Question

Which of the following mechanisms feature carbocation intermediates? (A) \(\mathrm{S}_{\mathrm{N}} \mathrm{l}\) only (B) \(S_{N} 2\) only (C) \(\mathrm{E}^{2}\) only (D) both \(S_{N} 1\) and \(E^{1}\)

Short answer

The correct answer is (D) both S_N1 and E1 mechanisms feature carbocation intermediates. S_N2 and E2 mechanisms do not involve the formation of carbocation intermediates.

Step-by-step solution

Analyzing S_N1 mechanism

The S_N1 mechanism involves two steps: 1. Formation of a carbocation intermediate by the leaving group departing from the substrate molecule. 2. Nucleophilic attack by the nucleophile on the carbocation. Since S_N1 involves the formation of a carbocation intermediate, it qualifies as one of the mechanisms that feature carbocation intermediates.

Analyzing S_N2 mechanism

The S_N2 mechanism is a one-step reaction where the nucleophile attacks the substrate molecule while the leaving group simultaneously leaves the molecule. Carbocation intermediates are not formed in S_N2 mechanisms. Thus, S_N2 does not qualify as a mechanism that features carbocation intermediates.

Analyzing E2 mechanism

In the E2 mechanism, both the base and the leaving group act simultaneously in a single concerted step, with the base removing a proton from the substrate while the leaving group departs with the electron pair. There is no formation of a carbocation intermediate in the E2 mechanism. Hence, the E2 mechanism does not qualify as a mechanism that features carbocation intermediates.

Analyzing E1 mechanism

The E1 mechanism involves two steps: 1. Formation of a carbocation intermediate by the leaving group departing from the substrate molecule. 2. Deprotonation of the carbocation intermediate by the base, resulting in the formation of an unsaturated product. Since E1 involves the formation of a carbocation intermediate, it qualifies as one of the mechanisms that feature carbocation intermediates.

Drawing conclusions

Considering our analysis of the given mechanisms, we can conclude that both S_N1 and E1 mechanisms involve carbocation intermediates. Therefore, the correct answer is (D) both S_N1 and E1.

Key concepts

S_N1 Mechanism

The S_N1 mechanism, which stands for nucleophilic substitution first-order, is a fundamental process in organic chemistry where a molecule undergoes substitution through a two-step pathway.

Firstly, a leaving group departs from the substrate, resulting in the formation of a carbocation intermediate. This step is the rate-determining one, meaning it's the slowest and therefore dictates the overall rate of the reaction. The stability of the carbocation is a crucial factor, with tertiary carbocations being the most stable due to hyperconjugation and inductive effects.

Following the formation of the carbocation, a nucleophile attacks the positively charged carbon to complete the substitution. It's essential to note the possibility of rearrangements in carbocations, which can lead to unexpected products, as the molecule seeks a more stable structure.

• Rate depends on the concentration of substrate only
• Reactions often occur in polar protic solvents
• Commonly observed in tertiary alkyl halides

E1 Mechanism

The E1 mechanism stands for unimolecular elimination, signifying that the rate of reaction depends only on the concentration of the substrate. Similar to the S_N1 mechanism, E1 also proceeds via a carbocation intermediate and is characterized by two main events.

Initially, the leaving group exits the molecule, forming a carbocation. This first step determines the reaction rate. Then, a base abstracts a proton (usually a hydrogen atom) from the carbon atom adjacent to the carbocation, leading to the formation of a double bond and the final elimination product.

This mechanism is influenced by factors such as the stability of the carbocation and the strength of the base. E1 reactions are often observed alongside S_N1 reactions, especially in scenarios involving a potential for both substitution and elimination.

• Favors highly substituted alkyl halides
• Usually competes with the S_N1 mechanism
• Produces an alkene as the final product

Organic Reaction Mechanisms

Understanding organic reaction mechanisms is vital for predicting and explaining the outcomes of chemical reactions in organic chemistry. These mechanisms provide a step-by-step illustration of the movement of electrons that leads to the conversion of reactants to products.

There are several types of reaction mechanisms, including addition, elimination, substitution, and rearrangement reactions. Each mechanism involves its distinct steps and intermediates - such as carbocations in the case of S_N1 and E1 reactions - and varying degrees of complexity. A firm grasp of these concepts allows chemists to manipulate reactions to produce desired compounds efficiently.

• Electron movements are depicted using arrow-pushing notation
• Stability of intermediates plays a crucial role in determining reaction pathways
• Reaction mechanisms are often confirmed through experimental observation and analysis

Nucleophilic Substitution

Nucleophilic substitution is a core concept in organic chemistry where a nucleophile - an electron-rich species - replaces a leaving group in a molecule. The two main types are the S_N1 and S_N2 mechanisms, which differ in their reaction order, steps, and type of substrates they typically involve.

While S_N1 reactions feature a carbocation intermediate and are common with tertiary substrates, S_N2 reactions proceed with a backside attack by the nucleophile, displacing the leaving group in a single, concerted step, and are favored with primary substrates. Steric hindrance and solvent effects play significant roles in determining the mechanism of nucleophilic substitution.

• S_N2 reactions are stereospecific, resulting in inversion of configuration
• S_N1 reactions can lead to racemization due to the planar structure of carbocations
• Both mechanisms are instrumental in synthetic organic chemistry