Stereochemistry

Stereochemistry is the branch of chemistry that studies the three-dimensional arrangement of atoms in molecules and the effects of this arrangement on chemical behavior. It plays an important role in determining the properties and reactions of molecules, particularly in organic chemistry.

1.0Concepts in Stereochemistry

Isomers: Isomers are compounds that have the same molecular formula but differ in the arrangement of atoms. They can be divided into two broad categories:

Structural isomers and Stereoisomers:

2.0Types of Stereoisomers

Stereoisomers can be further classified into two main types:

1. Geometrical Isomers (Cis-Trans Isomers):

• • Geometrical Isomerism arises due to the restricted rotation around a double bond or a ring structure.
  • Cis: Substituents are on the same side of the double bond or ring.
  • Trans: Substituents are on opposite sides.
  • Example: In but-2-ene, the cis isomer has both methyl groups on the same side, while the trans isomer has them on opposite sides.

2. Optical Isomers (Enantiomers and Diastereomers):

• • Optical Isomerism occurs in molecules with chirality, where the molecule and its mirror image are non-superimposable.
  • Enantiomers: Mirror images of each other that cannot be superimposed. These isomers have identical physical properties except for the direction in which they rotate plane-polarized light.
  • Diastereomers: Stereoisomers that are not mirror images of each other. They differ in their physical and chemical properties.

Chirality and Chiral Centers:

• Chirality refers to molecules that have non-superimposable mirror images, much like how your left and right hands are mirror images but cannot be perfectly aligned on top of each other.
• A chiral center is typically a carbon atom bonded to four different groups. The presence of one or more chiral centers in a molecule leads to optical activity.

Optical Activity:

• Optical isomers rotate plane-polarized light. If the light is rotated to the right, the compound is called dextrorotatory (+), and if it is rotated to the left, it is called levorotatory (-).
• The degree of rotation is measured using a polarimeter.

Fischer Projections:

• Fischer projections are a way to represent 3D structures of molecules on a 2D plane. In these projections, horizontal lines represent bonds coming out of the plane of the paper, and vertical lines represent bonds going into the plane of the paper. This is particularly useful for representing sugars and amino acids.

3.0Importance of Stereochemistry in Reactions

Enantiomers in Reactions: Enantiomers can react differently in chiral environments (such as biological systems). For example, in pharmaceuticals, one enantiomer of a drug might be therapeutic while the other could be inactive or even harmful.

Geometrical Isomers in Reactions: Cis-trans isomers can have different physical and chemical properties, like boiling points, melting points, and reactivity. For example, cis-but-2-ene has a higher boiling point than trans-but-2-ene due to the dipole moment in the cis form.

Regioselectivity and Stereoselectivity in Elimination Reactions

1. Zaitsev's Rule: In elimination reactions, the most substituted alkene (with the highest number of alkyl groups attached to the double bond) is usually the major product.

Example

The dehydrohalogenation reaction of 2-bromopentane in the presence of a base (OH⁻, such as KOH in alcohol), leading to the formation of two possible alkene products: pent-2-ene and pent-1-ene. Where-

• Pent-2-ene is the major product due to its greater stability (more substituted alkene).
• Pent-1-ene is the minor product due to its lower stability (less substituted alkene).

${\text{H}}_3\text{C}-{\text{CH}}_2-\text{CHBr}-{\text{CH}}_2-{\text{CH}}_3$$\stackrel{{\text{OH}}^{-}}{\to }$

${\text{H}}_3\text{C}-{\text{CH}}_2-\text{CH}=\text{CH}-{\text{CH}}_3(81\%)+{\text{H}}_2\text{C}=\text{CH}-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3(19\%)$

2. Hofmann Product: Sometimes, with bulky bases, the least substituted alkene is favored, leading to the Hofmann product.

Examples: 

${\text{CH}}_3{\text{CONH}}_2+{\text{Br}}_2+4\text{NaOH}\stackrel{\Delta }{\to }{\text{CH}}_3{\text{NH}}_2+2\text{NaBr}+{\text{Na}}_2{\text{CO}}_3+2{\text{H}}_2\text{O}$

${\text{CH}}_3{\text{CH}}_2{\text{CONH}}_2+{\text{Br}}_2+4\text{NaOH}\stackrel{\Delta }{\to }{\text{CH}}_3{\text{CH}}_2{\text{NH}}_2+2\text{NaBr}+{\text{Na}}_2{\text{CO}}_3+2{\text{H}}_2\text{O}$

4.0Comparison Between E1, E2, and E1cB