Problem 97 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

In the reaction: \(\mathrm{CH}_{3} \mathrm{CHO}+\mathrm{HCN} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CN}\) a chiral centre is produced. The number of stereoisomers of the product is (a) 3 (b) 2 (c) 4 (d) none of these

Step-by-Step Solution

Answer

The number of stereoisomers is 2.

Step 1: Identifying the Chiral Center

In the given reaction, an aldehyde group in acetaldehyde \(\text{CH}_3\text{CHO}\) reacts with HCN. The carbon atom bonded to CN, H, OH, and CH\(_3\) in the product molecule, \(\text{CH}_3\text{CH(OH)}\text{CN}\), is a chiral center because it is attached to four different groups.

Step 2: Understanding Stereoisomers

Stereoisomers are molecules that have the same structural formula but differ in spatial arrangements. A chiral center can give rise to stereoisomers that are mirror images of each other, known as enantiomers.

Step 3: Calculating the Number of Stereoisomers

For a molecule with n chiral centers, the maximum number of stereoisomers is \(2^n\). Here, there is one chiral center, so the maximum number of stereoisomers is \2^1 = 2\.

Step 4: Conclusion

Since there is only one chiral center in the molecule \(\text{CH}_3\text{CH(OH)}\text{CN}\), there are two possible stereoisomers (enantiomers). Therefore, the correct answer is (b) 2.

Key Concepts

Chiral Centers

In organic chemistry, a chiral center is a carbon atom that is connected to four different groups. This unique arrangement allows for the possibility of chiral molecules, which can lead to the formation of stereoisomers. Identifying a chiral center is crucial, as it helps predict the number of possible stereoisomers a molecule can have.

For example, in the reaction given:

• Acetaldehyde (7_3 7 ext{CHO}) reacts with hydrogen cyanide (HCN).
• The resulting product is f_3 f ext{CH(OH)CN}.
• The carbon atom bonded to CN, H, OH, and CH ext{3} becomes the chiral center.

With a chiral center, the molecule can exist in different forms that are non-superimposable mirror images of each other. Recognizing these configurations helps further understand how the molecule will behave chemically and biologically.

Stereoisomers

Stereoisomers are compounds that have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of these atoms. These differences can affect the physical and chemical properties of the molecules significantly.

In molecules where chiral centers are present, the stereoisomers are categorized further:

• Enantiomers: Mirror images that are non-superimposable, like left and right hands.
• Diastereomers: Not all stereoisomers are enantiomers. If there's more than one chiral center, not all stereoisomers will be mirror images.

For the given reaction, the product f_3 ext{CH(OH)CN} has one chiral center leading to the formation of two stereoisomers. In this case, these are enantiomers.

Enantiomers

Enantiomers are specific types of stereoisomers that are non-superimposable mirror images of each other. An intuitive way to think about enantiomers is to consider how your hands are mirrors of each other; they have the same structure but can't be perfectly overlaid.

Enantiomers have identical physical properties such as melting point and boiling point, but they rotate plane-polarized light in equal and opposite directions. This property is called optical activity. They also might interact differently with other chiral molecules, which is critical in biological systems where specific interactions between molecules matter.

In the reaction involving the formation of f_3 ext{CH(OH)CN}, the presence of one chiral center allows for the production of two enantiomers. Essentially, these two enantiomers would be mirror images and could exhibit different behavior in chemical reactions, although their basic physical properties remain the same.

Aldehyde Reactions

Aldehydes are carbonyl-containing compounds and are reactive in various organic reactions, especially with nucleophiles. In the specific reaction given, an aldehyde (f_3 ext{CHO}) reacts with hydrogen cyanide (HCN), a nucleophile.

During this reaction:

• Hydrogen cyanide adds to the carbonyl group of the aldehyde.
• This forms a new carbon-carbon bond.
• The resulting compound, f_3 ext{CH(OH)CN}, contains the chiral center.

Aldehyde reactions are fundamental as they allow for the construction of more complex molecules from simpler starting materials. The ability of aldehydes to form chiral centers like in the given reaction opens a gateway for further modifications and synthesis of diverse organic molecules in laboratory and industrial settings.