Question

Reaction of 2 -butanone with HCN yields a chiral product. What stereochemistry does the product have? Is it optically active?

Short answer

The product is racemic and optically inactive.

Step-by-step solution

Understand the Reactants and Products

2-butanone is a ketone with the formula C4H8O. It reacts with hydrogen cyanide (HCN) to form a cyanohydrin. The general reaction involves the nucleophilic addition of the cyanide ion (CN-) to the electrophilic carbonyl carbon of the ketone, leading to a new carbon-carbon bond formation.

Analyze the Reaction Mechanism

In this reaction, the carbonyl carbon of 2-butanone is planar and sp2 hybridized. The attack by CN- occurs from either side of the planar carbonyl, leading to the formation of a new chiral center at this carbon. As a result, two enantiomers are formed.

Identify Stereochemistry

The addition of CN- to the planar carbonyl can occur from above or below the plane, producing two possible stereoisomers, known as enantiomers. They are non-superimposable mirror images of each other. The product is thus racemic, containing both R and S configurations at the new chiral center.

Determine Optical Activity

A racemic mixture contains equal amounts of both enantiomers. Since these enantiomers rotate plane-polarized light in opposite directions with equal magnitude, their effects cancel out. Thus, the racemic product is optically inactive.

Key concepts

Stereochemistry

Stereochemistry deals with the three-dimensional arrangement of atoms in molecules and the effects of this spatial configuration on their chemical behavior. In the reaction of 2-butanone with hydrogen cyanide (HCN), we observe a fascinating aspect of stereochemistry. The initial planar, sp2 hybridized carbonyl group in 2-butanone allows the nucleophilic cyanide ion (CN-) to attack from either the front or the back side. This results in the formation of a chiral center at the carbon atom, which is a new stereogenic center where two possible stereoisomers can be formed.

These two possible products are enantiomers, which are non-superimposable mirror images of each other. They're like your left and right hands—identical in form but oppositely oriented, and unable to align completely if overlaid directly.

• Chiral Centers: Atoms in molecules which, when bonded to four different groups, can create chiral stereoisomers.
• Chirality: A property of a compound to exist in two forms that are mirror images.

This is a core concept in stereochemistry, as it determines how these molecules react with other chiral molecules and dictate the properties of the compound's resulting products.

Optical Activity

Optical activity is a crucial property of chiral molecules, where they have the ability to rotate plane-polarized light. Enantiomers, the mirror image isomers, each rotate light in opposite directions. When 2-butanone reacts with HCN, the result is a racemic mixture, comprising equal amounts of the two enantiomers. A racemic mixture is interesting because it appears optically inactive even though individual enantiomers are optically active.

In simpler terms, each enantiomer rotates light but in opposite directions and to the same extent. Thus, the effects of one cancellation out the other, resulting in no net rotation of light.

• Polarimeter: An instrument used to measure the angle of rotation caused by passing polarized light through an optically active compound.
• Racemic Mixture: Equal amounts of left- and right-handed enantiomers; shows no net optical activity.

While the individual molecules can interact with polarized light, the overall mixture does not exhibit optical activity due to this cancelling effect.

Enantiomers

Enantiomers are a type of stereoisomer that are mirror images of each other, yet not superimposable, similar to how your hands are reflections yet distinct from each other. In the reaction example of 2-butanone with hydrogen cyanide, the creation of a new chiral center results in two enantiomeric forms of the product.

Each enantiomer can have different interactions with other chiral substances, which can be particularly important in biological systems where even slight differences in stereochemistry can lead to vastly different outcomes.

• Stereoisomers: Isomers differing only in spatial orientation of atoms.
• Biological Significance: Many biological processes are stereospecific, meaning the shape of a molecule can affect how it interacts in biological systems.

Moreover, enantiomers are designated by the R and S configuration based on the Cahn-Ingold-Prelog priority rules, which helps chemists specify which optical isomer they are dealing with.