5.3 ARENES: BENZENE Syllabus specification Arenes: benzene 2014 a. use thermochemical, x-ray diffraction and infrared data as evidence for the structure and stability of the benzene ring. Students may represent the structure of benzene as or as appropriate in equations and mechanisms b. describe the following reactions of benzene, limited to: i) combustion to form a smoky flame. treatment with: ii) bromine. iii) concentrated nitric and sulfuric acids. iv) fuming sulfuric acid. v) halogenoalkanes and acyl chlorides with aluminium chloride as catalyst (Friedel-Crafts reaction). vi) addition reactions with hydrogen. c. describe the mechanism of the electrophilic substitution reactions of benzene in halogenation, nitration and Friedel-Crafts reactions including the formation of the electrophile. d. carry out the reactions in 5.4.1b where appropriate (using methylbenzene or methoxybenzene). e. carry out the reaction of phenol with bromine water and dilute nitric acid and use these results to illustrate the activation of the benzene ring. Introduction: Arenes are hydrocarbons with a ring or rings of carbon atoms in which there are delocalised electrons. Benzene, the simplest arene with a molecular formula C6H6, is an important and useful chemical which is obtained by the catalytic reforming of fractions from crude oil. Arenes are sometimes called aromatic compounds. Study of the structure of benzene is an another example that shows how scientific models develop in response to new evidence. This links to further investigations of the models that chemists use to describe the mechanisms of organic reactions. VIHS/DEPARTMENT OF CHEMISTRY Page 1 5.3 ARENES: BENZENE General properties of benzene It is a Colourless liquid with a characteristic odour. Boils at 80oC and freezes at 6oC. Immiscible with water but soluble in organic solvent. Gives smoky luminous sooty flame on burning. 2014 Structure of benzene: Benzene, C6H6, is a cyclic compound that has six carbon atoms in a hexagonal ring. Several structures for benzene have been proposed. Early theories suggested that there were alternative single and double bonds between the carbon atoms (fig 5.3.1), but this did not fit with later experimental evidence. It was shown that all the carbon-carbon bonds are the same length and that the molecule is planar. Fig. 5.3.1 Simplified structure of benzene Two modern theories are used to explain the structure. The Kekule’ version assumes that benzene is a resonance hybrid between the two structures as given below. This model can be used to explain many chemical properties and reaction of benzene. Fig 5.3.2 The displayed formula of kekule’s benzene ring structure The other theory assumes that each sp2 hybridized carbon atom is joined by a σ- (sigma) bond to each of its two neighbours, and by a third σ- sigma bond to s-orbital of hydrogen atom forming a hexagonal planar ring. The fourth bonding electron is in p-orbital(called as non-hybrid p—orbital) in the right angle to the planar of σ- (sigma) bonds. This p-orbital overlap side way, and the six p-orbitals overlap above and below the plane of the ring of carbon atoms. This produces a delocalised π-(pi)bonding system of electrons, as in: VIHS/DEPARTMENT OF CHEMISTRY Page 2 152 kJ less energy is given out because of benzene’s unique structure. 5. ∆H = -120 kJ mol—1 + H2(g) Cyclohexane Cyclohexene Therefore . For example. which is the theoretical compound with three single and three localised double carbon-carbon bonds. ∆H for the addition to three localised double bonds in ‘cyclohexatriene’ would be 3 x (-120) = -360 kJ mol—1. However for benzene: + 3H2(g) Benzene cyclohexane ∆H = -208kJ mol—1 Thus. VIHS/DEPARTMENT OF CHEMISTRY Page 3 .3. the enthalpy of hydrogenation of one mole cyclohexene is -120 kJ. The amount by which it is stabilised can be calculated from the enthalpies of hydrogenation.5. Benzene is more stable than ‘cyclohexatriene’.3 The delocalisation of the electrons in the π -bonds of the symmetrical six-membered ring structure of benzene Evidences for structure and extra stability of benzene (i) Thermochemical evidence: via enthalpy of hydrogenation. This is called the delocalisation stabilisation energy or resonance energy and can be shown in an enthalpy-level diagram.3 ARENES: BENZENE 2014 Fig. 3. which is calculated as below Three C—C = 3 x (-348) = -1044 kJ Three C=C = 3 X (-612) = -1836 kJ Six C—H = 6 x (-412) Total = -2472 kJ = .3 ARENES: BENZENE 2014 ‘Cyclohexatriene’ Enthalpy kJmol-1 ∆H = -360 kJ mol-1 ∆H = -152 kJ (resonance energy) Benzene ∆H = -208 kJ mol-1 Cyclohexane. The enthalpy of formation of gaseous benzene is +83 kJ mol -1.5362 kJ VIHS/DEPARTMENT OF CHEMISTRY Page 4 . C6H12 Fig.4 Enthalpy-level diagram for the hydrogenation of benzene and cyclohexatriene.5. Thermo-chemical evidence: via bond enthalpies The amount by which benzene is stabilised can also be calculated from average bond enthalpies. The value for the theoretical molecule ‘cyclohexatriene’ can be found using the Hess’s law cycle below: 6C(s) + 3H2(g) ∆Hf C6H6(g) Step 1 Step 2 6C(g) + 3H2(g) Step 3 6C(g) + 6H(g) Step 1 equals 6 x enthalpy of atomisation of carbon(∆Hatm[C(s)]) = 6 x (+715) = +4290 kJ Step 2 equals 3 x H—H bond enthalpy = 3 x (+436) = + 1308kJ Step 3 equals enthalpy change of bonds made.5. 2014 The actual enthalpy of formation of gaseous benzene is +83 kJ mol-1.3 ARENES: BENZENE Hence the ∆Hf of ‘cyclohexatriene’ = ∆Hstep 1 + ∆Hstep 2 + ∆Hstep 3 = +4290 + 1308 +(-5352) = +246 kJ mol-1.5 Electron density map of benzene.5. If the diffraction pattern of benzene is analysed.bonding electron system. equal distribution of electrons cannot be seen on the carbon ring. Hence. Thus.140 0. Fig. comparison of bond length in benzene and cyclohexene. If benzene has cyclohexatriene structure. Bond All the six carbon-carbon bonds in benzene Carbon-carbon single bond in cyclohexene Carbon-carbon double bond in cyclohexene 0.134 0. VIHS/DEPARTMENT OF CHEMISTRY Page 5 . benzene is thermodynamically more stable due to its delocalized pibonding system. X-ray diffraction evidence X-ray diffraction shows the position of the centre of atoms.3. Table 1.154 Bond length/nm 0. The value calculated above is 163 kJ more and approximately equals the resonance energy of benzene.5.140 nm Electrons are equally distributed over six carbon atoms due to delocalisation of the pi. it clearly shows that all the bond lengths between the carbon atoms are the same. the structure with the delocalised electron system is energetically more stable. Which is not the same in the case of cyclohexene. 3 ARENES: BENZENE Infra red evidence: 2014 Comparison of the infrared spectrum of aromatic compounds with those of aliphatic compounds containing a C=C group showed slight differences.-COCH3 Methanol. The derivatives of benzene are named either as substituted products of benzene or as compounds containing the phenyl group. The names and structures of some derivatives of benzene are given below. In some cases.5. The C —H stretching vibration in benzene is at 3036cm-1 and the C=C stretching is at 1479cm-1.3.-CH2OH Structure C6H5-Cl C6H5-NO2 C6H5-CH3 C6H5-OH C6H5-NH2 C6H5-COCH3 C6H5-CH2OH When more than one hydrogen atom is substituted. -NO2 Methyl. Naming benzene derivatives. C6H5—. VIHS/DEPARTMENT OF CHEMISTRY Page 6 . -OH Amine. -Cl Nitro. the ring is numbered anticlockwise to get the lowest possible numbers. whereas the equivalent vibrations in an aliphatic compound such as cyclohexene are at 3023 and 1438cm-1. 5. Systematic name Chlorobenzene Nitrobenzene Methylbenzene Phenol Phenylamine Phenylethanone Phenylmethanol Substituent group Chloro. -NH2 Ethanone.-CH3 Hydroxyl. The ring is usually numbered clockwise and the numbers used are the lowest ones possible. numbers are used to indicate the positions of substituent on the benzene ring.6 IR spectra for (a) cyclohexzene and (b) benzene. Fig. 5. 2014 In phenyl compounds. Reactions of benzene (i) combustion: Benzene burns in a limited amount of air with a smoky flame. groups Fig.3 ARENES: BENZENE are assumed to occupy the 1 position.3. 2C6H6(l) + 15O2(g) (ii) Addition: 12CO2(g) + 6H2O(l) The double bond in benzene is not as susceptible to addition as is the double bond in alkenes. However. Reaction type: Electrophilic addition. Conditions: In the presence of Raney nickel(finely divided with a very large surface area and very high catalytic activity)catalyst at high temperature(about 150 oC). + 3H2 Benzene Reagents: Hydrogen gas.5. cyclohexane VIHS/DEPARTMENT OF CHEMISTRY Page 7 .7 Naming substituted benzene compounds. the –OH and –NH2. it does react with hydrogen in the presence of a hot nickel catalyst to form cyclohexane. Combustion is incomplete and particles of carbon are formed. The complete combustion of benzene requires large volume of oxygen. such as phenol and phenylamine. Finally.5. the [FeCl4]— ion removes an H+ ion from benzene. Cl + Cl2(g) chlorobenzene + HCl(g) Benzene Reagents: Chlorine gas. Reaction type: Electrophilic substitution. Steamy fumes of hydrogen chloride are given off and chlorobenzene(C6H5Cl) is formed. Step 1: The catalyst. Conditions: Room temperature and pressure. Mechanism: Heterolytic electrophilic substitution.3 ARENES: BENZENE Electrophilic substitution: (iv) Halogenation Dry benzene reacts with chlorine gas in the presence of iron (or a catalyst of 2014 anhydrous iron(III) chloride). in the presence of anhydrous FeCl 3. The mechanism for this reaction is as follows. forming an intermediate with a positive charge. producing chlorobenzene(C6H5Cl) and reforming the catalyst(FeCl3) H+ + [FeCl4]— HCl + FeCl3 VIHS/DEPARTMENT OF CHEMISTRY Page 8 . anhydrous iron(III) chloride . is made by the reaction of iron with chlorine Fe + 1½ Cl2 FeCl3 This reacts with more chlorine. forming the electrophile Cl+ Cl+ + [FeCl4]— Cl2 + FeCl3 electrophile Step 2: The Cl+ attacks the π-electrons in the benzene ring. a nitro-group(NO2) replaces a hydrogen atom in the benzene ring. Conc. forming an intermediate with a positive charge.H2SO4 + HNO3(conc. 2H2SO4 + Acid HNO3 base 2HSO4¯ + H3O+ + NO2+ electrophile Step 2: The NO2+ attacks the π-electrons in the benzene ring. thus regaining the stability of the delocalised system.5. Mechanism: Heterolytic electrophilic substitution. The temperature must not go above 50 oC or some dinitrobenzene(C6H4(NO2)2) is formed. because the activation energy of the first step is higher than that for the addition to ethene. The benzene intermediate loses an H+. whereas the intermediate with ethene adds Cl—ion. When benzene is warmed with a mixture of concentrated nitric and sulfuric acid.H2SO4 and Conc. Reaction with nitric acid: Nitration. A catalyst must be present for the addition of Cl+ to benzene. Step 1:The sulfuric acid reacts with the nitric acid to form the electrophile NO2+. Nitrobenzene and water are produced. producing nitrobenzene(C6H5NO2) and reforming the catalyst(H2SO4).) Benzene 50oC NO2 + H 2O nitrobenzene Reagents: A mixture of Conc. the HSO4— ion removes an H+ ion from benzene. Reaction Type: Electrophilic substitution. Finally.HNO3(nitrating mixture) Conditions: Warm under reflux at 50oC. VIHS/DEPARTMENT OF CHEMISTRY Page 9 . The mechanism for this reaction is as follows.3 ARENES: BENZENE 2014 Note: The addition of Cl+ to benzene is similar to the first step of the addition of chlorine to ethene. The difference arises at the next step. VIHS/DEPARTMENT OF CHEMISTRY Page 10 .NO2+. as it increases the rate of reaction and remains chemically unchanged as it is being regenerated at the end of the reaction. Role of HNO3 It generates nitronium ion. which acts as an electrophile in the mechanism. Explain. Acts as an acid(proton donor). as it donates protons in the reaction. (02) In the nitration of benzene sulphuric acid acts as an acid whereas nitric acid acts as a base.3 ARENES: BENZENE 2014 HSO4— + Role of H2SO4 H+ H2SO4 Acts as a catalyst. Show by an equation how this is so. It acts as a base by accepting protons. Exercise (01) Benzene prefers to undergo substitution reaction rather than addition reactions.5. 5. VIHS/DEPARTMENT OF CHEMISTRY Page 11 . (05) Write an equation for the bromination of benzene.3 ARENES: BENZENE 2014 (03) Why Raney nickel is used in the manufacture of cyclohexane from benzene? (04) Explain why smoky flame are seen during the combustion of benzene. By using appropriate arrow draw the mechanism of this reaction. SO3H + SO3 Benzene Reagents: fuming sulphuric acid Conditions: Heat under reflux Reaction Type: Electrophilic substitution. Fuming sulphuric acid is a solution of sulphur trioxide in sulphuric acid. benzenesulfonic acid The mechanism for this reaction is as follows. Mechanism: Heterolytic electrophilic substitution. benzenesulfonic acid is 2014 produced.3 ARENES: BENZENE Reaction with fuming sulphuric acid: Sulfonation. When benzene is warmed with fuming sulfuric acid.5. alkyl group(eg C 2H5) can be substituted into the ring. Step 1 Step 2 This reaction is important in the manufacture of detergents. where a substituted benzene ring is sulfonated and the final product is neutralised. The electrophile is the SO3 molecule. Friedel-Crafts reaction: (i) Reaction with halogenoalaknes In the presence of an anhydrous aluminium chloride catalyst. VIHS/DEPARTMENT OF CHEMISTRY Page 12 . C2H5 + C2H5 Cl benzene Reagents: Halogenoalkanes ethylbenzene + HCl 2014 For example. Finally. +CH2CH3.3 ARENES: BENZENE ethylbenzene and hydrogen chloride. Step 1: The electrophile. is produced by the reaction of the catalyst with the halogenoalkane: + CH3CH2Cl + Chloroethane AlCl3 CH2CH3 + [AlCl4]— electrophile Step 2: The positive carbon atom attacks the π–system in the benzene ring: Step 3: The intermediate loses a H+ ion so as to regain the stability of the benzene ring. the catalyst is regenerated by the reaction: H+ + [AlCl4]— HCl + AlCl3. in the presence of anhydrous AlCl3 as a catalyst.5. Reaction Type: Electrophilic substitution. Note:The reaction mixture must be dry. In the reaction between benzene and chloroethane. VIHS/DEPARTMENT OF CHEMISTRY Page 13 . The mechanism for this reaction is as follows. Mechanism: Heterolytic electrophilic substitution. the products are Conditions: Heat under reflux at 50oC. COCH3 + CH3COCl Benzene Reagents: Acyl(acid) chlorides Conditions: Heat under reflux at 50oC. Mechanism: Heterolytic electrophilic substitution. a catalyst is used to increase the positive nature of the electrophile and make it better at attacking benzene rings. In the presence of an anhydrous aluminium chloride catalyst. Reaction Type: Electrophilic substitution. For example.3 ARENES: BENZENE 2014 In this reaction. the products are phenylethanone and hydrogen chloride.5. in the presence of anhydrous AlCl3 as a catalyst. VIHS/DEPARTMENT OF CHEMISTRY Page 14 . In the reaction between benzene and ethanoyl chloride. Step 3: The intermediate loses a H+ ion so as to regain the stability of the benzene ring. Friedel-Crafts reaction: (ii) Reaction with acyl(acid) chlorides. CH3C+O is produced by the reaction of the acylchloride with the catalyst: CH3C+O + [AlCl4]— electrophile CH3COCl + AlCl3 ethanoyl chloride Step 2: The positive carbon atom attacks the π-system in the benzene ring. phenylethanone + HCl The mechanism for this reaction is as follows. Step 1: The electrophile. benzene reacts with acylchlorides to form ketones. AlCl3 acts as a Lewis Acid and helps break the C—Cl bond. Phenol Phenol(C6H5OH) contains an –OH group on a benzene ring. Properties of phenol. 5. Phenol is less acidic than carboxylic acid but more acidic than alcohol(-COOH > phenol > -OH). A lone pair of electron on the oxygen atom becomes part of the delocalised π -system and makes phenol much more susceptible to attack by electrophiles.3. simple structure of phenol.5. Therefore it can easily loses a proton and form stable phenoxide ion. the catalyst is regenerated by the reaction: H+ + [AlCl4]— HCl + AlCl3. VIHS/DEPARTMENT OF CHEMISTRY Page 15 .8 Orbital structure of phenol. Fig. H+(aq) C6H5OH(aq) C6H5O¯(aq) Phenoxide ion + It is a solid at room temperature.3 ARENES: BENZENE 2014 Finally. (ii) Reaction with sodium metal. It can be used as antiseptic compounds.sodium phenoxide. This reaction is similar to that with aliphatic alcohols such as ethanol 2C6H5O¯ Na+(s) 2C6H5OH(s) + 2Na(s) + H2(g) (iii) Reaction with carbonates and hydrogen carbonates.9 p-orbitals in the system. (i) Reaction with aqueous sodium hydroxide. Phenol reacts with sodium hydroxide to form a salt . The p orbital on the Oxygen overlaps with the p orbitals in the ring. VIHS/DEPARTMENT OF CHEMISTRY Page 16 .5. 2014 It is partially soluble in water due to the formation of hydrogen bond with water.sodium phenoxide and hydrogen.3.3 ARENES: BENZENE It is more reactive than benzene. Reactions of phenol. it is ionic and water soluble C6H5O¯ Na+(aq) + H2O(l) C6H5OH(aq) + NaOH(aq) This reaction is an evidence for the acidic character of phenol. Phenol does not react with carbonates and hydrogen carbonates as is is weakly acidic. (iv) Electrophilic substitution: The OH group in phenol is electron releasing therefore it increases the electron density of the delocalised system which makes substitution much easier compared to benzene as a p orbital on the oxygen overlaps with the p orbitals in benzene Fig. 5. Phenol reacts with sodium to form an ionic salt . 4. The brown bromine water is decolorised and a white precipitate of 2. in an electrophilic substitution reaction. Phenol 2.4. The electron rich ring in phenol is attacked by bromine water.4 and 6 positions which results in the substitution 2 6 6 6 6 4 6 Reaction with aqueous bromine.4 and 6 positions. OH OH NO2 + HNO3(aq) + OH + H 2O NO2 Phenol 2-nitrophenol 4-nitrophenol VIHS/DEPARTMENT OF CHEMISTRY Page 17 . At room temperature. Reaction with nitric acid: The ring is sufficiently activated for nitration to take place with dilute nitric acid.3 ARENES: BENZENE takes place at the 2. No catalyst is needed.6tribromophenol and a solution of hydrogen bromide are formed. Reaction type: Electrophilic substitution Observation: Orange colour decolourises/ formation of white ppt.6-tribromophenol (white precipitate) Reagents : Conditions: Aqueous bromine Room temperature and pressure. the organic product is a mixture of 2-nitrophenol and 4-nitrophenol. 2014 The electron density is greatest at the 2./ misty fumes.5. .......................4-dimethylbenzene reacts with 2-bromobutane.....................6 dinitrophenol are formed as well.................. Practice questions (01) Explain why phenol can be nitrated under much milder conditions than those required to nitrate benzene. you should be able to: Define electrophile. 2.6-trinitrophenol is the product... Write equations and state conditions for the reactions of benzene and phenol with bromine and nitric acid and benzene with sulphuric acid and the friedel....5..................................................... If concentrated nitic acid is used...................................................................... AlCl3................................. which dissolves in the reaction mixture........ VIHS/DEPARTMENT OF CHEMISTRY Page 18 .....................crafts reactions of benzene.. Draw mechanisms for the halogenations.... The reaction is catalysed by aluminium chloride.................................................................................................. (02) In the reaction shown below... nitration and friedel............ ...........4 and 2...................................crafts reactions........... Explain why the ring in methylbenzene is slightly activated and that in phenol very activated.... .............................................4... .....3 ARENES: BENZENE Reagents : Conditions: Dilute nitric acid Room temperature and pressure........................................ 2014 Reaction type: Electrophilic substitution If the mixture is heated 2................ Checklist After studying this topic...... the aromatic compound 1......................... ........................... Estimate resonance energy of benzene from hydrogenation and bond enthalpy data......... ............5...........................................................................................3 ARENES: BENZENE 2014 (a) (i) Name the type of reaction and the mechanism................. ................ Equation: Mechanism: VIHS/DEPARTMENT OF CHEMISTRY Page 19 ........ (ii) Write the equation to show how the attacking species forms and give the mechanism for the reaction....... .... (b) The methylbenzene formed in reaction 1 generally reacts in a similar way to benzene but faster.................... reacts with reagent A to form the species which attacks the benzene ring...................... VIHS/DEPARTMENT OF CHEMISTRY Page 20 ..... AlCl3................5...... (ii) Draw the structure of the intermediate ion formed when the species in (ii) attacks the benzene ring........................................... 2014 (a) (i) Write the equation to show how the catalyst.......................... (i) Explain how the presence of a methyl group activates the benzene ring................................................ ...........................................................................................3 ARENES: BENZENE (03) Some reactions of benzene are shown below................. .. as the ring is said to be activated.... ..... ............................................................................................ ......................................................3 ARENES: BENZENE 2014 ........................................................................................ .........................................................5............................................................................................................. Suggest two reasons why this technique may be considered ‘greener’....................................................................................................................................................................................................................... ................ (ii) The organic product of reaction 2 is also formed when the same reactants............................. ..................... formed in reaction 2......................................... ........................................................................................................................................................................................................................................................................................................................................................................................................................................................ VIHS/DEPARTMENT OF CHEMISTRY Page 21 ................................................................................................................. are heated using microwave radiation.................................................. (c) (i) Draw the structural formula of compound X..................................................................................................................... (d) Name reagent B needed for reaction 3..... ............................................ ....... (ii) Use your answer to (i) to explain why methylbenzene reacts faster............................................................................................................ but with an aluminium catalyst..... ......... .. ....................... ................................................................................................................................................... ..................................................................................................................................................................................... ................. ........................... ................................................5................................................................................................................................ (05)(i) Explain why phenol................................................................................ VIHS/DEPARTMENT OF CHEMISTRY Page 22 ............................................. .................... State symbols are not required........ and methoxybenzene............................................................................................ .................................. ...................................................................................................... ............ (ii) Write the equation for the reaction between phenol and bromine water...................................... .. in terms of the bonding in the benzene ring............................................................................................................................................................................................................................................................................................................................. are much more reactive than benzene with bromine.................. why the enthalpy of hydrogenation is less exothermic than would be expected from a molecule with three double bonds................................. ................................................................................................................................................................................................................. C6H5OCH3.................................................................................................................................................................................................................................................................................................................... ................. ................3 ARENES: BENZENE 2014 (04) Explain............................................................. C6H5OH.......................... ............................................................