Question

2\. For compound shown, the incorrect statement is : $$ \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\stackrel{\alpha}{\mathrm{CH}}_{2}-\mathrm{Br} $$ (A) \(\mathrm{CH}_{3}\) substitution at \(\alpha\) decreases rate of \(\mathrm{S}_{\mathrm{N}} 2\) (B) \(\mathrm{CH}_{3}\) substitution at \(\beta\) decreases rate of \(\mathrm{S}_{\mathrm{N}} 2\) (C) \(\mathrm{CH}_{3}\) substitution at \(\alpha\) decreases rate of \(\mathrm{S}_{\mathrm{N}} 1\) (D) \(\mathrm{OCH}_{3}\) substitution at \(\alpha\) increases rate of \(\mathrm{S}_{\mathrm{N}} \mathrm{N}\)

Short answer

The incorrect statement is: (B) CH3 substitution at 𝛽 decreases the rate of S_N2.

Step-by-step solution

Understand SN1 and SN2 reactions

First, let's understand what S_N1 and S_N2 reactions are. S_N1 (substitution nucleophilic unimolecular) and S_N2 (substitution nucleophilic bimolecular) are two different types of nucleophilic substitution reactions. In an S_N1 reaction, the leaving group (in this case, Br) departs first, forming a carbocation intermediate. The nucleophile then attacks the carbocation to form the product. In an S_N2 reaction, the nucleophile attacks the molecule simultaneously with the departure of the leaving group, with no intermediate formed.

Analyze the effect of substitutions on SN1 and SN2 reactions

Let's analyze the effect of substitutions on the rate of S_N1 and S_N2 reactions: (A) CH3 substitution at 𝛼 decreases the rate of S_N2: Substitution of a CH₃ group at the 𝛼 position increases the steric hindrance, making it more difficult for the nucleophile to reach the leaving group, and thus decreasing the rate of S_N2 reaction. (B) CH3 substitution at 𝛽 decreases the rate of S_N2: Substitution of a CH₃ group at the 𝛽 position might slightly affect the S_N2 reaction but not significantly since it doesn't affect the steric hindrance directly at the reaction site. Hence, it won't significantly decrease the rate of an S_N2 reaction. (C) CH3 substitution at 𝛼 decreases the rate of S_N1: Substitution of a CH₃ group at the 𝛼 position increases the steric hindrance, which might slightly decrease the rate of S_N1 reaction. However, it also increases the stability of the resulting carbocation due to hyperconjugation, which may actually favor the S_N1 reaction. (D) OCH3 substitution at 𝛼 increases the rate of SNN: There is a typo here, and we can assume it meant S_N1 or S_N2. Substituting an OCH₃ group at the 𝛼 position makes the reaction more polar, which may favor S_N1 and S_N2 reactions. Putting aside the typo, the overall idea behind the statement is true.

Identify the incorrect statement

Based on the analysis above, we can determine that the incorrect statement is: (B) CH3 substitution at 𝛽 decreases the rate of S_N2

Key concepts

SN1 reaction

Nucleophilic substitution can occur through two main mechanisms: SN1 and SN2. In SN1 reactions, the name stands for 'Substitution Nucleophilic Unimolecular', indicating that the process takes place in two steps. Initially, the leaving group detaches from the compound, creating a positively charged carbocation intermediate. This step is vital as the stability of this carbocation greatly affects the reaction rate. Afterward, the nucleophile attacks the carbocation, resulting in the final product.
Typically, SN1 reactions are faster when the carbocation formed is stable. Therefore, secondary or tertiary alkyl halides are more likely to undergo SN1 reactions, as they can better stabilize the carbocation through hyperconjugation or resonance. Since the nucleophile does not participate in the rate-determining step, SN1 reactions are insensitive to the strength or concentration of the nucleophile.

SN2 reaction

The SN2 mechanism represents the 'Substitution Nucleophilic Bimolecular' reaction. SN2 reactions involve a direct attack by the nucleophile on the substrate in a one-step process. Here, the nucleophile pushes the leaving group away from the substrate as it simultaneously attaches to the substrate.
Importantly, SN2 reactions involve a backside attack, which means that the nucleophile approaches from the opposite side of the leaving group. As a result, the steric hindrance around the reaction site plays a significant role in the reaction rate. Primary substrates are usually more favorable for SN2 reactions, as less steric congestion allows nucleophiles to approach more easily. On the other hand, bulky groups near the reaction center slow down or even prevent the reaction.

Carbocation stability

Carbocation stability is a critical factor in chemical reactions, especially SN1 reactions. A carbocation is a positively charged carbon atom with only six electrons in its valence shell, making it unstable. Stability of carbocations can significantly influence the reaction pathway and rate.
The stability of a carbocation can be enhanced by several factors:

• **Hyperconjugation:** Overlap of filled C-H orbitals with the empty p-orbital of the carbocation stabilizes the charge. Secondary and tertiary carbocations have more hydrogen atoms capable of hyperconjugation, thus more stability.
• **Inductive Effect:** Electron-donating groups near the carbocation can help to stabilize the positive charge.
• **Resonance:** Certain structures allow the positive charge to be delocalized across multiple atoms, also increasing stability.

Tertiary carbocations are more stable than secondary and primary ones, often making SN1 reactions more favorable in such situations.

Steric hindrance

Steric hindrance refers to the resistance that occurs when the size of substituent groups near a reactive site hinders, or blocks, the approach of a nucleophile. This is an essential factor in nucleophilic substitution, especially for SN2 reactions. Larger substituents around or near the reactive center can significantly slow down a reaction because they physically block the nucleophile from accessing the electrophilic carbon.
A primary alkyl halide has minimal steric hindrance and is thus more reactive in SN2 reactions. On the contrary, secondary and tertiary alkyl substrates exhibit increased steric bulk, making it harder for the nucleophile to attack. This is why SN2 reactions are less favored with bulky tert-butyl groups, for instance.

• **Resulting Effect:** More steric hindrance speeds up SN1 reactions by favoring carbocation formation and discouraging direct nucleophilic attack.
• **Practical Consideration:** When predicting the viability of SN1 vs. SN2 reactions, considering steric hindrance is crucial.

Overall, having a clear understanding of steric effects helps in predicting and controlling the course of nucleophilic substitution reactions.