Question

The number of possible enantiomeric pairs than can be produced during monochlorination of 2 -methyl butane is (a) 2 (b) 3 (c) 4 (d) 1

Short answer

The correct answer is (a) 2.

Step-by-step solution

Understanding the Structure of 2-Methyl Butane

2-Methyl butane, also known as isopentane, is a five-carbon chain (pentane) with a methyl group attached to the second carbon atom. It can be represented by the chemical formula \( C_5H_{12} \).

Identifying Possible Chlorination Sites

Monochlorination involves replacing one hydrogen atom with chlorine. In 2-methyl butane, various hydrogen atoms are equivalent forms: the primary hydrogen on carbon 1, the secondary hydrogen on carbons 2 and 3, and the primary hydrogen on carbon 4 and 5. We must identify the unique carbon atoms: 1, 2, 3, and 4 (carbon 5 is symmetrical with carbon 4).

Evaluating Chiral Centers Formation

A chiral center is typically formed when a carbon atom is bonded to four different groups. After chlorination, we evaluate if new chiral arrangements of atoms and chlorine result. Chlorination at carbons 2 and 3 can lead to new chiral centers because it creates a tetrahedral carbon bonded to four different groups.

Counting Enantiomer Pairs

Chlorination at carbon 2 or 3 would create chiral centers, thus producing enantiomers. Chlorination on carbon 1 or 4 does not create new chiral centers because the resulting carbon is not bonded to four different groups. Thus, chlorination at four different positions can potentially create two enantiomer pairs, coming from chlorination at carbon 2 and carbon 3.

Key concepts

Chirality

In organic chemistry, chirality refers to the geometric property of a molecule that makes it non-superimposable on its mirror image. Just like our left and right hands, chiral molecules have a distinct 'handedness'. A crucial aspect of chirality is the presence of a chiral center, which is typically a carbon atom bonded to four distinct groups. This tetrahedral configuration allows molecules to exist in two different forms called enantiomers. When examining reactions like monochlorination, identifying potential chiral centers helps us predict whether enantiomers can form. Each possible arrangement around a chiral center could lead to unique chemical and physical properties, making the study of chirality pertinent in chemical reactions and applications.

Isomerism

Isomerism is the phenomenon where compounds have the same molecular formula but differ in structure or spatial arrangement. In the context of monochlorination of alkanes like 2-methyl butane, we are primarily concerned with stereoisomerism, particularly enantiomeric pairs. These are isomers that are mirror images of each other but not identical. This kind of isomerism is crucial because each enantiomer might interact differently in a chemical environment, leading to different physical properties or biological activities. Recognizing isomerism in chemical reactions helps in predicting the outcomes, especially when new chiral centers might form.

Monochlorination

Monochlorination is a substitution reaction where one hydrogen atom in a hydrocarbon is replaced by a chlorine atom. This reaction leads to a variety of structural outcomes depending on the position of the hydrogen being replaced. In the case of 2-methyl butane, each carbon atom's hydrogen can potentially be substituted, leading to unique chlorinated products. The process can produce different structural isomers, some of which might have chiral centers, resulting in the formation of enantiomer pairs. The understanding of monochlorination is vital because it teaches us about positional selectivity and the resulting diversity in product formation.

Alkanes

Alkanes are simple hydrocarbons consisting of carbon and hydrogen atoms, with the general formula of C_nH_{2n+2} for saturated chains. They are referred to as saturated hydrocarbons because they feature single bonds between carbon atoms. 2-Methyl butane, specifically, is a branched alkane with five carbon atoms, creating a unique branching at carbon 2 with a methyl group. While alkanes are relatively unreactive due to stable C-C and C-H bonds, reactions like chlorination can alter their structure significantly. Understanding alkanes and their structure is critical in studying reactions that involve changes in their carbon-hydrogen framework, such as monochlorination.