Question

Explain optical activity of 2-chlorobutane.

Answer 1

1. The property of a substance by which it rotates the plane of polarization of incident plane-polarized light is known as optical activity.
2. 2-Chlorobutane has one chiral carbon atom and thus exists as a pair of enantiomers, which are non-superimposable mirror images of each other.
3. These mirror images are referred to as optical isomers, which differ from each other in terms of a measurable property called optical activity. The structure of 2-chlorobutane and its mirror image can be represented as,
   
4. Thus, 2-chlorobutane will show optical activity, and in accordance with the direction of optical rotation, one isomer will be dextrorotatory, and the other will be laevorotatory.

Answer 2

          Enantiomers of 2-Chlorobutane
(with structural and Fischer’s projection formula)

1. 2-Chlorobutane contains an asymmetric carbon atom (the starred carbon atom) which is attached to four different groups, i.e. ethyl (– CH₂ – CH₃), methyl (CH₃), chloro (Cl) and hydrogen (H) groups.
   \[\begin{array}{cc}
   \ce{Cl}\phantom{.............}\\
   |\phantom{..............}\\
   \ce{CH3 - \underset{∗}{C}H - CH2 - CH3}\\
   \end{array}\]
2. As seen in the image, these groups can be arranged around the carbon atom in two different ways. It therefore exists in two enantiomers. The two enantiomers cannot be superimposed; they are mirror images of one another.
3. The laevorotatory isomer (l-isomer) is one of the enantiomers that will rotate the plane of plane-polarized light to the left. The second enantiomer, known as the dextrorotatory isomer (d-isomer), will rotate the plane of plane-polarized light to the right.
4. The racemic mixture, also known as the racemate (dl-mixture), is an equimolar mixture of the d- and l-isomers that is optically inactive. External compensation is the cause of the racemic mixture's optical inactivity.