Organic Chemistry II Aromatic Hydrocarbons PDF

Chapter 1 Lect. 1 Organic Chemistry II Lec. 1 By: Dr. Samir Mohamed Awad Prof. Of Pharm. Org. Chem. 1 Sources and Names of Aromatic Compounds Sources: 1. Coal 2.Petroleum: during refining when alkenes are passed over a catalyst at about 500OC...

Chapter 1 Lect. 1 Organic Chemistry II Lec. 1 By: Dr. Samir Mohamed Awad Prof. Of Pharm. Org. Chem. 1 Sources and Names of Aromatic Compounds Sources: 1. Coal 2.Petroleum: during refining when alkenes are passed over a catalyst at about 500OC under high pressure. Nomenclature: 1. Common name: A) Monocyclic aromatic compounds: CH3 CH3 CH3 Benzene Benzene Toluene Xylene CHO OH COOH NH2 Benzaldehyde Phenol Benzoic acid Aniline O CH = CH2 CH3 Acetophenone Styrene 2 B) Polycyclic Aromatic Compounds: Phenanceren Isomerism of Substituted Benzene: 1. Monosubstituted Benzenes: -Benzene is named as a parent, thus: Br NO2 CH2CH2CH3 Bromobenzene Nitrobenzene n-propylbenzene Radicals of Benzene: CH2 - Phenyl group Benzyl group 2. Disubstituted benzenes: 3  ortho (o),meta (m), or para(p) are used as prefixes. X 1 1,2- 2 ortho X 1 2 1,3 meta 3 X para 1 1,4 2 3 4 Examples: Cl CHO H 3C CH3 Cl ortho-Dichlorobenzene meta-Dimethylbenzene Cl 1,2-disubstituted m-xylene para-Chlorobenzaldehyde 1,3-disubstituted 1,4-disubstituted 2. Trisubstituted benzenes: -They are named by choosing a point of attachment as carbon then the second and third substituens which are listed alphabetically. 3 Br 4 2 CH3 OH 1 CH3 6 CH3 1 CH3 2 O 2N 6 NO2 1 2 H 3C 3 4 5 3 4 4-Bromo-1,2-dimethyl benzene NO2 2,4,6-trinitrotoluene 2,5-Dimethylphenol 2. IUPAC-name: (TNT) The series is numbered to give the substituent the least possible number. 4  1 CH3 3 4 5 6 7 CH - CH2 - CH2 - CH2 - CH2 - CH3 2 2-phenylheptane Aromaticity & Huckel 's rule: Requirements of aromaticity: Requirements of aromaticity: 1) Monocyclic structure. 2) All carbons are sp2 hybridized.( fully conjugated) 3) Planner ( the ring & hydrogens lie in the same plane ) 4) The number of π electrons should obey Huckel s' rule. Huckel s' rule: For a monocyclic planar aromatic molecule, it should have (4n +2) π electrons, where n is an integer number = 0, 1 ,2,3,…. i.e Huckel s' number = ( 2,6,10,14,18……..etc ) 5) The molecule should obey Molecular Orbital Theory (MOT). - All π bonding molecular orbitals (B.M.Os) are completely filled. i.e all π electrons are paired & this allow maximum overlapping of orbitals. e.g Benzene 6 mol.orb. 3 B.M.Os 3 A.B.M.Os 6) The molecule should obey Resonance Theory: * Benzene is a regular hexagon, bond angle 1200 5 *All 6 hydrogens lie in the same plane of the ring & each C – C – H valence angle is 1200. * The C – C bond length is the same = 1.39 A0. 7) 1HNMR spectrum: Protons attached to the outside of an aromatic ring are highly deshielded and appear at low magnetic field i.e at high δ- value, so aromatic protons usually appear at δ 7 – 9 ppm. 8) The main reaction is electrophilic substitution reaction, not addition. ** To sum up: To determine if a compound is aromatic check the following practical &theoretical aspects. Practical: 1- No addition reactions. 2- 1HNMR : 7 – 9 ppm. Theoretical: 1 – Resonance 2 – Huckel s' rule 3 – M.O.T Anti aromaticity : -Compounds contain 4π electrons e.g Cyclobutadiene *1,3 – cyclobutadiene is planar,cyclic, conjugated, contain 4π electrons In- which n = 1 , so it is antiaromatic. * All p – orbitals are not completely filled. * Not stable, its aliphatic acyclic analogue (1,3-butadiene ) is more stable than it * It can not be separated except under very drastic conditions ( very low temperature, 6  bellow 35 K). Q: Mark the following sentences True or False: Cyclobutadiene is more reactive than 1,3- butadiene. * Non benzenoid aromatic compounds. 1 – Cyclopropenyl cation: + C 3H 3 H N.M.R: 9.2 ppm H highly deshielded bec. 2 electrons are distributed on 3 carbons cyclopropene It is not aromatic bec. it contains one sp3 carbon -. + -H -H -H H H H - +. - cyclopropenyl anion - cyclopropenyl cation -cyclopropenyl free radical - it contains 4 electrons. - it contains 2 pi- electrons. - it contains 3 pi - electrons. ( not Huckel number ) ( Huckel number ) ( not Huckel number) Antiaromatic Aromatic Non aromatic all pi- electrons are paired + max. & complete overlapping resonance stabilized + + 2- Cyclopentadienyl anion: _ 7  cyclopentadiene not aromatic bec. it contains one sp3 carbon H H _ H + _ H- - H + H - cyclopentadienyl anion -cyclopentadienyl cation - it contains 6 pi - electrons ( Huckel number ) - it contains 4 pi -electrons ( not Huckel number ) - Pi- electrons are not paired. not max. & complete overlapping. Anti- aromatic Resonance stabilization of cyclopentadienyl anion. + - -H - H H _ - - Anion 3 – Cycloheptatrienyl ( Tropylium ) cation: 8  NMR: 9.2 ppm + H SP3 H cycloheptatriene not fully delocalized it is not aromatic bec. it contains one sp3 carbon + - -H -H - + cycloheptatrienyl anion Tropylium cation vaccant p- orbital - it contains 8 pi-electrons - it contains 6 pi -electrons ( not Huckel number ) ( Huckel number ) - stabilized by resonance - all pi - electrons are completely filled,. NMR:: 9.2 PPM (highly deshielded) P- orbitals are not completely filled,pi- electrons arenot completely Aromatic paired. not max. overlapping. Antiaromatic. + + + + + + ve charge is distributed all over carbon atoms causing deshielding of protons. * Cyclooctatetraene (COT): 9 Q: Explain the following in terms of Huckel s' rule, M.O.T, resonance & 1HNMR data. - Cyclooctatetraene undergoes addition reactions with HX &halogens. -Cyclooctatetraene is not aromatic ( no electrophilic substitution reactions), Thus reacts as polyenes by addition due to 1) The molecule is not planar (tub-shabed) 2) Angle strain ( 1350) which causes buckering or deformation of the molecule(loss of planarity). 3) It does not obey Huckel s' rule, since it contains 8 π electrons. 4) All π-orbitals are not completely filled i.e it does not obey M.O.T. - COT readily reacts with potassium toform the Salt K2COT which contains the dianion C8H82-, the dianion is planar in shape & aromatic with a Huckel electron count of 10. 2K __ + 2 k + H2 Q: Choose the most correct answer (2010 exam) Cyclooctatetraene is not aromatic because: a) It undergoes ESAr b) All carbons are sp2 hybridized. c) It has a tub-like shape & lacks planarity. Q: Which of the following features is not characteristic of aromatic compounds: (2011 exam) a) They are cyclic b) They have conjugated pi electrons c) They contain an odd number of pi electrons. Aromatic Heterocycles 10  Heterocycles: cyclic compounds with one or more heteroatom e.g. N,O,S,……etc. *Heterocycles may be aromatic, anti-aromatic, or non aromatic. Aromatic Heterocycles: NH2 NH2.. N N.. N N.. N N O N.. H OH.. Pyridine Pyrimidine Cytosine O OH O OH H 3C H 3C NH N NH N N O N OH N O N OH H H Uracil Thymine All these compounds contain 6-Pi electrons which is a Huckel number Preparation of Benzene: 1. By heating Carboxylic acid salt with soda lime ( CaO +NaOH ) O C - ONa heat + NaOH + Na2CO3 - CO2 Sodium benzoate 2. From benzene derivatives: 11  a) From phenol:(dehydroxylation, removal of OH) OH Zn -ZnO Phenol Q. Complete: OH CH3 Zn ? toluene CH3 p-Cresol b) From Chlorobenzene: ( via reduction ) Cl Ni-Al alloy + 2H + HCl NaOH c) Hydrolysis of Grignard Reagent of Chlorobenzene: OH Cl Mg - Cl dry ether H2O Cl + Mg + Mg H Chlorobenzene OH Phenyl magnesium chloride Complete: Mg H2O C6H5 - Cl ? ? ether 12 d) By desulphonation of benzene sulphonic acid: SO3H O 150-200 C + HOH + H2SO4 steam HCl-Pressure Benzene sulphonic acid e) From benzene diazonium chloride: + - H N N Cl SnCl2 + N2 + HCl + 2H H NaOH Benzene diazonium chloride f) From acetylene: Reaction of Benzene: Electrophilic Substitution of benzene: H E + + E 13 e.g Bromination: It should be carried out in presence of lewis acid such as FeBr3 Br FeBr3 + Br2 bromobenzene Mechanism: 1) Generation of Bromonium ion (electrophile). FeBr3 + - Br - Br : + Br + FeBr4 bromonium cation E + 2) π-electrons of benzene ring attack Br+ to form C – Br bond and leaving a non aromatic carbocation intermediate (ϭ-complex). + + Br + Br H non aromatic carbocation intermediate 3) A base (FeBr4) removes H+ from the carbocation intermediate to retain the aromaticity of the ring. 14  Br + Br H FeBr3 + HBr + FeBr3 + NO2 H2SO4| HNO3 Benzene Nitrobenzene Mechanism: + - protonation + - HO NO2 + H HSO4 H-O NO2 + HSO4 H H2SO4 + (dehydration) + - NO2 + H3O + HSO4 H NO2 NO2 + + + -H + NO2 - HSO4 Nitrobenzene What is the role of conc.H2SO4 ? 1. Protonation of nitric acid to increase the amount of nitronium ion. 2. Dehydrating agent. 3. Gives HSO4 which acts as a base to remove a proton from the intermediate carbocation to form nitrobenzene. 15 Q. Convert: Benzene into Nitrobenzene. Q. Complete: Benzene could be nitrated using ………….as a nitrating mixture. The answer: Conc. H2SO4| Conc HNO3 mixture. Mechanism: - O OH - + _ + + HSO4 S + H - HSO4 S O O O O + + SO3H SO3H - SO3H H HSO4 (base) Benzene sulphonic acid Notes: 1. Sulphonation is reversible process. sulphonation SO3H Conc H2SO4 dil acid desulphonation 16 2. Conc. H2SO4 is a self catalyst. Friedel-Crafts reaction: H 3C CH3 CH Cl AlCl3 + CH3 - CH - CH3 Isopropyl chloride Isopropyl benzene (Cumene) Mechanism:.. + isopropyl carbocation Cl + - CH3 - CH - CH3 + AlCl3 CH3 - CH - CH3 + AlCl4 Isopropyl chloride CH3 H 3C CH3 CH + CH CH3 H AlCl3 + HCl + - AlCl4 ( Cumene ) Q. Convert: benzene into toluene 17  CH3 CH3Cl | AlCl3 toluene Some drawbacks of F.C alkylation: *Un reactive benzene rings are not react by F.C-alkylation, rings with electron withdrawing groups such as : -NH3, -NO2, -CN, -SO3H, -CHO, - COOCH3, -COOH, -NH2, -NHR, -NR2 do not undergo F.C-alkylation, Because carbocation is a weak electrophile, & NO2 or +NR3 will deactivate benzene ring towards E+ ,therefore nitrobenzene can be used as a solvent in F.C reaction. thus, CN NO2 SO3H COOH CH3Cl | AlCl3 NH2 NHCH3 CHO COOCH3 No reaction H 3C + CH3 N(CH3)3 N COCH3 18  Ex. O COCH3 AlCl3 + CH3 - C - Cl acetophenone Mechanism: O.. + - CH3 - C - Cl + AlCl CH3 - C =O CH3 - C O + AlCl4 3 + O - O + AlCl4 CH3 C - CH3 + CH3 - C =O H + + HCl + AlCl3 * Effect of substituents on reactivity of benzene ring: * Activators olp directors: gps which electron density around benzene ring which will the reactivity of benzene ring, this occurs when the substituent is electron releasing gp. Electron releasing gps may be: 1) Strong activators by + M effect: NH2, OH, NHR,NR2 2) Moderate activators by + M effect: NHCOCH3, OCOCH3, OCH3 3) Weak activators by + I effect : -CH3, -CH2CH3 * Deactivators m- directors: 19 gps which electron density around benzene ring which will the reactivity of benzene ring, this occurs when the substituent is electron withdrawing gp. Electron withdrawing gps may be: 1) Strong deactivators by –I & -M effect: NO2, SO3H, COOH,CN 2) Moderate to weak deactivators: CF3, + NH3 * Deactivators olp directors: Cl and Br are exception deactivators olp director not m- director ( -I >>> +M effect ) Q: Arrange the following compounds in decreasing order towards ES Ar, Give mononitrated product. Nitrobenzene, trifluoromethylbenzene, aniline, N- acetylaniline &toluene. NHCOCH3 CF3 NO2 NH2 > > > Q: Complete the following equations: 1) NHCOCH3 NHCOCH3 NHCOCH3 HNO3 / H2SO4 NO2 ? + ? + Acetanilide NO2 Remember: NHCOCH3 is an activator, o / p director 2) NO2 NO2 HNO3 / H2SO4 m-Dinitrobenzene ? NO2 20 3) COOH COOH HNO3 / H2SO4 ? m-nitrobenzoic acid NO2 Benzoic acid NO2 NO2 4) m-Dinitrobenzene HNO3 / H2SO4 ? NO2 5) CH3 - CO - Cl / AlCl3 Conc. H2SO4 ? ? COCH3 COCH3 F.C acylation SO3H m-director 21 Chapter 2 Organic Chemistry II 22  Alkyl benzene ☺They are hydrocarbons consisting of aliphatic & aromatic groups. Structure & Nomenclature: CH3 CH2CH3 CH3 CH3 Toluene Ethylbenzene Xylenes ( o,m & p ) CH3 CH = CH2 CH2 - CH = CH2 H3C CH3 Styrene Allylbenzene Mysitylene Reactions: alkyl side chain reactions CH2 - R benzylic carbon SE reactions 1] Alkyl side chain reactions: 1. Oxidation Reactions: R COOH [O] + Oxidizing agents: KMnO /OH or HNO or K Cr O / H 4 3 2 2 7 Ex: 23  CH3 [O] COOH CH3 COOH o- xylene phthalic acid 2. Halogenation: Free radical halogenation is selective for benzylic carbons ,where benzylic radical is stabilized by resonance, it occurs in presence of light. CH3 CH2 - Cl CH Cl2 Cl2 Cl2 light light benzyl chloride light Cl2 CCl3 2] Reactions on the aromatic ring: ** Alkyl group is o/p activating group ( + I effect ). 1- Nitration: CH3 CH3 HNO3 / H2SO4 O 2N NO2 2,4,6- trinitrotoluene ( T.N.T ) NO2 TNT is one of the most commonly used explosives. 2- Halogenation: It occurs either in ring by ESAr or in benzylic position by free radical mechanism ( Br2 / peroxide or Br2 / hυ ). 24  Br CH - CH3 Br2 ROOR CH2CH3 or Br2 / light 1- bromo-1- phenyl ethane Br2 / FeBr3 CH2CH3 CH2CH3 ethyl benzene Br + o- bromoethyl benzene Br p- bromoethyl benzene 3- Sulphonation: It is a reversible process. CH3 CH3 CH3 H2SO4 SO3H p- toluene sulphonic acid o- toluene sulphonic SO3H acid Cl Halogenated Aromatic Hydrocarbons Ex. ** Chlorobenzene Preparation of D.D.T 25 Dichlorodiphenyltrichloroethane is an organochlorine insecticide, it was originally prepared in 1873, then in 1939 Paul Muller discovered its effectiveness as an insecticide ( Nobel Prize in Medicine and Physiology 1948 ). Cl Conc H2SO4 CCl3 - C - H + H Cl CCl3 - CH -H2O O H Cl Cl Chloral Chlorobenzene N.B para position of chlorobenzene should be free 26