Question

The reason of the loss of optically activity of lactic acid when \(O H\) group is exchanged by \(H\) is that (1) asymmetry of the molecule is destroyed. (2) symmetry of the molecule is destroyed. (3) structural change occurs. (4) spatial arrangement is changed.

Short answer

The loss of optical activity occurs because the asymmetry of the molecule is destroyed.

Step-by-step solution

- Understanding Optical Activity

Optical activity refers to a molecule's ability to rotate the plane of polarized light. This property is associated with the chirality of the molecule, meaning it must have a non-superimposable mirror image (lacks symmetry).

- Identifying Lactic Acid Structure

Lactic acid is a chiral molecule because it has a central carbon atom bonded to four different groups: a hydroxyl group (OH), a methyl group (CH3), a carboxyl group (COOH), and a hydrogen atom (H). This configuration makes it optically active.

- Exchanging OH with H

Exchanging the hydroxyl group (OH) with a hydrogen atom (H) means that the central carbon atom will now be bonded to two hydrogen atoms instead of four different groups.

- Assessing the Change

By substituting OH with H, the central carbon loses its four different groups, making it no longer chiral. The molecule becomes symmetric, destroying its optical activity.

- Conclusion

The replacement of the OH group with an H in lactic acid destroys the molecule's asymmetry, which is the reason for the loss of optical activity.

Key concepts

Chirality in Organic Chemistry

The concept of chirality is key in organic chemistry. Chirality occurs when a molecule cannot be superimposed on its mirror image, much like your left and right hands. For a molecule to be chiral, it typically needs a carbon atom (called a chiral center) bonded to four different groups or atoms. This lack of symmetry is crucial, as it allows the molecule to exist in two different forms (called enantiomers), each of which can rotate plane-polarized light in different directions.

Enantiomers are like mirror images: one might rotate light clockwise (dextrorotatory), and the other counterclockwise (levorotatory). The property that allows these molecules to rotate light is called optical activity. Without chirality, a molecule can't have this interesting behavior. This will be important when we examine why lactic acid loses its optical activity when an OH group is replaced by an H atom.

Lactic Acid Structure

Lactic acid is a perfect example to understand chirality and optical activity. Its structure includes a central carbon atom bonded to four different groups: a hydroxyl group (OH), a methyl group (CH3), a carboxyl group (COOH), and a hydrogen atom (H). Because of these four different groups, the central carbon is chiral. It means lactic acid has two enantiomers, and can, therefore, rotate plane-polarized light.

In practical terms, when you look at the structure, you’ll notice that lactic acid cannot be superimposed on its mirror image, confirming its chirality. These properties make lactic acid optically active. However, if we make changes to the structure, such as replacing the OH group with an H atom, the molecule loses this chirality.

Effect of Symmetry on Optical Activity

Optical activity heavily depends on the asymmetry of a molecule. Symmetry is essentially the opposite of chirality. If a molecule is symmetric, meaning one half mirrors the other, it cannot be optically active. When addressing the lactic acid example, replacing the OH group with an H atom makes the central carbon atom bonded to two hydrogen atoms. This change creates a plane of symmetry.

With symmetry restored, the molecule is no longer chiral. Because the molecule no longer has four different groups bonded to the central carbon atom, it loses the ability to rotate plane-polarized light. This loss of asymmetry, or symmetry restoration, is why the modified lactic acid is no longer optically active. Simply put, symmetry cancels out the special properties that allow a molecule to be optically active in the first place.