- Huckel Rube and Aromatic compounds

A. Hückel rule
            In the year 1931 a German chemist Erich Hückel, suggested that an aromatic compound to be flat, monocyclic (one ring) needs to have as many pi electron 4N + 2, where n is an integer . According to the Hückel rule, a ring with pi electrons as 2,6,10 or 14 can be aromatic, but the ring with 8 or 12 pi electrons, can not be. Siklooktatetraena (with 8 electrons pi) does not comply with Hückel rule for aromaticity.Why the 6 or 10 pi electrons are aromatic, whereas 8 pi electrons are not?In order to be aromatic, all pi electrons must be paired, so it is possible overlapping (overlapping) optimal resulting in delocalization perfect.

            If sikloo ktatetraena flat and has a system similar to pi pi system of benzene, the orbital p1, p2, and p3 will be filled with six pi electrons pi.Dua remaining electrons will each occupy degenerate orbitals p4 and p5 (Hund's rule). Then not all the pi electrons will pair up and will not overlap maximum so sikooktatetraena will not be aromatic.Aromatic compounds must meet the following criteria:- Cyclical- Contains a delocalized p electron clouds below and above the plane of the molecule- Double bonds alternate with single bonds- Have a total number of 4N +2 p electrons, where n must bulisal numbers: if the number of electrons in a ring of a cyclic p = 12, then n = 2.5 then instead of aromatic compounds 


B. Benzene 
Benzene and benzene derivative compounds first synthesized by Michael Faraday in 1825, from which the gas is used as fuel for lamps .Ten years later it was discovered that benzene has the molecular formula C6H6 thus concluded that benzene has a double bond more than alkenes.Benzene molecules form a flat hexagon with carbon atoms located on the sixth corner. All the same carbon-carbon bond length and strength, as well as all the carbon-hydrogen bonds, and all the angles CCC and HCC is 1200. Thus, each carbon atom sp2 hybridise, every atom that form three sigma bonds with two carbon atoms next to them and the hydrogen atom.
 This arrangement leaves one 2pz orbitals are not confined between two adjacent atoms bonded to each other, but also extends to three or more atoms.Of the oily residue is buried in the gas mains in London. Currently, the main source of benzene, substituted benzene and aromatic compounds are petroleum: formerly of nearly 90% coal tar compounds active ingredients are aromatic compounds: benzene core has the formula.

       Structure of BenzeneThe double bond in benzene is different from the alkene double bond. The double bond in alkenes can undergo addition reactions, whereas the double bond in benzene can not you get an addition, but benzene can react substitution. Example:Addition reaction: C2H4 + Cl2 -> C2H4Cl2Substitution reaction: C6H6 + Cl2 -> C6H5Cl + HClAccording to Friedrich August Kekulé, six carbon atoms arranged in a cyclic benzene irregular hexagonal shape with a bond angle of 120 ° respectively. Antaratom carbon bond is a double bond and a single alternating (conjugated).

 X-ray analysis of the structure of benzene indicates that the carbon bond lengths in benzene antaratom same, ie 0.139 nm. The length of a double bond C = C is 0.134 nm and the length of C-C single bond is 0.154 nm. Thus, the carbon-carbon bonds in the benzene molecule is between double bond and a single bond. This renders the structure of Kekulé.Kekulé describe the structure of benzene by carbon atoms linked to one another to form a ring.

• August Kekulé in 1865: The structure illustrates that the structure of benzene composed three double bonds in the ring 6 members.• The three double bonds can be shifted and return quickly so that the two forms may not be separated.

Orbital benzeneEach carbon atom in benzene tying 3 others using sp2 hybridized orbital forming a planar molecule.Benzene is a symmetrical molecule, hexagonal shape with a bond angle 120oEach C atom has four orbital into the p orbitals. P orbitals overlap will experience suh (overlapping) to form a cloud of electrons as the source of electrons.



C. AROMATIC COMPOUNDS
 heterocyclicAccording to Erich Hückel, a compound that contains five or six-membered ring is aromatic if:• all the constituent atoms lie in a flat (planar)• every atom that form a ring having a 2p orbital• have the pi electrons in the cyclic arrangement of the 2p orbitals as 4n +2 (n = 0, 1, 2, 3, ...)In addition to benzene and its derivatives, there are several other types of compounds exhibit aromatic properties, which have high unsaturation and showed no reactions like alkenes. Benzene homosiklik included in the class of compounds, ie compounds that have only one type of atom in the ring system. 

 There are heterocyclic compounds, are compounds that have more than one type of atom in the ring system, the ring is composed of one or more atoms that are not carbon atoms. For example, pyridine and pirimidina are aromatic compounds such as benzene. In pyridine one CH unit of benzene is replaced by a nitrogen atom sp2 hybridise, and in pirimidina two CH units replaced by nitrogen atoms are sp2 hybridise.Membered heterocyclic compounds of five such as furan, thiophene, pyrrole, and imidazole also includes aromatic compounds. 

Polycyclic aromatic hydrocarbons are carcinogenic particular one, meaning that there are cancerous. These compounds can produce tumors in mice within a very short time even though only a few are applied to the skin. This is not only carcinogenic hydrocarbons present in coal tar, but also the soot and tobacco smoke and can form in the meat baker. Biological effects have been known for a long time, ie since 1775, when the soot is defined as a cause of cancer of the penis chimney cleaning. Incidence of lip cancer and heart disease are also found in the smoker.

D. TERMS OF AROMATIC COMPOUNDS Aromatics Requirements: 
1. Molecules have cyclic and flat.
2. have p orbitals perpendicular to the ring plane (pi electron delocalization allow). 
3. have p orbitals perpendicular to the ring plane (pi electron delocalization allow)8 siklooktatetraena not aromatic pi electrons.USE AND IMPACT benzene in LIFE

a. Usefulness

1. Benzene is used as a solvent. 
2. Benzene is also used as a precursor in the manufacture of pharmaceuticals, plastics, rubber, and artificial dyes. 
3. Benzene is used to raise the octane number of gasoline
4. Benzene is used as a solvent for various types of substances. Besides benzene is also used as a raw material to make styrene (a type of synthetic rubber material made) and nylon-66. 
5. Acid SalicylateSalicylic acid is the common name of o-hydroxybenzoic acid. Esters of salicylic acid with acetic acid is used as a drug by the name of aspirin or aspirin
6. Acid BenzoateBenzoic acid is used as a preservative in many processed foods. 
7. AnilineAniline is the base material for the manufacture of diazo dyes. The reaction of aniline with nitrous acid will produce diazonium salt, and this process is called diazotized. 
8. TolueneUsability is important toluene as a solvent and as a raw material for making explosive substances trinitrotoluene (TNT)
9. StirenaJika styrene polymerizes be formed polystyrene, a type of plastic that is widely used to make electrical insulator, bonekaboneka, shoe soles, as well as plates and cups. 
10. BenzaldehydeBenzaldehyde is used as a preservative as well as raw material for perfume because it has a delicious smell.
11. Sodium BenzoateSuch as benzoic acid, sodium benzoate is also used as a preservative in canned foods.
12. PhenolPhenol (phenyl alcohol) in their daily lives better known as carbolic acid or lisol, and used as a disinfecting agent (bacteria-killing) could cause protein denaturation.


b. Impact
1. Benzene is highly toxic and cancer-causing (carcinogenic). 

2. Benzene can cause death if inhaled at high concentrations, whereas at low concentrations causes headaches and increase heart rate.