Free Radical Substitution Homolytic Fission

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

Mechanism of reaction Is the detailed step by step description of how the overall reaction occurs

Methane Chloromethane H Cl = + + Cl H Cl Cl Hydrogen Chloride Chlorine

Simple mechanism Substitution Cl Cl Chloromethane Methane Hydrogen and Chlorine have swapped places Substitution Cl Hydrogen Chloride Cl Chlorine

Stage 1 Initiation Getting Started

Ultra violet light breaks the bond Chlorine molecule Cl2 2 Chlorine radicals each with an unpaired electron Both species are the same Called Homolytic Fission

Stage 2 Propagation Keeping it going

The methyl radical is now free to react with a chlorine molecule Methane Chlorine radical Cl H The chlorine radical pulls the hydrogen and one electron across to it. Lets put in the 2 electrons in this bond Methyl radical Hydrogen chloride The methyl radical is now free to react with a chlorine molecule

Chlorine radical Cl Cl Chloromethane Methyl radical Chlorine Chloromethane Chlorine radical can now go and react with a methane molecule

Stage 3 Termination Grinding to a halt

Three different ways this can happen

No free radicals to keep it going Cl Cl Chlorine molecule Reaction stops No free radicals to keep it going Chlorine radical Chlorine radical

Because there are no free radicals to keep it going Methyl radical Cl Chlorine radical Chloromethane forms Reaction stops Because there are no free radicals to keep it going

Methyl radical Methyl radical Ethane 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.

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

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-Cl Cl°+ °CH3= CH3Cl CH3+ CH3= C2H6

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

Addition Reaction (pg. 367) When two substances react together to form a single substance

Addition Reaction Mechanism and evidence for Heterolytic Fission

Polarising of the bond in bromine Step 1 Polarising of the bond in bromine

Ethene Concentration of negative charge Because 4 electrons in this area Bromine Br2 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+ Moves in this direction

Heterolytic Fission Occurs Step 2 Heterolytic Fission Occurs

The two electrons of the bond have been forced across to the right Br making it Br- while the other is Br+ Br+ Br - 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+ The Br2 has been split into 2 different species i.e. Br+ and Br- this is called Heterolytic Fission

Formation of the Carbonium Ion Step 3 Formation of the Carbonium Ion

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 The two electrons of the bond have been forced across to the right Br making it Br- while the other is Br+ Br Br - Br+ Br - Let us put in the two electrons of this bond + The Br2 has been split into 2 different species i.e. Br+ and Br- this is called Heterolytic Fission Carbonium ion Cyclic bromium ion

Attack on carbonium ion by Br- Step 4 Attack on carbonium ion by 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 Br - + Br Carbonium ion 1,2 dibromoethane Called Ionic Addition because the species are ions when they add on

Step 5 Proof of mechanism

Br Br - + Cl Br - Cl - 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

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. Br+ molecule attacks double bond and forms cyclic bromonium ion/ carbonium ion Step 4: Termination Br- now attacks the carbonium ion

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

Evidence for the carbonium ion When bromine and chlorine ions present Ethene forms 1-bromo-2-chloroethane as well as 1, 2 dibromoethane

Polymerisation rxns Molecules that contain double bonds undergo addition to become less unsaturated (addition polymers such as polythene and polypropene)

Polymerisation Reactions Example of an addition reaction Ethene molecules add together Polymers are long chain molecules made by joining together many small molecules + =

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

Elimination reactions

Elimination reactions When a small molecule is removed from a larger molecule to leave a double bond in the larger molecule

AB  A + B Elimination rxns a compound breaks down into 2 or more simpler substances Double bond created only one reactant AB  A + B

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

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

Dehydration of ethanol Ethanol is dehydrated to ethene This reaction is used in the preparation of ethene

Dehydration of ethanol to ethene

Reaction conditions Heat Aluminium oxide catalyst

Preparation of ethene

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

C. Decomposition 2 H2O(l)  2 H2(g) + O2(g)

Redox reactions

Redox reactions These reactions involves oxidation and reduction reactions The removal or addition of lectrons from the molecule

-3 -2 -1 1 2 3 Receives electrons Looses electrons 1 2 3 Reduction Oxidation Receives electrons Looses electrons Reducing agents give electrons Oxidation agents take electrons

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

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

Reaction of ethanol with sodium dichromate(VI)

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

Preparation of ethanal

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.

Oxidation of ethanol

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

Preparation of ethanoic acid Reflux followed by Distillation

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

Oxidation of propan-2-ol

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

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

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

Reduction of ethanal to ethanol

Reduction of propanone to propan-2-ol

ENERGY PROFILE one step reaction transition state TS activation energy maximum activation energy Ea obtained from heat (collisions) E N R G Y heat of reaction DH starting material exothermic (releases heat) product opposite is endothermic REACTION COORDINATE ( follows the progress of the reaction )

Reactions as acids

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

Reaction of ethanol with sodium

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)

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

Reaction of ethanoic acid with magnesium Acid + metal → salt + hydrogen 2 CH3COOH + Mg → (CH3COO)2Mg + H2 ethanoic acid magnesium ethanoate

Reaction of ethanoic acid with sodium hydroxide Acid + Base → Salt + Water CH3COOH + NaOH → CH3COONa + H2O ethanoic acid sodium ethanoate

Reaction of ethanoic acid with sodium carbonate Acid + Carbonate → Salt + Water + Carbon dioxide 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2 ethanoic acid sodium ethanoate