Problem 35 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following compounds exhibits stereoisomerism? (a) 2-methylbutene-1 (b) 3-methylbutyne-1 (c) 3-methylbutanoic acid (d) 2-methylbutanoic acid

Step-by-Step Solution

Answer

Both 3-methylbutanoic acid and 2-methylbutanoic acid exhibit stereoisomerism.

Step 1: Understand Stereoisomerism

Stereoisomerism occurs when compounds have the same molecular formula and atom-to-atom connections but differ in the spatial arrangement of these atoms. Two common types of stereoisomerism are geometrical (cis-trans) isomerism and optical isomerism (chiral centers).

Step 2: Analyze Option (a) 2-methylbutene-1

The compound 2-methylbutene-1 features a double bond. For geometrical isomerism, typically the double bond must have two different groups that allow for cis/trans configurations. In 2-methylbutene-1, the double bond at position 1 does not allow for stereoisomerism as it does not meet these structural requirements.

Step 3: Examine Option (b) 3-methylbutyne-1

3-methylbutyne-1 is an alkyne with a triple bond, which does not allow for geometrical isomerism (cis-trans), as the linear arrangement of atoms around a triple bond restricts different spatial arrangements. Thus, 3-methylbutyne-1 does not exhibit stereoisomerism.

Step 4: Evaluate Option (c) 3-methylbutanoic acid

In 3-methylbutanoic acid, look for chiral centers, which are carbon atoms connected to four different groups. The third carbon in 3-methylbutanoic acid connects to CH₃, CH₂, a hydrogen, and a carboxylic acid group (-COOH), making it a chiral center. This allows 3-methylbutanoic acid to have optical isomers, i.e., it exhibits stereoisomerism.

Step 5: Review Option (d) 2-methylbutanoic acid

Checking for chiral centers in 2-methylbutanoic acid, the second carbon connects to a CH₃ group, CH, a carboxylic acid group (-COOH), and a hydrogen atom. This makes it also a chiral center, indicating that 2-methylbutanoic acid can exhibit optical isomerism as well.

Step 6: Conclusion

Both option (c) 3-methylbutanoic acid and option (d) 2-methylbutanoic acid exhibit stereoisomerism due to the presence of chiral centers.

Key Concepts

Geometrical Isomerism

Geometrical isomerism, also known as cis-trans isomerism, is a type of stereoisomerism that occurs due to the restricted rotation around a double bond or within a ring structure. This form of isomerism is characterized by the spatial arrangement of atoms or groups around this fixed bond. There are two main forms:

• Cis isomer: The same or similar groups are on the same side of the double bond.
• Trans isomer: The same or similar groups are on opposite sides of the double bond.

To exhibit geometrical isomerism, a compound typically requires a double bond between two carbons, each attached to different substituents, allowing for distinct configurations. In the original exercise, neither 2-methylbutene-1 nor 3-methylbutyne-1 can exhibit geometrical isomerism, as they do not meet the necessary criteria. The lack of different substituents required for cis-trans arrangement or the presence of a triple bond, which is linear, limits the potential for such isomerism in these compounds.

Optical Isomerism

Optical isomerism is another type of stereoisomerism, arising from the presence of chiral centers in a compound. This kind of isomerism results in two types of isomers, known as enantiomers, which are non-superimposable mirror images of each other. These enantiomers can rotate plane-polarized light in different directions:

• Levorotatory (l-): Rotates light to the left.
• Dextrorotatory (d-): Rotates light to the right.

Optical isomerism is seen in compounds with one or more chiral centers. A chiral center is a carbon atom connected to four different groups, allowing for distinct spatial arrangement that gives rise to optically active isomers. In the original solution, both 3-methylbutanoic acid and 2-methylbutanoic acid contain chiral centers, making them capable of exhibiting optical isomerism.

Chiral Centers

The concept of chiral centers is central when discussing optical isomerism. A chiral center, usually a carbon atom, is uniquely bonded to four different groups or atoms. This distinct arrangement makes the molecule non-superimposable on its mirror image, similar to how your left and right hands are mirror images but not identical. When identifying chiral centers:

• Look for carbons with four single bonds.
• Ensure that each of the carbon's four attachments differ from one another.

The presence of chiral centers enables the compound to exist in two enantiomeric forms, each affecting light's polarization differently. In the case viewed in the original exercise, both 3-methylbutanoic acid and 2-methylbutanoic acid have chiral centers, thus explaining their capacity to demonstrate optical isomerism. This makes them fascinating examples of isomers in organic chemistry.