A2 Chemistry F324 - Rings, Acids and Analysis Arenes Plymstock School P.J.McCormack

Empirical & Molecular Formulae Lesson 1 Empirical & Molecular Formulae 25 February 2019 A hydrocarbon is found to contain 92.3% carbon and has a molecular mass of 78. Work out the empirical and molecular formula of the hydrocarbon. Draw this structure. Empirical formula = CH Molecular formula = C6H6 C6H6 = benzene P.J.McCormack

Arenes 25 February 2019 Arenes are compounds that contain a benzene ring or derivatives of benzene obtained by replacing hydrogen’s by other groups or fusing benzene rings together, e.g. 2,4,6 Trinitrotoluene (TNT) P.J.McCormack

Learning Objectives By the end of the lesson I will be able to… 25 February 2019 Learning Objectives By the end of the lesson I will be able to… Draw the skeletal formula of benzene; Describe the bonding with benzene; Compare and contrast the Kekule structure of benzene with the delocalised model Key words: Delocalised, Kekule, orbitals, electrophilic P.J.McCormack

The Structure of Benzene 25 February 2019 Benzene Animation P.J.McCormack

Kekulé Structure. 25 February 2019 The molecular formula was found to be C6H6 therefore the structure must be unsaturated. In 1865 Kekulé proposed a ring structure and drew the structure cyclohexa-1,3,5-triene. Kekulé structure suggests that benzene; Has alternating double and single bonds Would react in the same way as cyclohexene (undergo electrophilic addition). Would have three short bonds (double) and three longer bonds (single) bonds. P.J.McCormack

25 February 2019 P.J.McCormack

Problems with Kekulé’s Model. 25 February 2019 Problems with Kekulé’s Model. X-ray diffraction shows that benzene is planar which concurs with Kekulé’s structure but:- The molecule is a regular hexagon of carbon atoms, with six equal bond lengths. C-C in cyclohexane = 0.154nm C=C in cyclohexene = 0.133nm C-C bond in benzene = 0.139nm We find that benzene has a constant bond length, somewhere between a single and double bond length. Benzene does not behave like an alkene. It reacts by electrophilic substitution rather than by electrophilic addition, even though it is unsaturated. P.J.McCormack

Problems with Kekulé’s Model. 25 February 2019 KeKulé Benzene (cyclohexa-1,3,5-triene) was drawn as, a highly symmetrical hexagon, but using the x-ray diffraction evidence is should be Benzene is drawn to represents the delocalisation of electrons within the ring system. P.J.McCormack

Problems with Kekulé’s Model. 25 February 2019 Problems with Kekulé’s Model. The theoretical heat of formation of benzene is +252 kJmol-1 (Kekulé’s structure). The experimental value is +82 kJmol-1. The structure is more stable (endothermic) than Kekulé’s structure. The extra stability and equivalent carbon-carbon bond length can be explained by delocalisation. P.J.McCormack

Properties of Benzene. Planar Highly symmetrical 25 February 2019 Properties of Benzene. Planar Highly symmetrical Non-polar (evenly distributed charge). Immiscible with water. Boiling point 80C Melting point 6C Reacts by electrophilic substitution. Carcinogenic. Charge Density P.J.McCormack

Benzene and its Derivatives 25 February 2019 Benzene and its Derivatives Give three properties of benzene. What is the empirical formula of benzene? What evidence suggests that the structure of benzene is not the Kekulé structure? What is the main mechanism of reaction of benzene? P.J.McCormack

25 February 2019 Bonding in Benzene. The carbon atoms are bonded to one another and to their hydrogen atoms by sigma bonds. This leaves one unused p orbital on each carbon, each containing a single electron. These p orbitals are perpendicular to the plane of the ring, with one lobe above and one below the plane. P.J.McCormack

Delocalisation 25 February 2019 Each p orbital overlaps sideways with two neighbouring orbitals to form a single  bond that extends as a ring of charge above and below the plane of the molecule. P.J.McCormack

Bonding in Arenes. 25 February 2019 The carbon atoms in the ring are bonded to one another and to their hydrogen atoms by  bonds. This leaves one unused p orbital on each carbon, each containing a single electron. The p orbitals are perpendicular to the plane of the ring above and below the plane of the molecule. Each p orbital overlaps sideways with the two neighbouring orbitals to form a single bond that extends as a ring. P.J.McCormack

25 February 2019 Delocalisation. Electrons tend to repel one another, so a system when they are far apart as possible will involve minimum repulsion and will therefore be stabilised. Delocalisation of the  electrons has a profound affect on the both physical and chemical properties. P.J.McCormack

Bonding in Benzene. 6 electrons in a delocalised  bond C 25 February 2019 Bonding in Benzene. C H 6 electrons in a delocalised  bond above and below the plane of the atoms P.J.McCormack

delocalisation energy Energy Diagram 25 February 2019 -152 kJmol-1 delocalisation energy Enthalpy (kJ mol-1) + 3H2 -360 theoretical value + 2H2 -240 + 3H2 + H2 + H2 -120 real value -208 -120 P.J.McCormack

Hydrogenation of Benzene 25 February 2019 Hydrogenation of Benzene Hydrogenation of cyclohexene – one double bond H = -120 KJmol-1 Hydrogenation of Kekule’s benzene – three double bonds Expected value H = -360 KJmol-1 (theoretical value) Hydrogenation benzene – delocalised system Hrxn = -208 KJmol-1 P.J.McCormack

25 February 2019 Benzene is more stable than the Kekule model as less energy is given out P.J.McCormack

THERMODYNAMIC EVIDENCE FOR STABILITY When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured. When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C6H10(l) + H2(g) ——> C6H12(l) 2 3 - 120 kJ mol-1

THERMODYNAMIC EVIDENCE FOR STABILITY When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured. When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C6H10(l) + H2(g) ——> C6H12(l) Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane C6H6(l) + 3H2(g) ——> C6H12(l) Theoretical - 360 kJ mol-1 (3 x -120) 2 3 - 120 kJ mol-1

THERMODYNAMIC EVIDENCE FOR STABILITY When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured. When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C6H10(l) + H2(g) ——> C6H12(l) Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane C6H6(l) + 3H2(g) ——> C6H12(l) Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale Theoretical - 360 kJ mol-1 (3 x -120) 2 3 Experimental - 208 kJ mol-1 - 120 kJ mol-1

THERMODYNAMIC EVIDENCE FOR STABILITY MORE STABLE THAN EXPECTED When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured. When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C6H10(l) + H2(g) ——> C6H12(l) Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane C6H6(l) + 3H2(g) ——> C6H12(l) Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale It is 152kJ per mole more stable than expected. This value is known as the RESONANCE ENERGY. MORE STABLE THAN EXPECTED by 152 kJ mol-1 Theoretical - 360 kJ mol-1 (3 x -120) 2 3 Experimental - 208 kJ mol-1 - 120 kJ mol-1

THERMODYNAMIC EVIDENCE FOR STABILITY MORE STABLE THAN EXPECTED When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured. When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C6H10(l) + H2(g) ——> C6H12(l) Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane C6H6(l) + 3H2(g) ——> C6H12(l) Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale It is 152kJ per mole more stable than expected. This value is known as the RESONANCE ENERGY. MORE STABLE THAN EXPECTED by 152 kJ mol-1 Theoretical - 360 kJ mol-1 (3 x -120) 2 3 Experimental - 208 kJ mol-1 - 120 kJ mol-1

Reactions of Arenes. 25 February 2019 An arene has a percentage composition of 94.38% carbon and 5.62% hydrogen. The molecular mass of this compound is 178. Determine the empirical, molecular masses and the structure of the arene. Answers; Empirical formula = C7H5 Molecular formula = C14H10 P.J.McCormack

Learning Objectives. By the end of the lesson I will be able to.... 25 February 2019 Learning Objectives. By the end of the lesson I will be able to.... All.. State the name of the mechanism by which benzene reacts; Draw the mechanism for the reactivity of benzene; State the equation for the formation of a nitronium ion Most.. Explain what a halogen carrier is Some.. State the equation with conditions for the alkylation of benzene Low High Key Words: Halogenation, benzene, halogen carrier, electrophile P.J.McCormack

General Electrophilic Substitution Mechanism. 25 February 2019 General Electrophilic Substitution Mechanism. E+ is the electrophile (an electron deficient species). The attack comes from the benzene ring (centre of high electron density). The mechanism is firstly an addition reaction then followed by an elimination reaction. P.J.McCormack

Key Terminology What is a nitronium ion 25 February 2019 Key Terminology What is a nitronium ion What the nitration mixture required for nitration of benzene? What are the conditions required for nitration of benzene? Give the equation (2) for the formation of the nitronium ion. What is the mechanism for nitration of benzene? NO2+ C. H2SO4 + c. HNO3 50 degree C H2SO4 + HNO3 = HSO4- + H2NO3+ H2NO3+ = NO2+ + H2O Electrophilic substitution P.J.McCormack

Electrophilic Substitution. Nitration of benzene C6H6 + HNO3 C6H5NO2 + H2O Conditions / Reagents concentrated HNO3 and concentrated H2SO4 50oC Mechanism 25 February 2019 P.J.McCormack

Electrophilic Substitution Mechanism (nitration) 1. Formation of NO2 + the nitronium ion HNO3 + 2H2SO4 NO2 + + 2HSO4- + H3O+ 2. Electrophilic attack on benzene NO2 + + NO2 H O SO3H- NO2 3. Forming the product and re-forming the catalyst H O SO3H 25 February 2019 Reaction Equation P.J.McCormack

Nitration of Benzene. Benzene undergoes electrophilic substitution reactions. The substitution of a hydrogen atom for a nitronium ion (NO2+). This reaction occurs in several steps. First the nitronium ion is formed. HNO3 + H2SO4  H2NO3+ + HSO4- H2NO3+  NO2+ + H2O The reaction mechanism is: Arenium ion 25 February 2019 P.J.McCormack

Friedel-Crafts Reactions. Alkylarenes are made by using a halogen carrier and an halogenoalkane to bring about the substitution reaction of a delocalised ring. As in the reaction with halogens, the halogen carrier (aluminium chloride) accepts an electron pair from the chlorine atom, polarising the chloromethane molecule. CH3---Cl:---AlCl3 + - The positively charged methyl group attacks the delocalised ring and electrophilic substitution occurs. This is an example of a Friedel-Crafts reaction. 25 February 2019 P.J.McCormack

Reaction Mechanism. Reflux A carbocation is formed CH3+ - + 25 February 2019 Reaction Mechanism. - Reflux + A carbocation is formed CH3+ P.J.McCormack

25 February 2019 Reactions of Arenes. What are the conditions and reagents needed for the following: Nitration of benzene? Alkylation of benzene 2. Give the reactions to show the formation of a nitronium ion. 3. Show the reaction sequence for the formation of propylbenzene. 4. What is a halogen carrier? P.J.McCormack

Alkenes & Benzene. 25 February 2019 Alkylbenzene can be made from a halogenoalkane using the Freidel-Crafts catalyst. A cheaper method is to use an alkene. Phenylethene is a very useful monomer. What is the industrial name for phenylethene? styrene P.J.McCormack

Arenes Revisited. Show the mechanism for the nitration of benzene. 25 February 2019 Show the mechanism for the nitration of benzene. Give a Freidel-Crafts reaction to show the alkylation of benzene using a halogenoalkane. Explain the relative stability of benzene compared to cyclohexene. Name the following arenes. P.J.McCormack

Learning Objectives. By the end of the lesson I will be able to.... 25 February 2019 Learning Objectives. By the end of the lesson I will be able to.... All.. State the mechanism for the monohalogenation of benzene; State a suitable halogen carrier required to form chlorobenzene and bromobenzene; Most.. Explain the function of a halogen carrier Some.. Explain the resistance of benzene to halogenation Low High Key Words: Halogenation, benzene, halogen carrier, electrophile P.J.McCormack

Reactivity of Benzene and Alkenes 25 February 2019 Reactivity of Benzene and Alkenes Give the mechanism for the reaction of cyclohexene with bromine water. Give the mechanism for the reaction of benzene with bromine Explain the relative resistance of benzene to halogenation in terms of its structure compared to the halogenation of cyclohexene P.J.McCormack

Halogenation of Benzene 25 February 2019 Halogenation of Benzene Benzene does not react with halogens on its own, but will react in the presence of a halogen carrier. Examples of halogen carriers catalysts are FeCl3, FeBr3, AlCl3, AlBr3. The reaction is an electrophilic substitution reaction as with the nitration of benzene where one halogen replaced a hydrogen on the benzene ring. P.J.McCormack

Halogenation of Benzene 25 February 2019 Halogenation of Benzene Benzene will react with bromine in a similar way except the halogen carrier catalyst is FeBr3, AlBr3. P.J.McCormack

Halogenation of Benzene 25 February 2019 Halogenation of Benzene The halogen carrier generates the bromonium ion Br+, which is a more powerful electrophile than Br2. Br2 + FeBr3  Br+ + FeBr4- H+ + FeBr4-  FeBr3 + HBr P.J.McCormack

Learning Objectives. All.. Draw the structure of phenol; Most.. 25 February 2019 Learning Objectives. By the end of the lesson I will be able to.... All.. Draw the structure of phenol; Most.. Describe the reactions of phenol with aqueous alkalis and with sodium to form salts; Describe the reaction of phenol with bromine to form 2,4,6-tribromophenol Some.. Compare and contrast the reaction of bromine with phenol and benzene; Low High Key Words: Phenol, phenoxide ion, bromine, TCP P.J.McCormack

Phenol & Sodium. Phenol reacts vigorously with sodium. What is formed? 25 February 2019 Phenol reacts vigorously with sodium. What is formed? The greater reactivity of phenol compared to ethanol is due to the weak acidity of phenol. P.J.McCormack

Electrophilic Reactions of Phenol. 25 February 2019 Electrophilic Reactions of Phenol. Phenol (in sodium hydroxide) reacts with aqueous bromine room temperature to give 2,4,6-tribromophenol, which is a white precipitate. The reaction does not require a catalyst. The reason why phenol reacts much faster than benzene is that the lone pair of electrons on the oxygen atom becomes part of the delocalised  system. The  system is extended over seven atoms rather than six, therefore electron density is increased making the ring more reactive. Highly activating P.J.McCormack

Phenol. Phenol is a weak acid but neutralises strong alkalis. What is the product of phenol and sodium hydroxide? Sodium phenoxide is an ionic compound. The phenol dissolves completely in NaOH, but is only sparingly soluble in water. What is formed when an acid is added to sodium phenoxide? 25 February 2019 P.J.McCormack

Acidity of Phenol. Phenol is slightly acidic, considerably more so than water and aliphatic alcohols. If we consider the following equilibrium we can explain why phenols are acidic. In the case of the phenoxide ion, the negative charge on the oxygen can to some extent be delocalised round the ring. One of the p orbitals on the oxygen atom, which contains two electrons, interacts slightly with the singly occupied p orbital on the adjacent carbon atom. Phenol C6H5OH Mpt. 43C Bpt 181C Mpt. ethanol = -117  C 25 February 2019 P.J.McCormack

25 February 2019 Phenol Phenols are a class of organic compounds where a hydroxyl group (-OH) is attached directly to the benzene ring. Phenol is a solid at room temperature and pressure It is slightly soluble in water as the –OH group can hydrogen bond with water but the benzene ring makes it less soluble than alcohols P.J.McCormack

Relative Ease of Bromination of Phenol 25 February 2019 Relative Ease of Bromination of Phenol The reaction of bromine with phenol does not involve a halogen carrier like benzene. This means that something in its structure must make in more reactive than benzene. The electron density in phenol is greater than that of benzene The difference between benzene and phenol is the OH group. The oxygen atom in the OH group has a lone pair of p orbital electrons. These are able to delocalise with the p electrons in the benzene ring: P.J.McCormack

25 February 2019 Acidity of Phenol. This reduces the tendency of the phenoxide ion to attract protons, in other words it reduces its strength as a base. Consequently, phenol is a stronger acid Ka = 1.3x10-10) than aliphatic alcohols, though it is still weak. (Weak acids have low Ka values). Acidic strength: Phenol (more acidic) > water > ethanol Ka = 1.3x10-10 mol dm-3 > 1x10-14 mol dm-3 > 1.0x10-18 mol dm-3 P.J.McCormack

Delocalised Phenol. 25 February 2019 Delocalisation also occurs in this ion. One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring. This overlap leads to a delocalisation which extends from the ring out over the oxygen atom. As a result, the negative charge is no longer entirely localised on the oxygen, but is spread out around the whole ion. P.J.McCormack

25 February 2019 Solubility of Phenol. Phenol is sparingly soluble in water. The –OH hydrogen bonds to water, whilst the benzene ring reduces the solubility because it only forms weak Van der Waals’ forces to other molecules. P.J.McCormack

Recap. What is a phenoxide ion? 25 February 2019 What is a phenoxide ion? Why is phenol more acidic than ethanol? Why is phenol more reactive than benzene with Br2? Describe the reactivity of phenol and benzene with aqueous bromine. Explain, and show how you would make cumene from benzene. P.J.McCormack

25 February 2019 Task 1 Produce a mind map summarising all the reactions of benzene and phenol and the bonding within each molecule. P.J.McCormack

STRUCTURE OF BENZENE - DELOCALISATION The theory suggested that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure. 6 single bonds

STRUCTURE OF BENZENE - DELOCALISATION The theory suggested that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure. 6 single bonds one way to overlap adjacent p orbitals

STRUCTURE OF BENZENE - DELOCALISATION The theory suggested that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure. 6 single bonds one way to overlap adjacent p orbitals another possibility

STRUCTURE OF BENZENE - DELOCALISATION The theory suggested that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure. 6 single bonds one way to overlap adjacent p orbitals another possibility delocalised pi orbital system

STRUCTURE OF BENZENE - DELOCALISATION The theory suggested that instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds were delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It also gave a planar structure. 6 single bonds one way to overlap adjacent p orbitals another possibility delocalised pi orbital system This final structure was particularly stable and resisted attempts to break it down through normal electrophilic addition. However, substitution of any hydrogen atoms would not affect the delocalisation.

STRUCTURE OF BENZENE

STRUCTURE OF BENZENE ANIMATION

WHY ELECTROPHILIC ATTACK? Theory The high electron density of the ring makes it open to attack by electrophiles HOWEVER... Because the mechanism involves an initial disruption to the ring, electrophiles will have to be more powerful than those which react with alkenes. A fully delocalised ring is stable so will resist attack.

WHY SUBSTITUTION? Theory Addition to the ring would upset the delocalised electron system Substitution of hydrogen atoms on the ring does not affect the delocalisation Overall there is ELECTROPHILIC SUBSTITUTION STABLE DELOCALISED SYSTEM ELECTRONS ARE NOT DELOCALISED AROUND THE WHOLE RING - LESS STABLE

ELECTROPHILIC SUBSTITUTION Theory The high electron density of the ring makes it open to attack by electrophiles Addition to the ring would upset the delocalised electron system Substitution of hydrogen atoms on the ring does not affect the delocalisation Because the mechanism involves an initial disruption to the ring, electrophiles must be more powerful than those which react with alkenes Overall there is ELECTROPHILIC SUBSTITUTION

ELECTROPHILIC SUBSTITUTION Theory The high electron density of the ring makes it open to attack by electrophiles Addition to the ring would upset the delocalised electron system Substitution of hydrogen atoms on the ring does not affect the delocalisation Because the mechanism involves an initial disruption to the ring, electrophiles must be more powerful than those which react with alkenes Overall there is ELECTROPHILIC SUBSTITUTION Mechanism • a pair of electrons leaves the delocalised system to form a bond to the electrophile • this disrupts the stable delocalised system and forms an unstable intermediate • to restore stability, the pair of electrons in the C-H bond moves back into the ring • overall there is substitution of hydrogen ... ELECTROPHILIC SUBSTITUTION

ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagents conc. nitric acid and conc. sulphuric acid (catalyst) Conditions reflux at 55°C Equation C6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzene

ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagents conc. nitric acid and conc. sulphuric acid (catalyst) Conditions reflux at 55°C Equation C6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzene Mechanism

ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagents conc. nitric acid and conc. sulphuric acid (catalyst) Conditions reflux at 55°C Equation C6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzene Mechanism Electrophile NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction... 2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2+ acid base

ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagents conc. nitric acid and conc. sulphuric acid (catalyst) Conditions reflux at 55°C Equation C6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzene Mechanism Electrophile NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction... 2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2+ acid base Use The nitration of benzene is the first step in an historically important chain of reactions. These lead to the formation of dyes, and explosives.

ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATION Reagents chlorine and a halogen carrier (catalyst) Conditions reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3) chlorine is non polar so is not a good electrophile the halogen carrier is required to polarise the halogen Equation C6H6 + Cl2 ———> C6H5Cl + HCl Mechanism Electrophile Cl+ it is generated as follows... Cl2 + FeCl3 FeCl4¯ + Cl+ a Lewis Acid

FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION Overview Alkylation involves substituting an alkyl (methyl, ethyl) group Reagents a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3 Conditions room temperature; dry inert solvent (ether) Electrophile a carbocation ion R+ (e.g. CH3+) Equation C6H6 + C2H5Cl ———> C6H5C2H5 + HCl

FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION Overview Alkylation involves substituting an alkyl (methyl, ethyl) group Reagents a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3 Conditions room temperature; dry inert solvent (ether) Electrophile a carbocation ion R+ (e.g. CH3+) Equation C6H6 + C2H5Cl ———> C6H5C2H5 + HCl Mechanism General A catalyst is used to increase the positive nature of the electrophile and make it better at attacking benzene rings. AlCl3 acts as a Lewis Acid and helps break the C—Cl bond.

FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION Catalyst anhydrous aluminium chloride acts as the catalyst the Al in AlCl3 has only 6 electrons in its outer shell; a LEWIS ACID it increases the polarisation of the C-Cl bond in the haloalkane this makes the charge on C more positive and the following occurs RCl + AlCl3 AlCl4¯ + R+

FRIEDEL-CRAFTS REACTIONS - INDUSTRIAL ALKYLATION Industrial Alkenes are used instead of haloalkanes but an acid must be present Phenylethane, C6H5C2H5 is made by this method Reagents ethene, anhydrous AlCl3 , conc. HCl Electrophile C2H5+ (an ethyl carbonium ion) Equation C6H6 + C2H4 ———> C6H5C2H5 (ethyl benzene) Mechanism the HCl reacts with the alkene to generate a carbonium ion electrophilic substitution then takes place as the C2H5+ attacks the ring Use ethyl benzene is dehydrogenated to produce phenylethene (styrene); this is used to make poly(phenylethene) - also known as polystyrene

FRIEDEL-CRAFTS REACTIONS OF BENZENE - ACYLATION Overview Acylation involves substituting an acyl (methanoyl, ethanoyl) group Reagents an acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3 Conditions reflux 50°C; dry inert solvent (ether) Electrophile RC+= O ( e.g. CH3C+O ) Equation C6H6 + CH3COCl ———> C6H5COCH3 + HCl Mechanism Product A carbonyl compound (aldehyde or ketone)

FURTHER SUBSTITUTION OF ARENES Theory It is possible to substitute more than one functional group. But, the functional group already on the ring affects... • how easy it can be done • where the next substituent goes Group ELECTRON DONATING ELECTRON WITHDRAWING Example(s) OH, CH3 NO2 Electron density of ring Increases Decreases Ease of substitution Easier Harder Position of substitution 2,4,and 6 3 and 5

FURTHER SUBSTITUTION OF ARENES Examples Substitution of nitrobenzene is... • more difficult than with benzene • produces a 1,3 disubstituted product Substitution of methylbenzene is… • easier than with benzene • produces a mixture of 1,2 and 1,4 isomeric products Some groups (OH) make substitution so much easier that multiple substitution takes place

RELATIVE POSITIONS ON A BENZENE RING ortho dichlorobenzene STRUCTURAL ISOMERISM RELATIVE POSITIONS ON A BENZENE RING 1,2-DICHLOROBENZENE ortho dichlorobenzene 1,3-DICHLOROBENZENE meta dichlorobenzene 1,4-DICHLOROBENZENE para dichlorobenzene Compounds have similar chemical properties but different physical properties