Free radical substitution
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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.

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Free Radical Substitution

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Free radical substitution

Free Radical Substitution

Homolytic Fission


Substitution rxn free radical substitution

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

Mechanism of reaction

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


Free radical substitution

H

H

Cl

Cl

Cl

Cl

Methane

Chloromethane

=

+

+

Hydrogen Chloride

Chlorine


Free radical substitution

Simple mechanism

H

Cl

Cl

Chloromethane

Methane

Hydrogen and Chlorine have swapped places

Substitution

Hydrogen Chloride

Chlorine


Stage 1

Stage 1

Initiation

Getting Started


Free radical substitution

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

Stage 2

Propagation

Keeping it going


Free radical substitution

H

Cl

Lets put in the 2 electrons in this bond

Chlorine radical

Methane

The chlorine radical pulls the hydrogen and one electron across to it.

Hydrogen chloride

Methyl radical

The methyl radical is now free to react with a chlorine molecule


Free radical substitution

Cl

Cl

Chlorine

radical

Methyl

radical

Chlorine

Chloromethane

Chlorine radical can now go and react with a methane molecule


Stage 3

Stage 3

Termination

Grinding to a halt


Free radical substitution

  • Three different ways this can happen


Free radical substitution

Cl

Cl

Chlorine molecule

Reaction stops

No free radicals to keep it going

Chlorine

radical

Chlorine

radical


Free radical substitution

Cl

Chlorine

radical

Methyl

radical

Chloromethane forms

Reaction stops

Because there are no free radicals to keep it going


Free radical substitution

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.


Free radical substitution

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


Tetramethyl lead is added to speed up the rxn

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

Addition Reaction (pg. 367)

  • When two substances react together to form a single substance


Addition reaction mechanism

Addition Reaction Mechanism

and evidence for

Heterolytic Fission


Step 1

Step 1

Polarising of the bond in bromine


Free radical substitution

Concentration of negative charge

Because 4 electrons in this area

δ+

δ-

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+

Ethene

Bromine Br2


Step 2

Step 2

Heterolytic Fission Occurs


Free radical substitution

The two electrons of the bond have been forced across to the right Br making it Br- while the other is Br+

δ+

δ-

The Br2 has been split into 2 different species i.e. Br+ and Br- this is called Heterolytic Fission

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-


Step 3

Step 3

Formation of the Carbonium Ion


Free radical substitution

The two electrons of the bond have been forced across to the right Br making it Br- while the other is 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

Br-

  • 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

Br-

+

Carbonium ion

Cyclic bromium ion


Step 4

Step 4

Attack on carbonium ion

by Br-


Free radical substitution

Br

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

+

Carbonium ion

1,2 dibromoethane

Called Ionic Addition because the species are ions when they add on


Step 5

Step 5

Proof of

mechanism


Free radical substitution

Br

Br-

Br-

Br-

Br-

Cl

Br-

Cl-

Br-

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


Hydrogenation

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

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

Polymerisation rxns

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


Polymerisation reactions

Polymerisation Reactions

  • Example of an addition reaction

  • Ethene molecules add together

  • Polymers are long chain molecules made by joining together many small molecules

+

=


Polymers

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


Elimination reactions1

Elimination reactions

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


Elimination rxns

Elimination rxns

  • a compound breaks down into 2 or more simpler substances

  • Double bond created

  • only one reactant

AB  A + B


Elimination reactions2

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

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

Dehydration of ethanol

  • Ethanol is dehydrated to ethene

  • This reaction is used in the preparation of ethene


Dehydration of ethanol to ethene

Dehydration of ethanol to ethene


Reaction conditions

Reaction conditions

  • Heat

  • Aluminium oxide catalyst


Preparation of ethene

Preparation of ethene


Elimination rxn

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

C. Decomposition

2 H2O(l)  2 H2(g) + O2(g)


Redox reactions

Redox reactions


Redox reactions1

Redox reactions

  • These reactions involves oxidation and reduction reactions

  • The removal or addition of lectrons from the molecule


Free radical substitution

Reduction

Oxidation


Redox reactions of primary alcohols

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

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)


Reaction of ethanol with sodium dichromate vi1

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

Preparation of ethanal


Oxidation of primary alcohols

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

Oxidation of ethanol


Reaction of ethanol with sodium dichromate vi2

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

Preparation of ethanoic acid

Refluxfollowed by Distillation


Oxidation of secondary alcohols

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

Oxidation of propan-2-ol


Combustion of organic compounds

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

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

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 ethanal to ethanol


Reduction of propanone to propan 2 ol

Reduction of propanone to propan-2-ol


Free radical substitution

ONE STEP

ENERGY PROFILE

one step reaction

transition

state TS

energy maximum

activation

energy Ea

obtained from

heat (collisions)

E

N

E

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 as acids


Reactions of alcohols with sodium

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

Reaction of ethanol with sodium


Acidic nature of the carboxylic acid group

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

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

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

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

Reaction of ethanoic acid with sodium carbonate

Acid + Carbonate → Salt + Water + Carbon dioxide

2CH3COOH + Na2CO3→ 2CH3COONa + H2O + CO2

ethanoic acid sodium ethanoate


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