Polynuclear Aromatic Hydrocarbons Ref. books Organic Chemistry, Vol.1 - I.L. Finar Organic Chemistry - Morrison and Boyd Advanced Organic Chemistry – Bahl and Bahl Organic Chemistry - Herbert Meislich

Polynuclear Hydrocarbons Polynuclear aromatic hydrocarbons are composed by two or more benzene rings Polynuclear Hydrocarbons Benzenoid Non- Benzenoid Isolated Fused rings Linear Angular

Benzenoid: Similar to benzene in structure or linkage; having an aromatic ring system. Fused or condensed ring system: When two rings share a pair of carbon atoms, the rings are said to be fused rings. Isolated ring o m o m 2 3 3 2 1 1 p p 4 4 5 6 5 6 m o o m Biphenyl or diphenyl

Naphthalene (C10H8) Shows aromatic properties Satisfy Huckel’s rule (4n+2) =(4*2+2)=10

All C=C are not same (X-ray diffraction study) Resonance energy of naphthalene is 61 Kcal/mol Benzene, 36 Kcal/mol 2nd aromatic ring is less stable (61-36)=25 Kcal/mol Naphthalene is less aromatic (more reactive) than benzene

Structure elucidation of naphthalene 1. Molecular Formula: C10H8 2. So naphthalene contains the skeleton

So nitro group is present in benzene ring 3. So nitro group is present in benzene ring 4. The benzene ring in phthalic acid produced by oxidation of aminonaphthalene is not the same ring is that obtained by oxidation of nitronaphthalene.

i.e. Naphthalene contains two benzene rings and we can explain this by this equation

The structure of naphthalene is confirmed by method of its synthesis Howarth method

Other way of cyclization

The reaction occurs if R is o- or p- directing group such as NH2, NHR, OH, OR, R, halogen. If R is m- directing group (e.g. NO2, CN, COOH, COCH3, SO3H) no reaction occur. The above reaction gives -substituted naphthalene.

Synthesis of 1-alkyl naphthalene

From -benzylidene – propenoic acid

Reduction

Oxidation

Addition of Cl2

Electrophilic substitution reaction Naphthalene undergoes ES mostly at alpha-position Resonance forms determine higher reactivity at C-1 C-1 attack has 2 resonance structures with benzene rings C-2 attack has only 1 resonance structure with a benzene ring The most stable intermediate (C-1 attack) gives faster reaction Attack at C-1 Attack at C-2

At position 1; carbocation intermediate stabilize by two resonance So carbocation is more stable position 1 than 2

Sulfonation The lower stability of 1-S is attributed to the steric interaction between the sulfonic group and the hydrogen atom in the 8-position.

Substituted naphthalene Activating groups direct the electrophile to the same ring; i.e. Elctrodonating group (EDG): NH2, OH, OR, alkyl Deactivating groups direct it to the other ring; i.e. Electrowithdrawing group (EWG): NO2, CO, COOH, CN, SO3H

Homonuclear attack Heteronuclear attack

Examples

Examples

Summary of naphthalene reactions

Anthracene (C14H10)  8 9 1 7 2 6 3 5 10 4 

Anthracene (C14H10)   monosubstitution (C14H9X) = 3 isomers 8 2 7 3 6 4 10 5  monosubstitution (C14H9X) = 3 isomers Disubstitution (C14H8X2) = 15 isomers

Anthracene (C14H10) C1-C2 bond to have more double bond character (shorter bond length) C2-C3 bond to have more single bond character (longer bond length) From X-ray diffraction study: C1-C2 bond = 1.37 Å C2-C3 bond = 1.42 Å Resonance energy 84 kcal mol-1, average 28, less aromatic than benzene

Synthesis of anthracene (i) By Friedel Crafts reaction (a)

Synthesis of anthracene (b) (c)

Synthesis of anthracene (ii) By Haworth synthesis

Synthesis of anthracene (iii) By Diels-Alder reaction

Chemical reactions Attack at C-1 Attack at C-2 Leaves naphthalene intact Loss of RE=84-61=23 kcal Attack at C-2

Chemical reactions Reactions preferentially occur at C-9 & C-10 Attack at C-9 Leaves two benzene intact Loss of RE=84-72 =12 kcal Substitution product Addition product Reactions preferentially occur at C-9 & C-10

Chemical reactions Diels Alder reaction Addition of one molecule of O2

[HNO3+H2SO4 is not used, leads formation of 9,10 anthraqunone by oxidation]

Phenanthrene C14H10 6 5 7 4 8 3 2 9 1 10

Phenanthrene C14H10 3 4 2 5 1 6 10 7 8 9

Phenanthrene C14H10 monosubstitution (C14H9X) = 5 isomers Disubstitution (C14H8X2) = 25 isomers 3 4 2 5 1 6 9 10 10 7 8 1 8 9 7 2 6 5 4 3 8 9

Position of double bond 3 2 4 1 5 6 10 7 9 8 C9-C10 bond to have more double bond character RE 92 kcal/mole, 92-72=20 Kcal/mole to remove the aromaticity of the middle ring

Preparation of phenanthrene 1) Howrth method

2) Posher synthesis

Preparation of 1- alkyl phenanthrene:

Oxidation: Reduction:

EAS in anthracene or phenanthrene yields mixtures and is not generally useful. For example, in sulfonation:

Diphenyl methane (C13H12) o o m m 2 2 3 3 7 1 1 p p 4 4 6 6 5 5 o o m m Biphenyl methane or diphenyl methane

Methods of preparation 1. Friedel- Crafte 2. From benzophenone

Nitration

Halogenation Oxidation

Stilbene (C6H5-CH=CH-C6H5) Trans-stilbebe stable Cis-stilbebe unstable

Syntheis of trans-stilbene C6H5CHOHCH2C6H5

Syntheis of trans-stilbene (II) C6H5CHOHCOC6H5 (III) -Phenylcinnamic acid

Reactions of trans-stilbene C6H5CH2CH2C6H5 bibenzyl Stilbebe dibromide Dphenyl acetylene

Synthesis of cis-stilbene Cis-stilbebe is readily converted into trans-stilbebe under the catalytic influence of traces of hydrogen bromide and peroxides