senyawa bensena atau senyawa yang mempunyai sifat kimia seperti bensena.

• Benzene is a colorless compound, m.p. 6o C and b.p. 80o C, discovered in 1825 by Michael Faraday.

• It is a six membered ring, with the formula C6H6 indicating three double bonds and a ring.

• The structure of benzene was a puzzle for chemists in the 19th century as its formula indicated a high degree of saturation, like alkenes. However, it did not give addition reactions as did alkenes.

• The first structure of benzene was proposed by August Kekulé in 1865. He proposed that benzene was composed of three double bonds in a six-membered ring.

• These double bonds, he hypothesized, shift back and forth rapidly such that the two possible forms cannot be separated.

Theorie of the Structure of Benzene

›The Resonance Explanation of the Structure of Benzene Structures I and are equal resonance contributors to

the real structure of benzene

Benzene is particularly stable because it has two equivalent and important resonance structures

Each carbon-carbon bond is 1.39 Å, which is between the length of a carbon-carbon single bond (1.47Å) and a carbon-carbon double bond (1.33 Å)

Often the hybrid is represented by a circle in a hexagon (III)

Benzene is much more stable than would be expected based on calculations for “cyclohexatriene”

A reasonable prediction for the heat of hydrogenation of hypothetical cyclohexatriene is -360 kJ mol-1 (3 times that of cyclohexene, -120 kJ mol-1 )

The experimentally determined heat of hydrogenation for benzene is -280 mol-1, 152 kJ mol-1 more stable than hypothetical cyclohexatriene

This difference is called the resonance energy

THE STABILITY OF BENZENE

KESTABILAN BENSENA

Tidak seperti senyawa-senyawa yang mengandung ikatan rangkap lainnya, bensena lebih mudah mengalami reaksi substitusi daripada adisi. Hal ini dapat dilihat dari data berikut.

Reagen Sikloheksena Bensena

KMnO4 encer Terjadi Oksidasi, cepat Tidak bereaksi

Br2/CCl4 (dlm

gelap)

Terjadi Adisi, cepat Tidak bereaksi

HI Terjadi Adisi, cepat Tidak bereaksi

H2 + Ni Terjadi hidrogenasi, 25oC, 20 lb/in.2

Terjadi hidrogenasi, lambat,

100-200oC, 1500 lb/in.2

•Hückel’s criteria for aromaticity are as follows:

1.The compound must be cyclic.

2.It must have one p orbital on each atom of the ring.

3.It should be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals.

4.It must have a closed loop of (4n+2) electrons in the cyclic arrangement of p orbitals.

Note: n adalah bilangan bulat

AROMATISITASMENURUT ATURAN HUCKEL

H

H H

HH

H

H H

HH

H

H H

HH

Kation siklopentadienil Radikal siklopentadienil Anion siklopentadienil4 elektron 5 elektron 6 elektron

Aromatis

HH

H

HH

H

H

HH

H

HH

H

H

HH

H

HH

H

H

Kation sikloheptatrienil Radikal sikloheptatrienil Anion sikloheptatrienil

6 elektron Aromatis

7 elektron 8 elektron

TATANAMA DERIVAT BENSENA

1. menambahkan awalan gugus substituen diikuti nama bensena, misal : klorobensena, bromobensena, nitrobensena, dll

2. beberapa derivat bensena mempunyai nama spesifik yang mungkin tidak menunjukkan nama dari substituen yang terikat pada bensena, misal : metilbensena dikenal sebagai toluene, aminobensena sebagai aniline, dll

3. Apabila bensena mengikat lebih dari satu substituen, maka nama substituen dan letak substituen harus dituliskan. Ada 3 (tiga) isomer yang mungkin untuk bensena yang tersubstitusi oleh 2 gugus. Penamaan digunakan nama orto (1,2-); meta (1,3-); para (1,4-)

4. Apabila 2 atau lebih substituen yang terikat pada bensena berbeda, maka penamaannya diawali dengan nama substituen berturut-turut dan diikuti dengan nama bensena atau diberi nama khusus/spesifik.

In molecules containing other functional groups, the phenyl group and others are named as substituents.

H2CCH3

benzene toluenephenyl group, Ph- benzyl group, Bn-

ELECTROPHILIC AROMATIC SUBSTITUTION

• The characteristic reaction of benzene is electrophilic aromatic substitution—a hydrogen atom is replaced by an electrophile.

• Benzene does not undergo addition reactions like other unsaturated hydrocarbons, because addition would yield a product that is not aromatic.

• Substitution of a hydrogen keeps the aromatic ring intact.

Reaction Coordinate Diagrams for the Two Benzene Reactions

Y+

+

H

Y

H

Y

H

Y

Y

B:

slow

fast

All electrophilic aromatic substitution reactions occur by the same two-step mechanism :1.addition of the electrophile E+ to form a resonance-stabilized carbocation

2.followed by deprotonation with base, as shown below:

-H+

The energy changes in electrophilic aromatic substitution are shown below:

• In halogenation, benzene reacts with Cl2 or Br2 in the presence of a Lewis acid catalyst, such as FeCl3 or FeBr3, to give the aryl halides chlorobenzene or bromobenzene respectively.

• Analogous reactions with I2 and F2 are not synthetically useful because I2 is too unreactive and F2 reacts too violently.

HALOGENATION

• Chlorination proceeds by a similar mechanism.

• Nitration and sulfonation introduce two different functional groups into the aromatic ring.

• Nitration is especially useful because the nitro group can be reduced to an NH2 group.

NITRATION and SULFONATION

• Generation of the electrophile in both nitration and sulfonation requires strong acid.

• In Friedel-Crafts alkylation, treatment of benzene with an alkyl halide and a Lewis acid (AlCl3) forms an alkyl benzene.

Friedel-Crafts Alkylation and Friedel-Crafts Acylation

• In Friedel-Crafts acylation, a benzene ring is treated with an acid chloride (RCOCl) and AlCl3 to form a ketone.

• Because the new group bonded to the benzene ring is called an acyl group, the transfer of an acyl group from one atom to another is an acylation.

• In Friedel-Crafts acylation, the Lewis acid AlCl3 ionizes the carbon-halogen bond of the acid chloride, thus forming a positively charged carbon electrophile called an acylium ion, which is resonance stabilized.

• The positively charged carbon atom of the acylium ion then goes on to react with benzene in the two step mechanism of electrophilic aromatic substitution.

Three additional facts about Friedel-Crafts alkylation should be kept in mind.

[1] Vinyl halides and aryl halides do not react in Friedel-Crafts alkylation.

[2] Rearrangements can occur.

These results can be explained by carbocation rearrangements.

Rearrangements can occur even when no free carbocation is formed initially.

[3] Other functional groups that form carbocations can also be used as starting materials.

Starting materials that contain both a benzene ring and an electrophile are capable of intramolecular Friedel-Crafts reactions.