Problem 51 - A Complete Resource Book in Chemistry for JEE Main

Question

The optically active tartaric acid is named as \(\mathrm{D}-(+)=\) tartaric acid because it has a positive (a) optical rotation and is derived from D-glucose (b) \(\mathrm{pH}\) in an organic solvent (c) optical rotation and is derived from D-(t)= glyceraldehyde (d) optical rotation only when substituted by deuterium

Step-by-Step Solution

Answer

The correct answer is (c).

Step 1: Understanding Optical Activity

Optical activity refers to a compound's ability to rotate plane-polarized light. An optically active compound like D-(+)-tartaric acid rotates the light in a positive direction.

Step 2: Nature of D-(+)-tartaric acid

The compound is identified as D-(+), where D indicates its configuration and (+) denotes the direction of its optical rotation. The positive sign means it rotates light clockwise.

Step 3: Analyze Option (a)

Option (a) suggests D-(+)-tartaric acid has positive optical rotation and derives from D-glucose. The compound indeed has positive optical activity, but its derivation is not from D-glucose.

Step 4: Examine Option (b)

Option (b) relates optical activity with pH in an organic solvent, which is incorrect since pH is not related to optical isomerism or optical rotation.

Step 5: Evaluate Option (c)

This option indicates positive optical rotation and derivation from D-(+)-glyceraldehyde. This statement is consistent with what is known about D-(+)-tartaric acid, as D-glyceraldehyde is often used as a reference for defining D- and L- configurations.

Step 6: Consider Option (d)

Option (d) suggests positive rotation only when substituted by deuterium. This is unrelated to standard descriptions of chirality in tartaric acid, making the option incorrect.

Step 7: Conclusion

After analyzing all options, option (c) aligns with the characteristics of D-(+)-tartaric acid, confirming it has positive optical activity and correct derivation.

Key Concepts

Chiral Compounds

Chiral compounds play a pivotal role in chemistry, especially when discussing molecules like D-(+)-tartaric acid. A molecule is defined as chiral if it cannot be superimposed on its mirror image. Just like our hands, which are mirror images but not identical, chiral molecules exist in two forms: left-handed and right-handed, also known as enantiomers.
Chirality arises from the presence of an asymmetric carbon atom, often referred to as a chiral center. This carbon atom is bonded to four different groups, leading to two non-superimposable mirror images. Chiral molecules exhibit unique features, one of which is the ability to rotate plane-polarized light, giving rise to a property known as optical activity.

Optical Isomerism

Optical isomerism is a fascinating phenomenon arising from the presence of chiral centers in a molecule. Molecules exhibiting optical isomerism have identical physical properties, such as melting point and boiling point. However, they differ in the direction they rotate plane-polarized light. The two enantiomers of a chiral compound can rotate light either to the right (clockwise) or to the left (counterclockwise). This property makes optical isomers essential in fields like pharmaceuticals, where different enantiomers can have vastly different effects.
In the case of D-(+)-tartaric acid, its ability to rotate polarized light to the right is what identifies it as the "+" isomer. Understanding optical isomerism helps in determining the specific version or enantiomer of a molecule you are dealing with.

D-(+)-tartaric Acid

D-(+)-tartaric acid is a well-known example of an optically active compound. The designation "D" refers to the molecule's absolute configuration, derived from its structural similarity to D-glyceraldehyde. The "+" sign indicates the direction of light rotation, which is clockwise. This compound is naturally occurring, commonly found in grapes and other fruits, contributing to the tart taste in wines.
Interestingly, D-(+)-tartaric acid played a historical role in the development of stereochemistry, largely through the pioneering work of Louis Pasteur. By understanding D-(+)-tartaric acid's nature and optical activity, researchers can better comprehend chiral molecules' effects in biological systems and organic synthesis.

Positive Optical Rotation

When a chiral compound like D-(+)-tartaric acid rotates plane-polarized light to the right, it is said to have a positive optical rotation. This characteristic is crucial for identifying the enantiomer of a compound and understanding its behavior in chemical reactions. The actual measurement is performed using a polarimeter, an instrument that accurately detects the angle of rotation. Positive optical rotation is indicated by a "+" symbol and represents the clockwise rotation caused by the molecule.
This property is not only important for identifying compounds like D-(+)-tartaric acid but also plays a significant role in determining the purity and concentration of the compounds used in various industries. Optical rotation can provide insight into specific structural features of a molecule, crucial for various applications.