Question

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

Short answer

2-Methylpentane and 3-Methylheptane are optically active.

Step-by-step solution

Understanding Optical Activity

An optically active compound can rotate the plane of polarized light, typically because it contains a chiral center—a carbon atom bonded to four different groups.

Analyze Butane

Butane is a straight-chain alkane with no branches or chiral centers. All carbon atoms in butane are attached to two hydrogen atoms or two carbon atoms, which do not satisfy the condition for chirality.

Analyze 2-Methylpentane

2-Methylpentane has the structure CH extsubscript{3}-CH(CH extsubscript{3})-C extsubscript{3}H extsubscript{7}. We need to check each carbon: the second carbon is bonded to a methyl group, a hydrogen, and differing groups on the other branches, making it a chiral center, hence optically active.

Analyze 4-Methylpentane

4-Methylpentane is structured so the carbon backbone doesn't allow for any carbon to have four different substituents. Thus, it lacks a chiral center and is not optically active.

Analyze 3-Methylheptane

In 3-Methylheptane, the third carbon is bonded to a hydrogen, a methyl group, an ethyl group, and a propyl group, creating a chiral center. Therefore, it can be optically active.

Key concepts

Chiral Center

A chiral center, also known as a stereocenter, is a specific type of carbon atom that is bonded to four different atoms or groups. This unique configuration imparts chirality, meaning the molecule can exist in two non-superimposable mirror images, much like human hands.
Each of these mirror images is referred to as an enantiomer. Enantiomers are significant in the world of chemistry because they can have entirely different effects in biological systems.

• To identify a chiral center, look for carbon atoms connected to four distinct substituents.
• Without a chiral center, a molecule typically cannot be optically active.

This concept is crucial in determining whether a compound can rotate plane-polarized light and thus be considered "optically active."

Optically Active Compounds

Optically active compounds have the special ability to rotate the plane of polarized light. This property arises due to the presence of chiral centers within the molecule.
When light passes through an optically active substance, its direction of vibration gets altered. This change is measurable using an instrument called a polarimeter.

• A compound is considered optically active if it contains at least one chiral center that cannot be counteracted by another chiral center (in cases where two centers may exist).
• Enantiomers will rotate plane-polarized light in equal amounts but opposite directions.

The optical activity of a compound is a hallmark feature used in stereochemistry to understand the spatial arrangement of atoms in molecules.

Stereochemistry

Stereochemistry studies the arrangement of atoms in molecules and how they affect the physical and chemical properties of a substance. Understanding stereochemistry is key for recognizing chirality and optical activity in organic compounds.
The 3D configuration of molecules is crucial for many chemical processes, including the effectiveness of pharmaceuticals, as one enantiomer may be active while another may not.

• Stereochemistry involves understanding concepts such as enantiomers, diastereomers, and conformations.
• Determining whether a compound is optically active involves assessing its stereochemistry, especially the presence and interaction of chiral centers.

In practical applications, stereochemistry helps chemists predict how different molecular structures interact with biological systems or other chemicals.

Chirality in Organic Compounds

Chirality is a fundamental concept in organic chemistry, referring to a property where a molecule is not superimposable on its mirror image. This property is attributed to chiral centers.
Chiral molecules are essential in the production of specific optical isomers, which are necessary for various chemical applications and reactions.

• The presence of chirality in compounds can affect how they react in different environments, leading to variations in biological activity and chemical properties.
• Understanding chirality helps chemists design compounds with desired optical activities, particularly in pharmaceuticals, where the chirality can significantly impact a drug's efficacy and safety.

Therefore, identifying chirality in organic compounds is vital for applications in both research and industry, as it impacts everything from synthesis to application.