Question

Optically active 2 -iodooctane, upon standing in an acetone solution containing \(\mathrm{Nal}^{131}\), loses its chirality and exchanges its \(\mathrm{I}^{127}\) for \(\mathrm{I}^{131}\). In addition, while the rate of reaction is dependent on both [RI] and \(\left[\mathrm{I}^{-}\right]\) racemization proceeds twice as fast as isotopic exchange. Explain. vent Affects

Short answer

The reaction of optically active 2-iodooctane with NaI involves two processes: racemization and isotopic exchange. The racemization, which is the loss of chirality, occurs through the formation of an achiral carbocation intermediate. The reaction mechanism involves dissociation of the carbon-iodine bond in the presence of nucleophilic I- ion, followed by either I²⁷ attack (racemization without isotopic exchange) or I¹³¹ attack (racemization with isotopic exchange). Racemization happens faster than isotopic exchange because it involves both processes, whereas isotopic exchange only involves the latter.

Step-by-step solution

Understanding the reaction mechanism

In this reaction, 2-iodooctane undergoes two processes: racemization and isotopic exchange. Racemization refers to the loss of the chiral nature of the compound, and isotopic exchange refers to the swap of \(\mathrm{I}^{127}\) by \(\mathrm{I}^{131}\). First, we need to understand how these two processes take place on a molecular level.

Identifying the key species in the reaction

It's given that the reaction rate depends on the concentration of RI and the concentration of \(\mathrm{I^{-}}\). Since the reaction involves the iodination of 2-iodooctane, we can deduce that RI stands for 2-iodooctane, and \(\mathrm{I^{-}}\) stands for iodide ion.

Determining the relationship between racemization and isotopic exchange

It's mentioned that racemization occurs at twice the rate of isotopic exchange, which means that the loss of chirality happens at a faster rate compared to the exchange of \(\mathrm{I}^{127}\) for \(\mathrm{I}^{131}\). We will need to examine the mechanism to understand the reason behind this observation.

Reaction mechanism

To investigate the reaction mechanism, we need to consider the stereochemistry and nature of the 2-iodooctane reactant. The likely explanation for the racemization, in this case, is due to the formation of a carbocation intermediate, which is achiral owing to its planar structure. This intermediate can be formed by the dissociation of the carbon-iodine bond in the presence of nucleophilic I- ion. The proposed mechanism involves the following steps: 1. The iodine atom in 2-iodooctane leaves the molecule, and a carbocation intermediate is formed. 2. The carbocation intermediate undergoes one of the following processes: a. The \(\mathrm{I}^{127}\) ion attacks and forms a new bond, resulting in racemization without isotopic exchange. b. The \(\mathrm{I}^{131}\) ion attacks and forms a new bond, resulting in racemization with isotopic exchange.

Explaining the rate difference

Based on the proposed steps, the rate of racemization is the sum of the rates of processes a and b. Therefore, if we denote the rate of racemization as R(racemization) and the rate of isotopic exchange as R(isotopic exchange), we have: \[R(\text{racemization}) = R(a) + R(b)\] And given that the racemization rate is twice the isotopic exchange rate, we get: \[R(a) + R(b) = 2R(b)\] This equality indicates that process a (the racemization step without isotopic exchange) occurs at the same rate as process b (the racemization step with isotopic exchange). Thus, the reason behind the faster rate of racemization as compared to isotopic exchange is due to the fact that racemization involves both processes a and b, whereas isotopic exchange only involves process b.

Key concepts

Optically Active Compounds

Optically active compounds are molecules that have the ability to rotate plane-polarized light. This unique characteristic is due to the presence of an asymmetric carbon atom—often referred to as a chiral center—which has four different groups attached to it.

The significance of optical activity lies in the fact that the spatial arrangement of atoms in such compounds can affect how they interact with other substances, including biological molecules like enzymes and receptors. In the example of 2-iodooctane, the compound starts as optically active but loses its chirality in the reaction. This indicates that a change in the molecular structure has occurred, specifically at the chiral center.

Isotopic Exchange

Isotopic exchange is a process where an atom in a molecule is replaced by another atom of the same element but with a different number of neutrons—essentially, a different isotope of the same atom. The reaction between 2-iodooctane and \(\mathrm{NaI}^{131}\) involves the swapping of the iodine isotope \(\mathrm{I}^{127}\) with \(\mathrm{I}^{131}\).

This process is particularly interesting for studying reaction mechanisms as it can occur without altering the overall structure of the organic compound. The rate of isotopic exchange provides valuable insights into the stability of bonds and the nature of interaction between atoms in a molecule.

Reaction Mechanism

A reaction mechanism is a step-by-step description of how a chemical reaction occurs at the molecular level. It identifies the sequence of elementary steps, including bond breakages and formations, leading to the final products of the reaction. Understanding a reaction mechanism allows chemists to predict the outcome of a reaction and manipulate conditions to achieve desired products.

In our specific case, elucidating the mechanism helps us to understand why racemization occurs at a different rate than isotopic exchange. It reveals the underlying structural changes and interactions that lead to racemization and how it is possible for the racemization to proceed without isotopic exchange in some instances.

Carbocation Intermediate

A carbocation intermediate is a short-lived species with a positively charged carbon atom. It occurs during certain organic reactions and is usually very reactive due to the positive charge, which confers a high energy state on the molecule. In the dissolution of 2-iodooctane, the formation of a carbocation intermediate is crucial as it allows the molecule to lose its three-dimensional spatial configuration, leading to racemization.

The planar structure of a carbocation is non-chiral, meaning it cannot rotate plane-polarized light. This characteristic is central to understanding why racemization occurs in this reaction and why it happens faster than isotopic exchange. Since the carbocation is symmetrical, the subsequent attack by iodide can lead to the formation of two enantiomers, effectively resulting in a racemic mixture.