Reactivity of Alkyl Halides Purpose The student will study the relationship of structure and reactions reactivity in nucleophilic substitution Introduction In lecture we discussed the SNl and Sy2 organic reaction mechanisms. S1 reactions are given this abbreviation because they are substitution reactions (S), which involve a nucleophilic reactant (N). The “1” denotes that only one reactant is involved in the rate- limiting step. Similarly, the S2reaction is a substitution(s) involving a nucleophile (N) except that two reactants (2) are involved in the rate-limiting step. The Sy1 mechanism is a two-step process of forming a carbocation by the breaking of the carbon-leaving group bond. Once the carbocation is formed then the nucleophile can attack the positively charged carbon to form a new bond. An example of the mechanism is shown below :Br: Nu; Figure 1. Curved arrow mechanism for a typical Syl reaction. Note: the first step is the rate-determining step The rates of reaction for an SN1 reaction are related to carbocation formation. The rate of carbocation formation is related to the stability of the carbocations: tertiary> secondary> primary > methyl The solvent also influences the reaction rates. Generally, solvents that are polar and protic ravor Swl reactions because these solvents stabilize charged species. In addition, these polar solvents are poor nucleophiles that will not react with the substrate
The Correct Answer and Explanation is :
Correct Answer
In an SN1 (Substitution Nucleophilic Unimolecular) reaction, the rate-determining step is the formation of a carbocation intermediate. This step depends on the stability of the carbocation, which in turn influences the reaction rate. The greater the stability of the carbocation, the faster the reaction will proceed. The stability of the carbocation follows the order: tertiary > secondary > primary > methyl. Tertiary carbocations are the most stable due to hyperconjugation and inductive effects from the surrounding alkyl groups, which help to distribute the positive charge. Methyl carbocations are the least stable because they lack such stabilizing effects.
The solvent plays a key role in an SN1 reaction. Polar protic solvents (those capable of hydrogen bonding) generally favor SN1 reactions. These solvents stabilize the charged carbocation intermediate and the leaving group through solvation, making the carbocation more likely to form. Examples of polar protic solvents include water, alcohols, and acetic acid. These solvents are not good nucleophiles because the hydrogen bonding makes it harder for nucleophiles to attack the substrate, allowing the nucleophile to attack the carbocation after it has formed.
Thus, for an SN1 reaction to occur efficiently, the substrate must form a stable carbocation, and the solvent must be polar and protic. These factors work together to ensure that the reaction proceeds smoothly.
Explanation
In SN1 reactions, the first step involves the breaking of the carbon-leaving group bond, which leads to the formation of a carbocation. The stability of the carbocation is a major factor that affects the reaction rate. More stable carbocations form faster, leading to a faster overall reaction rate. The stability of the carbocation is highest for tertiary carbocations, which are stabilized by alkyl groups, and lowest for methyl carbocations, which have no such stabilizing effects.
In addition to carbocation stability, the solvent used in the reaction can influence the rate. Polar protic solvents, which can hydrogen bond, stabilize the carbocation and leaving group, making it easier for the carbocation to form. Since polar protic solvents are poor nucleophiles, they do not interfere with the nucleophilic substitution process once the carbocation is formed.
Image of SN1 Mechanism
I will now generate an image depicting the curved arrow mechanism for a typical SN1 reaction.
Here is the detailed diagram illustrating the SN1 reaction mechanism. It shows the two-step process: the departure of the leaving group (Br), the formation of the carbocation intermediate, and the nucleophile (Nu) attacking the positively charged carbon to form the final product. The diagram is labeled for clarity.
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