ELECTROPHILIC SUBSTITUTION OF BENZENE

Benzene is an aromatic compound that shows exceptional but explainable behavior. Being a highly saturated compound,it ought to undergo addition reactions but instead undergoes substitution.
The reason for this is the conjugated system of pi bonds in benzene that is further explained in "Modern Concept Of Benzene".

Electrophilic substitution reactions involve an electron-poor species(atom ,ion or molecule)which maybe a positive ion or partial positive end of a polar molecule and is called the ELECTROPHILE and an electron-rich species.The electrophile replaces a hydrogen atom in the ring.

TYPES OF ELECTROPHILIC REACTIONS IN BENZENE ON THE BASIS OF ELECTROPHILE:

• HALOGENATION:

The process in which a ring hydrogen is replaced by a halogen in the presence of Lewis Acid catalyst is called halogenation.

The electrophillic reaction in which a halogen takes the place of a hydrogen atom in the ring in the 

presence of a Lewis acid catalyst.

CATALYST:

Iron reacts with some of the chlorine in the following manner:

REAGENT:
Taking CHLORINE as an example of a halogen,it is difficult to form a positive ion since that requires too much energy,hence it is merely polarized.

FORMATION OF ELECTROPHILE:
As the chlorine molecule approaches a benzene ring, the delocalised electrons in the ring repel the electrons in the chlorine-chlorine bond. That induces a dipole in the chlorine.

Also nearby is the FERRIC chloride, and this encourages the polarization of the chlorine.The FERRIC is strongly attracted to the slightly negative end of the chlorine molecule, and pulls electrons even more towards that end.

The Electrophilic substitution mechanism:

STAGE 1:
Formation of intermediate arenium ion:
Two electrons from the delocalization form a new bond with the partial positive end of the chlorine molecule.This breaks the delocalization partly and the ring attains a positive charge.It is now called arenium or benzene ion.

The breaking of the chlorine molecule transfers 1 atom to the benzene and one to the ferric chloride forming AlCl4

STAGE 2:
Formation of HCl and regeneration of catalyst:
One of the aluminium-chlorine bonds breaks and both electrons from it are used to join to the hydrogen. Removing the hydrogen from the ring forms the HCl which is also produced in the reaction, and the ferric chloride catalyst is re-generated. The electrons which originally joined the hydrogen to the ring are now used to re-establish the delocalised system.(3)

• SULPHONATION:

The process in which a sulphonic acid group replaces a ring hydrogen atom.

The formation of the electrophile:
The electrophile is sulphur trioxide, and this arises in one of two ways depending on which sort of acid you are using.
1-DISSOCIATION OF SULPHURIC ACID:
Due to dissociation of concentrated Sulphuric acid ,it contains traces of SO3.(1)

2-FUMING SULPHURIC ACID:
It is a solution of SO3 in H2SO4. Hence, contains more SO3.

Sulphur trioxide is highly polar with all the oxygen atoms pulling the electrons towards themselves leaving a partially positive sulphur atom which attacks the benzene.

The electrophilic substitution mechanism:

STAGE 1:
Formation of intermediate arenium ion:
Two electrons from the delocalization form a new bond with the partial positive end of the chlorine molecule.This breaks the delocalization partly and the ring attains a positive charge.It is now called arenium or benzene ion.

To make room for the new bond between the ring and benzene, two of the electrons joining the oxygen to the sulphur(in the double bond) are transferred entirely to the oxygen making it a negative ion.

STAGE 2:
formation of Benzene Sulphonic acid:
The removal of hydrogen from the benzene ring is affected by the oxygen with the lone pairs.It forms a bond with the hydrogen and releases the electrons previously holding hydrogen into the ring to re-establish the delocalization. The benzene sulphonic acid is formed.

• NITRATION:

The process in which a nitro group replaces a hydrogen atom in benzene is called Electrophilic nitration.

Formation of electrophile:
NO2+!  or nitronium or nitryl cation is the electrophile.  It is formed by the reaction of sulphuric acid and nitric acid.

The electrophilic substitution mechanism:

STAGE 1:
Formation of intermediate arenium ion:
As the nitronium ion approaches the benzene ring ,the delocalized electrons are attracted to the positive ion and two of them form a new bond with the nitronium ion thus breaking the delocalization.(2)

STAGE 2:

Formation of nitro benzene and Regeneration of catalyst:

The second stage involves the negative hydrogen sulphate ion, formed along with the nitronium ion.
The lone pairs of one of its oxygen atoms forms a bond with the hydrogen of the ring thus regenerating the catalyst by forming sulphuric acid and at the same time releasing the bonding electrons of hydrogen to re-establish the delocalization.

• FRIEDEL-CRAFTS ACYLATION OF BENZENE:

The reaction in which hydrogen of the benzene ring is replaced by an acyl group.

FORMATION OF ELECTROPHILE:
The electrophile is CH3CO+.It is formed by reaction with catalyst aluminium chloride.(1)Alongside negative ion AlCl4-1 is also formed.

The electrophilic substitution mechanism:

STAGE 1:Formation of intermediate arenium ion:

Two electrons from the delocalised system are used to form a new bond with the CH3CO+ ion affectively breaking part of the delocalization.(2)

STAGE 2:
Formation of ketone and regeneration of catalyst:
The second stage involves the AlCl4 -1 formed alongside the positive ion.One of the aluminium chlorine bonds break and the chlorine with both electrons forms a bond with hydrogen from the ring.This results in the formation of phenyl ethanone and regeneration of AlCl3 catalyst.

• FRIEDEL-CRAFTS Alkylation OF BENZENE

The reaction involving replacement of a hydrogen of the benzene ring with an alkyl radical.

The formation of the electrophile:
The electrophile is CH3+. It is formed by reaction between the chloromethane and the aluminium chloride catalyst.

The electrophilic substitution mechanism:

STAGE 1:
Formation of intermediate arenium ion:
Two electrons from the delocalized system are used to form a new bond with the ion CH3+.The delocalization is partly broken.Arenium ion is formed.(2)

STAGE 2:
Formation of alkyl benzene and regeneration of catalyst:
The second stage involves the AlCl4 -1 formed alongside the positive ion.One of the aluminium chlorine bonds break and the chlorine with both electrons forms a bond with hydrogen from the ring.This results in the formation of methyl benzene and regeneration of AlCl3 catalyst.

oh LORD! the stories we spin...............

DISTINGUISHING TESTS

FOR ALKENE AND ALKANE:

Bayer's Test:

It is a quantitative test for the presence of unsaturation (double or triple bonds).
a sample is treated with small amount of elemental bromine either

• as aqueous solution
• as a solution in CCl4
• as solution in CH2Cl2

REACTION:
Deep brown colouration of bromine rapidly disappears as it is consumed by reaction with sample containing alkenes.

Potassium permanganate test:

It is the quantitative test for the presence of unsaturation.
Dilute KMnO4 is purple and the test depends on its ability to act as an oxidizing agent.

REACTION:
When alkenes are oxidized with cold, alkaline KMnO4, dihydroxy compounds (diols or glycols) are formed. The KMnO4 gets decolorized and in its place brown ppt of MnO2 is seen.

A more compressed form in which to express the reaction in is:

ALKANE AND ALKYNE:

Potassium permanganate test:

At room temperature:
Alkynes react with KMnO4 to give "dioics" and decolorize the purple hue of  the oxidizing agent.

At low temperature:
Alkynes may react to give an 'oic' at low temperatures.

ALKENE AND ALKYNE:

The formation of acetylides is used as mark of identification of alkynes.
ACETYLIDES are the dianions with the formula C2−2.They are formed by deprotonation of alkynes and subsequent addition of metals to the carbon atom.

Alkenes do not react as such with metals.

                                                                                         DI SODIUM ACETYLIDE

The above forms DISILVER ACETYLIDE which cools to form a white solid and DICOPPER ACETYLIDE which cools to form a red solid.

ALKANE AND ALKYL HALIDES:

The most efficient way of distinguishing alkyl halides is by a substitution reaction which would convert the halogen into halide ions.The ions can then be tested by Silver Nitrate solution.

REACTION:

Haloalkanes are warmed with sodium hydroxide solution in a mixture of ethanol and water.Everything will    dissolve.

The nitric acid is added to remove other ions that give a confusing precipitate.
Silver Nitrate solution is added.
It may form white ,pale yellow or yellow ppt depending on the halogen.

ALDEHYDE AND KETONE:

Aldehydes are easily oxidized by all sorts of different oxidizing agents: Ketones aren't.
For Aldehydes ,Oxidation under :
Acidic conditions---gain of oxygen
Alkaline conditions---lose of hydrogen and gain of oxygen

Fehling's Test:

Fehling's solution contains copper (II) ions complexed with tartrate ions in sodium hydroxide solution.

Aldehydes reduce the complexed copper (II) ion to copper (I) oxide. 

Because the solution is alkaline, the aldehyde itself is oxidized to a salt of the corresponding carboxylic acid.

ketone         :No change in the colourless solution.  

Aldehyde     :Red ppt of Cu2O.

Tollen's Test:

Tollens' reagent contains the diamminesilver (I) ion, [Ag (NH3)2] +.

Aldehydes reduce the diamminesilver(I) ion to metallic silver. 

Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.

• ketone     : No change in the colourless solution.  
• aldehyde  : The colourless solution produces a Grey precipitate of silver, or a silver mirror on the test tube.