Electrophilic Aromatic Substitution Benzene and Electrophilic Aromatic Substitution New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Aromatic Hydrocarbons (Arenes) Simplest member : Benzene, C6H6 Characteristic aroma Alkylbenzene, Cn+6H2n+6 E.g. C6H5CH3 C6H5C2H5 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Aromatic Hydrocarbons (Arenes) Generally less dense than water Insoluble in water but soluble in many organic solvents New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Aromatic Hydrocarbons (Arenes) PAHs : Polycyclic aromatic hydrocarbons Fused aromatic rings New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Aromatic Hydrocarbons (Arenes) No heteroatoms Not PAH New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Aromatic Hydrocarbons (Arenes) PAHs : Polycyclic aromatic hydrocarbons Sources : - incomplete combustion of wood, coal, diesel, fat, or tobacco Q.38 benzocyclobutadiene New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A PAHs : Polyaromatic hydrocarbons Toxic and carcinogenic Benzo(a)pyrene : - first carcinogen discovered Found in : - tobacco smoke, - char-grilled food - burnt toast, - edible oils New Way Chemistry for Hong Kong A-Level 3A

Preparation of Benzene New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 1. Destructive Distillation of Coal Gives coal gas, ammoniacal liquor, coal tar and coke as products The coal tar produced is a mixture of many organic compounds (mainly aromatic ones) benzene and methylbenzene can be obtained by fractional distillation New Way Chemistry for Hong Kong A-Level 3A

1. Destructive Distillation of Coal A laboratory set-up of the destructive distillation of coal New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Industrial Preparation 2. Catalytic trimerization of ethyne New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Industrial Preparation 3. Catalytic Reforming of Petroleum Converts alkanes and cycloalkanes into aromatic hydrocarbons C6H14  C6H6 + 4H2 500 oC, 10 – 20 atm Pt New Way Chemistry for Hong Kong A-Level 3A

Laboratory Synthesis 1. Decarboxylation of Sodium Salt of Benzoic Acid When sodium benzoate is fused with sodium hydroxide  the carboxylate group is removed volatile Benzene is separated by fractional distillation

Passing phenol vapour over heated zinc dust (reducing agent) 2. Reduction of Phenol Passing phenol vapour over heated zinc dust (reducing agent)  produce benzene and zinc(II) oxide volatile Benzene is separated by fractional distillation

New Way Chemistry for Hong Kong A-Level 3A Reactions of Benzene New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Reactivity of Benzene Unreactive towards addition reactions due to stabilization of the system by delocalization of -electrons New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Reactivity of Benzene Not oxidized by KMnO4 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Reactivity of Benzene Resistant to electrophilic addition New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Reactivity of Benzene Resistant to catalytic hydrogenation New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Addition reactions occur only under drastic conditions. New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A No reaction Occur only under drastic conditions New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A The -electron cloud is susceptible to electrophilic attack. Substitution is preferred to addition since the former retains aromaticity. Electrophilic aromatic substitution (SE) New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Electrophilic Aromatic Substitution Reactions E+ : electrophile New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A General Reaction Mechanism Step 1: Rate determining step New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A General Reaction Mechanism Stabilized by delocalization of  electrons + New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A General Reaction Mechanism Step 2: The carbocation loses a hydrogen ion  forms the substitution product New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 1. Halogenation No apparent reation Catalysts : AlCl3, FeCl3 or FeBr3 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 1. Halogenation - Mechanism Step 1: The catalyst (FeBr3) combines with bromine to give a complex New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Step 2: Formation of carbocation intermediate Rate determining step Stabilized by delocalization of -electrons New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Step 3: The catalyst (FeBr3) is regenerated Fumes of HBr(g) are produced, indicating substitution rather than addition has occurred. New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 2. Nitration Benzene reacts readily with a mixture of conc. HNO3 and conc. H2SO4 Conc. H2SO4 increases the rate of reaction by increasing the concentration of the electrophile, NO2+ New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A acid base electrophile 2HNO3 NO3 + H2O + NO2+ New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Optional meta-directing New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.39 H2SO4 + HNO3 NO2+ + H2O + HSO4 New Way Chemistry for Hong Kong A-Level 3A

+ H2SO4

New Way Chemistry for Hong Kong A-Level 3A 3. Sulphonation (H2SO4 + SO3) Benzene reacts with fuming sulphuric acid at room temp  form benzenesulphonic acid + New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 3. Sulphonation Sulphonation is a reversible process By heating an aqueous solution of benzenesulphonic acid to above 100 oC  benzene and sulphuric acid are formed heat New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.40 H2SO4 + H2SO4 SO3 + H3O+ + HSO4 New Way Chemistry for Hong Kong A-Level 3A

+ H2SO4

New Way Chemistry for Hong Kong A-Level 3A 4. Friedel-Crafts Alkylation When benzene is warmed with a haloalkane in the presence of AlCl3 as a catalyst  alkylbenzene is formed New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 4. Friedel-Crafts Alkylation Important starting step in the manufacture of styrene, phenol and detergents The alkyl group introduced activates the ring towards further alkylation by +ve I-effect.  Friedel-Crafts alkylation is not a good way to prepare alkylbenzene. New Way Chemistry for Hong Kong A-Level 3A

Friedel-Craft acylation Clemensen reduction deactivating + Friedel-Craft acylation Clemensen reduction 45 New Way Chemistry for Hong Kong A-Level 3A

Devise a reaction pathway of not more than four steps for the following conversion. 6B 09/07/2011 46

47

New Way Chemistry for Hong Kong A-Level 3A Chlorination Bromination or Friedel-Craft reaction Soda lime to absorb HBr or HCl fumes Br2, RCl or RCOCl or AlCl3 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.41 electrophile CH3Cl + AlCl3  [CH3]+[AlCl4] New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.42(a) 2C6H6 + X C6H5CH2C6H5 + 2HCl AlCl3 X : CH2Cl2 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A -CH2Cl deactivate the ring Susceptibility to SE reaction : New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.42(b) 3C6H6 + Y (C6H5)3CH + 3HCl AlCl3 Y : CHCl3 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.42(c) 2C6H6 + Z C6H5CH2CH2C6H5 + 2HCl AlCl3 Z : CH2ClCH2Cl New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Industrial preparation of toluene New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Laboratory Preparation of Methylbenzene 1. Friedel-Crafts acylation of benzene followed by Clemensen reduction. 2. Friedel-Crafts alkylation of benzene is not recommended as further alkylation will take place giving a mixture of alkylbenzenes that are difficult to be separated. New Way Chemistry for Hong Kong A-Level 3A

Reactions of methylbenzene and Other Alkylbenzenes 1. Electrophilic aromatic substitution rxs more susceptible to electrophilic attacks ∵ alkyl groups activate the ring by positive inductive effect.

New Way Chemistry for Hong Kong A-Level 3A 2. Reactions of the side chain (a) Free radical substitution vs electrophilic substitution at the ring Free radical substitution Electrophilic substitution New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Benzylic hydrogens are much more susceptible to free radical substitution than hydrogens at other positions. Major products New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A (b) Oxidation at the benzylic carbon Absence of benzylic H New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A If alkaline KMnO4 is used, the resulting mixture must be acidified in order to obtain the benzoic acid. heat Or, New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Structural determination of isomeric alkylbenzene alkylbenzenes with very close boiling points C8H10 acids with different melting points New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Structural determination of isomeric alkylbenzene alkylbenzenes with very close boiling points acids with different melting points New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.43(a) 66 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.43(b) Terminal alkene gives CO2 + H2O 67 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.43(b) 68 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.43(b) 69 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.43(b) 70 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A Q.43(c) + H2O 71 New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A The END New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 29.2 Nomenclature of the Derivatives of Benzene (SB p.191) Back Example 29-2 Draw the structural formula for each of the following compounds: (a) 1,3,5-Trichlorobenzene (b) 2,5-Dibromophenol (c) 2,4-Dinitrobenzoic acid Answer (a) (b) (c) New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 29.2 Nomenclature of the Derivatives of Benzene (SB p.192) Check Point 29-2 Give the IUPAC name for each of the following compounds: (a) (b) (a) 1,2-Dimethylbenzene (b) 1-Methyl-2-nitrobenzene or 2-nitrotoluene Answer New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 29.2 Nomenclature of the Derivatives of Benzene (SB p.192) Check Point 29-2 Back Give the IUPAC name for each of the following compounds: (c) (d) (c) 3-Bromo-5-chlorobenzoic acid (d) 4-Bromo-2,6-dinitrophenol Answer New Way Chemistry for Hong Kong A-Level 3A

29.3 Structure of Benzene and Aromaticity (SB p.195) Let's Think 1 The basic structural requirement for aromatic compounds is that the molecule must be planar, cyclic and with (4n + 2)  electrons delocalized in the ring. n must be a natural number (i.e. n = 1, 2, 3, and so on). There are aromatic compounds without benzene ring. An example is the 1,3-cyclopentadienyl anion. Can you draw its structure and explain its aromaticity? Answer New Way Chemistry for Hong Kong A-Level 3A

29.3 Structure of Benzene and Aromaticity (SB p.195) Back Let's Think 1 Cyclopentadienyl anion is an aromatic anion. It has six π electrons delocalized over a completely conjugated planar monocyclic system of five sp2 hybridized carbon atoms. Cyclopentadienyl anion New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 29.4 Physical Properties of Aromatic Hydrocarbons (SB p.197) Let's Think 2 PAHs are formed from partial combustion and pyrolysis of aromatic compounds. They are in common occurrence in our environment. List some important uses of aromatic hydrocarbons and how they release PAHs to our environment. Answer New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 29.4 Physical Properties of Aromatic Hydrocarbons (SB p.197) Let's Think 2 Aromatic hydrocarbons are the raw materials for the manufacture of monomers and plasticizers in polymers, commonly used as solvents and important constituents of lead-free gasoline. Incomplete combustion and pyrolysis process favour the production of PAHs. These compounds are encountered abundantly in the atmosphere, soil and elsewhere in the environment from sources that include engine exhaust, wood stove smoke, cigarette smoke and charbroiled food. Coal tar and petroleum residues such as road and roofing asphalt have high levels of PAHs. Back New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 29.6 Reactions of Benzene (SB p.203) Back Example 29-6 Complete each of the following by supplying the missing reactant or product as indicated by the question mark: (a) (b) (c) (a) conc. H2SO4, conc. HNO3 fuming H2SO4 Answer New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 29.6 Reactions of Benzene (SB p.203) Check Point 29-6 (a) One mole of benzene reacts with three moles of chlorine under special conditions. What are the conditions required for the reaction? Answer (a) UV radiation or diffuse sunlight must be present for the free radical addition reaction to take place. New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 29.6 Reactions of Benzene (SB p.203) Check Point 29-6 (b) Draw the structure of the reaction product in (a). Answer (b) New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 29.6 Reactions of Benzene (SB p.203) Check Point 29-6 (c) Methylbenzene undergoes two different types of chlorination reaction by different mechanisms. Compare the two different types of chlorination reaction in terms of reaction conditions as well as the products formed. Answer New Way Chemistry for Hong Kong A-Level 3A

New Way Chemistry for Hong Kong A-Level 3A 29.6 Reactions of Benzene (SB p.203) Check Point 29-6 Back (c) The two different types of chlorination reaction of methylbenzene are: Type I: free radical substitution reaction Type II: electrophilic aromatic substitution reaction New Way Chemistry for Hong Kong A-Level 3A

Cyclohexane (a saturated alicyclic hydrocarbon) Comparison of some reactions of cyclohexane, cyclohexene and methylbenzene Reaction Cyclohexane (a saturated alicyclic hydrocarbon) Cyclohexene (an unsaturated alicyclic hydrocarbon) Methylbenzene (an aromatic hydrocarbon) Action of bromine in 1,1,1-trichloro-ethane (in dark) No reaction Bromine is decolourized and no hydrogen bromide is evolved No reaction with bromine alone. In the presence of iron(III) bromide, bromine is decolourized and hydrogen bromide fumes are evolved New Way Chemistry for Hong Kong A-Level 3A

Cyclohexane (a saturated alicyclic hydrocarbon) Comparison of some reactions of cyclohexane, cyclohexene and methylbenzene Reaction Cyclohexane (a saturated alicyclic hydrocarbon) Cyclohexene (an unsaturated alicyclic hydrocarbon) Methylbenzene (an aromatic hydrocarbon) Action of hydrogen (with nickel catalyst) No reaction One mole of cyclohexene reacts with one mole of hydrogen at room temperature One mole of methylbenzene reacts with three moles of hydrogen at high temperature New Way Chemistry for Hong Kong A-Level 3A

Cyclohexane (a saturated alicyclic hydrocarbon) Comparison of some reactions of cyclohexane, cyclohexene and methylbenzene Reaction Cyclohexane (a saturated alicyclic hydrocarbon) Cyclohexene (an unsaturated alicyclic hydrocarbon) Methylbenzene (an aromatic hydrocarbon) Action of acidified potassium manganate(VII) No reaction Acidified potassium manganate(VII) solution is decolourized New Way Chemistry for Hong Kong A-Level 3A

Cyclohexane (a saturated alicyclic hydrocarbon) Comparison of some reactions of cyclohexane, cyclohexene and methylbenzene Reaction Cyclohexane (a saturated alicyclic hydrocarbon) Cyclohexene (an unsaturated alicyclic hydrocarbon) Methylbenzene (an aromatic hydrocarbon) Action of concentrated nitric acid and concentrated sulphuric acid No reaction Cyclohexene is oxidized and the colour darkens A yellow liquid is formed New Way Chemistry for Hong Kong A-Level 3A