Organic Mechanisms Chapter 23 Free Radical Substitution CH

Organic Mechanisms Chapter 23

Free Radical Substitution CH 4 + Cl 2 CH 3 Cl + 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 Substitution The mechanism involved in the chlorination of Methane is believed to consist of the following steps.

Initiation Cl 2 uv light 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 CH 4 + Cl * CH 3* + Cl 2 CH 3* + HCl CH 3 CL + 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, CH 3 Cl. 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. 2 Cl·. . Cl 2 CH 3· + Cl·. . CH 3 Cl CH 3· + CH 3·. . CH 3

Evidence * Tetramethyl-lead greatly speeds up the reaction. * Molecular oxygen slows down the reaction. Studies have shown that tetramethyl-lead, Pb(CH 3)4, decomposes to give lead, Pb, and four CH 3· 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, O 2, combines with methyl radicals, CH 3·, to form the less reactive peroxymethyl radical, CH 3 OO·. This slows down the reaction as a single oxygen molecule prevents thousands of CH 3 Cl 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 CH 3+ CH 3 CH 3 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

C ELECTROPHILIC ADDITION OF BROMINE Reagent Bromine. (Neat liquid or dissolved in tetrachloromethane, CCl 4 ) Conditions Room temperature. No catalyst or UV light required! Equation C 2 H 4(g) + Br 2(l) ——> CH 2 Br(l) 1, 2 - dibromoethane Mechanism It is surprising that bromine should act as an electrophile as it is non-polar. CONVERSIONS

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 C 2 H 4 molecule. Carbonium ion 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.

Step 3 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 (CH 2 Br. CH 2 OH) OR addition with bromine water containing a chloride (sodium chloride) gives 1 -bromo-2 -chloroethane (Allow 1 -chloro-2 -bromoethane) (CH 2 Br. CH 2 Cl) 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 HCl B Reagent Hydrogen Chloride. . . it is electrophilic as the H is slightly positive Condition Room temperature. Equation C 2 H 4(g) + HCl(g) ———> C 2 H 5 Cl(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

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 Na. OH or KOH. Ethyl Ethanoate + Sodium Hydroxide Sodium Ethanoate + Ethanol CH 3 COOC 2 H 5 + Na. OH CH 3 COONa + C 2 H 5 OH

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.

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.

Soaps are formed by the hydrolysis of fatty acid esters to produce salts of the fatty acids. Glycerine Tri. Sterate + Na. OH 3 C 17 H 35 COOCH 2 + 3 Na. OH Sodium Sterate + 3 C 17 H 35 COONa Glycerol

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 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 n represents a large number ethene poly(ethene) MONOMER POLYMER

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

L 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 Equation alkene e. g. C 2 H 5 OH(l) ————> CH 2 = CH 2(g) + H 2 O(l) CONVERSIONS

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 ALCOHOLS N Primary alcohols are easily oxidised to aldehydes e. g. CH 3 CH 2 OH(l) + [O] ———> CH 3 CHO(l) + H 2 O(l) it is essential to distil off the aldehyde before it gets oxidised to the acid CH 3 CHO(l) + [O] ———> OXIDATION TO ALDEHYDES DISTILLATION CH 3 COOH(l) 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

O OXIDATION OF ALDEHYDES Aldehydes are easily oxidised to carboxylic acids e. g. CH 3 CHO(l) + [O] • • ———> CH 3 COOH(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 2+ + e- ——> Cu+ CONVERSIONS

OXIDATION OF SECONDARY ALCOHOLS Secondary alcohols are easily oxidised to ketones e. g. CH 3 CHOHCH 3(l) + [O] ———> CH 3 COCH 3(l) + H 2 O(l) Propan-2 -ol is oxidised to propanone CONVERSIONS

REDUCTION OF ALDEHYDES R Reagent H 2 / Nickel catalyst Conditions Product Equation primary alcohol e. g. CH 3 CHO(l) + 2[H] ———> Ethanal is reduced to Ethanol CONVERSIONS C 2 H 5 OH(l)

Q REDUCTION OF CARBOXYLIC ACIDS Reagent/catalyst H 2 Nickel catalyst Conditions reflux in ethoxyethane Product aldehyde Equation e. g. CH 3 COOH(l) + 2[H] ———> CONVERSIONS CH 3 CHO(l) + H 2 O(l)

S REDUCTION OF KETONES Reagent H 2 / Nickel catalyst Conditions warm in water or ethanol Product secondary alcohol Equation e. g. CH 3 COCH 3(l) + 2[H] ———> CH 3 CH(OH)CH 3(l) Propanone is reduced to Propan-2 -ol CONVERSIONS

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 CH 3 COOCH 3 METHYL ETHANOATE ETHYL METHANOATE

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 CH 3 COOCH 3 METHYL ETHANOATE ETHYL METHANOATE Preparation From carboxylic acids or acyl chlorides Reactivity Unreactive compared with acids and acyl chlorides

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 CH 3 COOCH 3 METHYL ETHANOATE ETHYL METHANOATE Preparation From carboxylic acids or acyl chlorides Reactivity Unreactive compared with acids and acyl chlorides Isomerism Esters are structural isomers of carboxylic acids

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

PREPARATION OF ESTERS - 1 Reagent(s) alcohol + carboxylic acid Conditions reflux with a strong acid catalyst (e. g. conc. H 2 SO 4 ) Equation Notes e. g. CH 3 CH 2 OH(l) + CH 3 COOH(l) ethanol ethanoic acid CH 3 COOC 2 H 5(l) + H 2 O(l) ethyl ethanoate Conc. H 2 SO 4 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’

HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification ESTER + WATER CARBOXYLIC ACID + ALCOHOL HCOOH METHANOIC ACID ETHYL METHANOATE + C 2 H 5 OH ETHANOL

HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification ESTER + WATER CARBOXYLIC ACID + ALCOHOL HCOOH METHANOIC ACID ETHYL METHANOATE METHYL ETHANOATE + C 2 H 5 OH ETHANOL

HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification ESTER + WATER CARBOXYLIC ACID + ALCOHOL HCOOH + METHANOIC ACID C 2 H 5 OH ETHANOL ETHYL METHANOATE CH 3 COOH ETHANOIC ACID METHYL ETHANOATE + CH 3 OH METHANOL

HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification ESTER + WATER CARBOXYLIC ACID + ALCOHOL The products of hydrolysis depend on the conditions used. . . acidic CH 3 COOCH 3 + H 2 O alkaline CH 3 COOCH 3 + Na. OH CH 3 COOH + CH 3 OH ——> CH 3 COO¯ Na+ + CH 3 OH

HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification ESTER + WATER CARBOXYLIC ACID + ALCOHOL The products of hydrolysis depend on the conditions used. . . acidic CH 3 COOCH 3 + H 2 O alkaline CH 3 COOCH 3 + Na. OH CH 3 COOH + CH 3 OH ——> CH 3 COO¯ Na+ + CH 3 OH If the hydrolysis takes place under alkaline conditions, the organic product is a water soluble ionic salt

HYDROLYSIS OF ESTERS Hydrolysis is the opposite of esterification ESTER + WATER CARBOXYLIC ACID + ALCOHOL The products of hydrolysis depend on the conditions used. . . acidic CH 3 COOCH 3 + H 2 O alkaline CH 3 COOCH 3 + Na. OH CH 3 COOH + CH 3 OH ——> CH 3 COO¯ Na+ + CH 3 OH 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 CH 3 COO¯ Na+ + HCl ——> CH 3 COOH + Na. Cl

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) CH 2 OH 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

Hydrolysis of Esters to produce soap 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.