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Difference between Sn1 and Sn2 reactions is that Sn1 is made up of one molecule, whereas Sn2 is made up of two. Sn1 and Sn2 are two types of nucleophilic substitution reactions, the former being the unimolecular reaction and the latter bimolecular. The two reactions are associated with the terminologies and working of nucleophilic substitution reactions.
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Key Terms: Sn1, Sn2, Nucleophilic Substitution Reaction, Nucleophiles, Electrons, Anions, Hydroxide Ion, Atoms, Molecules, Electron pair
What is a Nucleophile?
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An atom or molecule that has an electron pair(s) to donate is referred to as a nucleophile. In other words, it contains extra unused electrons, which cause it to be negatively charged.
Nucleophilic Atom
Nucleophiles are classified into two types:
- Neutral nucleophiles are molecules that have a single pair of electrons but have an overall neutral charge.
- Anions- Anions are chemical species that have a negative charge. Hydroxide ion, or OH negative, is an example.
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| Topic Related Concepts | ||
|---|---|---|
| Electrophilic Substitution | Potassium Ferrocyanide | Gattermann Reaction |
| Electrophile | Difference between Cations and Anions | Chemical Reactions of Haloalkanes and Haloarenes |
The reaction of Nucleophilic Substitution
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Any substitution reaction in which an atom or functional group is replaced by a negatively charged ion or by an atom or functional group with a lone pair of electrons. Negatively charged ions or atoms/molecules with a single pair of electrons will be drawn to the positively charged area of an atom or compound and will attempt to replace the functional group or atom that is already attached to the positive area.
Examples of Nucleophilic Substitution
Let's take a look at bromomethane. It has the chemical formula CH3Br. The alkyl CH3 atom is positive, whereas the atom of bromine is negative. When it reacts with the cyanide anion, it replaces the negatively charged Br with CN-. The reaction will be as follows:
CH3BR + CN- → CH3CN + BR-
Again, consider chloromethane or CH3Cl. In this case, CH3 is positive and Cl is negative. When CH3Cl reacts with hydroxide ions, the Cl is replaced by the negatively charged hydroxide ion. The reaction would be as follows:
CH3OH + Cl- → CH3Cl + OH-
In each of the two reactions, one negatively charged atom/molecule is replaced by another negatively charged atom/molecule.
Sn1 vs Sn2 Reaction
Difference between Sn1 and Sn2
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Sn1 is a unimolecular reaction, whereas Sn2 is a bimolecular reaction. The primary differences between Sn1 and Sn2 reactions are given below;
| Sn1 | Sn2 |
|---|---|
| Sn1 reaction is unimolecular. | Sn2 reaction is bimolecular. |
| 1st Order Kinetic Mechanism is followed. | 2nd Order Kinetic Mechanism is followed. |
| Two-step process | Single-Step Process |
| The concentration of the substrate is a necessary factor for the rate of reaction. | The concentration of both the substrate and the nucleophile is the necessary factor for reaction. |
| In Sn1, the substrate forms a carbocation intermediate. | In Sn2, the reaction happens in a single transition state. |
| It involves optically active substrates becoming inactive and the other half becoming similar. | It involves an inversion reaction. |
Previous Year Questions
- A compound of molecular formula… [NEET 2001]
- Z in the above reaction sequence is… [NEET 2002]
- Reactivity order of halides for dehydrohalogenation is… [NEET 2022]
- Consider the reaction… [NEET 2016]
- How many stereoisomers does this molecule… [NEET 2008]
- If there is no rotation of plane polarised light by a compound… [NEET 2007]
- In an SN1 reaction on chiral centres… [NEET 2015]
- Which one is most reactive towards… [NEET 2010]
- An organic compound on heating with… [KCET 2009]
- Following is the substitution reaction in which… [KCET 2011]
Things to Remember
- The nucleophilic substitution reactions Sn1 and Sn2 are two types of nucleophilic substitution reactions.
- An electron-rich atom or molecule is referred to as a nucleophile. Sometimes it takes the form of an anion, and other times it takes the form of a compound or atom with at least one lone pair of electrons.
- A nucleophilic substitution reaction is one in which one functional group or atom is replaced by another negatively charged functional group or atom.
- There is a stage in Sn1 where carbocation forms. The anion, or negatively charged atoms or compounds, is then drawn to the carbocation. In Sn2, there is only a transition stage and no intermediates are formed.
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Sample Questions
Ques: What is the definition of a Nucleophilic Substitution Reaction? (2 Marks)
Ans: The nucleophilic substitution reaction occurs when negatively charged atoms or molecules in a compound are replaced by another negatively charged atom or molecule. In this type of reaction, one functional group or atom/molecule is replaced by another functional group, atom, or molecule, thus the name substitution. Because negatively charged atoms or molecules are involved, it is referred to as nucleophilic.
Ques: Define Sn1 Reaction. (2 Marks)
Ans: Sn1 is a single-molecular substitution reaction. It starts with the separation of negatively charged functional groups or atoms. A carbocation is formed as a result of this. The carbocation is then attached to an anion or another negatively charged functional group or atoms.
Ques: Define Sn2 Reaction. (2 Marks)
Ans: A bimolecular nucleophilic substitution reaction is the Sn2 reaction. The attachment of the new negatively charged functional group or anion occurs concurrently with the detachment of the original negatively charged functional group, resulting in the formation of an intermediate. There is a second step. In the end, the replacement is finished.
Ques: Why can't CH3Br react with OH Negative in the same way that Sn1 does? (2 Marks)
Ans: If CH3BR follows the Sn1 pattern, it will be split into CH3 and BR-. CH3, on the other hand, is extremely unstable and cannot wait for the OH to form a bond. As a result, it can only form a bond in the Sn2 reaction because Sn2 is a transitional reaction in which the detachment and attachment processes occur simultaneously.
Ques: What factors influence Sn1 and Sn2 reaction rates? (2 Marks)
Ans: The rate of reaction in Sn1 is determined by the substrate concentration. In contrast, the rate of reaction in Sn2 is affected by both the concentration of the substrate and the concentration of the functional group.
Ques: What Is the Distinction Between Nucleophile and Electrophile? (2 Marks)
Ans: A nucleophile is an electron-rich chemical species. As a result, it is negatively charged. The electrophile, on the other hand, is a chemical species that lack electrons and thus is positively charged.
Ques: What happens during a substitution reaction? (2 Marks)
Ans: A substitution reaction, also known as a single displacement reaction or a single replacement reaction, is a chemical reaction in which one functional group in a chemical compound is replaced by another functional group. A good example of a replacement reaction is halogenation.
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