1. Organic Mechanisms
Chapter 23

2. Free Radical Substitution
CH Cl2  CH3Cl + HCl
An example of a substitution reaction is the
chlorination of methane.
A chlorine atom replaces an atom of Hydrogen
in a molecule of methane.

3. Free radical Substitution
The mechanism involved in the chlorination of Methane is believed to consist of the following steps.

4. Initiation
uv light
Cl Cl* Cl*
The reaction mechanism begins with the homolytic fission of the chlorine molecule by UV light.
Two atoms of chlorine with unpaired electrons are formed. These are very reactive and, as stated above, are called free radicals.

5. Propagation CH4 + Cl * CH3* + HCl CH3* + Cl2 CH3CL + Cl*
A chlorine atom attacks the methane molecule to form Hydrogen chloride and a methyl free radical. The methy free radical attacks a chlorine molecule and gives us one of the desired products, CH3Cl. In so doing it yields another chlorine free radical. If this follows the same pathway it will yield more products and more free radicals.
We now have a chain reaction initiated by chlorine radicals and ending with new chlorine radicals. This also explains why a large number of chloromethane molecules are produced for every photon absorbed.

6. Termination 2Cl· .... Cl2 CH3· + Cl· .... CH3Cl
As the number of free radicals is increasing and the concentrations
of methane and chlorine are falling. A single free radical has
caused many thousands of methane and chlorine molecules to be
broken down.
Eventually, the probability of one of these reactions occurring increases.
2Cl· .... Cl2
CH3· + Cl· .... CH3Cl
CH3· + CH3· .... CH3CH3

7. Evidence * Tetramethyl-lead greatly speeds up the reaction.
* Molecular oxygen slows down the reaction.
Studies have shown that tetramethyl-lead, Pb(CH3)4, decomposes to give lead, Pb, and four CH3· radicals. This would greatly increase the concentration of methyl radicals, thus increasing the reaction rate, i.e it serves as an accelerator.
On the other hand oxygen, O2, combines with methyl radicals, CH3·, to form the less reactive peroxymethyl radical, CH3OO·. This slows down the reaction as a single oxygen molecule prevents thousands of CH3Cl molecules being formed. Oxygen is an inhibitor and the slowing down of a reaction by small amounts of a substance is a sure indication that a chain reaction is involved.

8. Evidence for free radical substitution
Free Radical Substitution Mechanism
Halogenation reactions with alkanes involve replacement of one or all of the hydrogens in the alkane. These reactions may produce many products due to the high reactivity of the free radical species. The substitution reaction needs energy to be supplied before the reaction can proceed. Heating or shining ultraviolet light on the reaction mixture may supply this energy.
(a) Chlorination of Methane
Evidence for the mechanism occurs at all steps
• For the initiation step
1. The reaction will not occur in the dark at room temperature. It will occur at room temperature if ultraviolet light is shone on the reactants.

9. 2. The energy supplied is not sufficient to break a C-H bond
2. The energy supplied is not sufficient to break a C-H bond. Sufficient energy isupplied to break a Cl-Cl bond however. The energy of the radiation needs to be at least that required to homolytically spilt the chlorine molecule. 3. No molecular hydrogen produced – hence no hydrogen free radicals have been formed. For the propagation steps 1. Thousands of chloromethane molecules are produced for every one photon of light used. This suggests a chain reaction consistent with theproposed mechanism. 2. No molecular hydrogen produced – hence no hydrogen free radicals have been formed. For the termination steps 1. Ethane is produced in small amounts. Its occurrence can only be explained by CH3+ CH3 CH3CH3 If the reaction is left run with excess chlorine and uv light di- tri- and tetra-chloro methane are produced as are minute amounts of a range of chloroethanes. 2. The presence of tetramethyl-lead greatly speeds up the reaction as it a source of methyl free radicals

10. Ionic Addition
An addition reaction is one in which 2 substances react together to form a single substance.
The mechanism involved is different from that between methane and chlorine

11. ELECTROPHILIC ADDITION OF BROMINE
Reagent Bromine. (Neat liquid or dissolved in tetrachloromethane, CCl4 )
Conditions Room temperature. No catalyst or UV light required!
Equation C2H4(g) + Br2(l) ——> CH2BrCH2Br(l) ,2 - dibromoethane
Mechanism
It is surprising that bromine
should act as an electrophile
as it is non-polar.
CONVERSIONS

12. Ionic Mechanism of Bromination of Ethene
Step 1
The first stage in the mechanism involves a bromine molecule becoming momentarily polarised on approach to the region of high electron density of the double bond. The bromine molecule undergoes heterolytic fission (unequal splitting), forming a bromonium ion (Br+) and a bromide ion(Br–),

13. Step 2
The Br+, in order to gain the 2 electrons it needs, attacks the C2H4 molecule.
The Br+ forms a covalent bond with one of the carbon atoms.
The other carbon atom is left with a positive charge since it lost one of its outer electrons. This positively charged atom is called a carbonium ion.
Carbonium ion

14. Step3
The carbonium ion is then attacked by the Br- ion. This results in the formation of 1,2-dibromoethane.

15. Evidence of ionic addition
Evidence: addition using bromine water gives 2-bromoethanol
(CH2BrCH2OH) OR
addition with bromine water containing a chloride (sodium chloride)
gives 1-bromo-2-chloroethane (Allow 1-chloro-2-bromoethane)
(CH2BrCH2Cl)
Another specified anion / chlorine water / HCl in water (HCl(aq), hydrochloric acid)
Product where that anion has added in place of the chlorine (e.g. 2-chloroethanol for chlorine water, and ethanol for HCl(aq))

16. ELECTROPHILIC ADDITION OF HClB
ELECTROPHILIC ADDITION OF HCl
Reagent Hydrogen Chloride... it is electrophilic as the H is slightly positive
Condition Room temperature.
Equation C2H4(g) HCl(g) ———> C2H5Cl(l) chloroethane
Mechanism
Step 1 As the HCl nears the alkene, one of the carbon-carbon bonds breaks
The pair of electrons attaches to the slightly positive H end of H-Cl.
The HCl bond breaks to form a chloride ion.
A carbocation (positively charged carbon species) is formed.
Step 2 The chloride ion behaves as a nucleophile and attacks the carbocation.
Overall there has been addition of HCl across the double bond.
CONVERSIONS

17. Esterification-Formation of an Ester
An Ester is formed when an alcohol and a carboxylic acid react together. This is called a condensation reaction.
Alcohol + Carboxylic Acid  Ester + Water
The reverse reaction is called a Hydrolysis.
Esters may be Hydrolysed easily in the presence of a Base like NaOH or KOH.
Ethyl Ethanoate + Sodium Hydroxide  Sodium Ethanoate + Ethanol
CH3COOC2H NaOH  CH3COONa C2H5OH

18. Soap formation
Soaps are salts of fatty acids (long chain carboxylic acids). Fats are esters formed by the condensation of fatty acids and glycerol (propane-1,2,3-triol).
Soaps are manufactured by the base hydrolysis of these fats (esters). In this experiment the fat is hydrolysed using sodium hydroxide in ethanol solution. The ethanol is then removed by distillation.
Soaps are formed by the hydrolysis of fatty acid esters to produce salts of the fatty acids.

19. How Soap Works
The hydrocarbon end of the molecule is hydrophobic (water repelling) and the carboxylate end is hydrophilic (water attracting). The hydrophobic end dissolves in grease and the hydrophilic end dissolves in the water.

20. Soap
Soaps are formed by the hydrolysis of fatty acid esters to produce salts of the fatty acids.
Glycerine TriSterate + NaOH  Sodium Sterate Glycerol
3C17H35COOCH NaOH C17H35COONa

21. Preparation of Soap
The ethanol solvent is removed by distillation
Reflux apparatus used in the preparation of Soap

22. Polymerisation reactions
Polymers are long chain molecules made by joining together many small molecules called monomers.
The polymers that we study are Addition polymers because their manufacture involves addition reactions.

23. POLYMERISATION OF ALKENES
ADDITION POLYMERISATION
Process • during polymerisation, an alkene undergoes an addition reaction with itself
• all the atoms in the original alkenes are used to form the polymer
• long hydrocarbon chains are formed
the equation shows the original monomer and the repeating unit in the polymer
ethene poly(ethene)
MONOMER POLYMER
n represents a large number

24. POLYMERISATION OF ALKENES
EXAMPLES OF ADDITION POLYMERISATION
ETHENE
POLY(ETHENE)
PROPENE
POLY(PROPENE)
CHLOROETHENE
POLY(CHLOROETHENE)
POLYVINYLCHLORIDE PVC
TETRAFLUOROETHENE
POLY(TETRAFLUOROETHENE)
PTFE “Teflon”

25. ELIMINATION OF WATER (DEHYDRATION)
An elimination reaction is one in which a small molecule is removed from a larger molecule to leave a double bond in the larger molecule.
Example. The removal of water from an alcohol is an example of an elimination reaction
Product alkene
Equation e.g. C2H5OH(l) ————> CH2 = CH2(g) H2O(l)
CONVERSIONS

26. Redox reactions
When a primary alcohol reacts with an oxidising agent the primary alcohol is converted to an aldehyde.
When a secondary alcohol reacts with an oxidising agent the secondary alcohol is converted to a ketone.

27. OXIDATION OF PRIMARY ALCOHOLS
Primary alcohols are easily oxidised to aldehydes
e.g. CH3CH2OH(l) [O] ———> CH3CHO(l) H2O(l)
it is essential to distil off the aldehyde before it gets oxidised to the acid
CH3CHO(l) [O] ———> CH3COOH(l)
OXIDATION TO
ALDEHYDES
DISTILLATION
OXIDATION TO
CARBOXYLIC ACIDS
REFLUX
Aldehyde has a lower boiling point so distils off before being oxidised further
Aldehyde condenses back into the mixture and gets oxidised to the acid
CONVERSIONS

28. OXIDATION OF ALDEHYDES
Aldehydes are easily oxidised to carboxylic acids
e.g. CH3CHO(l) [O] ———> CH3COOH(l)
one way to tell an aldehyde from a ketone is to see how it reacts to mild oxidation
ALDEHYES are EASILY OXIDISED
KETONES are RESISTANT TO MILD OXIDATION
reagents include TOLLENS’ REAGENT and FEHLING’S SOLUTION
TOLLENS’ REAGENT
Reagent ammoniacal silver nitrate solution
Observation a silver mirror is formed on the inside of the test tube
Products silver + carboxylic acid
Equation Ag e- ——> Ag
FEHLING’S SOLUTION
Reagent a solution of a copper(II) complex
Observation a red precipitate forms in the blue solution
Products copper(I) oxide + carboxylic acid
Equation Cu e- ——> Cu+
CONVERSIONS

29. Secondary alcohols are easily oxidised to ketones
OXIDATION OF SECONDARY ALCOHOLS
Secondary alcohols are easily oxidised to ketones
e.g CH3CHOHCH3(l) [O] ———> CH3COCH3(l) H2O(l)
Propan-2-ol is oxidised to propanone
CONVERSIONS

30. REDUCTION OF ALDEHYDES
Reagent H2 / Nickel catalyst
Conditions
Product primary alcohol
Equation e.g. CH3CHO(l) [H] ———> C2H5OH(l)
Ethanal is reduced to Ethanol
CONVERSIONS

31. REDUCTION OF CARBOXYLIC ACIDSQ
REDUCTION OF CARBOXYLIC ACIDS
Reagent/catalyst H2 Nickel catalyst
Conditions reflux in ethoxyethane
Product aldehyde
Equation e.g. CH3COOH(l) [H] ———> CH3CHO(l) H2O(l)
CONVERSIONS

32. REDUCTION OF KETONES Reagent H2 / Nickel catalyst
Conditions warm in water or ethanol
Product secondary alcohol
Equation e.g. CH3COCH3(l) [H] ———> CH3CH(OH)CH3(l)
Propanone is reduced to Propan-2-ol
CONVERSIONS

33. ESTERS
Structure Substitute an organic group for the H in carboxylic acids
Nomenclature first part from alcohol, second part from acid
e.g. methyl ethanoate CH3COOCH3
METHYL ETHANOATE
ETHYL METHANOATE

34. ESTERS
Structure Substitute an organic group for the H in carboxylic acids
Nomenclature first part from alcohol, second part from acid
e.g. methyl ethanoate CH3COOCH3
Preparation From carboxylic acids or acyl chlorides
Reactivity Unreactive compared with acids and acyl chlorides
METHYL ETHANOATE
ETHYL METHANOATE

35. ESTERS
Structure Substitute an organic group for the H in carboxylic acids
Nomenclature first part from alcohol, second part from acid
e.g. methyl ethanoate CH3COOCH3
Preparation From carboxylic acids or acyl chlorides
Reactivity Unreactive compared with acids and acyl chlorides
Isomerism Esters are structural isomers of carboxylic acids
METHYL ETHANOATE
ETHYL METHANOATE

36. STRUCTURAL ISOMERISM – FUNCTIONAL GROUP
Classification CARBOXYLIC ACID ESTER
Functional Group R-COOH R-COOR
Name PROPANOIC ACID METHYL ETHANOATE
Physical properties O-H bond gives rise No hydrogen bonding
to hydrogen bonding; insoluble in water
get higher boiling point
and solubility in water
Chemical properties acidic fairly unreactive
reacts with alcohols hydrolysed to acids

37. PREPARATION OF ESTERS - 1
Reagent(s) alcohol + carboxylic acid
Conditions reflux with a strong acid catalyst (e.g. conc. H2SO4 )
Equation e.g. CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)
ethanol ethanoic acid ethyl ethanoate
Notes Conc. H2SO4 is a dehydrating agent - it removes water
causing the equilibrium to move to the right and thus
increases the yield of the ester
For more details see under ‘Reactions of carboxylic acids’

38. Hydrolysis is the opposite of esterification
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER CARBOXYLIC ACID + ALCOHOL
HCOOH C2H5OH
METHANOIC ETHANOL
ACID
ETHYL METHANOATE

39. Hydrolysis is the opposite of esterification
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER CARBOXYLIC ACID + ALCOHOL
HCOOH C2H5OH
METHANOIC ETHANOL
ACID
ETHYL METHANOATE
METHYL ETHANOATE

40. Hydrolysis is the opposite of esterification
HYDROLYSIS OF ESTERS
Hydrolysis is the opposite of esterification
ESTER + WATER CARBOXYLIC ACID + ALCOHOL
HCOOH C2H5OH
METHANOIC ETHANOL
ACID
ETHYL METHANOATE
CH3COOH CH3OH
ETHANOIC METHANOL
ACID
METHYL ETHANOATE

41. HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification
ESTER + WATER CARBOXYLIC ACID + ALCOHOL
The products of hydrolysis depend on the conditions used...
acidic CH3COOCH H2O CH3COOH CH3OH
alkaline CH3COOCH NaOH ——> CH3COO¯ Na CH3OH

42. HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification
ESTER + WATER CARBOXYLIC ACID + ALCOHOL
The products of hydrolysis depend on the conditions used...
acidic CH3COOCH H2O CH3COOH CH3OH
alkaline CH3COOCH NaOH ——> CH3COO¯ Na CH3OH
If the hydrolysis takes place under alkaline conditions,
the organic product is a water soluble ionic salt

43. HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification
ESTER + WATER CARBOXYLIC ACID + ALCOHOL
The products of hydrolysis depend on the conditions used...
acidic CH3COOCH H2O CH3COOH CH3OH
alkaline CH3COOCH NaOH ——> CH3COO¯ Na CH3OH
If the hydrolysis takes place under alkaline conditions,
the organic product is a water soluble ionic salt
The carboxylic acid can be made by treating the salt with HCl
CH3COO¯ Na HCl ——> CH3COOH NaCl

44. NATURALLY OCCURING ESTERS - TRIGLYCERIDES
• triglycerides are the most common component of edible fats and oils
• they are esters of the alcohol
glycerol (propane-1,2,3-triol)
Saponification
• alkaline hydrolysis of triglycerol esters produces soaps
• a simple soap is the salt of a fatty acid
• as most oils contain a mixture of triglycerols, soaps are not pure
• the quality of a soap depends on the oils from which it is made
CH2OH
CHOH

45. Hydrolysis of Esters to produce soap
Soaps Soaps are formed by the hydrolysis of fatty acid esters to produce salts of the fatty acids. The hydrocarbon end of the molecule is hydrophobic (water repelling) and the carboxylate end is hydrophilic (water attracting). The hydrophobic end dissolves in grease and the hydrophilic end dissolves in the water.