Determining SN1 Versus SN2 Reactions | Penji - The Easy-to-Use Student Services Platform (2024)
Substitution reactions may seem like a difficult concept to understand. However, as long as you follow these guidelines, you will be successful. When you first see a substitution reaction, you should answer these questions.
I will explain the questions in detail and then I will show you a practice problem.
Is the carbon bonded to the leaving group primary, secondary, or tertiary?
Observe the carbon bonded to the leaving group. To figure out if it is primary, secondary, or tertiary, look at the carbon bonded to the leaving group and count how many carbons are attached to it:
If 1 carbon is attached, we have a primary carbon.
If 2 carbons are attached, we have a secondary carbon.
If 3 carbons are attached, we have a tertiary carbon.
Primary carbons can only be SN2 substitutions. Tertiary carbons can only be SN1. Secondary, benzyllic, or allylic carbons can be either SN1 or SN2.
Here are examples of the types of carbons to look for.
Strong nucleophiles have negative charges but exceptions to this rule are halogens with negative charges and resonance stabilized negative charges. Strong nucleophiles indicate SN2 reactions while weak nucleophiles indicate SN1 reactions.
Strong nucleophile examples are CN-, OR-, OH-, RS-, NR2-, R-.
Weak nucleophile examples are RCO2–, HOR, H2O, HSR, HNR2, I–, Br–, or Cl–
Is this reaction SN1 or SN2?
Now that we have gone through the steps to answer a SN1 versus SN2 question, we will now go over a practice problem and answer the questions listed.
Is the carbon bonded to the leaving group primary, secondary, or tertiary? Looking at the leaving group Br, we see that the carbon bonded to the leaving group is a benzylic carbon. This means that this reaction can either be SN1 or SN2, so let's move on to the next question.
Is the nucleophile strong or weak? The nucleophile is CN- in the reaction. CN- is a strong nucleophile which indicates a SN2 reaction.
Is the nucleophile strong or weak? Strong nucleophiles have negative charges but exceptions to this rule are halogens with negative charges and resonance stabilized negative charges. Strong nucleophiles indicate SN2 reactions while weak nucleophiles indicate SN1
SN1
The unimolecular nucleophilic substitution (SN1) reaction is a substitution reaction in organic chemistry. The Hughes-Ingold symbol of the mechanism expresses two properties—"SN" stands for "nucleophilic substitution", and the "1" says that the rate-determining step is unimolecular.
Since an SN2 reaction depends on the concentration of nucleophile, while SN1 does not, set up two experiments exactly the same (same concentration of electrophile, same solvent, same temperature, etc) but double the amount of nucleophile in one of the experiments.
The two main nucleophiles are water and alcohols. In addition to the nucleophile, the solvent also plays a role in determining the major mechanism in nucleophilic substitution reactions. Here, you need to remember that polar aprotic solvents favor the SN2 mechanism, while polar protic solvents favor the SN1 mechanism.
SN1 reactions prefer polar protic solvents, proceed through carbocation intermediates, and are unimolecular in rate-determining steps. On the other hand, SN2 reactions favor polar aprotic solvents, proceed through concerted mechanisms, and are bimolecular in rate-determining steps.
In an SN1 reaction, the rate determining step is the loss of the leaving group to form the intermediate carbocation. The more stable the carbocation is, the easier it is to form, and the faster the SN1 reaction will be.
The relative reactivity of haloalkanes in SN1 reactions corresponds to the relative stability of carbocation intermediates that form during the reaction. We recall from Chapter 4 that the order of stability of carbocations is tertiary > secondary > primary.
This only happens when the base (if the mechanism is E2) or the nucleophile (if SN2) is strong – very reactive. If the base/nucleophile is weak, then the mechanism is unimolecular – E1 or SN1. Let's summarize this again: if strong – SN2 or E2, if weak – SN1 or E1.
Hence in SN2, there is a complete inversion of configuration. Stereochemistry of SN1: In the SN1 reaction, if the alkyl halide is optically active, then the product is a racemic mixture. In the SN1 reaction, nucleophile attacks occur from both the backside and front side.
2) The nucleophile: powerful nucleophiles, especially those with negative charges, favor the SN2 mechanism. Weaker nucleophiles such as water or alcohols favor the SN1 mechanism. 3) The solvent: Polar aprotic solvents favor the SN2 mechanism by enhancing the reactivity of the nucleophile.
Formation of Tosylates: An example of an SN1 reaction used in synthetic chemistry is the conversion of alcohols to tosylates (OTs) using p-toluenesulfonyl chloride (TsCl) in the presence of a base. The tosylate group is an excellent leaving group, making the subsequent SN1 reaction more efficient.
The reaction requires a collision between the nucleophile and the molecule, so increasing the concentration of either will increase the rate of the reaction. Since the unique geometry of back side attack is required, the most important factor in determining whether an SN2 reaction will occur is steric effects.
As the reaction is a single step, it is the rate-determining step as well and has one transition state. Now let's understand the SN2 reaction mechanism by an example of SN2 reaction- bromide (nucleophile, Br-) attacks on ethyl chloride (the electrophile) and results in ethyl bromide and chloride ions as products.
The SN2 reaction — A nucleophilic substitution in which 2 components are included in the rate-determining stage. -SN2 reactions are bimolecular with bond and bond-breaking steps simultaneously.
Examples: dimethylsulfoxide, dimethylformamide, acetone, etc. In parallel, solvation also has a significant impact on the intrinsic strength of the nucleophile, in which strong interactions between solvent and the nucleophile, found for polar protic solvents, furnish a weaker nucleophile.
An SN2 reaction occurs if a good nucleophile that is a weak bases is used in a polar aprotic solvent. An SN1 reaction along with an E1 reaction occurs if a poor nucleophile that is a weak bases is used in a protic solvent.
Lucas test is based on the difference in reactivity of alcohols with hydrogen halide. Primary secondary and tertiary alcohols react with hydrogen halide (hydrochloric acid) at different rates. It follows the SN1 reaction mechanism.
Introduction: My name is Horacio Brakus JD, I am a lively, splendid, jolly, vivacious, vast, cheerful, agreeable person who loves writing and wants to share my knowledge and understanding with you.
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