Define Finkelstein reaction with examples.
This is the very important reaction of organic chemistry for the preparation iodo alkane.
When chloro or bromo alkane are heated with the concentrated mixture solution of acetone and NaI,iodo alkane is obtained, then the reaction is called Finkelstein reaction .
This is an example of halogen or halide exchange reaction .
The reaction
follow the SN2 reaction mechanism
and I– ion acts as anucleophile.
Since the reaction follow the SN2 reaction mechanism, therefore the starting alkyl halide should be primary or 1ᵒ so that the reaction will proceed easily and the product is satisfactory.
Since the reaction follow the SN2 reaction mechanism, therefore the starting alkyl halide should be primary or 1ᵒ so that the reaction will proceed easily and the product is satisfactory.
In Finkelstein reaction, dry acetone is used as a solvent.Acetone is a polar aprotic solvent. Example of Finkelstein reaction is shown below,
Mechanism of
What is the
function of acetone in Finkelstein reaction
?
In Finkelstein reaction ,dry acetone is used as a solvent. Acetone is a polar aprotic solvent .
In acetone medium, NaI is soluble but NaCl is insoluble because the negative charge on oxygen atom of acetone is repelled with the negative charge of Cl– atom .
As a result NaCl is easily precipitate in acetone .This precipitate of NaCl is removed from the product mixture by which forward reaction takes place. The example is given above.
Which halogen will be replace in Finkelstein reaction between bromine and chlorine in 1-bromo-4-chloro methyl benzene?
In 1-bromo-4-chloro methyl benzene, bromine is a good leaving group than chlorine. So, in Finkelstein reaction normally the replace of bromine should be more feasible than chlorine.
But the lone pair of bromine atom takes part in
resonance with the pi electron of benzene ring .
As a result C –Br gets doublebond character . So the replace of bromine by nucleophile is not possible easily.
As a result C –Br gets doublebond character . So the replace of bromine by nucleophile is not possible easily.
Therefore instead of bromine ,chlorine atom is replaced by nucleophile , resulting in the formation of 1-bromo-4-iodo methyl benzene .
Why chlorine or bromine is replaced by iodine in Finkelstein reaction in spite of C –Cl, C –Br bond is more stronger than C –I bond?
The C –Cl, C –Br bond is more stronger than C –I bond, yet in Finkelstein reactions ,breaking of strong bond C – Cl or C –Br and making of weak bond C – I takes place.
Because in dry acetone solvent
NaI is soluble, but NaCl and NaBr are insoluble. Being insoluble in acetone
NaCl and NaBr are precipitated.
This precipitate continuously removed from the
reaction mixture . So the reaction change into forward reaction . That is the
reaction is thermo-dynamically feasible .
For this reason, chlorine or bromine
is replaced by iodine in Finkelstein reaction
in spite of C–Cl, C–Br bond is more stronger than C –I bond.
Is Finkelstein reactions are thermodynamically favorable ?
In Finkelstein reactions ,breaking of strong bond C – Cl or C–Br and making of weak bond C – I takes place.
So the reaction should be thermo-dynamically unfavorable , but actually the reaction is thermo-dynamically favorable.
This fact depends on the solubility of NaI andNaCl in acetone solvent . It has been experimentally found that NaI is completely soluble in acetone solvent but NaCl is insoluble.
So NaCl is
precipitated which is continuously removed from the solution .
Consequently, the forward reaction takes place . Therefore Finkelstein reactions are thermo-dynamically favorable.
Consequently, the forward reaction takes place . Therefore Finkelstein reactions are thermo-dynamically favorable.
Stereo chemistry of Finkelstein reactions.
The Finkelstein reactions follow the SN2 reaction mechanism. So the product must be inversion of configuration .
For example, when S-2-chloro butane under goes Finkelstein reactions the expected product is R-2-iodo butane.
What will be happen when S-2-iodo butane under goes Finkelstein reactions ?
The Finkelstein reactions follow the SN2 reaction mechanism . So when S-2-iodo butane under goes Finkelstein reactions , the resulting product would be R-2-iodo butane .
At the starting
point the concentration of R-2-iodo butane is zero . But after some time the
concentration of R-2-iodo butane gradually increases and the concentration of S-2-iodo
butane gradually decreases.
Because after few minute the nucleophile I–attack the product R-2-iodo butane and forms S-2-iodo butane. That the reaction
is reversible .
After a certain time the concentration of reactants and products will be same .
After a certain time the concentration of reactants and products will be same .
This condition is called equilibrium condition. Under
this condition the product is racemic mixture . So overall the reaction mixture
is optically inactive.
Summary What is Finkelstein reaction with examples ?
What is the
function of acetone in Finkelstein reaction
?
Which
halogen will be replace in Finkelstein reaction between bromine and chlorine in
1-bromo-4-chloro methyl benzene ?
Why chlorine
or bromine is replaced by iodine in Finkelstein reaction in spite of C –Cl, C –Br bond is more
stronger than C –I bond ?
Is
Finkelstein reactions are thermodynamically favorable ?
What will be
happen when S-2-iodo butane under goes Finkelstein reactions?
No comments:
Post a Comment