Question

Simplest alkane which can exhibit optical activity is (a) 3 -methyl hexane (b) 3 -methyl heptane (c) 2 -methyl butane (d) neopentane

Short answer

Answer: (c) 2-methyl butane

Step-by-step solution

Analyze the structure of each alkane options

We need to determine if any of these alkanes have a carbon atom with four different substituents attached to it. We may also identify chiral centers by looking for a carbon atom with no plane of symmetry. Let's analyze the structure of each of the given alkanes: Option (a) 3-methyl hexane: H3C-CH2-CH(CH3)-CH2-CH2-CH3 Option (b) 3-methyl heptane: H3C-CH2-CH(CH3)-CH2-CH2-CH2-CH3 Option (c) 2-methyl butane: H3C-CH(CH3)-CH2-CH3 Option (d) neopentane: H3C-C(CH3)3

Identify chiral centers

Now, we need to identify the presence of chiral centers in each of these alkanes. If we find a carbon atom with four different substituents, it will be considered a chiral center. Option (a) 3-methyl hexane: There is no carbon within this structure that meets the criteria for a chiral center. Option (b) 3-methyl heptane: There is no carbon within this structure that meets the criteria for a chiral center. Option (c) 2-methyl butane: In this structure, the second carbon has the following substituents: 1. An H atom 2. A CH3 group 3. A CH2-CH3 group 4. A CH3 group coming from the branch Since all four substituents are different, this carbon is a chiral center. Option (d) neopentane: All carbons in the neopentane have the same substituents, which are CH3 groups. Therefore, there is no chiral center in this structure.

Identify the alkane with the chiral center

We have identified that only the alkane in option (c), 2-methyl butane, has a chiral center, which is necessary for exhibiting optical activity. So, the correct answer is: (c) 2-methyl butane

Key concepts

Chiral Centers

Chiral centers are pivotal in the field of stereochemistry and are essential for molecules to exhibit optical activity. A chiral center, often referred to as a stereocenter, is a carbon atom that's bonded to four different groups. This unique structural feature makes it possible for that carbon atom—and thus the entire molecule—to not be superimposable on its mirror image, much like a person's left and right hands.

When we look at a molecule like 2-methyl butane, we see that it fits this criterion: the second carbon has four different groups attached, including hydrogen, a methyl group attached to the main chain, another methyl on the side branch, and an ethyl group. These diverse substituents ensure that the chiral center can indeed create two non-superimposable versions, or enantiomers, of the molecule. Despite being mirror images, enantiomers can have very different biological and chemical properties.

Alkane Isomers

Alkanes are organic compounds made up entirely of carbon and hydrogen atoms, bonded together in a tree-like structure without rings, known for their vast number of isomers. Structural isomers, which are variations in the arrangement of atoms in the molecule, can greatly influence the properties of these compounds.

In the case of alkanes, structural isomers come in various forms, such as chain isomers, which involve branching in different positions. This rearrangement can lead to different physical properties like boiling points and densities, even though the chemical formula remains unchanged. Positional isomers, where functional groups are attached at different locations on the main carbon chain, also fall under this category but are more relevant to other types of organic compounds with functional groups.

The task of distinguishing isomers falls into the realm of organic chemistry and shapes our understanding of complex biological systems and material science, where slight molecular changes can have significant impacts.

Stereochemistry

Stereochemistry is a sub-discipline of chemistry that studies the spatial arrangement of atoms in molecules and the effects of these arrangements on the physical and chemical properties of those molecules. Its principles are vital for understanding topics like optical activity, where molecules with chiral centers can rotate plane-polarized light either to the right or to the left, called 'dextrorotation' or 'levorotation' respectively.

Importantly, stereochemistry isn't just limited to single chiral centers. It also encompasses other types of stereoisomers such as diastereomers—molecules with two or more stereocenters that are not mirror images of each other—and cis-trans isomers, where groups are positioned differently around a double bond. The study of stereochemistry is crucial in fields like pharmaceuticals, agrochemicals, and materials science, as the 3D arrangement of atoms within a molecule can significantly alter its interaction with other molecules, including biological systems.