reflux followed by Distillation
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Oxidation of secondary alcohols
• Secondary alcohols such as propan-2-ol are oxidised to the corresponding ketones, such as propanone
• Unlike aldehydes, ketones are not easily oxidised, and so no further oxidation takes place
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Oxidation of propan-2-ol
CCH3
CH3
H
OH
CH3 C
O
CH3
N a 2 C r 2 O 7 / H +
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Combustion of organic compounds
• Most organic compounds burn in air, forming carbon dioxide and water
• The structure of the compounds’ molecules is completely destroyed, with the carbon and hydrogen atoms in each molecule being oxidised
• Combustion is exothermic, and ethanol is used as a fuel where it can be produced cheaply
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Non-flammable organic compounds
• Fully halogenated alkanes such as bromochlorodifluoromethane are non-flammable
• Because of this they can be used in fire extinguishers and as flame retardants
• For environmental reasons, the use of many of these substances is being phased out
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Reduction of aldehydes and ketones
• Aldehydes and ketones can be reduced to the corresponding alcohols, using hydrogen passed over the heated surface of a nickel catalyst
• For example, ethanal is reduced to ethanol
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Reduction of ethanal to ethanol
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Reduction of propanone to propan-2-ol
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ENERGY PROFILEENERGY PROFILEone step reaction
product
startingmaterial
transition state TS
activation
energy Ea
heat ofreactionH
ENERGY exothermic
(releases heat)
ONE STEP
REACTION COORDINATE
energy maximum
( follows the progress of the reaction )
obtained fromheat (collisions)
opposite isendothermic
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Reactions as acids
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Reactions of alcohols with sodium
• Alcohols react with the reactive metal sodium, forming a sodium salt and hydrogen
• For example, ethanol reacts with sodium forming sodium ethoxide and hydrogen
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Reaction of ethanol with sodium
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Acidic nature of the carboxylic acid group
• Ethanoic acid is a far stronger acid than ethanol
• This is because its anion is much more stable than that of ethanol
• This enables it to lose a hydrogen ion more readily
• The stability of the ethanoate ion is due to electron delocalisation (as in benzene)
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Reactions of carboxylic acids as acids
• Carboxylic acids react with:
• Magnesium, forming a magnesium salt and hydrogen
• Sodium hydroxide, forming a sodium salt and hydrogen
• Sodium carbonate, forming a sodium salt , carbon dioxide and water
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Reaction of ethanoic acid with magnesium
• Acid + metal → salt + hydrogen
2 CH3COOH + Mg → (CH3COO)2Mg + H2
ethanoic acid magnesium ethanoate
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Reaction of ethanoic acid with sodium hydroxide
• Acid + Base → Salt + Water
CH3COOH + NaOH → CH3COONa + H2O
ethanoic acid sodium ethanoate
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Reaction of ethanoic acid with sodium carbonate
Acid + Carbonate → Salt + Water + Carbon dioxide
2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2
ethanoic acid sodium ethanoate