Question

When \((R)-2\) butanol is left standing in aqueous acid, it slowly loses its optical activity. Account for this observation.

Short answer

Question: Explain how optical activity is lost when (R)-2 butanol is left standing in aqueous acid. Answer: When (R)-2 butanol reacts with aqueous acid, a planar carbocation intermediate is formed at the chiral center (C-2). The intermediate can then react with a water molecule with equal probability to form either the (R)-2 butanol or (S)-2 butanol enantiomer. This results in a racemic mixture, where the optical activities of both enantiomers cancel each other out, leading to a loss of optical activity.

Step-by-step solution

Understand the structure of \((R)-2\) butanol

The structure of \((R)-2\) butanol contains a chiral center at the second carbon atom (C-2). The four groups attached to this carbon are H, OH, methyl (CH3) and ethyl (CH2CH3). The \((R)\) notation indicates the absolute configuration of the chiral center.

Identify the reaction conditions

The molecule \((R)-2\) butanol is left standing in aqueous acid. This implies that \((R)-2\) butanol reacts with aqueous acid, typically H3O+ ions.

Reaction Mechanism

The reaction mechanism for the loss of optical activity can be outlined as follows: 1. The OH group on C-2 is protonated by the H3O+ ion in aqueous acid, which makes the oxygen a better leaving group (as H2O). 2. The bond between C-2 and the oxygen atom breaks, forming a planar carbocation intermediate at C-2. 3. A nearby water molecule in the solution attacks the C-2 carbocation, reforming a bond with the carbon atom in either the original configuration or the opposite configuration, having equal probability. 4. If the bond forms in the original configuration, the \((R)\) absolute configuration is retained. However, if the bond forms in the opposite configuration, an \((S)-2\) butanol is formed. This results in an equal mixture of both \((R)-2\) butanol and \((S)-2\) butanol called a racemic mixture.

Loss of optical activity

Since the reaction results in an equal mixture of both enantiomers (called racemic mixture), the optical activity is lost. This is because the optical rotation caused by the \((R)\) enantiomer cancels out the rotation caused by the \((S)\) enantiomer, and hence the net rotation of light passing through the mixture is zero.

Key concepts

Chiral Center

In organic chemistry, a chiral center, also known as a stereocenter, is a carbon atom that is attached to four different groups. The existence of a chiral center in a molecule creates asymmetry, allowing for the possibility of isomers that are mirror images of each other, known as enantiomers.

Considering the molecule of \(R)-2\) butanol, we find that the second carbon atom (C-2) is attached to four different groups: hydrogen (H), hydroxyl (OH), a methyl group (CH3), and an ethyl group (CH2CH3). This configuration makes it chiral, and therefore, capable of rotating plane-polarized light, a property known as optical activity. The enantiomer of this molecule, which has an opposite configuration at the chiral center, is designated as \(S)-2\) butanol. Together, the \(R)-\text{2\) butanol and \(S)-2\) butanol showcase the concept of chirality in action.

Racemic Mixture

A racemic mixture is a 50:50 combination of two enantiomers of a chiral molecule. Because these enantiomers have equal and opposite optical activities, the overall effect is a cancellation of the ability to rotate plane-polarized light; thus, the mixture appears optically inactive.

In the case of \(R)-2\) butanol reacting in an aqueous acid solution, the reaction ultimately generates a racemic mixture consisting of equal amounts of \(R)-\text{2\) butanol and \(S)-2\) butanol. This equilibrium between the two enantiomers leads to the loss of the solution's optical activity, as the rotations from the two forms counterbalance each other. Racemic mixtures are significant in many biological systems and pharmaceutical applications because often only one enantiomer is biologically active or desired for a particular treatment.

Reaction Mechanism

The reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs.

For the aqueous acid reaction with \(R)-2\) butanol, the phenomenon starts with the protonation of the hydroxyl group, making water, a better leaving group. Subsequently, the loss of water creates a planar carbocation at the second carbon. Since the carbocation is planar (flat), either face of the molecule can be attacked by water leading to reformation of the OH group. This process has no preference for the original configuration, resulting in replenishing both \(R)-\text{2\) and \(S)-\text{2\) butanol in equal amounts. The symmetrical nature of the carbocation intermediate and the non-discriminatory attack by water are the key factors that lead to the formation of the racemic mixture.

Aqueous Acid Reactions

Aqueous acid reactions involve the interaction of organic compounds with acids in water. In these reactions, the acid usually acts as a proton donor, transforming the reactants through processes like protonation, which can lead to increased reactivity of leaving groups.

The reaction between \(R)-2\) butanol and aqueous acid highlights the typical stages of an acid-catalyzed process. The presence of H3O+ ions in the solution allows for the protonation of alcohol, facilitating the departure of a water molecule and leading to the formation of the carbocation intermediate. This reaction scenario exemplifies the nature of aqueous acid reactions, showcasing protonation, departure of a leaving group, creation of an intermediate, and subsequent attack by a nucleophile (in this case, water), leading to the final products.