Question

Tertiary alkyl halides are practically inert to substitution by \(\mathrm{S}_{\mathrm{N}^{2}}\) mechanism because of [2005] (a) insolubility (b) instability (c) inductive effect (d) steric hindrance

Short answer

Tertiary alkyl halides are inert to SN2 due to steric hindrance.

Step-by-step solution

Understanding the SN2 Mechanism

The SN2 mechanism is a bimolecular nucleophilic substitution reaction where the nucleophile attacks the electrophilic carbon from the opposite side of the leaving group, simultaneously displacing it. This creates a single, concerted reaction step.

Analyzing Tertiary Alkyl Halides

Tertiary alkyl halides contain a central carbon atom attached to three alkyl groups and one halide (such as chloride, bromide, or iodide). The central carbon is highly substituted and surrounded by bulky alkyl groups.

Exploring Factors Affecting SN2 Reactions

For an SN2 reaction to occur, the nucleophile must have unhindered access to the electrophilic carbon atom. The steric crowding around this carbon, particularly with bulky groups, can inhibit this access.

Identifying the Main Barrier to SN2 in Tertiary Alkyl Halides

The correct explanation for why tertiary alkyl halides are practically inert to SN2 reactions is steric hindrance. The bulky alkyl groups prevent the nucleophile from approaching the electrophilic carbon effectively, thereby impeding the SN2 mechanism.

Conclusion

Given the steric hindrance created by the bulky groups in tertiary alkyl halides, the SN2 mechanism is not viable for these substances.

Key concepts

Tertiary Alkyl Halides

Tertiary alkyl halides are unique organic molecules in which the central carbon—which is bound to the halogen atom—is attached to three alkyl groups. These groups can be long or short carbon chains. What distinguishes tertiary alkyl halides from primary and secondary ones is their higher degree of substitution at the central carbon. This means, unlike primary or secondary alkyl halides that have fewer bulky groups, tertiary alkyl halides are surrounded by a crowd of carbon chains.

These structures have a profound effect on their chemical behavior, especially in substitution reactions. The crowded environment around the central carbon makes them relatively inert in certain reactions, particularly those requiring a direct approach, such as the SN2 mechanism. Overall, this characteristic structural arrangement makes tertiary alkyl halides a principal focus when studying organic reaction patterns, especially in the realm of nucleophilic substitution processes.

Steric Hindrance

Steric hindrance is a concept that refers to the restriction of a chemical reaction due to the physical size and proximity of groups within a molecule. In the case of tertiary alkyl halides, steric hindrance plays an inhibiting role because the three alkyl groups occupy space around the electrophilic carbon.

This spatial crowding obstructs potential attacking nucleophiles from directly reaching and reacting with the electrophilic carbon. Imagine a crowded room, where crossing it would be difficult without bumping into people. Similarly, in chemical terms, the bulky groups "get in the way" of the reaction. This makes it hard or sometimes impossible for the nucleophilic substitution, especially in the SN2 mechanism, to proceed. The larger and bulkier the surrounding groups, the increased effect of steric hindrance, which is a significant factor in making tertiary alkyl halides inert to SN2 reactions.

Nucleophilic Substitution

Nucleophilic substitution reactions are a fundamental type of reaction in organic chemistry where a nucleophile, an electron-rich species, attacks an electrophilic center, often leading to the replacement of a group or atom in the molecule. The SN2 reaction, in particular, is a type of nucleophilic substitution mechanism that involves a direct displacement of the leaving group by the nucleophile.

During an SN2 reaction, the nucleophile approaches the electrophilic carbon from the side opposite the leaving group, leading to a concerted mechanism where bonds are simultaneously broken and formed in a single step. The SN2 reaction is favored in substrates where the electrophilic carbon is less hindered or crowded, making it mostly suitable for primary or secondary alkyl halides. However, due to steric hindrance, tertiary alkyl halides do not readily undergo SN2 reactions, highlighting the importance of understanding molecular structure and reaction compatibility in organic chemistry.

Electrophilic Carbon

The electrophilic carbon in a molecule is the carbon atom that serves as the center of the reaction with nucleophiles. It has a partial positive charge, usually as a result of being bonded to a more electronegative element such as a halogen. This creates a polar bond where the carbon acts as a target for nucleophiles.

In the SN2 mechanism, this electrophilic carbon is the site where the nucleophile aims to form a new bond. The design of the molecule surrounding this carbon is crucial since it affects the accessibility of the site for reaction. For tertiary alkyl halides, the electrophilic carbon is significantly blocked by the bulky alkyl groups, making it difficult for nucleophiles to access and react. Understanding the role of the electrophilic carbon is critical for predicting the outcomes of nucleophilic substitution reactions and designing effective synthetic routes in organic chemistry.