Question

The correct order of rate of following compound with NaI in acetone is (I) Cis-4-iodo-t-butyl cyclohexane (II) Trans-4-iodo-t-butyl cyclohexane (A) \(\mathrm{I}>\mathrm{II}\) (B) II > I (C) \(\mathrm{I}=\mathrm{II}\) (D) Can't compare

Short answer

The correct order of rate for the reaction of cis-4-iodo-t-butyl cyclohexane (I) and trans-4-iodo-t-butyl cyclohexane (II) with NaI in acetone is I > II, due to the lower steric hindrance around the iodine atom in compound I than in compound II, allowing it to react faster in an SN2 mechanism. Hence, the answer is \((A) \mathrm{I}>\mathrm{II}\).

Step-by-step solution

Identify the reaction mechanism

The reaction of compounds (I) and (II) with NaI in acetone is an example of nucleophilic substitution reactions, specifically, an SN2 mechanism. In SN2 reactions, the nucleophile (NaI in this case) attacks the substrate (compounds I and II), displacing the leaving group (iodide ion).

Analyze the steric hindrance for compound (I)

In cis-4-iodo-t-butyl cyclohexane (compound I), the t-butyl group and iodine atom are on the same side of the ring. In its chair conformation, the axial position of the iodine atom makes it less sterically hindered. Thus, the iodine atom has less steric hindrance and is more accessible for attack by the nucleophile.

Analyze the steric hindrance for compound (II)

In trans-4-iodo-t-butyl cyclohexane (compound II), the t-butyl group and iodine atom are on opposite sides of the ring. In its chair conformation, the iodine atom is in a more sterically hindered position due to the presence of t-butyl group on the opposite side of the cyclohexane ring. This steric hindrance makes it more difficult for the nucleophile to attack the iodine atom.

Determine the reaction rate order

Since the iodine atom in compound (I) has less steric hindrance than in compound (II), the reaction of compound (I) with NaI (the nucleophile) will occur at a faster rate than the reaction of compound (II) with NaI. Therefore, the rate order is I > II.

Identify the correct answer

Based on our analysis, the correct order of rate is I > II, which corresponds to option (A). Thus, the answer to the problem is (A) I > II.

Key concepts

Nucleophilic Substitution Reactions

When exploring the fascinating realm of organic chemistry, we often encounter nucleophilic substitution reactions. These reactions are pivotal for understanding many processes in both natural and synthetic organic chemistry.

In a nucleophilic substitution reaction, a nucleophile, which is rich in electrons and often carries a negative charge, seeks out a positive or partially positive carbon atom in a molecule. The nucleophile then forms a new bond with this carbon atom. In the process, another group, called the leaving group, is expelled.

The SN2 mechanism is a type of nucleophilic substitution characterized by two major features: a single-step reaction with the simultaneous bonding of the nucleophile and release of the leaving group, and a distinct backside attack pattern. The 'S' stands for substitution, 'N' for nucleophilic, and '2' for bimolecular, which means the rate of the reaction depends on the concentration of both the nucleophile and the substrate.

Given its sensitivity to factors such as steric hindrance and solvent, understanding SN2 reactions is crucial for students preparing for competitive exams like the JEE Advanced. Mastery of how these reactions are influenced by the structure of the substrate and the nature of the nucleophile can translate directly to better problem-solving skills on the exam.

Steric Hindrance in Cyclohexane

Steric hindrance is an important concept that often dictates the outcome of many organic reactions, particularly in the rigid structure of cyclohexane.

This hindrance refers to the prevention or slowing down of chemical reactions due to the physical space occupied by substituents attached to the carbon atoms of a molecule. In the cyclohexane ring, the spatial arrangement is key. Cyclohexane adopts a chair conformation that minimizes torsional strain, and substituents can take on either equatorial or axial positions. In general, larger substituents prefer the equatorial position as it affords them more room and causes less steric strain.

In the case of substituted cyclohexanes, the size and position of these substituents dramatically affect the reactivity of the molecule in nucleophilic substitution reactions. This becomes especially relevant when considering substituents of considerable bulk, such as the t-butyl group, which can generate significant steric clashes that impede the approach of nucleophiles to the desired reactive sites.

The ability to accurately assess how steric hindrance influences reaction rates is a valuable skill tested in organic chemistry sections of exams such as the JEE Advanced. An intuitive grasp of spatial arrangements within a cyclohexane ring can greatly aid in predicting reaction outcomes.

Organic Chemistry JEE Advanced

For students preparing for the JEE Advanced, organic chemistry is a crucial subject to master. This section of the exam tests not only the student's memorization of organic compounds and reaction mechanisms but also their ability to apply these concepts in complex problem-solving situations.

One area where students can showcase their deep understanding is in dealing with mechanistic pathways such as the SN2 reaction. For such questions, knowledge of aspects like steric hindrance, electronic effects, and reaction kinetics isn't just helpful—it's essential.

It's important to approach study with a strategy that includes practicing numerous problems, visualizing molecules in three dimensions, and understanding the underlying principles that govern their reactivity. A common pitfall in exams like the JEE Advanced is the rote memorization of reactions without understanding the 'why' behind them - a strategy that often leads to confusion and mistakes when faced with unfamiliar problems.

Emphasizing conceptual clarity and the analysis of reaction conditions, like in the problem given about the rate of nucleophilic substitution in different cyclohexane derivatives, is a proven method for success. By mastering these topics, students drastically increase their chances of excelling in the organic chemistry section of the JEE Advanced.