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Chapter 32 Halogeno-compounds 32.1 32.2 32.3 32.4 32.5 32.6 32.7 32.8 1 Introduction Nomenclature of Halogeno-compounds Physical Properties of Halogeno-compounds Preparation of Halogeno-compounds Reactions of Halogeno-compounds Nucleophilic Substitution Reactions Elimination Reactions Uses of Halogeno-compounds New Way Chemistry for Hong Kong A-Level Book 3A 32.1 Introduction (SB p.169) • Haloalkanes are organic compounds having one or more halogen atoms replacing hydrogen atoms in alkanes • Haloalkanes are classified into primary, secondary and tertiary, based on the number of alkyl groups attached to the carbon atom which is bonded to the halogen atom 2 New Way Chemistry for Hong Kong A-Level Book 3A 32.1 Introduction (SB p.169) Halobenzenes are organic compounds in which the halogen atom is directly attached to a benzene ring e.g. not a halobenzene, because the chlorine atom is not directly attached to the benzene ring 3 New Way Chemistry for Hong Kong A-Level Book 3A 32.2 Nomenclature of Halogeno-compounds (SB p.170) • Naming haloalkanes are similar to those for naming alkanes • The halogens are written as prefixes: fluoro- (F), chloro(Cl), bromo- (Br) and iodo- (I) e.g. 4 New Way Chemistry for Hong Kong A-Level Book 3A 32.2 Nomenclature of Halogeno-compounds (SB p.170) When the parent chain has both a halogen and an alkyl substituent, the chain is numbered from the end nearer the first substituent regardless of what substituents are e.g. 5 New Way Chemistry for Hong Kong A-Level Book 3A 32.2 Nomenclature of Halogeno-compounds (SB p.171) In case of halobenzenes, the benzene ring is numbered so as to give the lowest possible numbers to the substituents e.g. 6 New Way Chemistry for Hong Kong A-Level Book 3A 32.2 Nomenclature of Halogeno-compounds (SB p.171) Example 32-1 Draw the structural formulae and give the IUPAC names of all isomers with the following molecular formula. (a) C4H9Br Solution: Answer (a) 7 New Way Chemistry for Hong Kong A-Level Book 3A 32.2 Nomenclature of Halogeno-compounds (SB p.171) Example 32-1 Solution: Draw the structural formulae and give the IUPAC names (b) of all isomers with the following molecular formula. (b) C4H8Br2 Answer 8 New Way Chemistry for Hong Kong A-Level Book 3A 32.2 Nomenclature of Halogeno-compounds (SB p.172) Check Point 32-1 Draw the structural formulae and give the IUPAC names for all the structural isomers of C5H11Br. Answer 9 New Way Chemistry for Hong Kong A-Level Book 3A 32.2 Nomenclature of Halogeno-compounds (SB p.172) 10 New Way Chemistry for Hong Kong A-Level Book 3A 32.3 Physical Properties of Halogeno-compounds (SB p.173) Name Formula Chloro-derivatives: Chloromethane Chloroethane 1-Chloropropane 1-Chlorobutane 1-Chloropentane 1-Chlorohexane (Chloromethyl)benzene Chlorobenzene CH3Cl CH3CH2Cl CH3(CH2)2Cl CH3(CH2)3Cl CH3(CH2)4Cl CH3(CH2)5Cl C6H5CH2Cl C6H5Cl 11 Melting Boiling Density at point (°C) point (°C) 20°C (g cm–3) –97.7 –136 –123 –123 –99 –83 –39 –45.2 New Way Chemistry for Hong Kong A-Level Book 3A –23.8 12.5 46.6 78.5 108 133 179 132 — — 0.889 0.886 0.883 0.878 1.100 1.106 32.3 Physical Properties of Halogeno-compounds (SB p.173) Name Formula Bromo-derivatives: Bromomethane Bromoethane 1-Bromopropane 1-Bromobutane 1-Bromopentane 1-Bromohexane (Bromomethyl)benzene Bromobenzene CH3Br CH3CH2Br CH3(CH2)2Br CH3(CH2)3Br CH3(CH2)4Br CH3(CH2)5Br C6H5CH2Br C6H5Br 12 Melting Boiling Density at point point 20°C (g cm–3) (°C) (°C) –93.7 –119 –109 –113 –95 –85 –3.9 –30.6 New Way Chemistry for Hong Kong A-Level Book 3A 3.6 38.4 70.8 101 129 156 201 156 — 1.460 1.354 1.279 1.218 1.176 1.438 1.494 32.3 Physical Properties of Halogeno-compounds (SB p.173) Name Formula Iodo-derivatives: Iodomethane Iodoethane 1-Iodopropane 1-Iodobutane 1-Iodopentane 1-Iodohexane (Iodomethyl)benzene CH3I CH3CH2I CH3(CH2)2I CH3(CH2)3I CH3(CH2)4I CH3(CH2)5I C6H5CH2I 13 Melting Boiling Density at point (°C) point (°C) 20°C (g cm–3) –66.5 –108 –101 –103 –85.6 — 24.5 42.5 72.4 102 130 155 181 decompose New Way Chemistry for Hong Kong A-Level Book 3A 2.279 1.940 1.745 1.617 1.517 1.437 1.734 32.3 Physical Properties of Halogeno-compounds (SB p.174) Boiling Point and Melting Point 14 New Way Chemistry for Hong Kong A-Level Book 3A 32.3 Physical Properties of Halogeno-compounds (SB p.174) Haloalkanes have higher b.p. and m.p. than alkanes ∵ dipole-dipole interactions are present between haloalkane molecules m.p. and b.p. increase in the order: RCH2F < RCH2Cl < RCH2Br < RCH2I ∵ larger, more polarizable halogen atoms increase the dipole-dipole interactions between the molecules No. of carbon m.p. and b.p. 15 New Way Chemistry for Hong Kong A-Level Book 3A 32.3 Physical Properties of Halogeno-compounds (SB p.174) Density • Relative molecular mass density ∵ closer packing of the smaller molecules in the liquid phase • Bromo and iodoalkanes are all denser than water at 20°C 16 New Way Chemistry for Hong Kong A-Level Book 3A 32.3 Physical Properties of Halogeno-compounds (SB p.174) Solubility Although C — X bond is polar, it is not polar enough to have a significant effect on the solubility of haloalkanes and halobenzenes Immiscible with water Soluble in organic solvents 17 New Way Chemistry for Hong Kong A-Level Book 3A 32.4 Preparation of Halogeno-compounds (SB p.175) Preparation of Haloalkanes Substitution of Alcohols • Prepared by substituting –OH group of alcohols with halogen atoms • Common reagents used: HCl, HBr, HI, PCl3 or PBr3 • The ease of substitution of alcohols: 3° alcohol > 2° alcohol > 1° alcohol > CH3OH • This is related to the stability of the reaction intermediate (i.e. stability of carbocations) 18 New Way Chemistry for Hong Kong A-Level Book 3A 32.4 Preparation of Halogeno-compounds (SB p.175) Reaction with Hydrogen Halides • Dry HCl is bubbled through alcohols in the presence of ZnCl2 catalyst • For the preparation of bromo- and iodoalkanes, no catalyst is required 19 New Way Chemistry for Hong Kong A-Level Book 3A 32.4 Preparation of Halogeno-compounds (SB p.176) • The reactivity of hydrogen halides: HI > HBr > HCl • e.g. 20 New Way Chemistry for Hong Kong A-Level Book 3A 32.4 Preparation of Halogeno-compounds (SB p.176) Reaction with Phosphorus Halides Haloalkanes can be prepared from the vigorous reaction between cold alcohols and phosphorus(III) halides 21 New Way Chemistry for Hong Kong A-Level Book 3A 32.4 Preparation of Halogeno-compounds (SB p.177) Addition of Alkenes and Alkynes Addition of halogens or hydrogen halides to an alkene or alkyne can form a haloalkane e.g. 22 New Way Chemistry for Hong Kong A-Level Book 3A 32.4 Preparation of Halogeno-compounds (SB p.177) Preparation of Halobenzenes Halogenation of Benzene Benzene reacts readily with chlorine and bromine in the presence of catalysts (e.g. FeCl3, FeBr3, AlCl3) 23 New Way Chemistry for Hong Kong A-Level Book 3A 32.4 Preparation of Halogeno-compounds (SB p.177) From Benzenediazonium Salts 24 New Way Chemistry for Hong Kong A-Level Book 3A 32.4 Preparation of Halogeno-compounds (SB p.178) Check Point 32-2 State the major products of the following reactions: (a) CH3CHOHCH2CH3 + PBr3 (b) CH3CH = CH2 + HBr (c) CH3C CH + 2HBr (d) (a) CH3CHBrCH2CH3 (b) CH3CHBrCH3 (c) CH3CBr2CH3 (d) 25 New Way Chemistry for Hong Kong A-Level Book 3A Answer 32.5 Reactions of Halogeno-compounds (SB p.178) • Carbon-halogen bond is polar • Carbon atom bears a partial positive charge • Halogen atom bears a partial negative charge 26 New Way Chemistry for Hong Kong A-Level Book 3A 32.5 Reactions of Halogeno-compounds (SB p.178) • Characteristic reaction: Nucleophilic substitution reaction • Alcohols, ethers, esters, nitriles and amines can be formed by substituting – OH, – OR, RCOO –, – CN and – NH2 groups respectively 27 New Way Chemistry for Hong Kong A-Level Book 3A 32.5 Reactions of Halogeno-compounds (SB p.179) • Another characteristic reaction: Elimination reaction Haloalkane Base Alkene • Bases and nucleophiles are the same kind of reagents • Nucleophilic substitution and elimination reactions always occur together and compete each other 28 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.179) Reaction with Sodium Hydroxide The reactions proceed in 2 different reaction mechanisms: bimolecular nucleophilic substitution (SN2) unimolecular nucleophilic substitution (SN1) 29 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.180) Bimolecular Nucleophilic Substitution (SN2) Example: CH3 – Cl + OH– CH3OH + Cl– Experiment number 1 2 3 4 Initial Initial Initial rate [CH3Cl] [OH–] (mol dm–3 s–1) –3 –3 (mol dm ) (mol dm ) 0.001 0.002 0.001 0.002 1.0 1.0 2.0 2.0 4.9 10–7 9.8 10–7 9.8 10–7 19.6 10–7 Results of kinetic study of reaction of CH3Cl with OH– Rate = k[CH3Cl][OH–] Order of reaction = 2 both species are involved in rate determining step 30 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.181) Reaction mechanism of the SN2 reaction: • The nucleophile attacks from the backside of the electropositive carbon centre • In the transition state, the bond between C and O is partially formed, while the bond between C and Cl is partially broken 31 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.181) Transition state involve both the nucleophile and substrate second order kinetics of the reaction Energy profile of the reaction of CH3Cl and OHby SN2 mechanism 32 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.182) Stereochemistry of SN2 Reactions • The nucleophile attacks from the backside of the electropositive carbon centre • The configuration of the carbon atom under attack inverts 33 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.182) Unimolecular Nucleophilic Substitution (SN1) Example: • Kinetic study shows that: Rate = k[(CH3)3CCl] • The rate is independent of [OH–] • Order of reaction = 1 only 1 species is involved in the rate determining step 34 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.183) Reaction mechanism of SN1 reaction involves 2 steps and 1 intermediate formed Step 1: • Slowest step (i.e. rate determining step) • Formation of carbocation and halide ion 35 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.183) Step 2: • Fast step • Attacked by a nucleophile to form the product 36 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.183) • Rate determining step involves the breaking of the C – Cl bond to form carbocation • Only 1 molecule is involved in the rate determining step first order kinetics of the reaction Energy profile of the reaction of (CH3)3CCl and OH- by SN1 mechanism 37 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.184) Stereochemistry of SN1 Reactions • The carbocation formed has a trigonal planar structure • The nucleophile may either attack from the frontside or the backside 38 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.184) For some cations, different products may be formed by either mode of attack e.g. The reaction is called racemization 39 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.184) The above SN1 reaction leads to racemization ∵ formation of trigonal planar carbocation intermediate 40 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.185) The attack of the nucleophile from either side of the planar carbocation occurs at equal rates and results in the formation of the enantiomers of butan-2-ol in equal amounts 41 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.185) Factors Affecting the Rates of SN1 and SN2 Reactions Most important factors affecting the relative rates of SN1 and SN2 reactions: 42 1. The structure of the substrate 2. The concentration and strength of the nucleophile (for SN2 reactions only) 3. The nature of the leaving group New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.186) The Structure of the Substrate 1. SN2 reactions • The reactivity of haloalkanes in SN2 reactions: CH3X > 1° haloalkane > 2° haloalkane > 3° haloalkane • Steric hindrance affects the reactivity ∵ bulky alkyl groups will inhibit the approach of nucleophile to the electropositive carbon centre energy of transition state activation energy 43 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.186) Steric effects in the SN2 reaction 44 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.187) 2. SN1 reactions • Critical factor: the relative stability of the carbocation formed • Tertiary carbocations are the most stable ∵ 3 electron-releasing alkyl groups stabilize the carbocation by releasing electrons • Methyl, 1°, 2° carbocation have much higher energy activation energies for SN1 reactions are very large and rate of reaction become very small 45 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.187) The Concentration and Strength of the Nucleophile • Only affect SN2 reactions • Concentration of nucleophile rate 46 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.187) • Relative strength of nucleophiles can be correlated with two structural features: (I) A negatively charged nucleophile (e.g. OH–) is always a stronger nucleophile than a neutral nucleophile (e.g. H2O) (II)In a group of nucleophiles in which the nucleophilic atom is the same, the order of nucleophilicity roughly follows the order of basicity: e.g. RO– > OH– >> ROH > H2O • 47 Strength rate New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.188) The Nature of Leaving Group • Halide ion departs as a leaving group • For the halide ion, the ease of leaving: I– > Br– > Cl– > F– • This is in agreement with the order of bond enthalpies of carbon-halogen bonds 48 Bond Bond enthalpy (kJ mol–1) C–F +484 C – Cl +338 C – Br +276 C–I +238 C – I bond is weakest I– is the best leaving group New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.188) • Uncharged or neutral compounds are better leaving groups e.g. The ease of leaving of oxygen compounds: H2O >> OH– > RO– • Strongly basic ions rarely act as leaving group e.g. 49 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.188) When an alcohol is dissolved in a strong acid, it can react with a halide ion ∵ the acid protonates the –OH group, and the leaving group becomes a neutral water molecule e.g. 50 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.188) Comparision of Rates of Hydrolysis of Haloalkanes and Halobenzene 1. Experiment 1 : Comparison of the rates of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane (a) Objective To study the effect of the nature of the halogen leaving group on the rate of hydrolysis of haloalkanes 51 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.189) (b) Procedure 52 • Put 2 cm3 of ethanol and 1 cm3 of 0.1 M aqueous silver nitrate into each of three test tubes • Place them in a water bath at 60°C • After 5 mins, add 5 drops of 1-chlorobutane the test tube A, 5 drops of 1-bromobutane to B and 5 drops of 1-iodobutane to C • Shake each test tube and observe for 10 mins New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.189) (c) Result and Observation A precipitate of silver halide is formed in each of the three test tubes Test tube A AgCl(s) 53 Test tube B AgBr(s) New Way Chemistry for Hong Kong A-Level Book 3A Test tube C AgI(s) 32.6 Nucleophilic Substitution Reactions (SB p.190) (d) Discussion • Water molecule is the nucleophile of the reaction • Haloalkanes react with water by nucleophilic substitutions • The halide ion departs as the leaving group • The ease of leaving of halide ions decreases: I– > Br– > Cl– • The order of precipitates appeared tends to follow the order of ease of leaving of the halide ions, which subsequently form precipitates with Ag+ ions from AgNO3 Ag+(aq) + X–(aq) AgX(s) 54 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.190) 2. Experiment 2: Comparison of the rates of hydrolysis of primary, secondary and tertiary haloalkanes and halobenzene (a) Objective To study the effect of the structure of haloalkanes on the rate of hydrolysis of them and to compare the rates of hydrolysis of haloalkanes and halobenzene 55 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.190) (b) Procedure 56 • Put 2 cm3 of ethanol and 1 cm3 of 0.1 M aqueous silver nitrate into each of four test tubes • Add 5 drops of 1-chlorobutane the test tube D, 5 drops of 2-chlorobutane to E, 5 drops of 2-chloro-2-methylpropane to F and 5 drops of chlorobenzene to G • Shake each test tube well and observe for 10 mins New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.190) (c) Result and Observation Except test tube G, a white precipitate of silver chloride was formed in each of test tubes D, E and F. Test tube D 57 Test tube E Test tube F New Way Chemistry for Hong Kong A-Level Book 3A Test tube G 32.6 Nucleophilic Substitution Reactions (SB p.191) (d) Discussion • The halogen-compounds used in the experiment are of different classes: • The rate of formation of the white precipitate of silver chloride decreases in the order: 2-chloro-2-methylpropane > 2-chlorobutane chlorobutane >> chlorobenzene 58 New Way Chemistry for Hong Kong A-Level Book 3A 1- 32.6 Nucleophilic Substitution Reactions (SB p.191) • The rate of hydrolysis of halogeno-compounds is related to the structure of the substrate around the carbon which is being attacked • The experimental condition favours SN1 reactions ∴ tertiary haloalkane reacts at the fastest rate while primary haloalkane proceeds at a slower rate • 59 Chlorobenzene can be hydrolyzed to phenol under severe conditions (cannot be carried out in school laboratory) New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.192) Unreactivity of Halobenzene • Halobenzenes are comparatively unreactive to nucleophilic substitution reactions ∵ the p orbital on the carbon atom of the benzene ring and that on the halogen atom overlap side-by-side to form a delocalized bonding system 60 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.192) ∵ Delocalization of electrons throughout the ring and halogen atom • ∴ The C – X bond has partial double bond character stronger than that of haloalkane larger amount of energy is required to break the bond substitution reactions become more difficult to occur • 61 ∵ Delocalization of electrons makes the polarity of C – X bond electropositive carbon center is less susceptible to nucleophilic attack New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.192) • Delocalized electrons repel any approaching nucleophiles unreactive towards SN2 reactions • Benzene cations are highly unstable because of loss of aromaticity unreactive towards SN1 reactions 62 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.192) Example 32-2 The reactions between three bromine-containing compounds and aqueous silver nitrate at room conditions are summarized in the following table: Compound Reaction with aqueous silver nitrate Sodium bromide A pale yellow precipitate appears immediately 1-Bromobutane No reaction at first; a pale yellow precipitate appears after several minutes Bromobenzene No reaction even after several hours Solution: (a) What is the pale yellow precipitate produced in the Answer reaction silver nitrate and sodium bromide? (a)between Silver bromide 63 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.192) Example 32-2 (b) Write an ionic equation for the reaction. Answer Solution: (b) Ag+(aq) + Br–(aq) AgBr(s) 64 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.192) Example 32-2 (c) Why does silver nitrate produce no immediate precipitate with 1-bromobutane, even though it contains bromine? Why is there the formation of the pale yellow precipitate after several minutes? Answer Solution: (c) The hydrolysis of 1-bromobutane takes time. Precipitation of AgBr occurs only after OH– from water has replaced Br– from 1-bromobutane. 65 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.192) Example 32-2 (d) Briefly explain why bromobenzene does not give any precipitate with aqueous silver nitrate. Answer Solution: (d) The C – Br bond of bromobenzene is strengthened due to the delocalization of electrons throughout the benzene ring and the halogen atom. As the breaking of the C – Br bond of bromobenzene requires a larger amount of energy than 1-bromobutane, the substitution reaction becomes more difficult to occur. Thus, bromobenzene does not give any precipitate with aqueous silver nitrate. 66 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.193) Example 32-3 Which is the stronger nucleophile in each of the following pairs? Explain your choice briefly. (a) OH– and H2O (b) OH– and CH3CH2O– Answer Solution: (a) OH– is a stronger nucleophile than H2O because it carries a negative charge while H2O is electrically neutral. (b) CH3CH2O– is a stronger nucleophile than OH–. It is because the ethyl group (CH3CH2–) is an electronreleasing group, this increases the electron density on the oxygen atom. This makes CH3CH2O– to be a stronger nucleophile than OH–. 67 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.194) Check Point 32-3 Predict whether the following substitution reaction follows mainly SN1 or SN2 pathway. Briefly explain your answer. (a) CH3I + OH– CH3OH + I– Answer (a) The reaction follows mainly the SN2 mechanism because of the following reasons. The haloalkane (CH3I) is a methyl halide. There is little steric hindrance for the nucleophile to attack the carbon atom of the molecule. On the other hand, if the reaction follows the SN1 mechanism, the carbocation (CH3+) formed is not stabilized by the inductive effects of alkyl groups. Thus the SN1 mechanism for this reaction is unfavourable. 68 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.194) Check Point 32-3 Predict whether the following substitution reaction follow mainly SN1 or SN2 pathway. Briefly explain your answer. (b) 69 (b) The reaction follows mainly the SN1 mechanism. It is because the haloalkane is a secondary haloalkane with a bulky phenyl group attached directly to the carbon atom bearing the halogen atom. The bulky phenyl group exerts a dramatic steric hindrance to the approaching nucleophile. Therefore, the SN2 mechanism for this reaction is not favoured. On the other hand, the carbocation formed inAnswer the SN1 reaction is stabilized by both the inductive effect of the electron-releasing ethyl group and the resonance effect of the phenyl group. New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.194) Reaction with Potassium Cyanide A nitrile is formed when a haloalkane is heated under reflux with an aqueous alcoholic solution of potassium cyanide e.g. 70 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.194) • Cyanide ion (CN–) acts as a nucleophile • Halobenzenes do not react with potassium cyanide • The reaction is very useful because the nitrile can be hydrolyzed to carboxylic acids which can be reduced to alcohols • 71 A useful way of introducing a carbon atom into an organic molecule, so that the length of the carbon chain can be increased New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.195) Reaction with Ammonia When a haloalkane is heated with an aqueous alcoholic solution of ammonia under a high pressure, an amine is formed e.g. 72 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.195) • Ammonia is a nucleophile because the presence of a lone pair of electrons on the nitrogen atom • As the lone pair electrons on nitrogen atom in ethylamine are still available, the ethylamine will compete with ammonia as the nucleophile. • A series of further substitutions take place • A mixture of products is formed 73 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.195) • The reaction stops at the formation of a quaternary ammonium salt • The competing reactions can be minimized by using an excess of ammonia 74 New Way Chemistry for Hong Kong A-Level Book 3A 32.6 Nucleophilic Substitution Reactions (SB p.195) Example 32-4 Give the reagents and reaction conditions needed for each of the following conversions: (a) (CH3)3CBr (CH3)3COH (b) CH3I CH3OC2H5 (c) CH3I (CH3)4N+I– Solution: (a) Dilute NaOH (b) C2H5O–Na+ or Na in C2H5OH (c) NH3 in excess CH3I 75 New Way Chemistry for Hong Kong A-Level Book 3A Answer 32.6 Nucleophilic Substitution Reactions (SB p.196) Check Point 32-4 Give the name(s) and structural formula(e) of the major organic product(s) formed in each of the following reactions. (a) (a) (b) (b) CH3NH2 (c) Methylamine (c) Answer 76 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.196) Formation of Alkenes The elimination of HX from adjacent atoms of a haloalkane is widely used for synthesizing alkenes e.g. 77 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.196) The elements of a hydrogen halide are eliminated from a haloalkane in this way, the reaction is called dehydrohalogenation 78 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.196) Dehydrohalogenation of most haloalkanes yields more than one product e.g. 79 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.197) • The major product will be the more stable alkene • The more stable alkene has the more highly substituted double bond • Elimination follows the Saytzeff’s rule when the elimination occurs to give the more highly substituted alkene as the major product • The stabilities of alkenes: 80 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.197) Elimination Versus Substitution • Nucleophiles are potential bases • Bases are potential nucleophiles • In SN2 pathway, elimination and nucleophilic substitution compete each other 81 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.198) • Substitution is favoured when the substrate is primary alcohol and the base is hydroxide ion • Elimination is favoured when the substrate is secondary alcohol 82 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.198) • With tertiary haloalkanes, SN2 reactions cannot take place Elimination is highly favoured especially at high temperatures Substitution occurs through SN1 mechanism only 83 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.198) Eliminations will be favoured when using: 1. higher temperatures 2. strong sterically hindered bases (e.g. (CH3)3CO–) 84 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.199) CH3X Methyl RCH2X 1° R2CHX 2° R3CX 3° Gives SN2 reactions only Gives mainly SN2 and gives mainly E with a strong sterically hindered base (e.g. (CH3)3CO–) Gives mainly SN2 with a weak base (e.g. I–, CN– , RCO2–) and gives mainly E with a strong base (e.g. RO–) No SN2 reaction. In hydrolysis, gives SN1 or E. At low temperatures, SN1 is favoured. When a strong base (e.g. RO–) is used or at high temperatures, E predominates. Summary of the reaction pathways for the substitution and elimination reactions of simple haloalkanes 85 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.199) Formation of Alkynes • Alkynes can be produced by dehydrohalogenation of dihaloalkanes • Two molecules of hydrogen halides are eliminated e.g. 86 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.199) Example 32-5 (a) Hot and concentrated alcoholic potassium hydroxide can eliminate hydrogen iodide from the compound CH3CH2CHICH3. Suggest and name two possible products. Answer Solution: (a) 87 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.199) Example 32-5 (b) Draw the structural formulae and give the names of all Solution: possible products formed by elimination of hydrogen (b) bromide from the dibromoalkane, CH3CHBrCHBrCH3. Answer 88 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.200) Check Point 32-5 (a) Notice how the hydrogen and halogen atoms come off from adjacent carbon atoms in an elimination reaction. Could (iodomethyl)benzene undergo an elimination to give a HI molecule? Why? (a) No, because there is no hydrogen available on the carbon atom adjacent to the carbon atom that is directly bonded to the iodine atom. 89 New Way Chemistry for Hong Kong A-Level Book 3A Answer 32.7 Elimination Reactions (SB p.200) Check Point 32-5 (b) 2-Iodo-2-methylbutane gives two elimination products: one is 2-methylbut-2-ene, what is the other one? Answer (b) 2-Methylbut-1-ene 90 New Way Chemistry for Hong Kong A-Level Book 3A 32.7 Elimination Reactions (SB p.200) Check(c)Point 32-5 (c) Arrange the following compounds in order of increasing tendency towards elimination reactions: 2-bromo-2-methylbutane, 1-bromopentane and 2-bromopentane Answer The rate of elimination depends on the stability of the alkene formed. A more highly substituted alkene is more stable and is formed more readily. 91 New Way Chemistry for Hong Kong A-Level Book 3A 32.8 Uses of Halogeno-compounds (SB p.200) As Solvents in Dry-cleaning • Chlorinated hydrocarbons are good solvents for oil and greases widely used in the dry-cleaning industry e.g. trichloroethene, CCl2 = CHCl tetrachloroethene, CCl2 = CCl2 • Properties that favour the use: 1. Relatively non-flammable 92 2. Volatile 3. Little or no structural effect on fabrics New Way Chemistry for Hong Kong A-Level Book 3A 32.8 Uses of Halogeno-compounds (SB p.201) As Raw Materials for Making Addition Polymers Poly(chloroethene) (also known as PVC): • Produced by means of the addition polymerization of the chloroethene monomers in the presence of a peroxide catalyst 93 New Way Chemistry for Hong Kong A-Level Book 3A 32.8 Uses of Halogeno-compounds (SB p.201) • Polar C – Cl bond results in dipole-dipole interactions between polymer chains, making PVC hard and brittle and used to make pipes and bottles Products made of PVC without plasticizers 94 New Way Chemistry for Hong Kong A-Level Book 3A 32.8 Uses of Halogeno-compounds (SB p.201) • PVC becomes flexible when plasticizer is added • Used to make shower curtains, raincoats, artificial leather, insulating coating of electrical wires Products made of PVC with plasticizers 95 New Way Chemistry for Hong Kong A-Level Book 3A 32.8 Uses of Halogeno-compounds (SB p.201) Poly(tetrafluoroethene) (PTFE, ‘Teflon’): • Produced through addition polymerization of the tetrafluoroethene monomers under high pressure and in the presence of catalyst 96 New Way Chemistry for Hong Kong A-Level Book 3A 32.8 Uses of Halogeno-compounds (SB p.201) • Teflon has a high melting point and is chemically inert • Used to make non-stick frying pans 97 New Way Chemistry for Hong Kong A-Level Book 3A The END 98 New Way Chemistry for Hong Kong A-Level Book 3A