Question

An optically active alkene with molecular formula \(\mathbf{C}_{6} \mathrm{H}_{12}\) which upon hydrogenation gives optically inactive alkane is (1) 2 -methyl- 2 -pentene (2) 3-methyl-2-pentene (3) 2 -hexene (4) 3-methyl- 1 -pentene

Short answer

3-methyl-2-pentene

Step-by-step solution

- Identify the structural formulas

Write the structural formulas for each given compound: 2-methyl-2-pentene, 3-methyl-2-pentene, 2-hexene, and 3-methyl-1-pentene.

- Determine optical activity

Determine which given compound is optically active. For a compound to be optically active, it must contain a chiral center (a carbon atom bonded to four different groups).

- Analyze each compound

1. 2-methyl-2-pentene: No chiral center.2. 3-methyl-2-pentene: Has a chiral center at carbon 3.3. 2-hexene: No chiral center.4. 3-methyl-1-pentene: No chiral center.

- Verify optical inactivity upon hydrogenation

Verify if the given optically active compound forms an optically inactive alkane upon hydrogenation. The compound should lose its chiral center upon hydrogenation.

- Confirm the correct compound

Among the four options, 3-methyl-2-pentene is the only compound that is optically active (chiral center at carbon 3), which upon hydrogenation forms 3-methylpentane (optically inactive due to symmetry). Thus, the answer is 3-methyl-2-pentene.

Key concepts

Chiral Center

A chiral center is essential for understanding optical activity. It is a carbon atom bonded to four distinct groups or atoms. Due to this unique bonding, the molecule is non-superimposable on its mirror image, much like your left and right hands.
Chiral molecules rotate plane-polarized light; this rotation is what we mean by optical activity.
In the exercise, we need to find a compound with at least one chiral center. 3-methyl-2-pentene has a chiral center at carbon 3, making it optically active. Other compounds, like 2-methyl-2-pentene, 2-hexene, and 3-methyl-1-pentene, lack this feature and are optically inactive.

Structural Formulas

Structural formulas represent the arrangement of atoms within a molecule. They show how atoms are connected and the types of bonds between them.
For example, in 3-methyl-2-pentene, the structure is:

• 5 carbon atoms in a chain (pentene)
• 3rd carbon has a methyl group attached (CH3)
• Double bond between the 2nd and 3rd carbon atoms

Drawing these structures helps us identify chiral centers and analyze molecules' properties. Understanding structural formulas is crucial for determining optical activity and predicting reactions like hydrogenation.

Hydrogenation

Hydrogenation is the process of adding hydrogen to a molecule, typically across a double bond. This reaction converts alkenes (with double bonds) into alkanes (single bonds only).
For the given exercise, hydrogenation of 3-methyl-2-pentene (with a chiral center) yields 3-methylpentane, an optically inactive alkane. The chiral center in the original molecule is lost, resulting in a symmetric molecule.
Hydrogenation helps in verifying if the original molecule loses optical activity, confirming the nature of the product formed.

Optical Isomerism

Optical isomerism arises when a molecule has one or more chiral centers. Isomers are molecules with the same molecular formula but different arrangements of atoms.
Specifically, optical isomers rotate plane-polarized light differently. This phenomenon not only helps in identifying chiral compounds but also in differentiating between isomers.
The given exercise illustrated optical isomerism in 3-methyl-2-pentene. Its optical activity confirmed it has a chiral center, unlike the other options provided. Thus, optical isomerism plays a critical role in analyzing and understanding molecular properties and behavior.