Question

Attempts to prepare optically active iodides by nucleophilic displacement on optically active bromides using I - normally produce racemic iodoalkanes. Why are the product iodoalkanes racemic?

Short answer

Answer: The product iodoalkanes are racemic because the reaction proceeds through an SN1 mechanism, wherein the iodide ion (I-) attacks the carbocation intermediate formed after the departure of the bromide ion. The intermediate is planar and can be attacked by the iodide ion from both sides, resulting in a racemic mixture of iodoalkanes, having equal proportions of both enantiomers, making the net optical rotation zero.

Step-by-step solution

Understand Chirality and Optical Activity

Optically active compounds are chiral, meaning they have a non-superimposable mirror image. Chirality generally arises when a carbon atom (called a stereocenter) has four different groups attached to it. Optically active compounds can rotate plane-polarized light, and the two enantiomers of a chiral compound will rotate the light in opposite directions.

Recognize the Reaction as SN2 or SN1

Nucleophilic displacement reactions are classified into two categories: SN2 and SN1. In SN2 reactions, the nucleophile attacks the substrate directly, causing an inversion of stereochemistry at the stereocenter (when there is one). In SN1 reactions, the nucleophile attacks an intermediate carbocation, formed after the leaving group departs. The nucleophile can attack the carbocation from either side, leading to racemization.

Identify the Mechanism in the Given Reaction

Optically active bromides are used as substrates for the nucleophilic displacement reaction with I- as the nucleophile. For primary or secondary alkyl bromides, the reaction is most likely an SN2 mechanism due to steric hindrance. However, substitution reactions of tertiary alkyl substrates are more likely to follow an SN1 mechanism because of the more stable carbocation intermediate.

Analyze the Reaction

If the given reaction follows the SN2 mechanism for primary or secondary alkyl bromides, there will be an inversion of configuration at the stereocenter due to a direct backside attack of the nucleophile. However, in the case of the SN1 mechanism with tertiary alkyl substrates, the carbocation intermediate can be attacked by the nucleophile from either side, resulting in racemization of the product.

Explain the Formation of Racemic Iodoalkanes

In the present case, the reaction proceeds through an SN1 mechanism, where the I- nucleophile attacks the carbocation intermediate formed by the departure of the bromide ion from the optically active bromide. Since the intermediate is planar and can be attacked by the nucleophile from both sides, it forms a racemic mixture of iodoalkanes, which have one half (50%) of its enantiomers rotated in one direction and the other half (50%) rotated in the opposite direction, making the net optical rotation zero. This is the reason why optically active iodides prepared by nucleophilic displacement on optically active bromides using I- are racemic.