Introduction
There are two major classes of organic chemicals aliphatic : straight or branched chain organic substances aromatic or arene: includes one or more ring of six carbon atoms with delocalised bonding. All of the organic substances we have looked at so far have been aliphatic Benzene belongs to the aromatic class. Benzene’s Structure The simplest arene is benzene. It has the molecular formula C6H6 Its basic structure is six C atoms in a hexagonal ring, with one H atom bonded to each C atom Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds. This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring. The Six p electrons are delocalised in a ring structure above and below the plane of carbon atoms H H H H H H H H H H H H In 1865 Kekule suggested the following structure for Benzene consisting of alternate single and double covalent bonds between the carbon atoms C C C C C C H H H H H H This structure is not correct. Evidence suggests that all the C-C bonds are the same length. Benzene is a planar molecule.The evidence suggests all the C-C bonds are the same and have a length and bond energy between a C-C single and C=C double bond In formulae we draw a circle to show this delocalised system Abbreviated formula Displayed formula The H-C-C bond angle is 120o in Benzene The six electrons in the pi bonds are delocalised and spread out over the whole ring. Delocalised means not attached to a particular atom. + H2 + 3H2 + 3H2 ∆H = -120 kJ/mol ∆H = -360 kJ/mol ∆H = -208kJ/mol Enthalpies of Hydrogenation cyclohexene cyclohexane Non delocalised structure delocalised structure Theoretically because there are 3 double bonds one might expect the amount of energy to be 3 times as much. x3 However, the real amount of energy is less. The 6 pi electrons are delocalised and not arranged in 3 double bonds -360 kJ/mol Theoretical value ∆H = -208kJ/mol actual value enthalpy ∆H = -152kJ/mol delocalisation energy This when represented on an energy level diagram shows that the delocalised benzene is more thermodynamically stable. The increase in stability connected to delocalisation is called the delocalisation energy Summary of evidence for why benzene has a delocalised structure. • Bond length intermediate between short C=C and long C–C • ΔH hydrogenation less exothermic than expected when compared to ΔH hydrogenation for kekule structure • Only reacts with Br2 at high temp or in presence of a halogen carrier
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6.1.1 Aromatic compounds
Benzene and aromatic compounds (a) the comparison of the Kekulé model of benzene with the subsequent delocalised models for benzene in terms of p-orbital overlap forming a delocalised π-system Learners may represent the structure of benzene in equations and mechanisms as: or HSW1,7 Development of the model for benzene over time. (b) the experimental evidence for a delocalised, rather than Kekulé, model for benzene in terms of bond lengths, enthalpy change of hydrogenation and resistance to reaction (see also 6.1.1 f) HSW11 Acceptance of the delocalised benzene model by the scientific community in light of supporting experimental evidence
Naming
Naming aromatic molecules Naming aromatic compounds can be complicated. The simplest molecules are derivatives of benzene and have benzene at the root of the name CH3 C2H5 Cl Br NO2 CO2H CHO Methylbenzene ethylbenzene chlorobenzene bromobenzene nitrobenzene benzenecarboxylic acid benzaldehyde N Goalby chemrevise.org If two or more substituents are present on the benzene ring, their positions must be indicated by the use of numbers. This should be done to give the lowest possible numbers to the substituents. When two or more different substituents are present, they are listed in alphabetical order and di, tri prefixes should be used. CH3 CH3 CH3 NO2 NO2 O2N COOH OH CH3 Cl 1,3-dimethylbenzene 1-chloro- 4-methylbenzene 4-hydroxybenzenecarboxylic acid 2,4,6-trinitromethylbenzene In other molecules the benzene ring can be regarded as a substituent side group on another molecule, like alkyl groups are. The C6H5 – group is known as the phenyl group. NH2 CH CH2 H3C CH CH2 CH3 C CH3 O H3C C O O phenylamine phenylethene 2-phenylbutane phenylethanone phenylethanoate
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6.1.1 Aromatic compounds
(c) use of IUPAC rules of nomenclature for systematically naming substituted aromatic compounds Use of locant numbers to identify positions of substitution e.g. 2,4-dinitromethylbenzene. HSW8 Introduction of systematic nomenclature.
Electrophilic substitution of benzene
Reactions of Benzene Benzene does not generally undergo addition reactions because these would involve breaking up the delocalised system. Most of Benzene’s reactions involve substituting one H for another atom or group of atoms. It reactions are usually electrophilic substitutions. Comparison of Benzene with alkenes: reaction with Bromine Alkenes react with Bromine easily at room temperature. Benzene does not react with Bromine without additional halogen carrier chemicals. In benzene, electrons in π-bond(s) are delocalised. In alkenes, π- electrons are localised between two carbons. Benzene therefore has a lower electron density than C=C. Benzene therefore polarises bromine less and induces a weaker dipole in bromine than an alkene would. Toxicity of Benzene Benzene is a carcinogen (cancers causing molecule) and is banned for use in schools. Methylbenzene is less toxic and also reacts more readily than benzene as the methyl side group releases electrons into the delocalised system making it more attractive to electrophiles. Nitration of Benzene Importance of this reaction Nitration of benzene and other arenes is an important step in synthesising useful compounds e.g. explosive manufacture (like TNT, trinitrotoluene/ 2,4,6- trinitromethylbenzene) and formation of amines from which dyestuffs are manufactured. (The reaction for this is covered in the amines section.) Change in functional group: benzene nitrobenzene Reagents: conc nitric acid in the presence of concentrated sulphuric acid (catalyst) Mechanism: Electrophilic Substitution Electrophile: NO2 + Equation for Formation of electrophile: (Learn!) HNO3 + 2H2SO4 NO2 + + 2HSO4 – + H3O+ + NO2 + NO2 + H + The horseshoe shape of the intermediate must not extend beyond C’s 2 to 6 Mechanism Overall Equation for reaction The H+ ion rejoins with the HSO4 – to reform H2SO4 catalyst. + H + This reaction is done at 60oC. On using higher temperatures a second nitro group can be substituted onto different positions on the ring N Goalby chemrevise.org H+ + HSO4 – H2SO4 If the benzene ring already has a side group e.g. methyl then the Nitro group can also join on different positions. A-level does not require knowledge of what positions the groups go on. Change in functional group: benzene Bromobenzene Reagents: Bromine Conditions: iron(III) bromide catalyst FeBr3 Mechanism: Electrophillic Substitution This reaction can be done with chlorine. The catalyst can be AlCl3 or FeCl3 Halogenation of Benzene + Br2 Br + HBr Overall Equation for reaction Equation for Formation of electrophiles: (Learn!) AlCl3 + Cl2 AlCl4 – + Cl+ FeBr3 + Br2 FeBr4 – + Br+ Cl+ Cl Mechanism The H+ ion reacts with the AlCl4 – to reform AlCl3 catalyst and HCl. H+ + AlCl4 – AlCl3 + HCl Cl H + N Goalby chemrevise.org 4 Friedel Crafts Alkylation Change in functional group: benzene alkylbenzene Reagents: chloroalkane in the presence of anhydrous aluminium chloride catalyst Conditions: heat under reflux Mechanism: Electrophilic Substitution Any chloroalkane can be used RCl where R is any alkyl group Eg –CH3 , -C2H5 . The electrophile is the R+ . Formation of the electrophile. AlCl3 + CH3CH2Cl CH3CH2 + AlCl4 – CH3CH2 + AlCl4 – + AlCl3 + HCl ethylbenzene + Overall Equation for reaction CH2CH3 +CH2CH3 The H+ ion reacts with the AlCl4 – to reform AlCl3 catalyst and HCl. H+ + AlCl4 – AlCl3 + HCl CH2CH3 + H CH2CH3 Mechanism Friedel Crafts Acylation Change in functional group: benzene phenyl ketone Reagents: acyl chloride in the presence of anhydrous aluminium chloride catalyst Conditions: heat under reflux (50OC) Mechanism: Electrophilic Substitution Any acyl chloride can be used RCOCl where R is any alkyl group e.g. –CH3 , -C2H5 . The electrophile is the RCO+ . Equation for Formation of the electrophile. AlCl3 + CH3COCl CH3CO+ AlCl4 – CH3CO+ AlCl4 – + AlCl + 3 + HCl C O CH3 phenylethanone Overall Equation for reaction These are important reactions in organic synthesis because they introduce a reactive functional group on to the benzene ring The H+ ion reacts with the AlCl4 – to reform AlCl3 catalyst and HCl. H+ + AlCl4 – AlCl3 + HCl
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6.1.1 Aromatic compounds
Electrophilic substitution (d) the electrophilic substitution of aromatic compounds with: (i) concentrated nitric acid in the presence of concentrated sulfuric acid (ii) a halogen in the presence of a halogen carrier (iii) a haloalkane or acyl chloride in the presence of a halogen carrier (Friedel–Crafts reaction) and its importance to synthesis by formation of a C–C bond to an aromatic ring (see also 6.2.4 d) Halogen carriers include iron, iron halides and aluminium halides. (e) the mechanism of electrophilic substitution in arenes for nitration and halogenation (see also 4.1.1 h–i) For nitration mechanism, learners should include equations for formation of NO2 +. Halogen carriers include iron, iron halides and aluminium halides. For the halogenation mechanism, the electrophile can be assumed to be X+. HSW1,2,8 Use of reaction mechanisms to explain organic reactions.(f) the explanation of the relative resistance to bromination of benzene, compared with alkenes, in terms of the delocalised electron density of the π-system in benzene compared with the localised electron density of the π-bond in alkenes (see also 4.1.3 a, 6.1.1 a) HSW2,5 Use of delocalised benzene model to explain reactivity. (g) the interpretation of unfamiliar electrophilic substitution reactions of aromatic compounds, including prediction of mechanisms Extra information may be provided on exam papers
Phenols PAG7
Phenols In a phenol the OH group is directly attached to the benzene ring. In a phenol the lone pair of electrons on the oxygen is delocalised with the electron charge cloud of the arene ring. The delocalised bonding changes the reactivity of the OH group and the arene ring. OH CH2OH This is not a phenol, but is an alcohol because the OH group is attached to an alkyl group rather than the Phenols are very weakly acidic. They are weaker acids than carboxylic acids. benzene ring. Both phenols and carboxylic acids will react with sodium metal and sodium hydroxide. Only carboxylic acids will react with sodium carbonate as a phenol is not strong enough an acid to react. O – OH Na+ + Na + ½ H2 O – OH Na+ + NaOH + H2O sodium phenoxide The sodium phenoxide compound is more soluble than the original phenol. So the solid phenol dissolves on addition of NaOH Reaction with Bromine Phenol does not need a FeBr3 catalyst like benzene and undergoes multiple substitution whereas benzene will only add one Br. Reagents: Br2 Conditions: room temp OH OH Br Br Br + 3 HBr 2,4,6 –tribromophenol + 3 Br2 In phenol the lone pair of electrons on the oxygen (p- orbital) is partially delocalised into the ring. The electron density increases and the Br2 is more polarised The product in this reaction is a white solid Phenols are used in the production of plastics, antiseptics, disinfectants and resins for paints. N Goalby chemrevise.org 6 Reaction of Phenol with Nitric acid In comparison with benzene, phenol does not need concentrated nitric acid or the concentrated sulphuric acid catalyst With 4M HNO3 single substitution occurs (in comparison to the conc HNO3 needed for benzene) Reagent 4M HNO3 Conditions: room temp OH NO2 NO2 OH with 4M HNO3 or 2 -nitrophenol 4 -nitrophenol OH Effect of side groups on substitution Side groups on a benzene ring can affect the position on the ring of substitution reactions. Electron-donating groups such as OH, NH2 will force further substitutions to occur on the 2- and 4- positions of the ring OH NO2 NO2 OH with 4M HNO3 or 2 -nitrophenol 4 -nitrophenol OH Electron-withdrawing groups (such as NO2 ) will have a 3-directing effect of in electrophilic substitution of aromatic compounds
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6.1.1 Aromatic compounds
Phenols (h) the weak acidity of phenols shown by the neutralisation reaction with NaOH but absence of reaction with carbonates (see also 5.1.3 b) PAG7 (see also 6.3.1 c) (i) the electrophilic substitution reactions of phenol: (i) with bromine to form 2,4,6-tribromophenol (ii) with dilute nitric acid to form 2-nitrophenol Note that nitration with phenol does not require concentrated HNO3 or the presence of a concentrated H2SO4 catalyst. (j) the relative ease of electrophilic substitution of phenol compared with benzene, in terms of electron pair donation to the π-system from an oxygen p-orbital in phenol (see also 4.1.3 a) Illustrated by reactions with bromine and with nitric acid. Explanation is only in terms of susceptibility of ring to ‘attack’ and not in terms of stability of intermediate. HSW2,5 Use of delocalised benzene model to explain reactivity. (k) the 2- and 4-directing effect of electrondonating groups (OH, NH2) and the 3-directing effect of electron-withdrawing groups (NO2) in electrophilic substitution of aromatic compounds Learners will not be expected to know further electron-donating or electron-withdrawing groups; relevant additional data will be supplied in examinations. HSW5 Correlation between substituted group and position of reaction. (l) the prediction of substitution products of aromatic compounds by directing effects and the importance to organic synthesis (see also 6.2.5 Organic Synthesis).
Credits: Neil Goalby