Sn2 Reaction Mechanism - Examples, Factors, Reaction Rate, FAQs

Q: What is SN2 reaction order?

The order of S N 2 reaction is two.

Q: Give a stereospecific reaction example?

S N 2 is an example for stereospecific reaction.

Sn2 Reaction Mechanism - Examples, Factors, Reaction Rate, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:46 PM IST

S_N2 Reaction

Substitution nucleophilic bimolecular reaction is a type of reaction mechanism commonly used in organic compounds. It is also a type of nucleophilic substitution reaction. The bimolecular in the name suggests that there are two species involved in the rate-determining step that is the slow step. S_N1 is another type of nucleophilic substitution reaction. S_N2 reaction also given the name interchange mechanism and associative substitution. S_N2 reaction is a single-step reaction in which a bond is broken and a new bond is formed suddenly.

This Story also Contains

SN2 Reaction
What is SN2 Reaction Mechanism?
Some other Examples of SN2 Nucleophilic Substitution
Factors Affecting SN2 Reaction Mechanism
SN2 Reaction Rate

The two stand for bimolecular which means there are 2 components included in the substitution reaction. In this reaction the nucleophilic substitution of a leaving group takes place and the leaving group generally used is halide groups or other electron-withdrawing groups and a new compound is formed after the reaction.

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What is S_N2 Reaction Mechanism?

The S_N2 reaction involves the attack of nucleophile from the backside of the carbon skeleton so there will be an inversion of configuration that is the stereochemistry just becoming opposite to that originally present. So, for this reaction, the role of the steric effect is more. It is also the best example of a stereospecific reaction.

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A bimolecular nucleophilic substitution reaction takes place in a single step so it is also called a concerted reaction. Here the bond breaking between the electrophilic carbon and the leaving group, and bond making between the nucleophile and electrophile carbon occurs at the same time. So the number of steps in S_N2 is one. The leaving group most commonly used is halides.

S_N2 Mechanism

The reaction mechanism involves the formation of a single transition state. The bond breaking occurs at the C-X bond and the carbon present is mostly aliphatic. After the breaking of the C-X bond nucleophile attacks and a new bond, C-Nu is formed. Where Nu represents nucleophile, a nucleophile is a species that is rich in electrons and has a tendency to attack electrophiles that are electron-deficient species. In the formation of the C-Nu bond, there is an intermediate state where the carbon is pentacoordinate. And after its formation, the leaving group leaves, and a new bond C-Nu is formed. All these reactions are taking place spontaneously.

S_N2 Reaction Examples:

S_N2 reaction examples are described in this portion. The following reaction shows the attack of the halogen Br^- which is the nucleophile on the carbon atom attached to a leaving group that is Cl. And correspondingly a pentacoordinate transition state is formed and is short-lived. After the formation of the transition state, it spontaneously dissociates to form the product with the breaking of the bond between C-Cl.

S_N2 The reaction mechanism in chloropropane using bromine as a nucleophile is shown below. And the stereochemistry of S_N2 reaction is shown below.

SN2 reaction mechanism in chloropropane.

In the ethyl chloride, the attack of nucleophile is from the backside so the stereochemistry gets inverted. This inversion in configuration is called Walden Inversion. Since it involves the attack from the backside, the backside should not be sterically hindered. That is primary and secondary carbon is more preferred on S_N2 reactions. If bulky substituents or tertiary carbon are present the reaction will not proceed. Tertiary carbon is more preferred in S_N1 reaction.

Some other Examples of S_N2 Nucleophilic Substitution

The nucleophilic substitution reaction taking place on 2-Bromobutane with the nucleophile OH^- is shown below. In the first step, an attack takes place from the backside and there is a formation of transition state which then suddenly dissociates to form the product. Here the leaving group is Br^-.

The S_N2 reaction mechanism taking place in 2-bromopropane using hydroxyl ion as a nucleophile is shown below.

SN2 reaction mechanism in 2-bromopropane.

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Factors Affecting S_N2 Reaction Mechanism

Effect of substrate

The substrate present should be free from bulky groups since the attack is taking place from the backside. And also the breaking and making of bonds are taking place in a single step the selection of substrate is very important. The carbon present should not be tertiary also.

Effect of nucleophile

The selection of nucleophiles has also a very good role in the reaction mechanism of S_N2. The nucleophile should also be in a sterically unhindered position. The strength of a nucleophile is greatly affected by its charge. With the increasing negative charge nucleophilicity also increases. For example OH^- is a better nucleophile in comparison to water it can act as a very good nucleophile in the S_N2 reaction.

Effect of solvent

The solvent will affect the rate of reaction in such a way that polar aprotic solvents are better solvents in comparison to polar protic solvents because polar protic solvents tend to form hydrogen bonds with the nucleophile thereby lowering its tendency to attack the carbon with the leaving group. Some of the best examples for solvents that can be used in S_N2 reaction mechanism is dimethyl sulfoxide, dimethylformamide, acetone, DMSO, etc.

Effect of leaving group

The very beginning of the reaction depends on the breaking of the bond between the carbon and the leaving group so the strength of the bond between carbon and the leaving group has a greater impact on the rate of S_N2 reaction. Halides are commonly used as a leaving group except for fluorine because fluorine forms a very strong bond with carbon due to its high electronegativity nature. Tosylate is also a very good example for leaving group but OH^- and NH2^- are not used as a leaving group because of its strong bond formation with the carbon.

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S_N2 Reaction Rate

The order of S_N2 reaction is second as it depends on both the concentration of substrate and the nucleophile and is present in the rate-determining step. The rate of S_N2 reactions can be written as,

r=k[RX][Nu^-]

Where r is the rate, and k is the reaction constant. Therefore the reaction rate will depend on the concentration of nucleophiles and the substrate which is undergoing attack. This means that the increasing concentration of them will increase the rate of S_N2 reaction. The figure below shows the formation of the transition state and the corresponding formation of the product.

Energy profile diagram and the formation transition state.

NCERT Chemistry Notes:

S_N1 and S_N2 Reactions Examples

S_N1 reaction is also a type of nucleophilic substitution reaction in which it is unimolecular that the order of this reaction is one. In comparison to S_N2 reaction the reaction occurs in two steps. The first step involves the formation of a carbocation then the nucleophilic attack takes place resulting in the formation of the corresponding product. The stereochemistry of S_N2 reaction is also different. The following reaction shows the S_N1 and S_N2 pathways.

SN1 and SN2 pathway.

The formation of the transition state is only present in the case of S_N2 reaction. And also there is no direct interaction between the given substrate and nucleophile; it only happens when the leaving group leaves. While in the case of S_N2 reaction there is a direct collision between the nucleophile and the substrate. The solvent used in S_N1 reaction is polar protic solvents. Water is the most commonly used solvent in S_N1 reaction.

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Frequently Asked Questions (FAQs)

1. Which is the best solvent for SN2?

Polar aprotic solvents like DMSO, acetone, sulfoxide, etc are used as a solvent in S_N2 reaction mechanism.

2. What is SN2 reaction order?

The order of S_N2 reaction is two.

3. Give a stereospecific reaction example?

S_N2 is an example for stereospecific reaction.

4. What types of substrates are most favorable for SN2 reactions?

SN2 reactions are most favorable with primary alkyl halides or other good leaving groups. They can also occur with secondary substrates, but are generally not observed with tertiary substrates due to steric hindrance.

5. Why are tertiary substrates usually unreactive in SN2 reactions?

Tertiary substrates are usually unreactive in SN2 reactions due to steric hindrance. The bulky groups around the reaction center prevent the nucleophile from approaching from the backside, which is necessary for the SN2 mechanism.

6. What is the difference between SN1 and SN2 reactions?

The main differences are: 1) SN2 is a one-step process while SN1 is two-step, 2) SN2 involves backside attack and inversion of configuration, while SN1 can lead to racemization, 3) SN2 is favored by primary substrates, while SN1 is favored by tertiary substrates, and 4) the rate law for SN2 is second-order, while for SN1 it's first-order.

7. Can SN2 reactions occur at sp2 hybridized carbons?

SN2 reactions typically do not occur at sp2 hybridized carbons. This is because the backside attack required for SN2 reactions is not possible due to the planar geometry of sp2 carbons. Additionally, the C-X bond in vinyl or aryl halides is usually stronger than in alkyl halides.

8. How does temperature affect SN2 reactions?

Increasing temperature generally increases the rate of SN2 reactions. This is because higher temperatures provide more energy for molecules to overcome the activation energy barrier. However, very high temperatures might favor elimination reactions over substitution.

9. What is the energy profile of an SN2 reaction?

The energy profile of an SN2 reaction shows a single transition state with no intermediate. The energy rises to a maximum at the transition state (where the nucleophile is attacking and the leaving group is departing) and then falls to the products. This single-hump profile is characteristic of a concerted mechanism.

10. Can SN2 reactions occur at neopentyl centers?

SN2 reactions are extremely slow or do not occur at neopentyl centers. The four methyl groups around the central carbon create significant steric hindrance, preventing the nucleophile from approaching for backside attack.

11. How does the leaving group's basicity affect SN2 reactions?

In general, weaker bases make better leaving groups in SN2 reactions. This is because weaker bases are more stable when they leave with the electron pair. For example, I- is a better leaving group than F- because it's a weaker base.

12. What is the Hammond Postulate and how does it apply to SN2 reactions?

The Hammond Postulate states that the transition state in a reaction resembles the species closest to it in energy. In SN2 reactions, the transition state is generally closer in energy to the reactants, making it an early transition state. This means the bond-making process is less advanced than the bond-breaking process in the transition state.

13. How do neighboring group effects influence SN2 reactions?

Neighboring group effects can significantly influence SN2 reactions. For example, a neighboring group capable of stabilizing a positive charge (like a phenyl group) can assist in the departure of the leaving group, potentially changing the mechanism from SN2 to SN1.

14. How does solvent polarity affect SN2 reactions?

Polar aprotic solvents, such as acetone or dimethyl sulfoxide (DMSO), generally increase the rate of SN2 reactions. These solvents don't hydrogen bond with the nucleophile, making it more reactive. Protic solvents tend to slow down SN2 reactions.

15. What is the rate equation for an SN2 reaction?

The rate equation for an SN2 reaction is: Rate = k[substrate][nucleophile], where k is the rate constant, and [substrate] and [nucleophile] are the concentrations of the substrate and nucleophile, respectively. This shows that the reaction is second-order overall.

16. How does the strength of the nucleophile affect SN2 reactions?

Stronger nucleophiles generally lead to faster SN2 reactions. The strength of a nucleophile is often related to its basicity, but factors like polarizability and solvent effects also play a role.

17. How does increasing the concentration of the nucleophile affect the rate of an SN2 reaction?

Increasing the concentration of the nucleophile will increase the rate of an SN2 reaction. This is because the rate is directly proportional to the concentration of both reactants, as shown in the rate equation: Rate = k[substrate][nucleophile].

18. How does the nature of the leaving group affect SN2 reactions?

Better leaving groups enhance the rate of SN2 reactions. Generally, weaker bases make better leaving groups. For example, iodide is a better leaving group than chloride, so alkyl iodides typically undergo SN2 reactions faster than alkyl chlorides.

19. Can SN2 reactions occur with chiral substrates?

Yes, SN2 reactions can occur with chiral substrates. In fact, when a chiral substrate undergoes an SN2 reaction, it results in inversion of configuration at the stereocenter, a phenomenon known as Walden inversion.

20. Can SN2 reactions occur in biological systems?

Yes, SN2 reactions can occur in biological systems. They are important in various enzymatic reactions and biochemical processes. For example, the methyl transfer reactions catalyzed by S-adenosyl methionine (SAM) often proceed through an SN2 mechanism.

21. What is meant by "inversion of configuration" in SN2 reactions?

"Inversion of configuration" in SN2 reactions refers to the change in stereochemistry at the reaction center. The nucleophile attacks from the side opposite to the leaving group, causing the other substituents to flip to the opposite side, like an umbrella turning inside out.

22. How do SN2 reactions compare to E2 reactions?

SN2 and E2 reactions are often competing pathways. Both are bimolecular and favor primary or secondary substrates. The main difference is that SN2 leads to substitution while E2 leads to elimination. Strong, hindered bases and higher temperatures tend to favor E2 over SN2.

23. How does an electron-withdrawing group on the substrate affect SN2 reactions?

Electron-withdrawing groups on the substrate generally increase the rate of SN2 reactions. They make the carbon center more electrophilic, facilitating the attack by the nucleophile. However, if they're too close to the reaction center, they might cause steric hindrance.

24. What is an SN2 reaction mechanism?

An SN2 reaction mechanism is a type of nucleophilic substitution reaction where a nucleophile attacks a molecule, displacing a leaving group in a single step. The "2" in SN2 stands for bimolecular, meaning the rate of reaction depends on the concentration of both the nucleophile and the substrate.

25. How does the stereochemistry change in an SN2 reaction?

In an SN2 reaction, the stereochemistry at the reaction center is inverted. This is because the nucleophile attacks from the back side, opposite to the leaving group, resulting in a complete inversion of configuration at the carbon center.

26. What is the transition state of an SN2 reaction like?

The transition state of an SN2 reaction is a pentacoordinate structure where the nucleophile is partially bonded to the substrate, and the leaving group is partially detached. This results in a trigonal bipyramidal geometry around the central carbon atom.

27. What is meant by "backside attack" in SN2 reactions?

"Backside attack" in SN2 reactions refers to the approach of the nucleophile from the side opposite to the leaving group. This is crucial for the SN2 mechanism and results in the inversion of stereochemistry at the reaction center.

28. Why are primary alkyl halides more reactive in SN2 reactions compared to secondary ones?

Primary alkyl halides are more reactive in SN2 reactions because they have less steric hindrance around the reaction center. This allows the nucleophile to more easily approach from the backside, which is necessary for the SN2 mechanism.

29. What role does the solvent play in SN2 reactions?

The solvent plays a crucial role in SN2 reactions. Polar aprotic solvents like acetone or DMSO are ideal as they don't solvate the nucleophile, making it more reactive. Protic solvents can hydrogen bond with the nucleophile, reducing its reactivity and slowing the reaction.

30. How does the size of the nucleophile affect SN2 reactions?

Smaller nucleophiles generally lead to faster SN2 reactions. This is because they can more easily approach the backside of the substrate without steric hindrance. Bulky nucleophiles may struggle to perform the backside attack necessary for SN2 reactions.

31. Can SN2 reactions occur in the gas phase?

Yes, SN2 reactions can occur in the gas phase. In fact, gas-phase SN2 reactions are often faster than those in solution because there's no solvent to interfere with the nucleophile's approach. However, most practical applications of SN2 reactions occur in solution.

32. How does deuterium labeling help in studying SN2 reactions?

Deuterium labeling is useful in studying SN2 reactions because it can help track the stereochemical course of the reaction. If a chiral carbon bonded to deuterium undergoes an SN2 reaction, the inversion of configuration can be easily detected, confirming the SN2 mechanism.

33. What is the difference between a good nucleophile and a good leaving group in SN2 reactions?

A good nucleophile is typically a strong base with a high electron density, while a good leaving group is usually a weak base that's stable when it leaves with an electron pair. For example, OH- is a good nucleophile but a poor leaving group, while Br- is a moderate nucleophile but an excellent leaving group.

34. How do SN2 reactions differ in protic vs. aprotic solvents?

SN2 reactions are generally faster in aprotic solvents. In protic solvents, the nucleophile is often hydrogen-bonded to the solvent, reducing its reactivity. Aprotic solvents don't form these hydrogen bonds, leaving the nucleophile "naked" and more reactive.

35. What is the role of entropy in SN2 reactions?

SN2 reactions have a negative entropy of activation because two species (nucleophile and substrate) come together to form one transition state. This unfavorable entropy change contributes to the activation energy of the reaction.

36. How do SN2 reactions compare in cyclic vs. acyclic systems?

SN2 reactions are generally slower in cyclic systems compared to acyclic ones. This is because the backside attack required for SN2 can be hindered by the ring structure. However, small, strained rings (like cyclopropyl) can sometimes undergo SN2 reactions more readily due to ring strain relief.

37. Can SN2 reactions occur at carbonyl carbons?

While traditional SN2 reactions don't occur at sp2 hybridized carbons like those in carbonyls, a related process called "addition-elimination" can occur. This is sometimes referred to as an SN2' mechanism and is common in nucleophilic acyl substitution reactions.

38. How does the presence of an electron-donating group on the substrate affect SN2 reactions?

Electron-donating groups on the substrate generally decrease the rate of SN2 reactions. They make the carbon center more electron-rich, which can repel the incoming nucleophile. However, if they're not too close to the reaction center, their effect may be minimal.

39. What is the significance of kinetic isotope effects in SN2 reactions?

Kinetic isotope effects, particularly secondary kinetic isotope effects, can provide valuable information about SN2 reactions. A small, normal secondary kinetic isotope effect is typically observed, consistent with the change in hybridization from sp3 to sp2 in the transition state.

40. How do SN2 reactions compare in intramolecular vs. intermolecular cases?

Intramolecular SN2 reactions (where the nucleophile and electrophile are part of the same molecule) can sometimes occur more readily than intermolecular ones due to proximity effects. However, they may also be subject to strain or unfavorable geometries depending on the molecular structure.

41. What is the effect of conjugate bases on SN2 reactions?

Conjugate bases are often better nucleophiles than their conjugate acids because they carry a negative charge. For example, RO- is a better nucleophile than ROH in SN2 reactions. This is why increasing the pH (which increases the concentration of conjugate bases) can sometimes accelerate SN2 reactions.

42. How do SN2 reactions behave with respect to the principle of microscopic reversibility?

The principle of microscopic reversibility states that the forward and reverse reactions must proceed through the same transition state. In SN2 reactions, this means that if the forward reaction proceeds with inversion of configuration, the reverse reaction must also proceed with inversion.

43. What is the role of frontier molecular orbitals in SN2 reactions?

In SN2 reactions, the key interaction is between the highest occupied molecular orbital (HOMO) of the nucleophile and the lowest unoccupied molecular orbital (LUMO) of the substrate. The LUMO of the substrate is typically the σ* antibonding orbital of the C-X bond, where X is the leaving group.

44. How do SN2 reactions compare in aqueous vs. non-aqueous conditions?

SN2 reactions are generally slower in aqueous conditions compared to non-aqueous aprotic solvents. Water, being protic, can hydrogen bond with and solvate the nucleophile, reducing its reactivity. Additionally, water can compete as a nucleophile, potentially leading to hydrolysis products.

45. What is the effect of crown ethers on SN2 reactions?

Crown ethers can significantly enhance the rate of SN2 reactions, especially those involving alkali metal salts as nucleophiles. They do this by complexing the metal cation, effectively "naked" the anion and making it a more potent nucleophile.

46. How do SN2 reactions behave with respect to the reactivity-selectivity principle?

The reactivity-selectivity principle generally doesn't apply strongly to SN2 reactions. Unlike SN1 reactions, where more reactive conditions lead to less selectivity, SN2 reactions maintain their stereospecificity (inversion of configuration) regardless of the reactivity of the system.

47. What is the significance of the linear free energy relationship in SN2 reactions?

Linear free energy relationships, such as the Hammett equation, can be useful in studying SN2 reactions. They allow us to quantify the effect of substituents on reaction rates and can provide insight into the nature of the transition state and the extent of bond formation/breaking.

48. How do SN2 reactions compare in terms of regio- and stereoselectivity?

SN2 reactions are highly stereospecific, always resulting in inversion of configuration at the reaction center. In terms of regioselectivity, SN2 reactions strongly favor attack at less hindered sites, typically primary over secondary carbons, and almost never occur at tertiary carbons.

49. What is the effect of phase-transfer catalysis on SN2 reactions?

Phase-transfer catalysis can greatly enhance the rate of SN2 reactions, especially when the nucleophile and substrate are in different phases. The catalyst helps transfer the nucleophile into the organic phase where the substrate is located, increasing the effective concentration of the reactants.

50. How do SN2 reactions behave with respect to the principle of hard and soft acids and bases (HSAB)?

According to HSAB theory, "soft" nucleophiles (those with high polarizability and low electronegativity) tend to favor the SN2 pathway, while "hard" nucleophiles are more likely to follow other mechanisms. This is because soft nucleophiles can more easily distort their electron clouds to approach the backside of the substrate.

51. What is the significance of α-effect in SN2 reactions?

The α-effect refers