Question
Define optical activity. Explain optical activity of lactic acid.
✓
Answer
Optical activity:
Optical activity is the property of certain organic substances to rotate the plane of plane polarised light towards the right $($clockwise$)$ or towards the left $($anticlockwise$).$
The optical activity of lactic acid can be discussed as:
$1.$ Presence of asymmetrical carbon atom:
$i.$ Lactic acid contains one asymmetrical carbon atom.
$ii.$ According to van’t Hoff’s rule: $a = 2^n,$ where, $a$ is the number of isomers and $n$ is the number of asymmetric carbon atom.
$iii.$ Thus, two isomers of lactic acid are possible.
$2.$ Non$-$superimposable mirror image structures:
Hence, lactic acid can exist as $d-$form and $l-$form which are non$-$superimposable mirror images of each other.
$3. \ (dl)$ Racemic mixture and its optical inactivity:
A mixture containing equal moles of the $d$ and $l$ forms of lactic acid is a racemic mixture which is optically inactive i.e., $dl$ or $(±)$ form. This inactivity arises due to external compensation.
Optical activity is the property of certain organic substances to rotate the plane of plane polarised light towards the right $($clockwise$)$ or towards the left $($anticlockwise$).$
The optical activity of lactic acid can be discussed as:
$1.$ Presence of asymmetrical carbon atom:
$i.$ Lactic acid contains one asymmetrical carbon atom.
$ii.$ According to van’t Hoff’s rule: $a = 2^n,$ where, $a$ is the number of isomers and $n$ is the number of asymmetric carbon atom.
$iii.$ Thus, two isomers of lactic acid are possible.
$2.$ Non$-$superimposable mirror image structures:
Hence, lactic acid can exist as $d-$form and $l-$form which are non$-$superimposable mirror images of each other.
$3. \ (dl)$ Racemic mixture and its optical inactivity:
A mixture containing equal moles of the $d$ and $l$ forms of lactic acid is a racemic mixture which is optically inactive i.e., $dl$ or $(±)$ form. This inactivity arises due to external compensation.
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