Types of reactions

1. Types of reactions Chapter 23

2. Substitution Reactions A substitution reaction is a chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms. Eg.Halogenation of Alkanes Chlorine and Methane

3. BREAKING COVALENT BONDS There are 3 ways to split the shared electron pair in an unsymmetrical covalent bond. UNEQUAL SPLITTING produces IONS known as HETEROLYSIS or HETEROLYTICFISSION EQUAL SPLITTING produces RADICALS known as HOMOLYSIS or HOMOLYTICFISSION • If several bonds are present the weakest bond is usually broken first • Energy to break bonds can come from a variety of energy sources - heat / light • In the reaction between methane and chlorine either can be used, however... • In the laboratory a source of UV light (or sunlight) is favoured.

4. FREE RADICALS TYPICAL PROPERTIES • reactive species (atoms or groups) which possess an unpaired electron • their reactivity is due to them wanting to pair up the single electron •formed by homolytic fission (homolysis) of covalent bonds • formed during the reaction between chlorine and methane • formed during thermal cracking • involved in the reactions taking place in the ozone layer

5. CHLORINATION OF METHANE Reagentschlorine and methane ConditionsUV light or sunlight - heat is an alternative energy source Equation(s) CH4(g) + Cl2(g) ——> HCl(g) + CH3Cl(g) chloromethane CH3Cl(g) + Cl2(g) ——> HCl(g) + CH2Cl2(l) dichloromethane CH2Cl2(l) + Cl2(g) ——> HCl(g) + CHCl3(l) trichloromethane CHCl3(l) + Cl2(g) ——> HCl(g) + CCl4(l) tetrachloromethane Mixturesfree radicals are very reactive - they are trying to pair their electron with sufficient chlorine, every hydrogen will eventually be replaced.

6. CHLORINATION OF METHANE Reagentschlorine and methane ConditionsUV light or sunlight - heat is an alternative energy source Equation(s) CH4(g) + Cl2(g) ——> HCl(g) + CH3Cl(g) chloromethane CH3Cl(g) + Cl2(g) ——> HCl(g) + CH2Cl2(l) dichloromethane CH2Cl2(l) + Cl2(g) ——> HCl(g) + CHCl3(l) trichloromethane CHCl3(l) + Cl2(g) ——> HCl(g) + CCl4(l) tetrachloromethane Mixturesfree radicals are very reactive - they are trying to pair their electron with sufficient chlorine, every hydrogen will eventually be replaced. MechanismMechanisms portray what chemists think is going on in the reaction, whereas an equation tells you the ratio of products and reactants. Chlorination of methane proceeds via FREE RADICAL SUBSTITUTION because the methane is attacked by free radicals resulting in hydrogen atoms being substituted by chlorine atoms. The process is a chain reaction. In the propagation step, one radical is produced for each one used

7. CHLORINATION OF METHANE InitiationCl2 ——> 2Cl•RADICALS CREATED The single dots represent UNPAIRED ELECTRONS During initiation, the WEAKEST BOND IS BROKEN as it requires less energy. There are three possible bonds in a mixture of alkanes and chlorine. 412 348 242 Average bond enthalpy kJ mol-1 The Cl-Cl bond is broken in preference to the others as it is the weakest and requires requires less energy to separate the atoms.

8. CHLORINATION OF METHANE PropagationCl• + CH4 ——> CH3• + HClRADICALS USEDand Cl2 + CH3• ——> CH3Cl + Cl• then RE-GENERATED Free radicals are very reactive because they want to pair up their single electron. They do this by abstracting a hydrogen atom from methane; a methyl radical is formed The methyl radical is also very reactive and attacks a chlorine molecule A chlorine radical is produced and the whole process can start over again

9. CHLORINATION OF METHANE TerminationCl• + Cl• ——> Cl2RADICALS REMOVED Cl• + CH3• ——> CH3Cl CH3• + CH3• ——> C2H6 Removing the reactive free radicals brings an end to the reaction. This is not very likely at the start of the reaction because of their low concentration.

10. CHLORINATION OF METHANE OVERVIEW InitiationCl2 ——> 2Cl• radicals created PropagationCl• + CH4 ——> CH3• + HCl radicals used and Cl2 + CH3• ——> CH3Cl + Cl• then re-generated TerminationCl• + Cl• ——> Cl2 radicals removed Cl• + CH3• ——> CH3Cl CH3• + CH3• ——> C2H6 Summary Due to lack of reactivity, alkanes need a very reactive species to persuade them to react Free radicals need to be formed by homolytic fission of covalent bonds This is done by shining UV light on the mixture (heat could be used) Chlorine radicals are produced because the Cl-Cl bond is the weakest You only need one chlorine radical to start things off With excess chlorine you get further substitution and a mixture of chlorinated products

11. CHLORINATION OF METHANE RADICALS PRODUCED Initiation Propagation Termination RADICALSUSED ANDREGENERATED RADICALS REMOVED

12. CHLORINATION OF METHANE Further propagationIf excess chlorine is present, further substitution takes place The equations show the propagation steps for the formation of... dichloromethaneCl• + CH3Cl ——> CH2Cl• + HCl Cl2 + CH2Cl• ——> CH2Cl2 + Cl• trichloromethane Cl• + CH2Cl2 ——> CHCl2• + HCl Cl2 + CHCl2• ——> CHCl3 + Cl• tetrachloromethane Cl• + CHCl3 ——> CCl3• + HCl Cl2 + CCl3• ——> CCl4 + Cl• Mixtures Because of the many possible reactions there will be a mixture of products. Individual haloalkanes can be separated by fractional distillation.

13. Esterification An Ester is formed when an alcohol and a carboxylic acid react together. This is called a condensation reaction since water is formed in the reaction. It can also be regarded as a substitution reaction since the H of the -OH group in the carboxylic acid molecule has been replaced by an alkyl group.

14. ESTERIFICATION T Reagent(s) carboxylic acid + strong acid catalyst (e.g conc. H2SO4 ) Conditions reflux Product ester Equation e.g. CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l) NotesConcentrated H2SO4 is also a dehydrating agent, it removes water as it is formed causing the equilibrium to move to the right and thus increasing the yield of ester. Uses of estersEsters are fairly unreactive but that doesn’t make them useless Used as flavourings Naming esters Named from the alcohol and carboxylic acid which made them... CH3OH + CH3COOHCH3COOCH3+ H2O fromethanoic acidCH3COOCH3from methanol METHYLETHANOATE CONVERSIONS

15. HYDROLYSIS OF ESTERS U Reagent(s) dilute acid or dilute alkali Conditions reflux Product carboxylic acid and an alcohol Equation e.g. CH3COOC2H5(l) + H2O(l)CH3CH2OH(l) + CH3COOH(l) Notes If alkali is used for the hydrolysis the salt of the acid is formed CH3COOC2H5(l) + NaOH(aq)———>CH3CH2OH(l) + CH3COO-Na+(aq) CONVERSIONS

16. POLYMERISATION GeneralA process in which small molecules called monomers join together into large molecules consisting of repeating units. There are two basic types ADDITION all the atoms in the monomer are used to form the polymer CONDENSATIONmonomers join up the with expulsion of small molecules not all the original atoms are present in the polymer

17. ADDITION POLYMERISATION • all the atoms in the monomer are used to form the polymer • occurs with alkenes • mechanism can be free radical or ionic

18. POLYMERISATION OF ALKENES ADDITION POLYMERISATION Preparation Many are prepared by a free radical process involving high pressure, high temperature and a catalyst. The catalyst is usually a substance (e.g. an organic peroxide) which readily breaks up to form radicals whichinitiate a chain reaction. Another famous type of catalyst is a Ziegler-Natta catalyst (named after the scientists who developed it). Such catalysts are based on the compound TiCl4. Properties Physicalvaried by changing the reaction conditions (pressure, temperature etc). Chemicalhave chemical properties based on the functional groups in their structure. poly(ethene) is typical; it is fairly inert as it is basically a very large alkane. This means it is resistant to chemical attack and non-biodegradable.

19. 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

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

21. POLYMERISATION OF PROPENE - ANIMATION AN EXAMPLE OF ADDITION POLYMERISATION PROPENE MOLECULES DO NOT ALWAYS ADD IN A REGULAR WAY THERE ARE THREE BASIC MODES OF ADDITION ISOTACTIC SYNDIOTACTIC ATACTIC Animation may not work in earlier versions of Powerpoint

22. POLY(PROPENE) ISOTACTIC CH3 groups on same side most desirable properties SYNDIOTACTIC CH3 groups alternate sided ATACTIC random most likely outcome

23. Addition reactions An addition reaction is one in which two substances react together to form a single substance. Eg The reaction of Ethene with Bromine

24. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION MECHANISM The main reaction of alkenes is addition Because of the extra electron density in a C=C double bond, alkenes are attacked by species which ‘like’ electrons. These species are called electrophiles; they possess a positive or partial positive charge somewhere in their structure. Examples include... hydrogen halides concentrated H2SO4

25. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION MECHANISM The electrophile, having some positive character is attracted to the alkene. The electrons in the pi bond come out to form a bond to the positive end. Because hydrogen can only have two electrons in its orbital, its other bond breaks heterolytically. The H attaches to one of the carbon atoms.

26. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION MECHANISM The electrophile, having some positive character is attracted to the alkene. The electrons in the pi bond come out to form a bond to the positive end. Because hydrogen can only have two electrons in its orbital, its other bond breaks heterolytically. The H attaches to one of the carbon atoms. A carbocation is formed. The species that left now has a lone pair. It acts as nucleophile and attacks the carbocation using its lone pair to form a covalent bond. Overall, there is ADDITION

27. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION OF HYDROGEN BROMIDE ReagentHydrogen bromide... it is electrophilic as the H is slightly positive Condition Room temperature. EquationC2H4(g) + HBr(g) ———> C2H5Br(l) bromoethane Mechanism

28. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION OF HYDROGEN BROMIDE ReagentHydrogen bromide... it is electrophilic as the H is slightly positive Condition Room temperature. EquationC2H4(g) + HBr(g) ———> C2H5Br(l) bromoethane Mechanism Step 1 As the HBr nears the alkene, one of the carbon-carbon bonds breaks The pair of electrons attaches to the slightly positive H end of H-Br. The HBr bond breaks to form a bromide ion. A carbocation (positively charged carbon species) is formed.

29. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION OF HYDROGEN BROMIDE ReagentHydrogen bromide... it is electrophilic as the H is slightly positive Condition Room temperature. EquationC2H4(g) + HBr(g) ———> C2H5Br(l) bromoethane Mechanism Step 1 As the HBr nears the alkene, one of the carbon-carbon bonds breaks The pair of electrons attaches to the slightly positive H end of H-Br. The HBr bond breaks to form a bromide ion. A carbocation (positively charged carbon species) is formed. Step 2 The bromide ion behaves as a nucleophile and attacks the carbocation. Overall there has been addition of HBr across the double bond.

30. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION OF HYDROGEN BROMIDE ANIMATED MECHANISM Animation repeats continuously after every 10 seconds

31. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION OF BROMINE ReagentBromine. (Neat liquid or dissolved in tetrachloromethane, CCl4 ) Condition Room temperature. No catalyst or UV light required! EquationC2H4(g) + Br2(l) ——> CH2BrCH2Br(l) 1,2 - dibromoethane Mechanism It is surprising that bromine should act as an electrophile as it is non-polar. SEE NEXT SLIDE FOR AN EXPLANATION OF THE BEHAVIOUR OF BROMINE

32. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION OF BROMINE It is surprising that bromine should act as an electrophile as it is non-polar. Explanation...as a bromine molecule approaches an alkene, electrons in the pi bond of the alkene repel the electron pair in the bromine-bromine bond thus inducing a dipole. NON-POLAR AS A NON-POLAR BROMINE MOLECULE APPROACHES AN ALKENE, ELECTRONS IN THE PI ORBITAL OF THE ALKENE REPEL THE SHARED PAIR OF ELECTRONS IN THE Br-Br BOND

33. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION OF BROMINE It is surprising that bromine should act as an electrophile as it is non-polar. Explanation...as a bromine molecule approaches an alkene, electrons in the pi bond of the alkene repel the electron pair in the bromine-bromine bond thus inducing a dipole. NON-POLAR POLAR AS A NON-POLAR BROMINE MOLECULE APPROACHES AN ALKENE, ELECTRONS IN THE PI ORBITAL OF THE ALKENE REPEL THE SHARED PAIR OF ELECTRONS IN THE Br-Br BOND THE ELECTRON PAIR IS NOW NEARER ONE END SO THE BROMINE MOLECULE IS POLAR AND BECOMES ELECTROPHILIC.

34. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION OF BROMINE TEST FOR UNSATURATION The addition of bromine dissolved in tetrachloromethane (CCl4) or water (known as bromine water) is used as a test for unsaturation. If the reddish-brown colour is removed from the bromine solution, the substance possesses a C=C bond. A B C PLACE A SOLUTION OF BROMINE IN A TEST TUBE ADD THE HYDROCARBON TO BE TESTED AND SHAKE IF THE BROWN COLOUR DISAPPEARS THEN THE HYDROCARBON IS AN ALKENE A BC Because the bromine adds to the alkene, it no longer exists as molecular bromine and the typical red-brown colour disappears

35. CHEMICAL PROPERTIES OF ALKENES ELECTROPHILIC ADDITION OF SULPHURIC ACID ReagentConcentrated sulphuric acid (85%) Conditions 0°C EquationC2H4(g) + H2SO4(conc) ——> C2H5OSO2OH(aq) ethyl hydrogensulphate Hydrolysis the product can be converted to ethanol by boiling with water. C2H5OSO2OH(aq) + H2O(l) ——> H2SO4(aq) + C2H5OH(l) Industrial method(s) Phosphoric acid (H3PO4) and steam are used - see later Ethanol can also be made by FERMENTATION

36. ADDITION TO UNSYMMETRICAL ALKENES MARKOWNIKOFF’S RULE A Russian scientist, Markownikoff, investigated the products of the addition of hydrogen halides to alkenes. He found that, when two products were formed, one was formed in a larger quantity. His original rule was based only on this reaction. The modern version uses carbocation stability as a criterion for predicting the products. In the electrophilic addition to alkenes the major product is formed via the more stable carbocation (carbonium ion)

37. 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.

38. OXIDATION OF PRIMARY ALCOHOLS N 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

39. OXIDATION OF ALDEHYDES O • 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 EASILYOXIDISED • KETONES are RESISTANTTOMILDOXIDATION • 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 Cu2+ + e- ——> Cu+ CONVERSIONS

40. OXIDATION OF SECONDARY ALCOHOLS P Secondary alcohols are easily oxidised to ketones e.g. CH3CHOHCH3(l) + [O] ———> CH3COCH3(l) + H2O(l) The alcohol is refluxed with acidified K2Cr2O7. However, on prolonged treatment with a powerful oxidising agent they can be further oxidised to a mixture of acids with fewer carbon atoms than the original alcohol. CONVERSIONS

41. REDUCTION OF CARBOXYLIC ACIDS Q Reagent/catalyst H2 Nickel catalyst Conditions reflux in ethoxyethane Product aldehyde Equation e.g. CH3COOH(l) + 2[H] ———> CH3CHO(l) + H2O(l) CONVERSIONS

42. REDUCTION OF ALDEHYDES R Reagent H2 / Nickel catalyst Conditions Product primary alcohol Equation e.g. CH3CHO(l) + 2[H]———> C2H5OH(l) CONVERSIONS

43. REDUCTION OF KETONES S Reagent H2 / Nickel catalyst Conditions warm in water or ethanol Product secondary alcohol Equation e.g. CH3COCH3(l) + 2[H]———> CH3CH(OH)CH3(l) CONVERSIONS

44. 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

45. 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

46. 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

47. STRUCTURAL ISOMERISM – FUNCTIONAL GROUP ClassificationCARBOXYLIC ACIDESTER Functional GroupR-COOHR-COOR NamePROPANOIC ACID METHYL ETHANOATE Physical propertiesO-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

48. PREPARATION OF ESTERS - 1 Reagent(s) alcohol + carboxylic acid Conditions reflux with a strong acid catalyst (e.g. conc. H2SO4 ) Equatione.g. CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l) ethanol ethanoic acid ethyl ethanoate NotesConc.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’

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

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