Chirality and Enantiomers in GOB Chemistry: Study Notes

Chirality in Organic Molecules

Definition and Properties of Chirality

Chirality is a fundamental concept in organic chemistry describing molecules that are not superimposable on their mirror images. Such molecules are called chiral, while those that are superimposable are achiral.

• Chiral molecules have at least one carbon atom (chiral center) bonded to four different groups.

• Achiral molecules lack this property and are superimposable on their mirror images.

• Optical isomers (enantiomers) are pairs of chiral molecules that are non-superimposable mirror images of each other.

• Chiral molecules are optically active: they rotate plane-polarized light.

Key Terms:

• Mirror image: The reflection of a molecule as seen in a mirror.

• Non-superimposable: The property that the mirror image cannot be placed over the original molecule to give the same structure.

• Chiral center: A carbon atom attached to four unique groups.

• Enantiomers: Stereoisomers that are non-superimposable mirror images.

Example: The molecule 2-butanol (CH3CH(OH)CH2CH3) has a chiral center at the second carbon, which is attached to four different groups: CH3, OH, CH2CH3, and H.

Comparison of Achiral and Chiral Molecules

The following table summarizes the differences between achiral and chiral molecules:

Property	Achiral	Chiral
Superimposable on mirror image?	Yes	No
Chiral center present?	No	Yes (at least one)
Optical activity	Inactive	Active
Example	Ethane (CH3CH3)	2-butanol (CH3CH(OH)CH2CH3)

Identifying Chiral Centers

How to Identify Chiral Centers

To determine if a molecule is chiral, look for carbon atoms bonded to four different groups. These carbons are called chiral centers or stereocenters.

• Scan the molecule for tetrahedral carbons (sp3 hybridized).

• Check if each of the four groups attached to the carbon is different.

• If so, the carbon is a chiral center.

Example: In lactic acid (CH3CH(OH)COOH), the central carbon is attached to CH3, OH, COOH, and H, making it a chiral center.

Optical Activity

Optical Properties of Chiral Molecules

Chiral molecules interact with plane-polarized light, causing it to rotate. This property is called optical activity.

• Optically active compounds rotate plane-polarized light either to the right (dextrorotatory, +) or to the left (levorotatory, -).

• Achiral compounds do not rotate plane-polarized light.

Application: Optical activity is used in laboratories to distinguish between enantiomers using a polarimeter.

Drawing Enantiomers

Methods for Drawing Enantiomers

There are two main methods for drawing the enantiomer of a chiral molecule:

• Method 1: Draw the mirror image as the molecule would appear in a mirror. This involves reflecting the structure across a vertical axis.

• Method 2: Switch the positions of the groups attached to the chiral center by changing solid wedges to dashed wedges and vice versa.

Example: For 2-butanol, the mirror image will have the OH and H groups swapped in three-dimensional space.

Practice: Drawing Enantiomers

• Given a molecule with a chiral center, use either method to draw its enantiomer.

• For complex molecules, identify the chiral center first, then apply the method.

Example: For the thalidomide molecule, the chiral center is indicated, and the enantiomer is drawn by switching the groups attached to that center.

Summary Table: Methods for Drawing Enantiomers

Method	Description	Application
Method 1	Draw the mirror image as seen in a mirror	Simple molecules, visualizing symmetry
Method 2	Switch solid and dashed wedges at the chiral center	Complex molecules, stereochemistry assignments

Key Equations and Concepts

• Number of possible stereoisomers: For a molecule with n chiral centers, the maximum number of stereoisomers is $2^n$.

• Chiral center definition: A carbon atom bonded to four unique groups.

Additional info:

• Enantiomers have identical physical properties except for the direction in which they rotate plane-polarized light and their interactions with other chiral substances.

• Thalidomide is a classic example of a drug where one enantiomer is therapeutic and the other is teratogenic.