Problem 33 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is optically active? (a) butane (b) 2 -methylpentane (c) 4-methylpentane (d) 3-methylheptane

Step-by-Step Solution

Answer

2-methylpentane and 3-methylheptane are optically active.

Step 1: Understanding Optical Activity

Optical activity refers to the ability of a compound to rotate the plane of polarized light. A compound is optically active if it has at least one chiral center, which is a carbon atom bonded to four different atoms or groups.

Step 2: Analyzing Butane

Butane (option a) is a straight-chain alkane with no chiral centers as all its carbon atoms are attached to either hydrogens or other carbon atoms in a symmetrical fashion.

Step 3: Analyzing 2-Methylpentane

In 2-methylpentane (option b), we identify the carbon chain and look at carbon atoms for diversity in bonding. The third carbon atom is a chiral center because it is connected to four different groups: a hydrogen, a methyl group (from the 2 substituent), an ethyl chain, and a main chain.

Step 4: Analyzing 4-Methylpentane

In 4-methylpentane (option c), the molecular structure is checked for chiral centers. All carbon atoms are symmetrically bonded and there are no carbons connected to four different groups so it is not optically active.

Step 5: Analyzing 3-Methylheptane

In 3-methylheptane (option d), we identify the carbon atoms and their connectivity. The third carbon atom acts as a chiral center being bonded to four different groups: a hydrogen, methyl, hexyl chain, and main chain, making it optically active.

Step 6: Conclusion

From the evaluation, both 2-methylpentane and 3-methylheptane have chiral centers making them optically active. Butane and 4-methylpentane lack such centers.

Key Concepts

Chiral Center

A chiral center is a crucial player in determining the optical activity of a compound. It is essentially a carbon atom bonded to four distinct atoms or groups. This unique configuration allows the carbon to become a center of asymmetry, like a crux where different paths meet, resulting in compounds that can rotate plane-polarized light. The ability to rotate this light is what labels a compound as optically active.

For a compound to exhibit optical activity, at least one of its carbon atoms must be a chiral center. Think of it like a carbon atom having different spokes like a wheel, each leading to a different route through a diverse bonding configuration.

• Without any chiral centers, a compound cannot rotate polarized light.
• With one or more chiral centers, the compound has the potential to be a molecule with optical activity unless canceled out by symmetry.

To identify a chiral center, always look for asymmetry in the carbon's attachments, where each attachment is different— much like identifying a unique fingerprint.

Alkanes

Alkanes are simple hydrocarbons with single bonds, embodying a straightforward structure often seen as the building blocks of organic chemistry. They are composed entirely of carbon (C) and hydrogen (H) atoms. Alkanes can be linear or branched, influencing their characteristics and functionalities.

The structural formula of alkanes is \(C_nH_{2n+2}\), which gives them properties like being saturated hydrocarbons—meaning they hold as many hydrogen atoms as the carbon backbone can support.

• Each carbon atom in an alkane typically forms four single bonds with hydrogen or other carbon atoms.
• Most simple alkanes are not optically active due to the lack of chiral centers.
• Branched alkanes, however, could potentially contain carbon atoms that act as chiral centers, if they are bound to four different groups.

If you are tasked with identifying optical activity in an alkane, your first duty is to search for these chiral centers within any branches or unique formations.

Stereochemistry

Stereochemistry is the study of the three-dimensional geometry of molecules which directly impacts how they interact with other molecules. It takes into account not just the bonds between atoms, but also the spatial orientation of these bonds. This field is pivotal for understanding molecular chirality and optical activity.

A key component of stereochemistry involves stereoisomers—molecules with the same molecular formula and sequence of bonded atoms but oriented differently in space.

• Stereoisomers can be either enantiomers or diastereomers, depending on their spatial configuration and symmetry.
• Enantiomers, in particular, are non-superimposable mirror images of each other and differ in the direction they rotate polarized light.

Stereochemistry gives insight into why two molecules with the same atoms might behave entirely differently in chemical reactions or biological environments, often due to the presence and arrangement of chiral centers.