Difference between SN1 and SN2 Reactions

Introduction

The two types of nucleophilic substitution reactions are SN1 and SN2. Sn2 has two molecules, while SN1 only has one. It's crucial to comprehend a nucleophilic substitution reaction to comprehend SN1 and SN2. Understanding the distinction between SN1 and SN2 is only possible if all terminology relevant to the nucleophilic substitution process has been mastered. In this section, we will talk about nucleophiles, substitution processes, nucleophilic substitution processes, and the distinction between Sn1 and Sn2.

What is a Nucleophile?

An atom or molecule with electron pairs to contribute is called a nucleophile. In other words, it is negatively charged because it has excess unused electrons. Two categories of nucleophiles exist:

Neutral: Neutral nucleophiles are molecules that contain one or more lone pairs of electrons but an overall neutral charge.

Example: Let's use NH3 as an example. This molecule's octet is fulfilled. It has a neutral charge all around. Yet, since it possesses a single pair of electrons, the nitrogen atom is negatively charged in and of itself. Hence, the N atom will still be drawn to an electron-deficient region of a molecule or an electron-deficient atom regardless of the overall charge of the molecule.

Anions: Anions are chemical particles with negative charges. Example: the OH^- or hydroxide ion.

Substitution Reaction

Any chemical process replaces a molecule's functional group or an atom with another functional group or atom. Nucleophilic or electrophilic substitution reactions are also possible. We shall concentrate on nucleophilic substitution reactions in this article.

Conditions of Substitution Reaction

Certain criteria must be met for a substitution reaction to take place, which are as follow:

• Maintaining low temperatures, such as that of a room
• The strong base, such as NaOH, must be diluted. Consider the possibility of dehydrohalogenation if the base is present at a higher concentration.
• The mixture must be in an aqueous condition, like water.

Types of Substitution Reactions

There are two kinds of substitution reactions: nucleophilic reactions and electrophilic reactions. The primary distinction between these two sorts of reactions is the type of atom that is joined to the initial molecule. Atoms are referred to as electron-rich species in nucleophilic reactions and electron-deficient species in electrophilic reactions.

Nucleophilic Substitution Reaction

Any substitution process in which an atom or functional group is changed for either with a single pair of electrons or a negatively charged ion or both. To replace the functional group or atom already linked to the positive region, the negatively charged ion or the atoms/molecules with lone pairs of electrons will be drawn to the positively charged area of an atom or complex.

Example- Bromomethane, CH₃Br is its chemical formula. Whereas bromine is negative, the alkyl CH3 is positive. The negatively charged Br will now be replaced by the positively charged CN^- if it interacts with the cyanide anion. The reaction is shown below:

CH₃CN + Br- CH₃Br + CN^-

Similarly, if we consider chloromethane or CH3Cl. In this case, Cl is negative, while CH₃ is positive. When the hydroxide ion and CH₃Cl interact, the negatively charged hydroxide ion replaces the Cl atom. The reaction takes place as follows:

CH₃Cl + OH CH₃OH + Cl-

One negatively charged atom or molecule is replaced by another negatively charged atom or molecule.

Nucleophilic Substitution Reactions: Types

The nucleophilic substitution process has two types:

1. SN1 Reaction
2. SN2 Reaction

SN1 includes a monomolecular reaction, while SN2 involves a bimolecular reaction.

What is SN1 Reaction?

The rate-determining step of the SN1 process, a nucleophilic substitution reaction, is unimolecular. Substitution nucleophilic unimolecular is referred to as SN1. Thus, the rate equation-which claims that the SN1 reaction depends on the electrophile but not the nucleophile-is valid when the quantity of the nucleophile is much higher than the amount of the carbocation intermediate.

A carbocation intermediate is created during this reaction. Generally speaking, it occurs when extremely acidic or strongly basic conditions are present when secondary or tertiary alkyl halides react with secondary or tertiary alcohols. In inorganic chemistry, the SN1 reaction is often called the dissociative mechanism. Examples of SN1-type nucleophilic substitution reactions are shown below.

The SN1 reaction mechanism proceeds step-by-step, starting with the carbocation formation via eliminating the leaving group. The nucleophile then attacks the carbocation. Ultimately, the protonated nucleophile is deprotonated to produce the desired product. The nucleophile does not affect the rate-determining step of this reaction, which solely relies on the electrophilicity of the leaving group.

Reaction Mechanism for SN1

The following stages help explain the SN1 reaction's mechanism using the hydrolysis of tertiary butyl bromide.

Step 1

• A polar covalent connection forms between carbon and bromine. This relationship may be severed, allowing the departing group to be eliminated (bromide ion).
• A carbocation intermediate is created when the bromide ion departs from the tertiary butyl bromide.
• This is the SN1 mechanism's rate-determining phase, as was already explained.
• It is essential to remember that the link between carbon and bromine breaks endothermically.

Step 2

• The nucleophile attacks the carbocation in the second phase of the SN1 reaction process.
• An intermediate called an oxonium ion is created since water is utilized as the solvent.
• Due to the neutral nature of the solvent, a third step involving deprotonation is required.

Step 3

• In the preceding phase, the oxygen on the carbocation received the positive charge.
• The necessary alcohol and a hydronium ion are produced due to the water solvent's new role as a base and its deprotonation of the oxonium ion.
• Steps 2 and 3 of this reaction happen quickly.

SN1 Reaction Stereochemistry

An sp2 hybridized carbon is the carbocation intermediate created in step 1 of the SN1 reaction process. Due to its trigonal planar molecular structure, it has two potential sites for the nucleophilic attack: left and right.

The carbocation is subsequently attacked equally from both sides of the reaction and occurs at a stereo enter. If neither route for the nucleophilic attack is favoured, generating an equal ratio of left and right-handed enantiomers as illustrated below.

What are SN2 Reactions?

The SN2 reaction process necessitates a nucleophile assault from the carbon atom's reverse side. Hence, the leaving group once occupied by the product is now opposite it in the stereochemical structure. This is referred to as configuration inversion. A stereospecific reaction is one in which several stereoisomers react to produce various stereoisomers of the result. The SN2 reaction is an excellent example of such a reaction. Moreover, the most typical instance of Walden inversion involves the SN2 reaction, in which an asymmetric carbon atom experiences a configuration inversion.

The SN2 reaction is a nucleophilic substitution process in which a bond is broken, and a new one is simultaneously created. The reaction phase that establishes the rate involves two responding species. Substitution Nucleophilic Bimolecular is what the abbreviation "SN2" means. Associative substitution, exchange mechanism, and bimolecular nucleophilic substitution are other names for this kind of reaction.

Here are some examples of SN2 reactions. The following elements have an impact on this sort of reaction's rate:

• The rear of the substrate, which is unobstructed, facilitates the development of the carbon-nucleophile connection. As a result, nucleophilic replacement of methyl and primary substrates is simple.
• Strong anionic nucleophiles hasten the reaction's pace. A stronger nucleophile may readily create the carbon-nucleophile bond because nucleophilicity rises with a larger negative charge.
• Although polar aprotic solvents do not obstruct the nucleophile, they create hydrogen bonds. Acetone works well as a solvent in this process.
• The stability of the leaving group's anion and the poor binding strength between the leaving group and carbon aid in accelerating SN2 reactions.

Since an SN2 Reaction is a second-order reaction, both the nucleophile concentration and the substrate concentration must be present for the rate-determining step to occur.

Reaction Mechanism for SN2

The nucleophile attacks the substrate from the rear for this reaction to continue. The angle of the nucleophile's approach to the supplied substrate concerning the carbon-leaving group bond is 180^o. Via a transition state, a bond between a carbon and a nucleophile develops, and a bond between a carbon and a leaving group dissolves simultaneously.

The leaving group is now forced out of the transition state on the other side of the carbon-nucleophile link, forming the necessary product. It's crucial to remember that the product is created by inverting the tetrahedral geometry at the central atom.

The following diagram shows how the SN2 reaction occurs when bromine acts as the nucleophile in the nucleophilic substitution of chloroethane.

Reactions of SN2 with Stereochemistry

The nucleophile may attack the stereocenter of the substrate in one of two ways:

The product's stereochemical conformation is retained due to a frontside assault, in which the nucleophile attacks from the same side as the departing group.

An inversion of the stereochemical configuration occurs in the end product due to a backside assault. The nucleophile hits the stereocenter from the side of the carbon-leaving group bond opposite to the stereocenter.

Pure SN2 reactions have a 100% inversion in stereochemical arrangement, indicating that a backside assault is how these reactions take place.

As a result, in the indicated substrates, the nucleophile displaces the leaving group. It should be emphasized that tertiary substrates cannot participate in SN2 reactions, although primary and secondary substrates may.

The Difference between SN1 and SN2 Reactions

Some of the major differences between SN1 and SN2 reactions are listed below:

Let us now closely examine the differences

→ Have a look at the earlier illustration of bromomethane. The CN- ion replaces Br once removed from the alkyl group. So there are two steps:

The first separation of the Br occurs as CH₃Br à CH₃ + BR.

This is referred to as the rate-limiting or slow step. The CN^- ion in the area is drawn to CH3 when the Br ion separates, and as it attacks CH3, it forms CH₃CN. Moreover, in SN1, the first step is regarded as the crucial phase. It is a unimolecular reaction as the first step only includes one particular kind of molecule.

Hence, it is a two-stage process, with the production of a cation being the first phase.

The OH anion assaults the CH₃Br molecule in the SN2 reaction as the Br attempts to escape from the CH₃Br compound. This transitional step culminates in a partly connected OH^- and a partially detached Br^-. The full dissociation of Br and the complete attachment of the OH anion mark the completion of the process.

Special Note

Have you noticed that in both situations, we utilized the CH₃Br example? That was just done for educational reasons. Indeed, CH₃Br generates methyl carbocation in SN1. Stage 2 of SN1 cannot come from this since it is unstable. Hence, rather than SN1, the CH₃Br reaction takes the form of SN2 substitution.

Conclusion

Hence, we may state that there are two nucleophilic substitution reactions, SN1 and SN2.

• A nucleophile is an atom or molecule with a lot of electrons.
• An electrophile is a nucleophile's opposite. A chemical species with a positive charge is called an electrophile.
• A process in which one functional group or atom is swapped out for another negatively charged functional group or atom is known as a nucleophilic substitution reaction.
• The SN1 reaction is monomolecular, while the SN2 reaction is bimolecular.
• There are two phases in SN1. One step is needed for SN2.

There is a phase in SN1 when carbocation formation occurs. The carbocation is subsequently drawn to the anion or negatively charged atoms or molecules. There is simply a transition step in SN2 and no intermediary formation occurs.