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Polynuclear Hydrocarbons: Synthesis and Reactions

The polynuclear aromatic hydrocarbon has fused aromatic rings and is formed from a hydrocarbon molecule.

Polynuclear Aromatic Hydrocarbon

The polynuclear aromatic hydrocarbon has fused aromatic rings and is formed from a hydrocarbon molecule. Each ring shares at least one side and possesses delocalized electrons. Two or more benzene rings are fused together to form polynuclear aromatic hydrocarbon molecules. They only have carbon and hydrogen atoms.

Properties

Lipophilic, nonpolar polynuclear aromatic hydrocarbons belong to the group of aromatic compounds. Neither water nor PAHs are very soluble in them, which means they tend to persist in the environment. As the molecular mass of these compounds increases, their solubility decreases nearly linearly in aqueous solution. Small molecules with two, three, or four rings are sufficiently volatile to exist as gases, whereas larger molecules can exist as solids. PAHs may appear white, yellow, or green, or be colourless. There are four types of pure PAHs: colorless, white, pale yellow, and pale green.

Sources

Natural and environmental pollutant reactions produce organic molecules called PAHs. They are formed in forests and volcanic eruptions naturally. Coal and petroleum are abundant in these compounds. Wood burning and incomplete combustion of fossil fuels are two ways in which humans release PAHs into the environment. Cooking at high temperatures, grilling, or smoking creates these compounds as a natural consequence. Smoke from cigarettes and burning of waste release the compounds.

Health Effects

It is extremely important to prevent the spread of genetic damage and diseases caused by polynuclear aromatic hydrocarbons. In addition, environmental contaminants persist, leading to an increase in problems over time. Aquatic life is highly susceptible to PAHs. These compounds are also often carcinogenic, mutagenic, and teratogenic in addition to their toxicity. Childhood asthma and lowered IQ are associated with prenatal exposure to these chemicals. Inhaling contaminated air, eating contaminated food, or coming into contact with contaminated skin exposes people to PAHs. Except in industrial settings, exposure usually occurs over long periods of time and at low levels, no medical treatments are available to treat the effects of these chemicals. Exposure to PAHs can have detrimental health effects, so becoming aware of situations that increase risk is beneficial. You may breathe smoke, eat charred meat, or touch petroleum products under such circumstances.

Classification



Naphthalene

Two benzene rings (C10H8) are fused together in ortho positions in naphthalene (C10H8). As a result, the positions are shown in figures for the purposes of naming its derivatives.



Synthesis

From 3-Benzoyl Propanoic Acid

Upon heating 3-benzoyl propanoic acid with sulphuric acid, naphthalene will be formed, which is then distilled with zinc dust.




From 4-phenyl-1-butene

The formation of naphthalene occurs when 4-phenyl-1 butene is passed over hot calcium oxide.



Howarth Synthesis

A five-step process is involved.

Step 1
The reaction of succinic anhydride with benzene produces 3-benzoylpropanoic acid.



Step 2
4-phenyl butanoic acid is produced by treating benzyl propanoic acid with amalgamated zinc.



Step 3
Tetralone is formed when 4- phenylbutanoic acid is combined with conc. sulphuric acid (ring closer reaction).



Step 4
HCl and amalgamated zinc are again added to tertralone to obtain teraline.



Step 5
Naphthalene is obtained by heating tetraline with palladium.



Reactions

The reactivity of naphthalene is greater than that of benzene. Oxidation, reduction, addition, nitration, halogenation, acylation, etc. are some of the reactions it undergoes.

Oxidation of naphthalene




Reduction of naphthalene





Addition Reaction

Naphthalene dibromide or dichloride is formed by adding bromine or chlorine to naphthalene. By adding bromine or chlorine to naphthalene tetra bromide or chloride, the compound becomes naphthalene tetra bromide or chloride.



Electrophilic Substitution Reaction

Naphthalene experiences most electrophilic substitution at its C1(α -position), similar to benzene. For the intermediate carbonium ion formed by the attack on atom C-1, we can write the following conformational forms, but only this form is obtained from the substitution at atom C-2. The electrophile E+ in the following equations represents the intermediate.



Thus, the substituted product at C-1 is dominant over the former intermediate. It is possible to substitute at C-2 (ß-position) when using bulkier solvents or higher temperatures.

Nitration Reaction

Due to its stability, the former intermediate leads to a dominant product with a C-1 substituent. Substitution at C-2 (ß-position) can occur at higher temperatures or in bulkier solvents.




Halogenation Reaction

α -Substituted naphthalene comes into existence when naphthalene reacts with iron catalyst.



Sulphonation

If Naphthalene is sulphonated at 80 degrees Celsius, it produces naphthalene-1-sulfonic acid. But if it is sulphonated at 120 degrees Celsius, it produces naphthalene-2-sulfonic acid.




Friedel-crafts Alkylation

Currently 1 methyl naphthalene and 2 methyl naphthalene are produced by Friedrichs craft alkylation under low temperature using naphthalene and iodomethane.




Friedel-crafts Acylation

Friedrichs craft alkylation using naphthalene and iodomethane at low temperatures produces 1 methyl naphthalene and 2 methyl naphthalene currently.




Phenanthrene

The apolycyclic aromatic hydrocarbon phenanthrene contains three fused benzene rings. Known to irritate and photosensitize the skin, it can be found in pure form in cigarette smoke. Phenanthrene is a white powder which emits blue fluorescence. Phenanthrene is the substance that gives morphine its effects.



Resonance Structure of Phenanthrene

Phenanthrene has a flattened structure similar to anthracene. Carbon atoms all have ap orbitals, which are parallel to the bonds plane. In addition to sp2 orbitals, carbon atoms also have an s orbital overlapping ten hydrogen atoms' s orbitals. A molecular orbital of Phenanthrene contains ten electrons due to the lateral overlap of its p orbitals. Because the resulting π molecular orbital conforms to Huckel*s rule (n=3, 4n+2), phenanthrene has aromatic properties.



Preparation of Phenanthrene

Haworth Phenanthrene Synthesis



Reaction of Phenanthrene

Reduction




Oxidation




Oxidized phenanthrene is phenantraquinone, which becomes diphenic acid by oxidation of chromic acid with acetic acid.



Bromination




Substitution by electrophilicity versus addition by electrophilicity






Due to the presence of aromatic sextets preserved in two of three rings, the electrophilic attack on the 9- and 10-positions, whether substitution occurs or addition occurs, would explain the reactivity of these positions to electrophilic attack. Following this carbocation, a choice can be made between
  • The substitution product is produced by giving up a proton.
  • The addition product will accept a nucleophile.
The low resonance energy of anthracene (12 kcal / mol) and phenanthrene (20 kcal / mol or less) is the reason for the tendency for these compounds to undergo addition.

Anthracene

On distillation of tar, it occurs in the high boiling fractions of anthracene oil with percentages ranging between 0.3 and 3.5 percent, hence its Greek name anthracene meaning coal. Three benzene nuclei are fused in an orthogonal position in the anthracene molecule. Thus, anthracene oil occurs during the distillation of coal tar. Polycyclic aromatic hydrocarbons are colorless solids.



Preparation of Anthracene

By Freidel-crafts Reaction

By combining two molecules of benzyl chloride with AlCl3, anthracene is produced.




By Haworth Synthesis

Step 1
AlCl3 stimulates the reaction between benzene and phthalic anhydride, resulting in O-benzoyl benzoic acid.



Step 2
9.10-anthraquinone is produced by heating O-benzoyl benzoic acid with conc.H2SO4.



Step 3
Anthracene will be produced when 9,10-anthraquinone is distilled with zinc dust.




Elbs Reaction

Anthracene can be produced by the pyrolysis of O-methylbenzophenone at 450°C.




By Diel-Alder Reaction

A reaction between 1, 4 napthaquinone and 1, 3 butadiene occurs in this reaction. Anthracene is produced upon distillation (Zn) by oxidizing 9, 10 anthraquinone with chromium trioxide in GAA.



Reactions of Anthracene

Electrophilic Substitution Reaction







Diels-Alder Reactions

Diels-Alder reaction forms maleic anhydride from endo anthracene at positions 9, 10.



Oxidation

In the reaction of sodium dichromate and sulfuric acid with anthracene, nine and ten anthracene quinones are formed.




Reduction

When ethyl alcohol and sodium are reduced together, anthracene produces 9, 10 dihydroanthracene.




Dimerization

In UV light, dianthracene is formed by dimerization of anthracene.

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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