Question

Two isomeric alkenes $\mathrm{A}$ and $\mathrm{B}$ having molecular formula ${\mathrm{C}}_5{\mathrm{H}}_9\mathrm{Cl}$ on adding ${\mathrm{H}}_2$, A gives optically inactive compound, whereas B gives a chiral compound. The two isomers are (1) $\mathrm{\Lambda }$ is 4 -chloro-2-pentene and $\mathrm{B}$ is 4 -chloro-1 pentene. (2) $\mathrm{\Lambda }$ is 3 -chloro-2-pentene and $\mathrm{B}$ is 2 -chloro-2 pentene. (3) $\mathrm{\Lambda }$ is 2 -chloro-3-methyl-2-butenc and $\mathrm{B}$ is 1-chloro-3-methyl-lbutene. (4) $\mathrm{\Lambda }$ is 3 -chloro-1-pentene and $B$ is 1 -chloro-2 pentene.

Short answer

A = 2-chloro-3-methyl-2-butene, B = 1-chloro-3-methyl-1-butene

Step-by-step solution

- Understand the Problem

Identify the key details given: Two isomeric alkenes $A$ and $B$ with molecular formula $C_5{H}_{9}Cl$. Upon hydrogenation, $A$ gives an optically inactive compound, while $B$ gives a chiral compound.

- Remember Optical Activity

Optically inactive compounds do not rotate plane-polarized light because they lack chirality centers or are symmetrical. Chiral compounds have chirality centers and are optically active.

- Hydrogenation of A

Upon hydrogenation, $A$ forms an optically inactive compound. This implies that $A$ must be symmetrical after hydrogenation.

- Identify Compound A

From the options, $A$ could be either 4-chloro-2-pentene, 3-chloro-2-pentene, 2-chloro-3-methyl-2-butene, or 3-chloro-1-pentene. Upon hydrogenation, these structures lead to a specific product.

- Check Symmetry with Hydrogenation

Assess each option: For example, upon hydrogenation of 4-chloro-2-pentene, the resulting molecule is 4-chloropentane which is symmetrical, hence optically inactive.

- Hydrogenation of B

Since $B$ gives a chiral compound upon hydrogenation, the remaining choices (a or b that doesn't meet the criteria of step 5) will be checked for chirality after hydrogenation.

- Identify Chirality

Upon hydrogenation, check which compound from remaining produces a chiral center. For example, 1-chloro-3-methyl-1-butene gives a chiral center post hydrogenation.

- Final Selection

Based on the analysis: You can confirm that $A$ is 2-chloro-3-methyl-2-butene (symmetrical, optically inactive after hydrogenation) and $B$ is 1-chloro-3-methyl-1-butene (results in chiral, optically active).

Key concepts

Optical Activity

Optical activity refers to a compound's ability to rotate plane-polarized light. A compound is optically active if it has chiral centers, which means it is not superimposable on its mirror image.
Not all molecules can exhibit optical activity.
Symmetrical molecules and those without chirality centers are optically inactive.
Imagine holding up your left hand to a mirror; the reflection you see is not the same as your right hand.
This distinction is at the heart of optical activity and chirality.

Chirality in Organic Compounds

Chirality is a property of a molecule that makes it non-superimposable on its mirror image. A chiral molecule has at least one chirality (or stereogenic) center, which is usually a carbon atom bonded to four different groups.
This unique arrangement causes two possible configurations, called enantiomers.
Enantiomers are mirror images but cannot be aligned perfectly with each other.
They play crucial roles in fields like pharmaceuticals where different enantiomers of a drug can have different effects.
Understanding and identifying chirality helps in predicting a molecule's behavior and interactions.

Hydrogenation Process

The hydrogenation process involves adding hydrogen (H₂) to compounds, typically alkenes, converting them into alkanes.
This reaction helps saturate double bonds within the molecule, meaning an alkene (with at least one double bond) becomes an alkane (with only single bonds).
In the context of this problem, when an alkene undergoes hydrogenation, its structure changes and this transformation can impact its optical properties.
If a symmetric molecule after hydrogenation is formed, it becomes optically inactive. But if the result has a chirality center, it becomes optically active.

Isomer Identification

Identifying isomers involves recognizing different compounds with the same chemical formula but different structures (and possibly properties).
For example, the isomers 2-chloro-3-methyl-2-butene and 1-chloro-3-methyl-1-butene have identical molecular formulas but differ in the arrangement of their atoms.
By analyzing the structures and using clues like optical activity post-hydrogenation, we can determine the specific type of isomer.
In this exercise, one isomer (A) must become optically inactive after hydrogenation, indicating a symmetrical structure.
The other isomer (B) becomes chiral, indicating the presence of a chirality center post hydrogenation.
This structural analysis helps identify which specific isomers the problem refers to.