14 Difference Between The SN1 And SN2 Reactions (SN1 Vs SN2)

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What Is SN1 Reaction?

The SN1 reaction is a substitution reaction in organic chemistry. ‘’SN’ stand for ‘’nucleophilic substitution’’ and ‘’1’’ says that the rate-determining step is unimolecular. Thus, the rate equation is often shown as having first-order dependence on electrophile and zero-order dependence on nucleophile. This relationship holds for situations where the amount of nucleophile is much greater than that of the intermediate. Instead, the rate equation may be more accurately described using steady-state kinetics. The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary halides under strongly basic conditions, with secondary or tertiary alcohols. In inorganic chemistry, the SN1 reaction is often referred to as the dissociative mechanism.

What You Need To Know About SN1 Reaction

  1. SN1 is a two step process reaction. There is a loss of the leaving group to form a carbocation intermediate followed by a nucleophilic attack.
  2. SN1 reaction is a first order reaction because the rate of reaction depends on the substrate only.
  3. In SN1 reaction, both inversion and retention of configuration takes place, because the nucleophile can attack the substrate either front side or back side of the planar structure of the carbocation.
  4. SN1 reaction is nucleophilic substitution uni-molecular, that is, only one molecule takes part in rate determining step.
  5. The SN1 reaction tends to proceed with weak nucleophiles-generally neutral compounds such as solvents like CH3OH, H2O, CH3CH2OH and so on.
  6. The rate of SN1 reaction depends on the stability of the carbocation. 3o >2o>1o carbocation.
  7. The rate of SN1 reaction does not depend on the concentration and strength of nucleophile.
  8. Polar Protic solvent such as water, alcohol and carboxylic acids fours SN1 reaction. Polar Protic solvents dissolve both cation and anions in it.
  9. In SN1 reaction, the rate of reaction is dependent on the stability of the carbocation, cation and anion.
  10. In SN1 reaction, the big barrier is carbocation stability since the first step of the SN1 reaction is loss of a leaving group to give a carbocation, the rate of the reaction will be proportional to the stability of the carbocation.
  11. In SN1 reaction involves the formation of a carbonium ion as an intermediate.
  12. The greater the stability of carbocation, the greater the tendency of SN1 reaction.
  13. In SN1reaction, rearrangement is possible.
  14. Racemic mixture is formed.

What Is SN2 Reaction?

SN2 stands for ‘’substitution nucleophilic bimolecular’’ which means it will lead to the displacement of a group on a molecule and its rate will depend on the active participation of two reactants.

The SN2 reaction involves displacement of a leaving group (usually a halide or a tosylate), by a nucleophile. This reaction works the best within methyl and primary halides because bulky alkyl groups block the backside attack of the nucleophile, but the reaction does work with secondary halides (although it is usually accompanied by elimination), and will not react at all with tertiary halides.

Whether an alkyl halide will undergo an SN1 or SN2 reaction depends upon a number of factors. Some of the more common factors include the nature of the carbon skeleton, the solvent, the leaving group and the nature of the nucleophile.

What You Need To Know About SN1 Reaction

  1. SN2 is a one-step process in which the addition of nucleophiles and the loss of the leaving group occur simultaneously.
  2. SN2 reaction is a second order reaction because the rate of reaction depends on both the substrate and nucleophile.
  3. In SN2 reaction, only inversion of configuration takes place, because the nucleophile can attack the substrate from the back side only.
  4. SN2 reaction is nucleophilic substitution bi-molecular, that is, two molecules (both substrate and nucleophile) takes part in rate determining step.
  5. The SN2 tends to proceed with strong nucleophiles-generally negatively charged nucleophiles such as CH3O (-), CN (-), RS (-), N3 (-), HO(-) and others.
  6. The rate of SN2 reaction depends on the steric effect of the alkyl halide. The rate of SN2 reaction increases 3o> 2o>1o alkyl halide.
  7. The rate of SN2 reaction depends on the concentration and strength of the nucleophile.
  8. Polar Aprotic solvents like DMSO, acetone, acetonitrile, DMF, DMA favors SN2 reactions, because Polar Aprotic doesn’t  dissolve cations, it dissolves only anions in solution, so by taking Polar Aprotic solvent cations are removed and only Nu (:) is only anion present to attack substrate.
  9. In SN2 reaction, the rate of reaction is inversely proportional to the bulkiness of C atom-attached groups.
  10. In SN2 reaction, the big barrier is steric hindrance since the SN2 proceeds through backside attack, the reaction will only proceed if the empty orbital is accessible. The more groups that are present around the vicinity of the leaving group, the slower the reaction will be.
  11. In SN2 reaction involves the formation of an activated complex as the intermediate.
  12. The greater the stability of possible carbocation, the poor the tendency of SN2 reaction.
  13. In SN2 reaction, rearrangement is not possible.
  14. Walden inversion take place.

Also Read: Difference Between Protic And Aprotic Solvents

Difference Between The SN1 And SN2 Reactions In Tabular Form

BASIS OF COMPARISON SN1 REACTION SN2 REACTION
Description SN1 is a two step process reaction. There is a loss of the leaving group to form a carbocation intermediate followed by a nucleophilic attack.   SN2 is a one-step process in which the addition of nucleophiles and the loss of the leaving group occur simultaneously.  
Reaction Order SN1 reaction is a first order reaction because the rate of reaction depends on the substrate only.   SN2 reaction is a second order reaction because the rate of reaction depends on both the substrate and nucleophile.  
Inversion/Retention Of Configuration In SN1 reaction, both inversion and retention of configuration takes place, because the nucleophile can attack the substrate either front side or back side of the planar structure of the carbocation.   In SN2 reaction, only inversion of configuration takes place, because the nucleophile can attack the substrate from the back side only.  
Nature SN1 reaction is nucleophilic substitution uni-molecular, that is, only one molecule takes part in rate determining step.   SN2 reaction is nucleophilic substitution bi-molecular, that is, two molecules (both substrate and nucleophile) takes part in rate determining step.  
Reaction Process The SN1 reaction tends to proceed with weak nucleophiles-generally neutral compounds such as solvents like CH3OH, H2O, CH3CH2OH and so on.   The SN2 tends to proceed with strong nucleophiles-generally negatively charged nucleophiles such as CH3O (-), CN (-), RS (-), N3 (-), HO(-) and others.  
Carbocation The rate of SN1 reaction depends on the stability of the carbocation. 3o >2o>1o carbocation.   The rate of SN2 reaction depends on the steric effect of the alkyl halide. The rate of SN2 reaction increases 3o> 2o>1o alkyl halide.  
Rate Of Reaction The rate of SN1 reaction does not depend on the concentration and strength of nucleophile.   The rate of SN2 reaction depends on the concentration and strength of the nucleophile.  
Solvent Polar Protic solvent such as water, alcohol and carboxylic acids fours SN1 reaction. Polar Protic solvents dissolve both cation and anions in it.   Polar Aprotic solvents like DMSO, acetone, acetonitrile, DMF, DMA favors SN2 reactions, because Polar Aprotic doesn’t  dissolve cations, it dissolves only anions in solution, so by taking Polar Aprotic solvent cations are removed and only Nu (:) is only anion present to attack substrate.  
Rate Of Reaction In SN1 reaction, the rate of reaction is dependent on the stability of the carbocation, cation and anion.   In SN2 reaction, the rate of reaction is inversely proportional to the bulkiness of C atom-attached groups.  
Barrier In SN1 reaction, the big barrier is carbocation stability since the first step of the SN1 reaction is loss of a leaving group to give a carbocation, the rate of the reaction will be proportional to the stability of the carbocation.   In SN2 reaction, the big barrier is steric hindrance since the SN2 proceeds through backside attack, the reaction will only proceed if the empty orbital is accessible. The more groups that are present around the vicinity of the leaving group, the slower the reaction will be.  
Intermediates In SN1 reaction involves the formation of a carbonium ion as an intermediate.   In SN2 reaction involves the formation of an activated complex as the intermediate.  
Tendency Of Reaction The greater the stability of carbocation, the greater the tendency of SN1 reaction.   The greater the stability of possible carbocation, the poor the tendency of SN2 reaction.  
Rearrangement In SN1reaction, rearrangement is possible.   In SN2 reaction, rearrangement is not possible.  
Product Racemic mixture is formed.   Walden inversion takes place.  

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