Question

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

Short answer

3-Methylheptane is optically active.

Step-by-step solution

Understanding Optical Activity

Optical activity occurs when a compound can rotate plane-polarized light. For a compound to be optically active, it must be chiral, meaning it has a non-superimposable mirror image. Generally, this requires a chiral center, usually a carbon atom attached to four different groups.

Analyzing Butane for Chirality

Butane is a straight-chain alkane with the formula C4H10. Each carbon is bonded to either hydrogen or other carbon atoms symmetrically. No carbon atom in butane is attached to four different groups, so butane is not chiral, and thus not optically active.

Checking 2-Methylpentane for Chiral Centers

2-Methylpentane is a branched alkane. Its structure shows that, when drawn, all carbon atoms are symmetrical or in a position where they are only attached to either hydrogen or indistinguishable groups compared to others. Hence, no chiral center exists in 2-methylpentane, making it not optically active.

Assessing 4-Methylpentane for Chirality

4-Methylpentane, another branched alkane, also lacks any carbon atom attached to four different groups in its structure. Similar to 2-methylpentane, all carbons either have equivalent groups or symmetrical structures, leading to the same conclusion of being non-chiral and not optically active.

Evaluating 3-Methylheptane for Optical Activity

Examining 3-Methylheptane, identify the potential chiral centers. The carbon at position 3 is bonded to a hydrogen, a methyl group, an ethyl group, and a propyl group — four different substituents, confirming a chiral center. This confirms that 3-methylheptane is chiral and therefore optically active.

Key concepts

Chirality

Chirality originates from the Greek word 'cheir,' meaning hand, which perfectly illustrates the concept. Just like our left and right hands, chiral molecules have non-superimposable mirror images. This means you can't align all parts of the object with its mirror image.
In organic chemistry, chirality often involves carbon atoms, leading to compounds that can rotate plane-polarized light. This property is called optical activity. For a molecule to be chiral, it generally must have at least one carbon atom bonded to four different substituents or groups.
Chirality is a crucial feature in many biological systems and can influence how molecules interact with enzymes and receptors. A molecule's chirality can significantly affect its biological activity, making it an essential aspect to consider in drug design and synthesis.

Chiral Center

A chiral center, often referred to as a stereocenter, is where the magic happens in chiral molecules. It is typically a carbon atom bonded to four distinct groups. These unique attachments allow the carbon to produce two different spatial arrangements or mirror images that cannot be aligned identically, known as enantiomers.
Recognizing chiral centers is essential in determining whether a molecule is optically active. A molecule isn't automatically chiral just because of a possible chiral center; the overall molecule must not be superimposable on its mirror image. Here's how you can spot a chiral center:

• Look for carbon atoms with four different groups attached.
• Check for non-superimposable mirror image arrangements.
• Remember that if any two groups are identical, it won't be a chiral center.

This understanding aids in figuring out whether a compound can have enantiomers, which are vital in diverse chemical reactions and processes.

Stereochemistry

Stereochemistry deals with the spatial arrangement of atoms within molecules and how these arrangements affect their chemical behavior. It is an exciting branch of organic chemistry because it explores how different spatial configurations influence the properties and interactions of molecules.
There are several aspects to stereochemistry:

• Diastereomers: These are isomers that have similar connections but different spatial orientations, excluding mirror images.
• Enantiomers: Non-superimposable mirror images, crucial for discussing optical activity and chirality.
• Conformational Isomers: Different spatial arrangements of a molecule that can be converted via rotation around single bonds.

Hands-on understanding of stereochemistry is critical for applications in pharmaceuticals, where the particular 3D arrangement of atoms in a molecule can affect how it interacts with biological systems. Understanding these principles gives critical insight into chemical reactions, leading to innovation in drug development, agrochemical applications, and beyond.