Monday, November 23, 2015

Study Guide TMH JEE Ch.24 Benzene

JEE syllabus



Electrophile Substitution Reactions
--- Sulphonation
--- Friedel-Crafts Alkylation
--- Friedel-Crafts Acylation

Effect of --, m- and p- directing groups in mono-substituted benzenes

Main Topics Covered in the TMH Book








Important themes of the Chapter/Topic

Benzene has the molecular formula C6H6. It has hexagonal ring of six carbon atoms with three double bonds in alternate positions.

Arenes are the aromatic hydrocarbons which contain one or more hexagonal rings of carbon atoms with double bonds in alternate positions.

I. Structure

1. Benzene is a liquid hydrocarbon of formula C-6H-6.
2. It is a cyclic molecule in the shape of a flat hexagon.
3. In the bonds among six carbon atoms three have to be double bonds so that tetravalent bonding arrangement is preserved and the double bonds have to be alternate bonds in the ring.
4. The resonance energy of benezene is about 150.6 kJ per mol.
5. All carbon-carbon bond distances in benzene are equal (139 pm) and are intermediate in length between single (154 pm) and double bonds (134 pm)
6. Benzene is a planaer molecule where each carbon is sp^2 hybridized.
7. Of the three hybrid orbitals, two are used in sigma-bonding with two other carbon atoms and the third is used in sigma-bonding with hydrogen atom.
8. In addition to hybrid orbitals, eahc carbon has one p orbital occupied by one electron. This orbital lies perpendiculars to the plane of benzene ring, and hence this electron is of pi-type.
9. The p orbital of each carbon atom can overlap with the adjacent p orbitals making additional bond of pi-type. But this bond is not localized between two carbon atoms. But forms two continuous doughnut-shaped electon cloud one lying above and the other below the plane of cyclic carbon skeleton. The delocalization of pi-electrons gives rise to resonance energy and makes the molecule more stable.
10. The overlap of p orbitals in both directions gives rise to resonance hybrid of two strctures known as Kekule structures.
(August Kekule presented the structure of Benzene on 27 January 1865)
II. Nomenclature

III. Aromaticity

Aromatic compound are those which resemble benzene om chemical behaviour. These compounds contain alternate double and singe carbon-carbon bonds in a cyclic structure. They are more stable compared to aliphatic hydrocarbons having double bonds and undergo substitution reactions rather than addition reactions. These two characteristic behaviours in combination is aromatic character or aromaticity.

1. A molecule acquires aromatic characteristics provided it has cyclic clouds of delocalized pi-electrons above and below the plane of the molecule and this pi- clouds have a total of (4n+2) pi-electrons. This requirement of 4n+2 pi-electrons is known as 4n+2 rule or Huckel rule.
2. Examples are benzene (n = 1), naphthalene (n = 2) and anthracene (n = 3).

The modern theory of aromaticity was advanced by Eric Huckel. For aromaticity, the molecule must be planar, cyclic system having delocalised (4n+2) pi elctrons.
IV. Isomerism

V. Preparation of benzene and its homologues

1. From alkynes: acetylene and other alkynes polymerise at high temperatures to give benzene and other arenes.

3C2H2 gives C6H6

Benzene was first synthesized by Berthelot by passing acetylene through red hot iron tube.

2 Decarboxylation of aromatic acids: by heating sodium benzoate with soda lime

Decarboxylation: Removal carboxyl group

3. From phenol: by distillation of phenol with zinc.

4.From diazonium salts reduction of benzene diazonium with hypophosphorus acid (H3PO2)

5. From aryl halides

6. From Friedel Craft's reaction
Benzene is treated with alkyl halide in the presence of anhydrous aluminium chloride. For example when the alkyl halide is monochloromethane, Toluene(methylbenzene - methyl group substituted for one H in benzene is obtained)

7. From Grignard's reagent: reacting aromatic Grignard reagent with alkyl halide

Ex: Phenyl magnesium bromide + Isopropyl bromide in ether give isopropyl benzene.

Va. Physical properties

i) colour less liquids up to eight carbon atoms
ii) aromatic hydrocarbons are insoluble in water ut soluble in organic solvents.
iii) They are inflammable and burn with sooty flame
iv) M.P. and B.P. increase with increasing molecular mass.
v) they are toxic and carcinogenic in nature.

VI. Chemical properties

Even though double bonds are present, benzene is quite stable and does not undergo common addition reactions undergone by alkenes.

Benzene and other arenes undergo following types of reactions.

1. substitution
2. addition
3. oxidation

VIa. Substitution Reactions in Benzene

1. The typlical reactions of the benzene ring are those in which the pi-electrons serve as a source of electrons for electrophylic (acidic) reagents. In these substitution reactions, hydrogen atoms attached to carbon atoms are replaced by another atom or group of atoms.
2. Because of delolcalization of pi-electrons, benzene does not show addition reactions as in the case of alkenes(presence of double bond).

3. Halogenation

benzene will react with a mixture of Cl-2 and FeCl-3.

The output is a combination of benzene with Cl, Cl diplacing one hydrogen atom from benzene(Chlorobenzene).

4. Nitration
A mixture of nitric acid and sulphuric acid is the nitrating agent.
(a) The reaction between nitric acid and sulphuric acid gives nitronium ion. Nitric acid acts as base in this reaction and donate OH giving No-2^+

(b) the nitronium ion makes the electrophylic attack on benzene. Benzonium ion is formed. This step is a slow reaction or step. Hence this is a rate determining step. Benzonium ion is a carbocation. It is a resonance hybrid.

(c) From the benzonium ion the proton is removed by the HSO-4^- which is a product of step (a). Thus a combination benzene and NO-2 that displaces one hydrogen atom from benzene is formed.

5. Sulphonation

The product is a combination Benzene and SO-3H that displaced one hydrogen atom from benzene.

For sulphonation we require excess of H-2SO-4 along with SO-3.

6. Friedel-Crafts Alkylation

Benzene reacts with a combination of alkyl halide and AlCl-3. AlCl-3 acts as a Lewis acid.

The alkyl group replaces one hydrogen atom in benzene.

7. Friedel-Crafts Acylation

Acylation is the term given to substituting an acyl group such as CH-3CO- into another molecule. An acyl group is a hydrocarbon group attached to a carbon-oxygen double bond.

The most commonly used example of an acyl group is the ethanoyl group, CH3CO-.

If you react benzene with ethanoyl chloride in the presence of an aluminium chloride catalyst, the equation for the reaction is:

C-6H-6 + CH-3CoCl ------> C-6H-5-COCH-3 + HCl

In the simplified formula for the product, the phenyl group is usually written on the left-hand side and the alkyl group to the right of the carbon-oxygen double bond.

The aluminium chloride is acting as a catalyst.

The product is called phenylethanone (old name, acetophenone).


Mechanism of electrophilic substitution reactions of benzene

VIb. Addition reactions

VIc. Oxidation reactions

VII. Effect of --, m- and p- directing groups in mono-substituted benzenes

In planning syntheses based on substitution reactions of mono-substituted benzenes, you must be able to predict in advance which of the available positions of the ring are most likely to be substituted.

Basically, three problems are involved in the substitution reactions of aromatic compounds: (a) proof of the structures of the possible isomers, o, m, p, that are formed; (b) the percentage of each isomer formed, if the product is a mixture; and (c) the reactivity of the compound being substituted relative to some standard substance, usually benzene.

a. the Pattern of Orientation in Aromatic Substitution

The reaction most studied in connection with the orientation problem is nitration, but the principles established also apply for the msot part ot the related reactions of halogenation, sulfonation, alkylation and acylation.
web sites
Notes on the Structure and Nomenclature-naming of Aromatic Compounds

Updated  23 Nov 2015, 14 Oct 2007

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