Question

Tertiary alkylhalides arepractically inerttosubstitution by \(\mathrm{SN}^{2}\) mechanism because of: (a) Insolubility (b) Instability (c) Inductive effect (d) Steric hindrance

Short answer

(d) Steric hindrance

Step-by-step solution

Understanding the SN2 Mechanism

The SN2 (Substitution Nucleophilic Bimolecular) mechanism involves a single-step reaction where a nucleophile attacks the electrophilic carbon at the same time as the leaving group departs. This type of reaction requires a direct backside attack by the nucleophile.

Role of Steric Hindrance

Tertiary alkyl halides have bulky groups surrounding the carbon bonded to the halide. This bulkiness creates significant steric hindrance, making it difficult for the nucleophile to access the carbon atom for a backside attack, hindering the SN2 reaction.

Evaluating Other Options

While inductive effects can influence reactivity, they do not directly impede the nucleophilic attack like steric hindrance does. Insolubility and instability are unrelated to the mechanistic pathway of nucleophilic substitution in this context.

Conclusion

Based on the analysis, steric hindrance is the primary reason tertiary alkyl halides are inert to SN2 reactions, as it blocks the required approach of the nucleophile.

Key concepts

Tertiary Alkyl Halides

Tertiary alkyl halides are a specific type of organic compound where the carbon atom, directly bonded to the halogen (like chlorine or bromine), is also bonded to three other carbon atoms. This arrangement is significant in the context of chemical reactivity, particularly in nucleophilic substitution reactions.
In organic chemistry, these compounds often appear in discussions of reaction mechanisms. Their structure leads to unique chemical behaviors because the three carbon groups create a highly substituted environment around the pivotal carbon. This substitution influences how these molecules interact with other reactive species like nucleophiles. Understanding these dynamics is crucial when predicting reaction outcomes and choosing appropriate reaction pathways in synthetic processes.

Nucleophilic Substitution

Nucleophilic substitution reactions are central themes in organic chemistry, involving an electron-rich nucleophile replacing a leaving group attached to a carbon atom in a molecule.
The SN2 mechanism, a classic type of nucleophilic substitution, is characterized by a one-step process where the nucleophile attacks the backend of the carbon while the leaving group departs. This synchronous action requires that the nucleophile accesses the carbon atom without being obstructed. In the case of simple alkyl halides, this process happens fairly smoothly. However, as the surrounding environment becomes bulkier, as in tertiary alkyl halides, the process is hindered, making SN2 reactions less feasible.

Steric Hindrance

Steric hindrance is a critical concept in organic chemistry that refers to the physical blocking around a reactive site in a molecule due to the presence of bulky groups. This crowding effect impedes the access of reactants like nucleophiles to the central reactive atom, thereby influencing the course and feasibility of a reaction.
In tertiary alkyl halides, steric hindrance plays a pivotal role in making SN2 reactions impractical. The bulkiness of the three surrounding carbon groups creates a barrier that inhibits the nucleophile from reaching the central carbon for the necessary backside attack, effectively preventing the substitution reaction from proceeding. Recognizing and understanding steric hindrance can be key to successfully predicting and rationalizing the reactivity and mechanism of organic reactions.

Organic Chemistry

Organic chemistry is the study of carbon-containing compounds and their interactions. It is a vast field encompassing a wide variety of chemical reactions and principles, which are essential to understanding both biological processes and synthetic chemistry.
Within this sphere, mechanisms like SN2 are fundamental for exploring how molecules transform and interact. Nucleophilic substitution, hindrance effects, and the structure of compounds like tertiary alkyl halides offer insights into predicting and controlling chemical behavior. Mastery of these concepts is critical for anyone interested in pursuing advanced study or a career in chemistry, as they form the building blocks for designing and executing complex synthetic pathways in both laboratory and industrial settings.