AROMATIC HYDROCARB

ONS

AROMATIC HYDROCARBONS

• An unsaturated cyclic hydrocarbon that does not readily undergo addition reactions.

• Aromatic Ring System-the functional group present in aromatic compounds.

Benzene

C6H6, a flat symmetrical molecule with a structural formula that is often

formalized as that of cyclohexatriene- in other words, as a structural formula that

involves a six-membered carbon ring which 3 double bonds are present.

(Benzene itself is implicated as cancer causing chemical)

This structure is one of two equivalent structures that can be drawn for benzene that differ only in the

locations of the double bonds(1,3,5, positions versus 2,4,6 positions):

Neither of these conventional structure, however is totally correct.

Experimental evidence indicates that all of the carbon-carbon bonds in

benzene are equivalent (identical), and these structure imply three bonds of one type (double bonds) and three

bonds of a different type (single bonds)

An alternative notation for denoting the bonding in benzene-a notation that involves single structure is

In this “circle-in-the-ring” structures for benzene, the circle denotes the electron associated with the double bonds that move around the ring.Delocalized bond-a covalent bond in which electrons are shared among more than two atoms.A highly unsaturated carbon ring system in which both localized and delocalized bonds are present.

The equivalent nature of the carbon-carbon bond in benzene is addressed by considering the correct bonding structure for benzene to be an average of the two “triene” structures. Related to this “avarage”-structure situation is the concept that electrons associated with the ring double bonds are not held between specific carbon atoms; instead they, they are free to move “around” the carbon ring.

Placing a double-headed arrow between the conventional structures that are averaged to obtain the true structure is one way to denote the average structure.

Physical Propertiesand Sources

Physical Properties• Insoluble in water• Good solvents for nonpolar material• Less dense than water• volatile• Colorless, flammable liquid (burns with a sooty

flame due to incomplete combustion• Several aromatic hydrocarbons are toxic

Sources• At one time, coal tar the main source of

aromatic hydrocarbon but now its petroleum due to a high temperature with special catalyst• Toluene from heptane• Situation in which a person can be exposed

to low level benzene vapors , benzene inhalation can cause nausea and respiratory problems• Inhaling gasoline vapors while refueling on

automobile(gasoline contain about 2% (v/v) C6H6.

• Being around a cigarette smoker

Chemical Reaction of Aromatic

Hydrocarbons:Benzene undergoes substitution reactions.

-Characterized by different atoms or groups of atoms replacing hydrogen atoms in a hydrocarbon molecule.

2 Important types of substitution reaction for benzene and other aromatic hydrocarbons :

• Alkylation-An alkyl group(R--) from an alkyl chloride (R—Cl) substitutes for a hydrogen atom on the benzene ring.

(most important industrial reaction of benzene)

• Halogenation- (bromination or chlorination)

Fused Ring Aromatic Hydrocarbons

Examples:NaphthaleneAnthracene

PhenanthreneAll three are solids at room temperature.

2 carbon rings that share a pair of carbon atoms ,an aromatic hydrocarbon whose structure

contains two or more rings fused together.

NaphthaleneA white crystalline volatile solid with a strong coal-tar odor. The solid is denser

than water and insoluble in water. Burns, but may be difficult to ignite.

In the molten form it is very hot. Exposure to skin must be avoided. Also

the vapors given off by the material may be toxic. Used as a moth repellent,

fumigant, lubricants, and to make other chemicals, and for many other uses

Naphthalene balls or Mothballs

Anthracenea colorless, crystalline powder, C14H10, obtained from coal  tar, used chiefly as a source of anthraquinone and alizarin.

PhenanthreneUses:1.Phenanthrene can produce phenanthrenequinone,polyester resin,alkyd resin,benzene anhydride,ke-tohexamethylene and pheno;2.In papermaking,it is antifogging agent of pulp;3.In medicine,it can produce alkaloid;4.In dye industry,it can be used in 2-amino phena-nthrenequinone,benzanthrone and sulphur vat dyes;5.Phenanthrene can be used in the production of the fuel of jet plane.

Fused-Ring Aromatic

Hydrocarbons and Cancer A number of fused-ring aromatic

hydrocarbons are known to be carcinogens-that is, to cause cancer. Three of the most potent carcinogens are:• 1,2-Benzanthracene• 1,2,5,6-Dibenzanthracene• 3,4-Benzpyrene

1,2-Benzanthracene

Benzanthracene is a crystalline, aromatic hydrocarbon consisting of four fused benzene rings, produced by

incomplete combustion of organic matter. Benzanthracene is primarily found in gasoline and diesel

exhaust, tobacco and cigarette smoke, coal tar and coal tar pitch, coal combustion emissions, charcoal-broiled foods,

amino acids, fatty acids and carbohydrate pyrolysis products, wood and soot smoke, and creosote, asphalt and mineral oils.

This substance is used only for research purposes. Benzanthracene is reasonably anticipated to be a human

carcinogen.

 

1,2,5,6-Dibenzanthracene

is a polycyclic aromatic hydrocarbon (PAH) and a

known carcinogen. Dibenzanthracene has produced positive results in bacterial DNA damage and

mutagenicity assays and in mammalian cell DNA damage, mutagenicity and cell transformation

assays.

3,4-Benzpyrene

Angular,fused-ring hydrocarbon systems are believed to be partially responsible for the high incidence of lung and lip cancer among cigarette smokers because tabacco smoke contains 3,4-Benzpyrene.

Carcinogenic fused-ring aromatic hydrocarbons share some structural

features. They also contain four or more fused rings, and they all have the same

“angle” in the series of rings.Fused-ring aromatic hydrocarbons are

often formed when hydrocarbon materials are heated to high

temperatures.

These resultant compounds are present in low concentrations in …

automobile exhaust

tobacco smoke

and sometimes in burned (charred) food

THANK YOU

Prepared by: MADONNA MIRAFLOR PAJOSOGOD NATIONAL HIGH SCHOOL