What is Swarts reaction and Why NaF is not used in Swarts reaction ?

Definition of Swarts  reaction .

When alkyl chloride or alkyl bromide reacts with transition metal fluorides [such as AsF₃, SbF₃, AgF, Hg₂F₂, CoF₂ etc], alkyl fluoride is obtained , then the reaction is called Swarts reaction . 

Like Finkelstein reaction , Swarts reaction is also called halogen or halide ion exchange reaction . Swarts  reaction follow SN² reaction mechanism . 

That is nucleophilic substitution bimolecular . F ^– acts as a nucleophile in this reaction.

Swarts  reaction is very important reaction to prepare alkyl fluoride indirectly . 

If we use direct method like other halogen to prepare fluoro alkane, fluorine attack the alkane to form carbon and HF .

 Because F₂ is strong electronegative and very active.So alkyl fluoride is not available .

Example of few Swarts reactions are shown below,

Mechanism of Swarts  reaction : 

Like Finkelstein reaction ,Swarts  reaction also follow the SN² reaction mechanism . 

Here F^– ion is behave as nucleophile . So in this reaction inversion of configuration takes place .

Why NaF is not used in Swarts  reaction ?

If we use NaF in stead of transition metals fluoride in Swarts reaction  with chloro alkane , NaCl is obtained along with fluoro alkane .But NaCl is very much soluble in water solvent.

So the forward reaction will not favorable.Besides , in polar protic solvent,( H2O), as F^– is more electronegative than Cl^– ion and F^– ion has smaller ionic radius, the extent of hydration of F^– ion is much more higher than Cl– ion. 

Hence nucleophilicity of F^– ion is very much extent decreases . Consequently, the rate SN² reaction highly decreases .

Therefore, if we use NaF ,in stead of transition metal  fluoride in Swarts reaction, the yield of the reaction will be very low, infact we will get back chloro alkane in very high amount . 

Besides, the formation of C–F bond is explosive which can controlled only transition metal  fluoride, not NaF . For the above reason , NaF is not used in Swarts reaction  .

Why acetone solvent is used in Swarts  reaction ?

In Swarts  reaction ,dry acetone is used as a solvent. Acetone is a polar aprotic solvent . In acetone medium , NaBr and NaCl  are  insoluble because the negative charge on oxygen atom of acetone is repelled with the negative charge of Cl^– or Br^–atom . 

As a result NaCl and NaBr are easily precipitate in acetone solvent.We have to remove that precipitate from the product mixture so that the reaction changes into forward reaction. 

If we used other solvent like water in which  NaCl and NaBr are soluble ,then the pure fluoro is not available.For the above reason acetone solvent is used in Swarts reaction .

Compare the rate of Swart reaction among 2-fluoro propane, 2-chloro propane and 2-bromo propane .

Swart reaction follow SN² reaction mechanism .  F^– ion is used as  nucleophile . Now among the fluorine,chlorine and bromine, bromine is a good leaving group and  fluorine is a weak leaving group . 

So the rate of 2-bromo propane is highest and the rate of 2-fluoro propane is lowest . So the order of rate of Swarts  reaction of the above three halo propane are as follows,

Which have highest rate of Swart reaction between 1-chloro propane and 1-chloro -2-methyl propane  ?

Swarts reaction follow SN² reaction mechanism .  F^– ion is used as  nucleophile . Now, in 1-chloro -2-methylpropane  due to steric crowding  the SN² reaction is prohibited , but steric crowding  is absent in 1-chloro propane .

Consequently, the rate of Swarts reaction of 1-chloro propane higher than 1-chloro -2-methyl propane  .

Which have highest rate of Swart reaction between 1-chloro propane and 3-chloro-prop-1-ene ?

Swarts reaction follow SN² reaction mechanism .  F^– ion is used as  nucleophile.

In 1-chloro propane the carbon atom which attached with chlorinated carbon is sp3 hybridised  . 

But in chloro-prop-1-ene the double bonded carbon which attached with chlorinated carbon is sp2hybridized .

Now ,due to +I effect of sp3 hybridized  carbon , the positive charge on C–Cl carbon decreases . So the nucleophilic attack on that carbon is not facilitated .

Besides, after breaking of C–Cl bond in 3-chloro-prop-1-ene , the pi bond is overlap with the p-orbital of chlorinated carbon . 

So the trasition state is more stable that is the transition energy is low. So the rate of reaction increases for 3-chloro-prop-1-ene . 

Hence the rate of Swarts reaction of 1-chloro propane  is less than 3-chloro-prop-1-ene .

Why chloro cyclo hexane show high rate of reaction than chloro cyclo propane  towards SN² reaction ?

When the nucleophile attack on chlorinated carbon atom in both chloro cyclo hexane and chloro cyclo propane then in each transition state chlorinated carbon atom is sp2 hybridized. 

Now, in case of the sp2 hybridised carbon in transition state, the bond angle is required 120ᵒ . 
But in transition state of chlorocyclo propane, the bond angle is 60ᵒ whereas in case of chloro cyclo hexane , the bond angle is 120ᵒ . 

Therefore, the transition state of chlorocyclo propane is highly unstable than that of chlorocyclo hexane.Consequently, chloro cyclo hexane show high rate of reaction than chloro cyclo propane  towards SN² reaction .

Compare the rate of SN² reaction among 1-chloropan-2-one, 3-chloro-prop-1-ene and 1-chloropropane .

1-chloropan-2-one is a alpha halogenated ketone . The carbon atom of C–Cl bond is more reactive than the other two chlorinated carbon atoms. 

Because in 1-chloropan-2-one the vacant σ* orbital of C–Cl bond interact with the vacant π*orbital of C = O bond . 

As a result of this constructive interference a bonding molecular orbital is obtained with lower energy level in which the entering of electron takes place easily . 

So the rate of SN2 reaction is highest for alpha-chlorinated ketone that is in for 1-chloropan-2-one .

On the other hand in 1-chloropropane , due to +I effect of sp3 hybridised carbon the positive charge on alpha-carbon decreases .

Hence the the nucleophilic attack is not easily takes place with respect to 3-chloro-prop-1-ene . So the rate of SN2 reaction of 1-chloropropane is lowest.

The order of rate of SN² reaction of 1-chloropan-2-one, 3-chloro-prop-1-ene and 1-chloropropane are shown below,

What is Swarts  reaction ?

Why NaF is not used in Swarts  reaction ?

Why acetone solvent is used in Swarts  reaction ?

Compare the rate of Swart reaction among 2-fluoro propane, 2-chloro propane and 2-bromo propane .

Which have highest rate of Swart reaction between 1-chloro propane and 1-chloro -2-methyl propane  ?

Which have highest rate of Swart reaction between 1-chloro propane and 3-chloro-prop-1-ene ?

Why chloro cyclo hexane show high rate of reaction than chloro cyclo propane  towards SN² reaction ?

Compare the rate of SN² reaction among 1-chloropan-2-one, 3-chloro-prop-1-ene and 1-chloropropane .

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