Deciphering Nucleophilic Substitution: SN1 vs. SN2 Reactions

Abstract

Nucleophilic substitution reaction is a chemical reaction that is replaced leaving the group by nucleophile to form a new bond.

There are two types of nucleophilic substitution reactions that include ( SN1 - SN2 ). SN1 reaction means substitution nucleophilic unimolecular that their mechanisms occurred in two steps; the first step is slow that includes ionization of reaction where leaving group departs and forming carbocation intermediate, while the second step is the fastest step that includes attraction of nucleophile and forming a new bond.

Product of reaction is a racemic mixture ( retention of configuration and inversion of configuration) SN2 reaction that means substitution nucleophilic bimolecular that their mechanisms occurred in one fast step where the time of leaving group departs the nucleophile attacks the reaction and forming a new bond. The product of the reaction is inversion. We are using factors to determine which reaction which is occurred that includes

1.  Nature of substrate: if the substrate is tertiary then itis highly favored to SN1 but if the substrate is methyl is highly favored to SN2.
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2.  Nature of nucleophile: If the nucleophile is strong then itis favored SN2 and SN1 but if nucleophile is weak is highly favored to SN1.
3.  Nature of solvent: If itis polar aprotic then it is highly favored to SN2 but if it is polar protic is highly favored to SN1
4.  Nature of leaving group: It determines the reaction occurred or not.

If it is a weak base, so the reaction undergoes to substitution reactions but if it is Alkoxide, Hydroxide, and amide so the reaction, not substitution.

Nucleophilic substitution reactions[NSR] are a type of chemical reaction in which the nucleophile replaces the interested atom, ion, or group that found in the substrate. There are two types of NSR, The first called SN1 reaction. The second called SN2 reaction. In this article, we will focus on the definition and mechanism of these types and support with some examples. SN1 reaction (Lambert M. Surhone, 2010)

SN1 is an abbreviation of Substitution Nucleophileunimolecular. It occurs into two steps:

The first step includes the ionization of reaction where an interested atom, ion, or group departs the whole compound it left a positive charge called carbonation intermediate. This step is the slowest one. The second step includes the attack of the nucleophile on carbonation intermediate and forming a coordinating bond. This is the fastest one. These two steps can be shown by this equation. The rate of this reaction based on the concentration of one reactant, so it called SN1.

Where, Rate of reaction = K [ Substrate ].

SN2 reaction (Edenborough, 1998, p. 155) SN2 is an abbreviation of Substitution Nucleophilic bimolecular. It occurs in only one step. In this step, the bond between the carbon atom and the interested atom, ion, or group is broken and forming a transition state, during this time nucleophile, attacks carbon atom in the side of the leaving group. This step can be illustrated by this equation The rate of this reaction based on the concentration of the two reactants, so it called SN2.

Where, Rate of reaction = K [ Substrate ] = [ Nu - ].

Factors used to determine the favored reaction (Yoshida, 2015, p. 60)

So now we should know which reaction has the Preference to occurs, SN1 or SN2. There are some factors used to determine which reaction should occur; these factors are

1.  Nature of substrate
2.  Nature of nucleophile
3.  Nature of solvent
4.  Nature of interested atom, ion, or group ( leaving part ).

Now we will illustrate each one in details, Firstly, the Nature of the substrate. Secondly, the Nature of nucleophile. It depends on the strength of the nucleophile.

If the nucleophile is strong so SN2 reaction is favored, but If the nucleophile is weak so SN1 is the favored reaction. To determine the strength of nucleophile there are some factors

1. 1. Charge density; where nucleophilic strength increase by increasing of charge density, so greatest charge density, the strongest nucleophile.
2. 2. Electronegativity; where nucleophilic strength increase by increasing of electronegativity, so the greatest electronegativity, the strongest nucleophile.
3. 3. Steric effect; it means how the size of the group is. If it's a large size that means high steric effect and low nucleophilic strength, on the other hand, if the group size is small that means low steric effect and high nucleophilic strength.
4. Note: The steric effect in simple words means the atomic crowding. Thirdly, the Nature of solvent. It means the type f solvent is it's polar aprotic or polar protic. If the solvent is polar aprotic that means SN2 reaction is favored, but if it polar protic that means The SN1 reaction is favored.

*Polar protic solvents such (Alcohols and acids).

*Polar aprotic solvents such (DMF and Acetone).

Fourthly, the nature of the leaving group. The type of leaving group determines the reaction if it undergoes a substitution reaction or not. Where, if the leaving group is a weak base or EWD or neutral molecule the reaction undergoes substitution reactions. But if it Alkoxide or hydroxide or Amide the reaction not undergoes substitution reaction. Some practice exams (Fawzy, 2020) (1)

From this equation, we can notice that

1. Nature of substrate: 1ry so its good to SN2 reaction.
2. Nature of nucleophile: Strong nucleophile so its good to SN2 reaction.
3. Nature of solvent: Polar aprotic so it's favored to SN2 reaction.
4. Nature of interested atom, ion, or group ( leaving part ): Chlore is favored to SN2 reaction.

So in general we can say that SN2 reaction is favored to this equation. (2)

From this equation, we can notice that

1. Nature of substrate: 3ry so its good to SN1 reaction.
2. Nature of nucleophile: Weak nucleophile so its good to SN1 reaction.
3. Nature of solvent: Polar protic so it's favored to SN1 reaction.
4. Nature of interested atom, ion, or group ( leaving part ): Brome is favored to SN1 reaction.

So in general we can say that SN1 reaction is favored to this equation.

Conclusions

We can conclude from the above that we have two types of nucleophilic substitution. The SN1 reaction is slow while SN2 is a fast reaction. There are four factors used to determine which reaction is occurred; SN2 reaction to occur needs good nucleophiles such as (I - ), solvent polar aprotic like DMF and substrate may be methyl, primary or secondary not tertiary because of steric effect. SN1 reaction to occur needs weak nucleophile such as (F - ), a solvent is polar protic like ethanol and substrate may be tertiary, secondary and primary Heat can be used for helping speed reaction.