What is the Difference Between SN1 SN2 E1 and E2 Reactions

SN1, SN2, E1, and E2 are all common types of organic reactions that deal with nucleophilic substitution and elimination. Substitutions involve a new atom or group replaces another while eliminations involve the removal of two functional groups to create a double bond.

What is the main difference between SN1 SN2 E1 and E2 reactions? SN1 is favored by tertiary (3°) or secondary (2°) substrates due to more stable carbocation intermediates. SN2 is favored by primary (1°) substrates with less steric hindrance for the nucleophile to approach, while E1 and E2 have similar preference for tertiary substrates for stable carbocation formation (E1) or ease of proton removal (E2).

Key Areas Covered 

1. What is SN1 Reaction
     – Definition, Features
2. What is SN2 Reaction
     – Definition, Features
3. What is E1 Reaction
     – Definition, Features
4. What is E2 Reaction
     – Definition, Features
5. Similarities Between SN1 SN2 E1 and E2 Reactions
     – Outline of Common Features
6. Difference Between SN1 SN2 E1 and E2 Reactions
     – Comparison of Key Differences
7. FAQ: SN1 SN2 E1 and E2 Reactions
     – Answers to Frequently Asked Questions

Key Terms

SN1 Reaction, SN2 Reaction, E1 Reaction, E2 Reaction

What is SN1 Reaction

The SN1 reaction is short for “substitution nucleophilic unimolecular.” It a type of organic reaction where a nucleophile (electron-donating molecule) replaces a leaving group (departing atom) in a substrate molecule.

The SN1 mechanism follows a two-step process. First, the leaving group departs, forming a positively charged carbocation intermediate. This step is slow because it requires breaking a bond and creating a high-energy carbocation. Then, the nucleophile attacks the positively charged carbon of the carbocation, forming a new bond and kicking out a lone pair of electrons. This step is fast because the carbocation is very reactive.

SN1 reactions favor substrates with tertiary or secondary carbons. These create more stable carbocations due to electron-donating effects from surrounding alkyl groups. It also favors groups like chloride, bromide, or iodide, which are good at leaving with their electron pair.

What is SN2 Reaction

The SN2 reaction, or “substitution nucleophilic bimolecular,” is another type of nucleophilic substitution reaction. Bimolecular means that both the nucleophile and the substrate participate in the rate-determining step, meaning their concentrations affect the reaction rate.

In the SN2 mechanism, the nucleophile first attacks the substrate’s carbon atom from the back side, opposite the leaving group, in a single concerted step. This backside attack is crucial for efficient bond formation and breaking. Primary substrates with minimal steric hindrance (bulky groups) around the carbon allow for a smooth nucleophilic approach.

What is E1 Reaction

E1 reactions, which stands for “elimination unimolecular,” involve the removal of two functional groups from a molecule to form a double bond.

The key step in the E1 mechanism is the slow dissociation of a leaving group (often a halide) to create a carbocation intermediate. A nearby hydrogen atom (usually beta to the carbocation) is removed by a base (electron-pair donor). This creates the double bond and eliminates a molecule (often HX-like water)

What is E2 Reaction

E2 reactions are a type of elimination reaction in organic chemistry. E2 means “elimination, bimolecular.” This means that two substances are involved in the rate-determining step of the reaction.

In an E2 reaction, a base abstracts a proton from a beta-carbon adjacent to a carbon bearing a leaving group. Simultaneously, the bond between the carbon and the leaving group breaks, forming a double bond. The reaction occurs in a single concerted step. Factors like the strength of the base, the nature of the leaving group, and the stereochemistry of the substrate influence the outcome of the E2 reaction.

Similarities Between SN1 SN2 E1 and E2 Reactions

1. All four reactions result in the formation of a new bond between the substrate and another molecule.
2. In all cases, the hybridization of the carbon atom undergoing the reaction changes.
3. All these reactions involve a nucleophile, a species with a lone pair of electrons that can attack a positively charged center.

Difference Between SN1 SN2 E1 and E2 Reactions

Definition

• SN1 reaction is a substitution reaction where a nucleophile replaces a leaving group in a substrate molecule while SN2 reaction is a substitution reaction where a nucleophile attacks the substrate in a single step, displacing the leaving group simultaneously. E1 reaction is an elimination reaction where the rate-determining step involves the formation of a carbocation intermediate followed by the loss of a proton to form a double bond. E2 reaction, on the other hand, is an elimination reaction where a base removes a proton while the leaving group simultaneously departs, forming a double bond in a single concerted step.

Mechanism

• In SN reactions, nucleophile replaces a leaving group on a molecule, whereas in E reactions, molecule loses a leaving group and a proton, forming a double bond.

Leaving Group

• In SN1 & E1 reactions, the leaving group departs first, forming a carbocation intermediate, whereas in SN2 & E2 reactions, the leaving group departs simultaneously with another process (nucleophilic attack in SN2, base abstraction in E2).

Rate Determining Step

• SN1 & E1 reactions depend on the substrate concentration (formation of carbocation) while SN2 & E2 reactions depend on the concentration of both substrate and nucleophile/base.

Conclusion

SN1 and SN2 are substitution reactions, while E1 and E2 are elimination reactions. This is the basic difference between SN1 SN2 E1 and E2 reactions. All reactions involve a nucleophile and lead to a change in the carbon’s hybridization.

FAQ: SN1 SN2 E1 and E2 Reactions 

1. Why is SN2 faster than SN1?

SN2 reactions are generally faster than SN1 due to a one-step process. SN2 involves simultaneous bond breaking and formation, while SN1 has a slow carbocation formation step.

2. What is the main difference between SN1 and SN2?

The main difference between SN1 and SN2 lies in the mechanism.SN1 has a two-step process with a carbocation intermediate, making it slow and favored by tertiary substrates. SN2 reacts in a single step and is faster.

3. What is an example of an SN1 reaction?

The hydrolysis of tert-butyl bromide is an example of an SN1 reaction. The weak C-Br bond breaks in water, forming a tert-butyl carbocation (slow). Water then acts as a nucleophile, bonding to the carbocation to form tert-butyl alcohol.

4. What is the main difference between E1 and E2?

E1 is a two-step process with a carbocation intermediate, favoring high temperatures and weak bases. E2 reacts in one step with a strong base nearby, leading to faster reactions with anti-periplanar geometry.

5. What is an example of an E1 reaction?

A common E1 reaction example is heating 2-propanol with concentrated sulfuric acid. Here, the acid protonates the alcohol first. Then, a water molecule leaves as a leaving group, forming a carbocation intermediate. Finally, another molecule removes a proton to create propene, an alkene.

Reference:

1. “Elimination Reaction.”  Wikipedia. Wikipedia Foundation.
2. “Substitution Reaction.” Encyclopedia Britannica. 

Image Courtesy:

1. “SN1 Reaction Mechanism” By Chemicalbonding – Own work (CC0) via Commons Wikimedia
2. “SN2 Mechanism with the change of Stereochemistry” By DNL.Lawliet – Own work (CC BY-SA 4.0) via Commons Wikimedia
3. “E1 Elimination Reaction” By V8rik at the English-language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
4. “E2 mechanism” By Jordi picart – Own work (CC BY-SA 3.0) via Commons Wikimedia