Benzene & Aromaticity Course Teacher Ashis Kumar Podder

Aromatic Compounds Aromatic term was first used to described some fragrant compounds in early 19 th century Not correct: later they are grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition) Current: distinguished from aliphatic compounds by electronic configuration

Discovery of Benzene Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C 6 H 6. Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic.

Straight Chain Structure Elemental analysis and molecular weight determination showed that benzene had the molecular formula C 6 H 6. So, it should be an unsaturated compound. BUT………..

Moreover, Benzene does not behave like Alkenes or Alkynes. Alkene + KMnO 4  diol (addition) Benzene + KMnO 4  no reaction. Alkene + Br 2 /CCl 4  dibromide (addition) Benzene + Br 2 /CCl 4  no reaction. With FeBr 3 catalyst, Br 2 reacts with benzene to form bromobenzene + HBr (substitution!). Double bonds remain. Benzene + H 2 O/H +  no reaction. Straight Chain Structure ….contd.

Evidence of Cyclic Structure Substitution of Benzene with Bromine Addition of Hydrogen

Kekulé Structure Proposed in 1865 by Friedrich Kekulé, shortly after multiple bonds were suggested. “Benzene consist of a cyclic planar structure of six carbons with alternative double and single bonds. To each carbon, one hydrogen atom is attached with.” Failed to explain existence of only one isomer of 1,2-dichlorobenzene.

Kekulé Structure ….contd. But there were 2 limitations. 1) There should exist two ortho isomers of Dibromobenzene. To overcome that, he further suggested that Benzene was a mixture of two forms in rapid equilibrium. 2) Although it contains three double bonds like alkenes but does not give addition reactions

Resonance Structure (Explanation from book) Carbon – Carbon bonds in benzene are neither single bonds nor double bonds, rather they are something halfway between them. All the six C-C bonds are identical in benzene. So, first drawback of Kekule’s structure was solved. Resonance hybrid is more stable than any of it’s contributing structures. In case of benzene, the stability due to resonance is so high that π-bonds of the molecule will normally resist breaking HBr/Br 2 in CCl 4. Thus, the second drawback of Kekule’s structure was solved. What is Resonance? The phenomenon in which two or more structures can be written for a substance which involves identical positions of atoms is called Resonance.

Molecular Orbital Structure (Explanation from book) All the six carbon atoms in benzene are sp 2 hybridized. The remaining one unbridized p-orbital of each six carbons contains 1 electron. These p-orbitals are perpendicular to the plane of σ-bonds. The lateral overlap of these p-orbitals produce a π- molecular orbital containing 6 electrons. These 6 electrons are said to be delocalized to cover all the 6 carbons. One half of this π-molecular orbital lies below the plane and the other half lies above the plane of σ- bonds.

Molecular Orbital Structure …contd. (Explanation from book) As all the six carbon atoms of benzene are sp 2 hybridized, all of them are identical. That’s why we get only one ortho isomer of dibromobenzene. So, first drawback of Kekule’s structure was solved. Due to delocalization a stronger π-bond is formed in case of benzene than alkenes/alkynes which results in a more stable compound. As a result it gives substitution reactions rather than addition reactions to maintain the π-molecular orbital. So, second drawback of Kekule’s structure was solved.

Sources and Names of Aromatic Hydrocarbons From high temperature distillation of coal tar Heating petroleum at high temperature and pressure over a catalyst

Naming Aromatic Compounds Many common names (toluene = methylbenzene; aniline = aminobenzene) Monosubstituted benzenes systematic names as hydrocarbons with –benzene C 6 H 5 Br = bromobenzene C 6 H 5 NO 2 = nitrobenzene, and C 6 H 5 CH 2 CH 2 CH 3 is propylbenzene

The Phenyl Group When a benzene ring is a substituent, the term phenyl is used (for C 6 H 5  ) You may also see “Ph” or “  ” in place of “C 6 H 5 ” “Benzyl” refers to “C 6 H 5 CH 2 ” What is the difference between Aryl group and Aralkyl group?

Disubstituted Benzenes Relative positions on a benzene ring ortho- (o) on adjacent carbons (1,2) meta- (m) separated by one carbon (1,3) para- (p) separated by two carbons (1,4) Describes reaction patterns (“occurs at the para position”)

Naming Benzenes With More Than Two Substituents Choose numbers to get lowest possible values List substituents alphabetically with hyphenated numbers Common names, such as “toluene” can serve as root name (as in TNT)

Structure and Stability of Benzene: Molecular Orbital Theory Benzene reacts slowly with Br 2 to give bromobenzene (where Br replaces H) This is substitution rather than the rapid addition reaction common to compounds with C=C, suggesting that in benzene there is a higher barrier

Heats of Hydrogenation as Indicators of Stability The addition of H 2 to C=C normally gives off about 118 kJ/mol – 3 double bonds would give off 356kJ/mol Two conjugated double bonds in cyclohexadiene add 2 H 2 to give off 230 kJ/mol Benzene has 3 unsaturation sites but gives off only 206 kJ/mol on reacting with 3 H 2 molecules Therefore it has about 150 kJ more “stability” than an isolated set of three double bonds

Benzene’s Unusual Structure All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bonds Electron density in all six C-C bonds is identical Structure is planar, hexagonal C–C–C bond angles 120° Each C is sp 2 and has a p orbital perpendicular to the plane of the six-membered ring

Aromaticity and the Hückel 4n+2 Rule Planar hexagon: bond angles are 120°, carbon–carbon bond lengths 139 pm Undergoes substitution rather than electrophilic addition Resonance hybrid with structure between two line-bond structures Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+ 2  electrons (n is 0,1,2,3,4) For n=1: 4n+2 = 6; benzene is stable and the electrons are delocalized

Q. What is aromaticity? Q. What are the criteria of aromaticity?

Compounds With 4n  Electrons Are Not Aromatic (May be Antiaromatic) Planar, cyclic molecules with 4 n  electrons are much less stable than expected (antiaromatic) They will distort out of plane and behave like ordinary alkenes 4- and 8-electron compounds are not delocalized (single and double bonds) Cyclobutadiene is so unstable that it dimerizes by a self-Diels- Alder reaction at low temperature. Cyclooctatetraene has four double bonds, reacting with Br 2, KMnO 4, and HCl as if it were four alkenes

Aromatic Heterocycles: Pyridine and Pyrrole Heterocyclic compounds contain elements other than carbon in a ring, such as N,S,O,P Aromatic compounds can have elements other than carbon in the ring There are many heterocyclic aromatic compounds and many are very common Cyclic compounds that contain only carbon are called carbocycles (not homocycles) Nomenclature is specialized

Pyridine A six-membered heterocycle with a nitrogen atom in its ring  electron structure resembles benzene (6 electrons) The nitrogen lone pair electrons are not part of the aromatic system (perpendicular orbital) Pyridine is a relatively weak base compared to normal amines but protonation does not affect aromaticity

Pyrrole A five-membered heterocycle with one nitrogen Four sp 2 -hybridized carbons with 4 p orbitals perpendicular to the ring and 4 p electrons Nitrogen atom is sp 2 -hybridized, and lone pair of electrons occupies a p orbital (6  electrons) Since lone pair electrons are in the aromatic ring, protonation destroys aromaticity, making pyrrole a very weak base

Methods of Preparation of Benzene (small scale) 1. By passing acetylene through red hot tube at 500°C 2. By heating the sodium salt of benzoic acid with sodalime 3. By heating phenol with zinc dust Substrate: Phenol Reactant: Zn Product: Benzene

Methods of Preparation of Benzene (Large scale) 1. From Petroleum (by aromatization) Substrate: C 6 – C 8 fraction of petroleum naphtha Reactant: Pt/Al 2 O 3 Temperature: 500°C Product: mixture of Benzene, Toluene & Xylene Isolation: By Fractional distillation at 81°C. 2. From light oil of coal tar: A distillation process along with treatment with reagents to remove impurities.

Characteristic of Benzene Electrophilic substitution reaction: Mechanism: 1. Formation of Electrophile 2. Electrophile attacks benzene ring 3. Loss of proton Example: 1. Halogenation 2. Nitration 3. Sulphonation 4. Friedel Crafts Alkylation/Acylation

Addition reactions of Benzene 1. Hydrogenation Substrate: Benzene Reactant: H 2 Temperature: 150°C Catalyst: Ni Product: Cyclohexane 2. Chlorination: Substrate: Benzene Reactant: Cl 2 Condition: UV light Product: BHC / Gammexane / Lindane (insecticide)

Ozonolysis reaction of Benzene Substrate: Benzene Reactant: O 3 and later by Zn/H 2 O Product: Benzene triozonide → Glyoxal

Toluene 1. From Benzene by Friedel Crafts alkylation 2. By aromatization of n-heptane (Platforming reaction) 3. By Wurtz-Fittig reaction: Substrate: Halobenzene and Alkyl halide Reactant: Na Condition: In dry ether Product: Toluene Problem: Alkane & Biphenyl compounds are formed by side reactions. So, Toluene needs to be separated by fractional distillation at 110°C.

Electrophilic Substitution Reaction of Toluene We will get ortho and para substituted products simultaneously from the reactions. Example: 1. Halogenation (Cl 2 in presence of FeCl 3 ) 2. Nitration (HNO 3 in presence of H 2 SO 4 ) 3. Sulphonation (fuming H 2 SO 4 ) 4. Friedel Crafts Alkylation/Acylation (CH 3 Cl / CH 3 COCl in presence of AlCl 3 )

Addition Reaction of Toluene 1. Hydrogenation Substrate: Toluene Reactant: H 2 Temperature: 150°C Catalyst: Ni Product: Methyl cyclohexane

Side Chain Reaction of Toluene 1. Side chain halogenation: [follows free radical mechanism] Substrate: Toluene (boiling) Reactant: Cl 2 Condition: UV light Product: Benzyl chloride, Benzal chloride and Benzo trichloride successively. Q. What is the significance of synthesizing these products? 2. Side chain oxidation: Substrate: Toluene Condition: Acidic Reactant: KMnO 4 / MnO 2 Product: Benzoic acid / Benzaldehyde Stronger Mild

Directive effects of Substituents on further substitution  What is Directive effect?  What is Activity effect?  What are ortho-para directors? Examples.  What are meta directors? Examples.  Theory of Directive effects  Theory of Activity effects  Limitations of the theory of activity effects

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