Question

Which of the following phrases is/are not correctly associated with \(\mathrm{S}_{\mathrm{N}} 1\) reaction? (a) Rate of the reaction is affected by polarity of solvent. (b) The strength of nucleophile is important in determining rate of the reaction. (c) The reaction proceeds with complete inversion of configuration. (d) Neopentyl halide gives no rearranged product by \(\mathrm{S}_{\mathrm{N}} 1\) mechanism.

Short answer

Options (b), (c), and (d) are not correctly associated with SN1 reactions.

Step-by-step solution

Understanding SN1 Mechanism

The SN1 reaction mechanism involves two main steps: the formation of a carbocation intermediate, and then the attack of the nucleophile. The rate of the reaction is dependent on the formation of the carbocation, meaning it is only affected by the initial substrate and not the nucleophile.

Assessing Option (a)

This statement claims that the rate of the reaction is affected by the polarity of the solvent. In an SN1 reaction, a polar solvent stabilizes the carbocation and the leaving ion, which can increase the rate. Thus, option (a) is correctly associated with SN1.

Assessing Option (b)

This statement suggests that the nucleophile strength affects the reaction rate. SN1's rate is independent of the nucleophile strength because the slowest step is the formation of the carbocation, not the nucleophilic attack. Hence, option (b) is NOT correctly associated with SN1.

Assessing Option (c)

This statement mentions that the reaction causes complete inversion of configuration. SN1 reactions often involve a racemization process, leading to partial inversion, not complete. Thus, option (c) is NOT correctly associated with SN1.

Assessing Option (d)

This statement claims that neopentyl halide does not give rearranged products in SN1. SN1 reactions can involve rearrangements due to carbocation intermediates, but neopentyl surfaces are highly sterically hindered, slowing the reaction. However, they can still form rearranged products under the right conditions. So, option (d) is NOT correct.

Key concepts

Carbocation Intermediate

One of the defining steps in an SN1 reaction is the formation of a carbocation intermediate. This occurs when the leaving group departs from the substrate, leaving behind a positively charged carbon atom. This carbocation is highly reactive and unstable. It needs to quickly react with nearby nucleophiles to stabilize itself. The stability of this carbocation is crucial because it directly influences the overall rate of the SN1 reaction.
This stability can depend on several factors:

• Alkyl Substitution: Tertiary carbocations are more stable than secondary or primary due to hyperconjugation and the inductive effect from surrounding alkyl groups.
• Resonance Stabilization: The presence of pi bonds or aromatic rings can help delocalize the carbocation's charge, enhancing its stability.
• Rearrangements: Sometimes, a carbocation may rearrange to form a more stable structure, leading to different reaction products. This is particularly important in explaining outcomes where unexpected products appear.

Whether or not a carbocation intermediate forms efficiently can make or break the SN1 reaction.

Nucleophile Strength

In contrast to what one might initially think, the strength of the nucleophile does not affect the rate of an SN1 reaction. This is because the reaction rate is determined entirely by the formation of the carbocation intermediate.

• The nucleophile only participates in the second step after the carbocation has formed.

During this step, the nucleophile attacks the positively charged carbon, but since the rate-determining step has already occurred, the strength of the nucleophile has no influence on how fast the reaction proceeds.
Instead, the nucleophile merely needs to be present in the vicinity to capture the carbocation prompt enough before it recombines with the leaving group. Therefore, even a weak nucleophile can successfully engage in an SN1 reaction.

Solvent Polarity

The polarity of the solvent is a key factor affecting the SN1 reaction rate. Since the SN1 mechanism involves the formation of a charged carbocation intermediate, polar solvents are instrumental in stabilizing these charged species.

• Polar Protic Solvents: These solvents can form hydrogen bonds and effectively stabilize ions. The stabilization effect helps speed up the decomposition of the substrate into a carbocation and its subsequent recombination with a nucleophile.
• Examples: Water, methanol, and ethanol are commonly used polar protic solvents that create better environments for SN1 reactions.

The solvent's polarity can profoundly influence the energetic favorability of the reactant and transition state, thereby impacting how quickly the reaction occurs. By stabilizing the transition state more than the reactants, polar solvents lower the activation energy needed for the reaction, promoting faster reaction rates.

Reaction Rate

The reaction rate in SN1 mechanisms is primarily dependent on the concentration and nature of the substrate because the slowest, rate-determining step is carbocation formation. Unlike other mechanisms where both the nucleophile and substrate concentrations influence the reaction rate, SN1 reactions depend solely on the concentration of the substrate.

• Rate Equation: The rate is expressed as rate = k[substrate], where 'k' is the rate constant. This indicates a first-order reaction, depending only on one reactant's concentration.

The nature of the substrate plays a significant role as well. Substrates more prone to forming stable carbocations will react faster in SN1 reactions.
In summary, while SN1 reactions are less influenced by the nucleophile, the structure of the substrate and stabilizing effects of the solvent are crucial determinants for how fast the reaction will proceed.