Animated Solution for Chemistry - Organic Chemistry: Which type of isomerism is shown by 2, 3-dichlorobutane?

The problem asks us to identify the type of isomerism exhibited by 2,3-dichlorobutane. To solve this, we need to systematically analyze the structure of the molecule and check the conditions for different types of isomerism.

Let's start by visualizing the molecule given to us. The name "butane" indicates a straight chain of four carbon atoms. The "2,3-dichloro" part tells us that there are chlorine atoms attached to the second and third carbon atoms in this chain.

When we draw the expanded structural formula, we get:

Right away, we can rule out a few options. Geometric isomerism requires restricted rotation, which typically happens around a carbon-carbon double bond or within a rigid ring structure. Since 2,3-dichlorobutane is an open-chain alkane with only single bonds, there is free rotation around all carbon-carbon bonds. Thus, it cannot show geometric isomerism.

Now, let's investigate the possibility of optical isomerism. The fundamental requirement for a molecule to show optical isomerism is the presence of chirality. A carbon atom is considered chiral (or asymmetric) if it is $s{p}^3$ hybridized and bonded to four completely different atoms or groups of atoms.

Let's look closely at the second carbon atom (C-2). What is it attached to? 1. A hydrogen atom ($-\mathrm{H}$) 2. A chlorine atom ($-\mathrm{Cl}$) 3. A methyl group ($-\mathrm{C}{\mathrm{H}}_3$) 4. The rest of the carbon chain ($-\mathrm{CH}(\mathrm{Cl})\mathrm{C}{\mathrm{H}}_3$)

Since all four of these groups are different, C-2 is a chiral center! We usually mark this with an asterisk ($C^{\ast }$).

If we apply the exact same logic to the third carbon atom (C-3), we find that it is also attached to the same four distinct groups. Therefore, C-3 is our second chiral center.

Because 2,3-dichlorobutane possesses chiral carbon atoms, the molecule can exist in different spatial arrangements that are non-superimposable mirror images of each other (known as enantiomers).

Molecules that have non-superimposable mirror images are optically active; they can rotate the plane of plane-polarized light. Therefore, the presence of these chiral centers confirms that 2,3-dichlorobutane exhibits optical isomerism.