AROMATIC HYDROCARBONS

The compound has the molecular formula C6H6 benzene, and belongs to a class of hydrocarbons. When compared to other hydrocarbon containing six carbon atoms, such as hexane (C6H14) and cyclohexane (C6H12), then it can be assumed that benzene has a high degree of unsaturation. On the basis of these allegations, it can be expected that benzene has characteristics like those of the alkene. The estimate was far different from the reality, because benzene can not react like alkenes (addition, oxidation and reduction). More specifically benzene can not react with HBr, and other reagents that typically can react with alkenes. Chemical properties exhibited by benzene indicate that the compound is not a party to the alkene or sikloalkena.
Benzene and its derivatives classified in a number of aromatic compounds, is first classification solely based on the flavor of the part of the said compounds. Development of the next stage chemistry chemists realize that the classification should be based on the chemical structure and reactivity, and not on the basis of physical properties. Currently, the term aromatic is retained, but it refers to the fact that all the aromatic compounds of high and stable degree ketidakjenuhannya when dealing with the attacking reagent bonding pi (π).

A. AROMATIC COMPOUNDS
Benzene is a member of a large group of aromatic compounds, ie compounds sufficiently stabilized by pi-electron delocalization. Resonance energy an aromatic compound is helping gained stability.
The easiest way to determine whether a compound is aromatic is to determine the position of the absorption in mspektrum number of protons bound to the atom-atom ring. Proton-bound outward terperisai aromatic rings are very strong and absorbs far down-field than most protons, usually more than 7 ppm.
(Fessenden and Fessenden .454-455: 1982).

B. Aromatic compounds and its structure
Which includes aromatic compounds is
· Benzene compounds
· Chemical compound with chemical properties such as benzene

1. Benzene
Benzene and benzene derivative compounds first synthesized by Michael Faraday in 1825, from which the gas is used as fuel for lamps penerang.Sepuluh years later it was discovered that benzene has the molecular formula C6H6 thus concluded that benzene has a double bond more than alkenes.
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.
a. Structure of Benzene
The 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 -> C2H4Cl2
Substitution reaction: C6H6 + Cl2 -> C6H5Cl + HCl
According 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 benzene
Each 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 120o
Each 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 heterocyclic
According to Erich Hückel, a compound that contains five or six-membered ring is aromatic if:
· All constituent atoms lie in a flat (planar)
· Any atoms 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.

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.
(Fessenden and Fessenden .463-464: 1982).
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 elketron 4N + 2, where n is an integer sebuahn. 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 siklooktatetraena flat and has a system similar to pi pi system of benzene, the orbital π1, π2, and π3 will be filled with six pi electrons pi.Dua remaining electrons will each occupy degenerate orbitals and π5 π4 (Hund's rule). Then not all the pi electrons will pair up and will not overlap maksimal.Jadi sikooktatetraena will not be aromatic. (Fessenden and Fessenden .464-464: 1982).
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. Ion cyclopentadiene
            Cyclopentadiene is a conjugated diene and aromatic aromatik.Alasan main reason why not is that one carbon atom is sp3, not sp2.Karbon sp3 has no p orbitals take tuk un pi bond, but when taken one of the hydrogen ions in the cyclopentadiene carbon hidrodisasi will change to sp2 and p orbitals will have that contains a pair of electrons.
                             
 Aromatic Aromatic Cation Anion
All the carbon atoms of the cyclopentadiene cation will also be sp2.
Is one or both of these ions are aromatic? Each ion has five π molecular orbitals (p orbitals formed from five, one per carbon). Anion cyclopentadiene with six pi electrons (4N +), filling three orbitals and all these pi electrons that are aromatik.Tetapi berpasangan.Maka cation anion it only has four electrons (4N) to fill three orbital.Maka pi electrons is not going all berpasangan.Jadi cation is not aromatic.
(Fessenden and Fessenden .465-466: 1982).

E. DERIVED COMPOUNDS Benzene
Ease of benzene undergo electrophilic substitution reactions causing benzene has many derivative compounds. All carbon compounds containing benzene rings are classified as derivatives of benzene.
 F. Benzene nomenclature
All compounds containing benzene rings are classified as compound benzene derivatives. Structuring name derived compounds like benzene in aliphatic compounds, there are common nomenclature (trivial) and according to the IUPAC nomenclature based on the numbering system. In IUPAC nomenclature, the carbon atoms in the ring substituents are numbered smallest binding. According to IUPAC, benzene with the substituents are named as in aliphatic compounds, the parent group is benzene.
Benzene with alkyl groups as substituents, are classified as class arena. Structuring the name of the arena was divided into two groups based on the alkyl chain length. If the alkyl group is small (atoms C6) is taken as the alkyl substituents and benzene as the parent.
If the large alkyl group (atom C 6) then expressed as benzene and alkyl substituents as the parent chain. Benzene as a substituent named phenyl-(C6-H5, abbreviated-ph). Example:
Benzene with two substituent groups are named with the prefix: ortho-(o-), meta-(m-), and para-(p-). rto-applied to adjacent substituents (positions 1 and 2), meta-for positions 1 and 3, and para-substituents to position 1 and 4.
If the substituent groups of three or more, using the numbering and arrangement of names written in the alphabet. Smallest number given to functional groups (alcohols, aldehydes, or carboxylic acid) or a group with the smallest number.
Meanwhile, if there are three or more substituents on the benzene ring, the system o, m, p can not be applied anymore and can only be expressed with numbers.
All aromatic compounds based on benzene, C6H6, which has six carbon. Every corner of the hexagon has a carbon atom bound with hydrogen.
1. Cases where the name is based on benzene.

a. Klorobenzen
This is a simple example where a halogen attached to the benzene ring. Naming is very clear. The simplified formula C6H5Cl. So you could (although maybe not!) Named fenilklorida. Any if you draw a benzene ring with something attached to it you actually draw phenyl. To tie something you have to throw a hydrogen to produce phenyl.
b. Nitrobenzen
Nitro classes, NO2, benzene attached to the chain. The simplified formula C6H5NO2.
c. Metilbenzen
One more obvious name. Benzene with methyl attached to it. Alkyl group are also follow the naming sama.Contoh, etilbenzen. The old name of metilbenzen is toluene, you may still see itu.nFormula simply C6H5CH3.
d. (Chloromethyl) benzene
Variations of metilbensen where one hydrogen atom is replaced with a chloride atom. Notice the sign in parentheses, (chloromethyl). This is so that you can understand that chlorine is a methyl group and not part of the ring. If more than one hydrogen replaced by chlorine, naming would be (diklorometil) benzene or (triklorometil) benzene. Once again note the importance of brackets.
e. benzoic acid (acid benzenacarboxylic)
Benzoic acid is the old name, but still in common use is easier said and written. Whatever it is called there is a carboxylic acid,-COOH, bound to a benzene ring.
2. Cases where the name is based on phenyl
Remember that the phenyl group is a benzene ring loses a hydrogen atom - C6H5.
a. fenilamine
Phenylamine is a primary amine containing-NH2 bound to a benzene ring. The old name of phenylamine is aniline, and you can also named them aminobenzena.
b. Phenylethene
Ethene molecule binds with phenyl him. Eten is a two-carbon chain with a bond rangap. The old name for styrene-monomer from which polystyrene.
c. feniletanon
Containing two carbon chain with no double bonds. Is a ketone group is that there is a C = O in the middle. Bound to the carbon chain is phenyl.
d. feniletanoat
Etanoik acid esters basis. Hydrogen atoms in the-COOH group is replaced by phenyl.
e. phenol
Phenol has an-OH group attached to a benzene ring that has a formula C6H5OH.

Aromatic compounds with more than one group bound to a benzene ring.
Numbered ring
One of the groups attached to the ring are numbered satu.Posisi others are numbered 2 through 6. You can renumbers clockwise or counter-clockwise direction. Resulting in the smallest number. See the example for more details
Example:
Adding chlorine atom in the ring
Everything is based on methyl metilbenzen and with it the number 1 position on the ring.
Why 2-6-chloromethylbenzene not chloromethylbenzene? The ring is clockwise jamdalam this case because the number 2 over kcil of figure 6.
acid 2-hidrobenzoik
Also referred to as acid 2-hydroxybenzenecarboxylic. There is a-COOH group attached to the ring, and as the name is based on benzoic acid, then the group is assigned the number one. In the position is a hydroxy group,-OH 2.
acid benzene-1 ,4-dicarboxylic
 "On" indicates the presence of two carboxylic acids and one in the 1 position while the other is at the 4 position.
2,4,6-trikloofenol
Based on the phenol-OH bound to the number 1 of the carbon chain and the chlorine at position # 2.4 and 6 of the carbon ring.
Triklorofenol 2,4,6-TCP is a well-known antiseptic.
methyl 3-nitrobenzoic
This name is a name that you will come across the practice questions nitrate me the benzene ring. From the name indicated that methyl 3-nitrobenzoic an ester group (suffix oats). And methyl separate written.
Esther is based on the acid, 3-nitrobenzoic acid and we start from there.
There will be a benzene ring with one-COOH on the number of rings and the nitro at number 3. ester to produce a hydrogen-COOH degantikan with methyl.
Methyl 3-nitrobenzoic be:

G. Benzene RESONANCE MODEL
- The length of carbon-carbon bonds in benzene are equal, ie: halfway between the length of a single bond and a double bond.
- Length of C = C bond is 1.34 Å
                                                       single bond C - C is 1.53 Å.
- If benzene is considered to have three double bonds and three single bonds as in Kekulé structure, it will be found three short bond (1.34 Å) and three long bonds (1.53 Å). However, analysis by X-ray diffraction showed that the length of the C - C in benzene same, ie 1.39 Å.


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H. Benzene STABILITY
In contrast to compounds containing double bonds other, benzene is not prone to addition reactions
Reagent

Cyclohexene

Benzene
Dilute KMnO4

Oxidation occurs, quickly

Not react
Br2/CCl4 (in the dark)

Addition occurs, quickly

Not react
HI

Addition occurs, quickly

Not react
H2 + Ni

Hydrogenation occurs, 25oC,
20 lb/in.2

Hydrogenation occurs, slow,
100-200oC, 1500 lb/in.2

- The stability of the benzene ring can be seen quantitatively from the heat of hydrogenation and combustion.
- Heat and burning benzene hydrogenation is lower than the price calculation.

I. REACTION benzene (R. Substitution)

1. Nitration Reaction
A mixture of concentrated nitric acid and concentrated sulfuric acid with the same volume known as the nitration mixture. If the mixture is added to the benzene, exothermic reaction will occur. If the temperature is controlled at 55 ° C the main reaction products are nitrobenzene, a pale yellow liquid.
2. Sulfonation Reaction
Sulfonation is a substitution reaction of H atoms in the benzene sulfonate group. This reaction occurs when benzene is heated with concentrated sulfuric acid as a reagent.
3. Benzene alkylation
The addition of anhydrous AlCl3 catalyst in the reaction of benzene and haloalkane or hydrochloric acid is very exothermic reaction will occur. This type of reaction is called reaction to Friedel-crafts. Example equation:
4. The reaction of halogenated

As is the electrophile X +, resulting from the reaction between X2 + FeX3.
FeX3 (eg FeCl3) is a Lewis acid that serves as a catalyst. Other Lewis acid catalysts that can be used is AlCl3, AlBr3.
5. Friedel-Crafts reaction
Friedel-Crafts reactions include alkylation reactions and acylation reactions.
As electrophile in Friedel-Crafts alkylation reaction is carbonium ions (R +). Because involving carbonium ions, the reaction often occurs rearrangements (rearrangement) to form a more stable carbonium.
As electrophile in Friedel-Crafts acylation reaction is ion asilium.
In reaction to Friedel-Crafts acylation reactions do not occur rearrangement. In alkylation reactions and acylation Friedel-Crafts Lewis acid catalysts are also used, such as FeCl3, FeBr3, AlCl3, AlBr3.
Example acylation reaction:

Thus, it can be more susceptible to benzene substitution reactions rather than addition reactions.

J. PHYSICAL AND CHEMICAL
Physical properties:
· Liquids colorless
· It has a distinctive odor
· Easy to evaporate
· Benzene is used as a solvent.
· Not soluble in polar solvents like water water, but soluble in solvents that are less polar or nonpolar, such as ether and tetraklorometana
· Soluble in many organic solvents.
· Benzene can form azeotropic mixture with water.
· Density: 0.88
Chemical properties:

    Are toxic-carcinogenic (be careful using benzene as a solvent, use only if no other alternatives such as toluene)
    Is a nonpolar compound
    Not so reactive, but it's easy to generate a lot of soot burn
    More susceptible to substitution reactions of the adduct.

Hückel rule

In organic chemistry, Hückel rule which estimates whether a planar ring molecule will have aromatic properties. The basic formulation of quantum mechanics was first carried out by physical chemist Erich Hückel in 1931. [1] [2] The phrase short as 4 n +2 rule has been associated with von Doering (1951), [3] although some authors use this form at around the same time. [4]A cyclic ring molecule follows Hückel's rule when the number of π-electrons equals 4 n +2 where n is zero or a positive integer, although clearcut examples really only defined for values ​​of n = 0 to about n = 6. [5] Hückel rule was originally based on calculations using the Hückel method, although it can also be justified by considering the particles in the ring system, the LCAO method [6] and by the method of Pariser-Parr-Pople.Aromatic compounds are more stable than the theoretical prediction by data hydrogenation of alkenes, the "extra" stability is due to the cloud delocalized electrons, called a resonance energy. Simple aromatic Criteria are:1. follow Hückel's rule, has 4 n +2 electrons in delocalized and conjugated p-orbital cloud;2. can be planar and cyclic;3. every atom in the ring to participate in delocalizing the electrons by having p-orbital or pair of unshared electrons.Hückel rule does not apply to many compounds containing more than three nuclei are fused cyclic aromatic. For example, it contains 16 electrons conjugated pyrene (8 bonds), and contains 24 electrons conjugated coronene (12 bonds). Both polycyclic aromatic molecules that although they failed 4 n +2 rule. Indeed, the Hückel rule can only theoretically justified for monocyclic systems. [6]In 2000, Andreas Hirsch and co-workers in Erlangen, Germany, formulated rules for determining when a fullerene be aromatic. They found that if there are 2 (n +1) 2 π-electrons, the fullerene which will display aromatic properties. This follows from the fact that the aromatic fullerene must have full icosahedral (or other measures) symmetry, so that the molecular orbitals must be filled completely. This is possible only if there is exactly 2 (n +1) 2 electrons, where n is a positive integer. In particular, for example, buckminsterfullerene, with 60 π-electrons, is a non-aromatic, since 60/2 = 30, which is not a perfect square. [7]