SN1 and SN2 | Nucleophilic Substitution Reactions

To understand the sn1 and sn2 reactions, it is necessary to understand the basic mechanism of nucleophilic substitution reactions. Nucleophilic substitution reactions are the reactions in which halogens are replaced by some other atoms or a group. It is also defined as the reaction in which a nucleus lover (nucleophile, having negative charge) may react with an electrophile (electron lover, having positive charge) and is replaced by a leaving group is called Nucleophilic substitution reaction. Nucleophilic substitution reactions on alkyl halides involve two main reactions sn1 & sn2 reactions. In the last part of this blog, i will explain a brief difference between Sn1 and Sn2 reactions as a conclusion.

Electrophile/Nucleophile

An electrophile is basically a species that reacts with the nucleophile. It is an electron lover and has a positive charge on it. It is electron-deficient species that has a positive charge on it. All the electrophile acts as a lewis acid. Electrophiles mainly interact with the nucleophiles by addition or substitution reaction. But on the other hand, Nucleophile is the species having a negative charge on them. It is an electron-rich species and acts as a Lewis base.

Leaving Group

In a heterolytic cleavage, molecular fragments that depart with the pair of an electron is called leaving group. This leaving group may be anion, cation, or neutral molecule. Also leaving group is the nucleophile which accepts a pair of electrons. More stable is the nucleophile good is the leaving group. Basically, all weak bases are the good leaving group. Bad leaving groups are the strong bases. The following conclusions are made:

  • Extreme weak bases are excellent leaving groups.
  • Weak bases are moderately leaving groups.
  • Strong bases are poor leaving group.
  • Very strong bases are extremly poor leaving groups.

Nucleophilic Substitution Unimolecular Reaction (Sn1)

The SN1 reaction is basically a neutrophilic unimolecular substitution reaction. Basically, it is unimolecular but occurs in two steps. The SN1 reaction is a step-by-step process that occurs in two steps. In this reaction carbocation, formation takes place then a nucleophile (having negative charge) on it reacts with carbocation formed and finally the product obtained is deprotonated. The Electrophicity of leaving group is purely dependent on the rate-determining step or vice versa. So, the rate-determining step is the slow step and is unimolecular.

The SN1 stands for the substitution nucleophilic Unimolecular reaction. When the amount of nucleophile is far greater than that of the amount of carbocation intermediate then the rate equation is satisfied which states that “the SN1 is dependent upon the electrophile not on the nucleophile”. This reaction undergoes when secondary or tertiary alkyl halide reacts with alcohol under the influence of strongly acidic or basic conditions.

The mechanism of the Sn1 reaction shows a first-order reaction and the rate of reaction depends upon the concentration of alkyl halide. The rate reaction of such reaction can be written as:

Rate = k[Alkyl halide]

Mechanism of Sn1 Reaction:

Nucleophilic substitution unimolecular reaction is a step-by-step process. Basically, it occurs in two steps but this mechanism occurs in three steps. The third step is deprotonation which is a very fast process so, it is included in the second step so that’s why it is a two-step process.

Step-1: Cleavage of C-Br bond slowly to form carbocation intermediate

C-Br Bond breakdown and Br- depart from the bonding electron pair. this is a highly endothermic process and is the slowest step so it is a rate-determining step. The product of the first step is intermediate and reactant for the next process. The carbocation form has a trigonal planar shape with the empty 2p orbital and has an incomplete octet so it is very reactive.

SN1 & SN2

Step 2: Rapid reaction between nucleophile and carbocation intermediate.

Now in this step, OH- use their lone pair to react with carbocation intermediate results in the production of protonated tertiary butyl alcohol. OH- act as a nucleophile that attacks either side due to planar geometry. Therefore, we observe 50% inversion of configuration and 50% retention of configuration.

SN1 & SN2

Step 3: Rapid deprotonation of protonated tertiary butyl alcohol.

A water molecule act as a bronsted lowry base to accept a proton from an oxonium ion. It is a fast step. Sometimes, it gathers with the 2nd step to form only a single step.

SN1 & SN2

Energy level diagram for Sn1 Reaction

sn1 and sn2

This graph represents that the intermediate formed is very reactive.

Effect of solvent, substrste structure, and leaving group on Sn1:

Effect of solvent:

  • Rate determining step of Sn1 reaction is speed up by the effect of solvent that form carbocation intermediate.
  • Prefered solvent for this type of reactions are both polar and protic.
  • Polar nature of solvents helps to stabilize ionic intermediate, whereas protic nature of solvent help solvate as the leaving group.

Effect of substrate structure:

The reactivity rate of the substrate towards Sn1 reactions can be written as:

Tertiary > Secondary > Primary > Methyl

By the relation, it is noticed that tertiary is most stable and is the best parameter for Sn1 reaction but the other i.e. secondary, primary, and methyl are the best parameters for the Sn2 reaction. This is due to the stability of the carbocation intermediate formed in the slow rate determination step. The stability of carbocation can also be given by the concept of hyperconjugation. It states that “Higher the number of alpha hydrogen means higher no bond resonating structure and higher stability“.

Effect of leaving group:

In both nucleophilic substitution reactions, the rate-determining step does not include nucleophiles so its health or strength does not produce or put any effect on the Sn1 reaction mechanism. But some of the neutral substances like H2O, ROH, RCOOH usually act as nucleophiles as well as solvents. The reaction in which these substances act as both is called the solvolysis reaction.

Stereochemistry for Sn1 reaction

In order to explain the stereochemistry of the sn1 reaction, we consider a reaction of H2O from either side with 3-Bromo-3-methyl hexane. The reaction between these two reactants produces the R-S enantiomers i.e. 3-methyl-3-hexanol. This is the racemic mixture product.

This is an equal possibility that it possesses the same amount of enantiomers. So, to explain stereochemistry for Sn1 reaction, the product form is 50% inversion of configuration and 50% retention of configuration. A racemization reaction is a reaction that converts optically active compounds into a racemic form.

sn1 and sn2
Figure: 50% retention and 50% inversion of configuration

Nucleophilic Substitution Bimolecular reaction (Sn2)

It is very common in organic chemistry. The SN2 reaction is the reaction in which Bond breakage and formation take place in only one step. It has only one step which is a slow step and is a rate-determining step. The rate-determining step depends that how to species are interacting with one another. The mechanism of the SN2 reaction comprises the taking of nucleophiles from the backside of the carbon atom. The SN2 reaction is the best example of a stereospecific reaction in which different stereoisomers direct give different stereoisomers. Its molecularity is two because two molecules participate in determinating step so the reaction rate is written as:

Reaction rate= k[Alkyl halide][Nucleophile]

Mechanism of Sn2 reaction:

Its mechanism involves transferring two electron pairs at the same time. This mechanism occurs when nucleophiles attack the backside of the carbon Centre opposite the leaving group. When OH- negative ions came close to the carbon center and are then attached to it. Br- negative ion leaves from that alkyl halide as a good leaving group. So, the new C-OH bond forms and C-Br  Bond breaks at the same time. When OH- negative ions came close to the alkyl halide the leaving group is attached to it but just going to detach from it. The state development is called transition state. In the transition state “pentacoordinate” condition develops when carbon is attached to the five groups. SN2 mechanism is said to be the concerted mechanism.

Sn1 and sn2

Factors affecting Sn2 reaction mechanism

Effect of alkyl halide

The sn2 mechanism is influenced basically by primary, secondary, and methyl.

Methyl > Primary > Secondary > Tertiary

This trend shows that the SN2 mechanism shown in methyl is at higher risk. Due to less distance in the methyl group of carbon, it is easy for the nucleophile to attack from the backside of carbon. increasing the size of a connecting group causes an increase in the size of the molecule and an increase in the size of the distance of a carbon atom.

So, it is very difficult for a nucleophile to accept the carbon atom. So, this access is totally blocked in tertiary alkyl halide. The less reactivity of nucleophile occurs due to an increase in steric hindrance which takes place due to an increase in the volume of size of alkyl halide for stocks increase in Bulky group before carbon causes decrease in reactivity of nucleophile and increase in steric hindrance.

Effect of leaving group

In the Sn2 mechanism, halogens are the excellent leaving group. The stability of leaving group depends upon the basicity. The strong base having a greater tendency to share the electron pair is a bad or poor leaving group means it has less stability. On the other hand, leaving group stability depends upon the weak base which has a low tendency to share the electron pair.

Basicity ∝ 1/leaving group stability

So compounds or alkyl halides which have a good leaving group can undergo nucleophilic substitution bimolecular reaction. Among halogens, I- is the best leaving group because it has the largest size as compared to fluorine due to less nuclear attraction. It has a great ability to share an electron pair. And will leave a molecule very quickly.

Effect of nucleophile

 The nucleophile is species that helps to speed up the SN2 reaction. SN2 reaction depends upon the nucleophilicity which is the relative strength of nucleophiles. The nucleophilicity may also depend upon the structural features:

  • Negatively charged nucleophile is more stable then neutral one. For example: OH- > H2O, and RO- > ROH
  • The periodic trends of nuclophilicity comprises decreasing across period and increasing across groups.
  • The nucleophile having small group is better then bulky group because there is less steric hindrance in smaller group.

So, the stable nucleophile is used in the Sn2 reaction mechanism.

Energy level diagram of Sn2 reaction

In the Sn2 mechanism reaction, there is only a single step so a single curve is obtained for us. This graph shows that the intermediate has high energy and is less stable. The overall reaction is highly exothermic. The transition state shows the development of partial bonds which break down to form a new product.

Sn1 and sn2

Stereochemistry for Sn2 reaction

In this reaction, the overall configuration of carbon is inverted just like an umbrella flipped over. Such kind of inversion is called Walden’s inversion. It starts with R 2-bromobutane enantiomer and produces S 2-butanol enantiomer. In this condition configuration for carbon is inverted completely.

Conclusion:

The conclusion of this blog suggested that there is a brief difference between the sn1 and sn2 reactions.

Unimolecular Reaction (Sn1)Bimolecular Reaction (Sn2)
It follows a first-order reaction kineticsIt follows a second-order reaction kinetics
Its molecularity is oneIts molecularity is two
It is a two-step processIt is a single-step process
Reaction rate = k[Alkyl halide]Reaction rate = k[Alkyl halide][Nucleophile]
It depends upon the concentration of substrateIt depends upon nucleophile and concentration of substrate
Racemization reaction occurs in this reactionInversion reaction occurs in this reaction
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