The reason of the loss of optically activity of lactic acid when \(OH\) group is exchanged by \(H\) is that

asymmetry of the molecule is destroyed.

The correct answer is option 1. asymmetry of the molecule is destroyed.

Let us delve into the details of why the replacement of the hydroxyl group \((-OH)\) by hydrogen \((-H)\) in lactic acid leads to the loss of optical activity:

Understanding Optical Activity and Chirality:

Chirality and Chiral Centers:

A molecule is chiral if it lacks an internal plane of symmetry and has at least one chiral center, which is a carbon atom bonded to four different groups. Chiral molecules exist in two non-superimposable mirror image forms (enantiomers), which rotate plane-polarized light in opposite directions.

Lactic Acid (CH₃CHOHCOOH):

Lactic acid has a chiral center at the carbon atom bonded to both the carboxyl group \((-COOH)\) and the hydroxyl group \((-OH)\). The presence of the \(-OH\) group introduces chirality because the carbon atom (\(C_2\) in lactic acid) has four different substituents: -\(H\), \(-COOH\), \(-CH_3\), and \(-OH\).

Loss of Optical Activity in Hydroxyacetic Acid \((CH_3CH_2COOH)\):

When the \(-OH\) group in lactic acid is replaced by \(-H\), the resulting molecule is hydroxyacetic acid \((CH_3CH_2COOH)\). In hydroxyacetic acid, the carbon atom \((C_2)\) that previously had the \(-OH\) group now has \(-H\), \(-COOH\), \(-CH_3\), and \(-H\) as substituents. Here, the carbon atom no longer has four different substituents; instead, it has two hydrogens, which are identical. This change results in the molecule becoming achiral (not chiral) because it now possesses a plane of symmetry through the carbon atom and the carboxyl group.

Reason for Loss of Optical Activity:

Asymmetry Destruction: The replacement of \(-OH\) by \(-H\) destroys the asymmetry of the molecule. Lactic acid is chiral due to the presence of the \(-OH\) group, which creates an asymmetrical arrangement of substituents around the chiral center. Hydroxyacetic acid lacks this asymmetry because the replaced \(-H\) at the chiral center results in two identical substituents (both \(-H\)). As a result, hydroxyacetic acid does not exhibit optical activity because it is superimposable on its mirror image.

Conclusion: The loss of optical activity in lactic acid when the \(-OH\) group is replaced by \(-H\) occurs because the replacement destroys the asymmetry (chirality) of the molecule. Optical activity depends on the presence of a chiral center with four different substituents, and the replacement of \(-OH\) with \(-H\) results in a molecule (hydroxyacetic acid) that no longer meets this criterion, thus becoming achiral. Therefore, option (1) Asymmetry of the molecule is destroyed accurately explains why the optical activity is lost in this chemical transformation.