Question

During reaction of (+)-2-Iodooctane with sodium iodide in acetone with radioactive iodio The correct statement regarding this reaction is/are (A) Racemisation phenomenon (B) \(S_{N} 2\) mechanism (C) Rate of racemisation is double to rate of inversion during reaction (D) Reversible reaction

Short answer

The correct statements regarding the reaction of (+)-2-Iodooctane with sodium iodide are (A) Racemisation phenomenon and (B) \(S_{N} 2\) mechanism.

Step-by-step solution

Understand Racemisation

Racemisation is a process where an optically active compound (like (+)-2-Iodooctane) gets converted into a racemic mixture of its enantiomers, meaning that it loses its optical activity. The racemic mixture contains equal amounts of the two enantiomers.

Understand \(S_{N} 2\) Mechanism

The \(S_{N} 2\) mechanism stands for a second-order nucleophilic substitution reaction. In this reaction, the nucleophile attacks the chiral center of the substrate and replaces the leaving group (in this case, iodide) with stereochemistry inversion (back-side attack).

Understand Rate of Racemisation and Inversion

Rate of racemisation is the rate at which the racemic mixture forms in the reaction, while the rate of inversion is the rate at which the stereochemistry of the chiral center inverts due to the nucleophilic attack. In the case of an \(S_{N} 2\) reaction, as a single event (nucleophilic attack) causes both racemisation and inversion, their rates are the same.

Understand Reversible Reaction

A reversible reaction is a reaction in which the products can react back to form the reactants. In the case of \(S_{N} 2\) reactions, they are considered typically irreversible, as a strong nucleophile is involved, which usually does not act as a leaving group.

Identify the Correct Statements

(A) Racemisation phenomenon: In the reaction of (+)-2-Iodooctane with sodium iodide, the product formed is a racemic mixture due to the \(S_{N} 2\) mechanism. So, this statement is true. (B) \(S_{N} 2\) mechanism: As discussed in Step 2, the reaction follows the \(S_{N} 2\) mechanism. So, this statement is true. (C) Rate of racemisation is double to the rate of inversion during the reaction: As mentioned in Step 3, the rates of racemisation and inversion in the given reaction are the same. Thus this statement is false. (D) Reversible reaction: As explained in Step 4, \(S_{N} 2\) reactions are typically irreversible, so this statement is false. So, the correct statements regarding the reaction of (+)-2-Iodooctane with sodium iodide are (A) and (B).

Key concepts

Racemisation in Organic Chemistry

Racemisation is an interesting phenomenon in the world of organic chemistry where an optically active compound becomes a racemic mixture. Imagine holding a pair of gloves, one for the left hand and one for the right; they are non-superimposable mirror images of each other, just like enantiomers in a racemic mixture.

A racemic mixture contains equal amounts of two enantiomers, which are molecules that are mirror images of one another but cannot be superimposed. This process results in a loss of optical activity, meaning that the mixture does not rotate plane-polarized light. In the context of the exercise, (+)-2-Iodooctane, an optically active compound, undergoes racemisation when exposed to certain conditions such as reacting with sodium iodide in acetone. This is a crucial concept in the creation and understanding of pharmaceutical drugs, as different enantiomers can exhibit drastically different biological activities.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions are at the heart of organic chemistry. They're akin to a dance between molecules, where one partner, the nucleophile, is quick to find an opportunity to replace another, known as the leaving group. An SN2 mechanism is a type of nucleophilic substitution involving a bimolecular reaction, where the rate depends on the concentration of both the nucleophile and the electrophile.

The unique characteristic of an SN2 reaction, represented by '2' in the name, is that it occurs in a single step – a direct attack from the nucleophile leads to the displacement of the leaving group. The nucleophile approaches the central carbon atom from the opposite side to the leaving group, causing an 'umbrella flip' inversion of the stereochemistry at that carbon; it's like turning an umbrella inside out on a windy day. This reaction is significant in our exercise where the nucleophile, an iodide ion from sodium iodide, displaces another iodide ion attached to the octane chain.

Rate of Racemisation vs Rate of Inversion

Same Journey, Different Pathways?

When we speak of the rate of racemisation and the rate of inversion, we're referring to the speed of two processes that could seem different but are intimately connected in certain reactions. Specifically, in SN2 reactions, the act that leads to inversion – the direct attack and replacement of a leaving group – also sows the seed for racemisation.

The handshake between the incoming nucleophile and the substrate results in a new configuration. However, since this is a one-step process and the nucleophile is just as likely to attack from either side, each event of inversion is equally an event of racemisation. Therefore, in contrast to what one might initially think, the rate of racemisation and the rate of inversion are identical in an SN2 mechanism because they are essentially two descriptions of the same molecular event.

Reversible Reactions

Now, let's delve into the world of reversible reactions, where the products have the ability to turn back into reactants. While some reactions gladly march forward and never glance back, others are more reminiscent of a pendulum, swinging back and forth between reactants and products.

However, in the case of the SN2 reactions, such as the one illustrated in our exercise, they have a strong preference for moving forward. Once the nucleophile forms a bond with the substrate, creating a strong new connection, the reaction doesn't generally reverse, because the nucleophile is now firmly entrenched in its new position and is not inclined to leave. This is why SN2 reactions are generally considered to be irreversible; yet, it's important to note that in the vast landscape of chemistry, there are always exceptions depending on the specific conditions and the nature of the reacting species.