Difference Between The SN1 And SN2 Reaction

What Is SN1 Reaction?

The S_N1 reaction is a substitution reaction in organic chemistry. ‘’S_N’’ stand for ‘’nucleophilic substitution’’ and ‘’1’’ says that the rate-determining step is unimolecular. Thus, the rate equation is often shown as having first-order dependence on electrophile and zero-order dependence on nucleophile.

This relationship holds for situations where the amount of nucleophile is much greater than that of the intermediate. Instead, the rate equation may be more accurately described using steady-state kinetics. The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary halides under strongly basic conditions, with secondary or tertiary alcohols. In inorganic chemistry, the SN1 reaction is often referred to as the dissociative mechanism.

The SN1 reaction begins with the ionization or dissociation of the substrate molecule, usually an alkyl halide (e.g., R-X), in a polar solvent. This ionization step is typically initiated by the departure of a leaving group (X) from the substrate, leading to the formation of a carbocation (R⁺) and a halide ion (X⁻).

The carbocation is a highly reactive species with a positively charged carbon atom. The stability of the carbocation intermediate is a critical factor in the SN1 reaction. Tertiary carbocations (those attached to three alkyl groups) are more stable than secondary carbocations, which are in turn more stable than primary carbocations. This stability order influences the rate of the reaction.

In the second step of the SN1 reaction, a nucleophile (Nu⁻) in the reaction mixture attacks the carbocation. This nucleophilic attack leads to the formation of the final substitution product. The nucleophile can attack the carbocation from either side, resulting in the possibility of racemization or retention of stereochemistry, depending on the specific conditions.

Common examples of SN1 reactions involve the substitution of alkyl halides (e.g., tert-butyl chloride) in the presence of a nucleophile. The rate of the reaction is influenced by factors such as the stability of the carbocation, the strength of the nucleophile, and the nature of the leaving group.

Characteristics SN1 Reaction

• S_N1 is a two step process reaction. There is a loss of the leaving group to form a carbocation intermediate followed by a nucleophilic attack.
• S_N1 reaction is a first order reaction because the rate of reaction depends on the substrate only.
• In S_N1 reaction, both inversion and retention of configuration takes place, because the nucleophile can attack the substrate either front side or back side of the planar structure of the carbocation.
• S_N1 reaction is nucleophilic substitution uni-molecular, that is, only one molecule takes part in rate determining step.
• The S_N1 reaction tends to proceed with weak nucleophiles-generally neutral compounds such as solvents like CH₃OH, H2O, CH₃CH₂OH and so on.
• The rate of S_N1 reaction depends on the stability of the carbocation. 3^o >2^o>1^o carbocation.
• The rate of S_N1 reaction does not depend on the concentration and strength of nucleophile.
• Polar Protic solvent such as water, alcohol and carboxylic acids fours SN1 reaction. Polar Protic solvents dissolve both cation and anions in it.
• In S_N1 reaction, the rate of reaction is dependent on the stability of the carbocation, cation and anion.
• In S_N1 reaction, the big barrier is carbocation stability since the first step of the SN1 reaction is loss of a leaving group to give a carbocation, the rate of the reaction will be proportional to the stability of the carbocation.
• In S_N1 reaction involves the formation of a carbonium ion as an intermediate.
• The greater the stability of carbocation, the greater the tendency of S_N1 reaction.
• In S_N1reaction, rearrangement is possible.
• Racemic mixture is formed.

What Is SN2 Reaction?

S_N2 stands for ‘’substitution nucleophilic bimolecular’’ which means it will lead to the displacement of a group on a molecule and its rate will depend on the active participation of two reactants.

The S_N2 reaction involves displacement of a leaving group (usually a halide or a tosylate), by a nucleophile. This reaction works the best within methyl and primary halides because bulky alkyl groups block the backside attack of the nucleophile, but the reaction does work with secondary halides (although it is usually accompanied by elimination), and will not react at all with tertiary halides.

Whether an alkyl halide will undergo an S_N1 or S_N2 reaction depends upon a number of factors. Some of the more common factors include the nature of the carbon skeleton, the solvent, the leaving group and the nature of the nucleophile.

The SN2 reaction begins with the approach of a nucleophile (Nu⁻) to the substrate molecule, usually an alkyl halide (R-X). The nucleophile attacks the carbon atom bonded to the leaving group (X) from the opposite side, leading to the displacement of the leaving group and the formation of a new bond between the nucleophile and the carbon atom. This concerted mechanism means that the nucleophilic attack and the departure of the leaving group happen simultaneously.

The SN2 reaction results in the inversion of stereochemistry at the carbon atom that undergoes substitution. This is because the nucleophile approaches the substrate from the backside of the leaving group, leading to a complete reversal of the configuration (enantioselectivity).

Common examples of SN2 reactions include the substitution of primary alkyl halides (e.g., methyl or ethyl halides) with nucleophiles like hydroxide (OH⁻) or cyanide (CN⁻). These reactions are typically fast and result in the inversion of stereochemistry.

Characteristics SN1 Reaction

• S_N2 is a one-step process in which the addition of nucleophiles and the loss of the leaving group occur simultaneously.
• S_N2 reaction is a second order reaction because the rate of reaction depends on both the substrate and nucleophile.
• In S_N2 reaction, only inversion of configuration takes place, because the nucleophile can attack the substrate from the back side only.
• S_N2 reaction is nucleophilic substitution bi-molecular, that is, two molecules (both substrate and nucleophile) takes part in rate determining step.
• The S_N2 tends to proceed with strong nucleophiles-generally negatively charged nucleophiles such as CH₃O (-), CN (-), RS (-), N₃ (-), HO(-) and others.
• The rate of S_N2 reaction depends on the steric effect of the alkyl halide. The rate of S_N2 reaction increases 3^o> 2^o>1^o alkyl halide.
• The rate of S_N2 reaction depends on the concentration and strength of the nucleophile.
• Polar Aprotic solvents like DMSO, acetone, acetonitrile, DMF, DMA favors S_N2 reactions, because Polar Aprotic doesn’t  dissolve cations, it dissolves only anions in solution, so by taking Polar Aprotic solvent cations are removed and only Nu (:) is only anion present to attack substrate.
• In S_N2 reaction, the rate of reaction is inversely proportional to the bulkiness of C atom-attached groups.
• In S_N2 reaction, the big barrier is steric hindrance since the S_N2 proceeds through backside attack, the reaction will only proceed if the empty orbital is accessible. The more groups that are present around the vicinity of the leaving group, the slower the reaction will be.
• In S_N2 reaction involves the formation of an activated complex as the intermediate.
• The greater the stability of possible carbocation, the poor the tendency of S_N2 reaction.
• In S_N2 reaction, rearrangement is not possible.
• Walden inversion take place.

Also Read: Difference Between Protic And Aprotic Solvents

SN1 And SN2 Reactions: Key Differences

Reaction Rate

• SN1: A unimolecular reaction, where the rate-determining step involves only one molecule, typically a carbocation formation. Rate depends on the concentration of the substrate.
• SN2: A bimolecular reaction, where the rate-determining step involves both the substrate and the nucleophile. Rate depends on the concentrations of both reactants.

Kinetic Order

• SN1: First-order kinetics, as it depends on the concentration of the substrate.
• SN2: Second-order kinetics, as it depends on the concentrations of both the substrate and the nucleophile.

Reaction Mechanism

• SN1: Two-step process involving carbocation formation followed by nucleophilic attack.
• SN2: One-step concerted process where the nucleophile displaces the leaving group.

Stereochemistry

• SN1: Generally leads to racemization or the formation of a mixture of enantiomers due to the formation of a planar carbocation intermediate.
• SN2: Typically results in inversion of configuration (inversion of stereochemistry) at the chiral center.

Substrate Configuration

• SN1: Often occurs with substrates bearing bulky or polarizable leaving groups.
• SN2: Typically occurs with substrates bearing less hindered leaving groups.

Solvent Effects

• SN1: Can take place in both polar protic and polar aprotic solvents.
• SN2: Generally occurs in polar aprotic solvents, which facilitate nucleophilic attack.

Rearrangements

• SN1: Susceptible to carbocation rearrangements, leading to various products.
• SN2: Typically does not involve rearrangements since it is a concerted process.

Rate of Reaction

• SN1: Faster in the presence of increased temperature due to the first-order kinetics.
• SN2: Faster at lower temperatures because it is a bimolecular reaction.

Tertiary vs. Primary Substrates

• SN1: Often favors tertiary substrates because they can form more stable carbocations.
• SN2: Favors primary substrates because they provide less steric hindrance for nucleophilic attack.

Nucleophile Strength

• SN1: Nucleophile strength is less critical since the rate-determining step is the formation of the carbocation.
• SN2: Requires a strong nucleophile, as it directly participates in the rate-determining step.

Concentration of Nucleophile

• SN1: Concentration of the nucleophile does not significantly affect the rate of the reaction.
• SN2: Rate of the reaction is directly proportional to the concentration of the nucleophile.

Side Reactions

• SN1: Prone to side reactions such as carbocation rearrangements, elimination, and racemization.
• SN2: Tends to have fewer side reactions, resulting in a cleaner, more predictable outcome.

SN1 vs SN2 Reaction: Key Takeaway