Chemistry Benzene

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Hassan Subhie GCE A-level Chemistry Unit 5 1 Benzene the basis of aromatic compounds: o The study of organic chemistry is divided into 2 main sections: 1. Aliphatic compounds: compounds based on carbon chains. 2. Aromatic compounds: compounds based on benzene ring structure. Structure of benzene: o The molecular formula of benzene is C6H6. o The molecule of benzene has a regular hexagonal shape and is planar. o The benzene ring doesnt consist of alternate double and single bonds. o All the bond lengths are the same are between single C-C and and double C=C. o Each C-C bond in benzene is therefore neither a single nor a double bond but and intermediate type lying between the 2 forms. o Each C-atom in benzene forms 2 bonds with 2 carbon atoms and 1 bond with hydrogen atom and will still have one p orbital with single electron. o These p orbitals overlap below and above the plane of the molecule forming a continuous or deloecallised system. o This gives stability to the benzene structure which is why it undergoes substitution reaction rather than addition reactions. o Benzene can be represented either by: Thermo chemical evidence for the structure of benzene: o The enthalpy change of hydrogenation of cyclohexene is u = AH KJmol 120 H2+ o The enthalpy of hydrogenation of benzene if it has 3 C=C double bonds is = KJmol 360 ) 120 ( 3 Organic Chemistry III Hassan Subhie GCE A-level Chemistry Unit 5 2 3 H2+ u = AH KJmol 120 o The actual uAH of hydrogenation of benzene is u = AH KJmol 208 . Benzene cant have cyclohexatriene structure. It has a more stable structure which is the delocalized structure. o The extra stability of benzene gained by delocalization is equal to 360-208 =152 KJmol- which is called delocalization enthalpy or stabilization enthalpy. 152 KJ -360 KJ -208 KJ Cyclohexane Naming compounds of benzene: NO2 Cl CH3 Benzene Nitro Benzene Chloro benzene methyl benzene (Toluene) Hassan Subhie GCE A-level Chemistry Unit 5 3 C2H5 NH2 OH CH2Cl Ethyl benzene phenyl amine Phenol Chloro methylbenzene SO3H CO2H CHO NO2NO2 Benzene sulphonic acid Benzoic acid Benzyldehyde 1,4 dinitrobenzene Benzene carboxylic acid NO2NO2 NO2NO2 CH2CO2H 1,2 dinitrobenzene 1,3 dinitrobenzene Phenyl ethanoic acid CO2HC2H5 NH2CHO BrBrBr 2-ethyl benzoic acid 2-amino benzyldehyde 1,3,5 tri bromo benzene Hassan Subhie GCE A-level Chemistry Unit 5 4 COCH3 OCOCH3 BrBrBrOH phenyl ethanone Phenyl ethanoate 2,4,6 tri bromo phenol Physical properties of benzene: o Colorless liquid with boiling point 80 0C and melting point 5.5 0C o Immiscible with water. o It burns with a luminous smoky flame. o It is toxic and its inhalation over a period of time leads to anaemia and leukemia. Types of reactions of benzene: o Benzene makes electrophilic substitution reaction rather than addition reaction although the benzene ring is unsaturated. Reason: Benzene ring is thermodynamically very stable due to the delocalization of the pi molecular orbitals. Addition reactions to benzene would result in disruption of this delocalization and so reduces the stability of the benzene ring. Substitution reactions can occur with only temporary disruption and so the stability of the benzene ring is maintained. Reactions of Benzene: Benzene reacts mainly by electrophilic substitution reactions. 1. Nitration: Benzene reacts with a mixture of nitric acid and sulphuric acid to form nitro benzene. Reaction: O H NO H C HNO H C SO H2 2 5 6 3 6 64 2+ + HNO3H2SO4NO2H 2ONitro benzene++ Hassan Subhie GCE A-level Chemistry Unit 5 5 Reagent: concentrated HNO3 Condition: mix with concentrated H2SO4 at 50 0C 2. Bromination: Benzene reacts with liquid Bromine in the presence of a catalyst of anhydrous iron(III) bromide. Reaction: HBr Br H C Br H C l + +5 6 ) ( 2 6 6 Br2BrH BrBromo benzene++ Reagent: Br2 (l) Condition: Liquid Br2 with iron catalyst. 3. Friedel-Craft alkylation: Benzene will react with halogeno alkanes in the presence of a catalyst of anhydrous aluminium chloride. Reaction: HCl R H C RCl H C + +5 6 6 6 Reagent: Halogeno alkane RCl Condition: Anhydrous AlCl3 catalyst. E.g. HCl H C H C Cl H C H C + +5 2 5 6 5 2 6 6 C2H5ClC2H5H ClEthyl benzene++ 4. Friedel-Craft acylation: Benzene will react with acid chlorides in the presence of a catalyst of anhydrous aluminium chloride. Reaction: HCl COR H C RCOCl H C + +5 6 6 6 Reagent: Acyl chloride RCOCl Condition: Anhydrous AlCl3 catalyst. Hassan Subhie GCE A-level Chemistry Unit 5 6 CH ClRC//O__Cl//O__R++ E.g. HCl COCH H C COCl CH H C + +3 5 6 3 6 6 Phenyl Ethan one CH Cl//O__Cl//O__CH3CH3C++ Reactions of compounds with a carbon containing side chain: o Compounds such as methylbenzene C6H5CH3 and ethyl benzene C6H5C2H5 can be oxidized, and the product will contain a COO- group on the benzene ring, regardless of the number of carbon atoms in the chain. On acidification benzoic acid is produced. Reaction: 2 2 5 6 5 63 ] [ 6 CO O H COO H C OH O R H C + + + + R6 [O]OH__COO3 H2OCO2+ + + + Reagent: Potassium manganate (VII) + NaOH Condition: heat under reflux E.g. 2 2 5 6 5 2 5 63 ] [ 6 CO O H COO H C OH O H C H C + + + + Hassan Subhie GCE A-level Chemistry Unit 5 7 Phenols: o These contain the OH group connected directly to the benzene ring. OH Phenol Naming derivatives of Phenols: OH||Cl OH|CH3 OH||NO2 4-chloro phenol 2-methyl phenol 4-nitro phenol BrBrBrOH 2,4,6 tri bromo phenol Physical properties of phenols: o Colorless solid o Its vapor is toxic o Burns the skin o Although it forms some hydrogen bonds with water, its only partially soluble and forms two layers when added to water at room temperature. o It is a weaker acid than carbonic acid, and so it will not liberate CO2 from sodium hydrogen carbonate, unlike carboxylic acid which will. Hassan Subhie GCE A-level Chemistry Unit 5 8 o Acidity of Phenol Phenol is weakly acidic dissociating in water C6H5OH C6H5O- + H+ Ka=1.3 x 10-10 mol dm-3 Phenol is too weak an acid to affect litmus paper or liberate carbon dioxide from carbonates and hydrogencarbonates, but it dissolves readily in alkalis. o It is soluble in alkali as NaOH, as it is a weak acid and reacts with NaOH to form colorless solution. ) ( 2 ) ( 5 6) () ( 5 6 l aqaql O H O H C OH OH H C + + H 2OOHOH__O+ + Phenate ion Reactions of Phenol: o The presence of the OH group makes substitution into the benzene ring easier. No catalyst is required. 1. With Bromine water: Reaction: HBr Br H OHC Br OH H C aq3 33 2 6 ) ( 2 5 6 + + OH||+3 Br2 +|OHBrBrBr3 HBr 2,4,6- tribromo phenol Reagent: Bromine. Condition: aqueous Observation: When red-brown bromine in H2O is added, a white ppt. rapidly forms and bromine decolorizes . Hassan Subhie GCE A-level Chemistry Unit 5 9 2. With acid chlorides: Phenol reacts as an alcohol, and an ester is formed. Reaction: HCl H RCOOC RCOCl OH H C + +5 6 5 6 OHRCOCl|OOCRH Cl++ Reagent: Acyl chloride. E.g. HCl H COOC CH COCl CH OH H C + +5 6 3 3 5 6 Phenyl ethanoate OH|OOCCH3H ClCH3COCl ++ Phenyl amine :( Aniline) o Phenyl amine has a NH2 group attached to the benzene ring. |NH2 Preparation:

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