Organic Mechanisms Chapter 23 Free Radical Substitution CH 4 + Cl 2 CH 3 Cl + HCl An example of a...

Organic Mechanisms

Chapter 23

Free Radical Substitution

CH4 + 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.

Free radical SubstitutionThe mechanism involved in the chlorination of

Methane is believed to consist of the following steps.

Initiation

uv lightCl2 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.

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.

Termination

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

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.

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.

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

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

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) 1,2 - dibromoethane

Mechanism

It is surprising that bromineshould act as an electrophileas it is non-polar.

CC ELECTROPHILIC ADDITION OF BROMINEELECTROPHILIC ADDITION OF BROMINE

CONVERSIONSCONVERSIONS

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–),

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

Step3

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

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)

OR

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))

ELECTROPHILIC ADDITION OF HClELECTROPHILIC 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 breaksThe 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.

BB


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

CH3COOC2H5 + NaOH CH3COONa + C2H5OH

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.

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.

How Soap Works

Soap

Glycerine TriSterate + NaOH Sodium Sterate + Glycerol

3C17H35COOCH2 + 3NaOH 3C17H35COONa

Soaps are formed by the hydrolysis of fatty acid esters to produce salts of the fatty acids.

Preparation of Soap

Reflux apparatus used in the preparation of Soap

The ethanol solvent is removed by distillation

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.

POLYMERISATION OF ALKENESPOLYMERISATION OF ALKENES

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

ADDITION POLYMERISATION

the equation shows the original monomer and the repeating unit in the polymer

ethene poly(ethene)

MONOMER POLYMER

n represents a large number

POLYMERISATION OF ALKENESPOLYMERISATION OF ALKENES

ETHENE

EXAMPLES OF ADDITION POLYMERISATION

PROPENE

TETRAFLUOROETHENE

CHLOROETHENE

POLY(ETHENE)

POLY(PROPENE)

POLY(CHLOROETHENE)

POLYVINYLCHLORIDE PVC

POLY(TETRAFLUOROETHENE)

PTFE “Teflon”

ELIMINATION OF WATER (DEHYDRATION)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)

LL


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.

OXIDATION OF PRIMARY ALCOHOLSOXIDATION 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)

NN

Aldehyde has a lower boiling point so distils off before being oxidised further

OXIDATION TOALDEHYDES

DISTILLATION

OXIDATION TOCARBOXYLIC ACIDS

REFLUX

Aldehyde condenses back into the mixture and gets oxidised to the acid


OXIDATION OF ALDEHYDESOXIDATION 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’ REAGENTReagent ammoniacal silver nitrate solutionObservation a silver mirror is formed on the inside of the test tubeProducts silver + carboxylic acidEquation Ag+ + e- ——> Ag

FEHLING’S SOLUTIONReagent a solution of a copper(II) complex Observation a red precipitate forms in the blue solution Products copper(I) oxide + carboxylic acidEquation Cu2+ + e- ——> Cu+

OO


OXIDATION OF SECONDARY ALCOHOLSOXIDATION 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


REDUCTION OF ALDEHYDESREDUCTION OF ALDEHYDESRR

Reagent H2 / Nickel catalyst

Conditions

Product primary alcohol

Equation e.g. CH3CHO(l) + 2[H] ———> C2H5OH(l)

Ethanal is reduced to Ethanol


REDUCTION OF CARBOXYLIC ACIDSREDUCTION OF CARBOXYLIC ACIDSQQ

Reagent/catalyst H2 Nickel catalyst

Conditions reflux in ethoxyethane

Product aldehyde

Equation e.g. CH3COOH(l) + 2[H] ———> CH3CHO(l) + H2O(l)


REDUCTION OF KETONESREDUCTION OF KETONESSS

Reagent H2 / Nickel catalyst

Conditions warm in water or ethanol

Product secondary alcohol

Equation e.g. CH3COCH3(l) + 2[H] ———> CH3CH(OH)CH3(l)

Propanone is reduced to Propan-2-ol


ESTERSESTERS

Structure Substitute an organic group for the H in carboxylic acids

Nomenclature first part from alcohol, second part from acide.g. methyl ethanoate CH3COOCH3

ETHYL METHANOATE METHYL ETHANOATE

ESTERSESTERS



Preparation From carboxylic acids or acyl chlorides

Reactivity Unreactive compared with acids and acyl chlorides


ESTERSESTERS



Preparation From carboxylic acids or acyl chlorides

Reactivity Unreactive compared with acids and acyl chlorides

Isomerism Esters are structural isomers of carboxylic acids


Classification CARBOXYLIC ACID ESTER

Functional Group R-COOH R-COOR

Name PROPANOIC ACID METHYL ETHANOATE

Physical properties O-H bond gives rise No hydrogen bondingto hydrogen bonding; insoluble in waterget higher boiling pointand solubility in water

Chemical properties acidic fairly unreactivereacts with alcohols hydrolysed to acids

STRUCTURAL ISOMERISM – STRUCTURAL ISOMERISM – FUNCTIONAL GROUPFUNCTIONAL GROUP

PREPARATION OF ESTERS - 1PREPARATION 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 thusincreases the yield of the ester

For more details see under ‘Reactions of carboxylic acids’

HYDROLYSIS OF ESTERSHYDROLYSIS OF ESTERS

Hydrolysis is the opposite of esterification

ESTER + WATER CARBOXYLIC ACID + ALCOHOL

HCOOH + C2H5OHMETHANOIC ETHANOL ACID

ETHYL METHANOATE





ETHYL METHANOATE

METHYL ETHANOATE





CH3COOH + CH3OHETHANOIC METHANOL ACID

ETHYL METHANOATE

METHYL ETHANOATE




The products of hydrolysis depend on the conditions used...

acidic CH3COOCH3 + H2O CH3COOH + CH3OH

alkaline CH3COOCH3 + NaOH ——> CH3COO¯ Na+ + CH3OH







If the hydrolysis takes place under alkaline conditions, the organic product is a water soluble ionic salt







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

NATURALLY OCCURING ESTERS - NATURALLY OCCURING ESTERS - TRIGLYCERIDESTRIGLYCERIDES

• 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

CH2OH

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.

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