Chirality

Handedness in molecules

Chirality

Molecules that possess "handedness" exist in two forms: a "left-handed" form and a "right-handed" form.

Chirality

These two forms are related to each other in the same way that a pair of hands are related to

each other. The relationship is that of mirror images.

Chirality

A left hand and a right hand are mirror images of each other:

Chirality

Any molecule that contains a carbon atom with four different groups bonded to it in tetrahedral orientation possesses handedness

Chiral Center

An atom in a molecule that has four different groups bonded to it in a tetrahedral orientation

Chiral

A molecule that contains a chiral center

Chiral Molecule

A molecule whose mirror images are not superimposable

Chiral Molecule

Posses handedness

Achiral Molecule

A molecule whose mirror images are superimposable

Achiral Molecule

Does not posses handedness

Mirror Images

the reflection of an object in a mirror

Superimposable mirror images

images that coincide at all points when the images are laid upon each other

Nonsuperimposable mirror images

images where not all points coincide when the images are laid upon each other

Guidelines for Identifying Chiral Centers

A carbon atom involved in a multiple bond (double or triple bond) cannot be a chiral center since it has fewer than four groups bonded to it. To have four groups present, all bonds about the chiral center must be SINGLE BONDS

Guidelines for Identifying Chiral Centers

A carbon atom that has two like groups bonded to it cannot be a chiral center since it does not meet the requirement of four different groups. The commonly encountered entities —CH3 and —CH2— in a structural formula never involve chiral centers because of the presence of two or more like hydrogen atoms.

Isomerism in Organic Chemistry

Order of Bonding (CCPF)

• Constitutional Isomers

• Chain Isomers

• Positional Isomers

• Functional Isomers

Isomerism in Organic Chemistry

Stereoisomerism (SGCC)

• Spatial Orientation

• Geometric Isomers

• Conformational Isomers

• Configurational Isomers

Guidelines for Identifying Chiral Centers

Carbon atoms in a ring system, if not involved in multiple bonding, can be chiral centers. Such carbon atoms have four bonds— two to neighboring atoms in the ring and two to substituents on the ring.

Chirality occurs when both

(I) the two substituents are different and

(2) the two "halves" of the ring emanating from the chiral center are different.

Importance of Chirality

In human body chemistry, right-handed and left-handed forms of a molecule often elicit different responses within the body.

Importance of Chirality

Monosaccharides, the simplest type of carbohydrate and the building block for more complex types of carbohydrates, are almost always "right-handed."

Importance of Chirality

Interestingly, the building blocks for proteins, amino acids, are always

left-handed molecules.

Importance of Chirality

It is a general phenomenon found in all classes of organic compounds.

Order of Bonding (CSPF)

Constitutional Isomers

Skeletal Isomers

Positional Isomers

Functional Group Isomers

Constitutional Isomers

Isomers in which the atoms have different connectivity

Skeletal Isomers

Isomers with different carbon atom arrangements and different hydrogen atom arrangements

Positional Isomers

Isomers that differ in the location of the functional group

Functional Group Isomers

Isomers that contain different functional groups

Stereoisomerism

Isomers that have same molecular and structural formulas but differ in the orientation of atoms in space

Number of Stereoisomerism

2ⁿ

• n: number of chiral carbon

Two Major Structural Features that Generate Stereoisomerism

• the presence of a chiral center in a molecule

• the presence of "structural rigidity" in a molecule

STEREOISOMERS

Differ from each other only in their configuration (three-dimensional shape)

CONFIGURATIONAL ISOMERISM - Two Types

1. Enantiomers

2. Diastereomers

Enantiomers

stereoisomers whose molecules are nonsuperimposable, mirror images of each other. Left- and right-handed forms of a molecule with a single chiral center

Diastereomers

are stereoisomers whose molecules are not mirror images of each other.

Enantiomers

The handedness of the two enantiomers is specified by using the designations D and L

Enantiomers

The enantiomer with the chiral center -OH group on the right in the Fischer projection

formula is the right-handed isomer (D-glyceraldehyde).

Enantiomers

The enantiomer with the chiral center -OH group on the left in the Fischer projection

formula is the right-handed isomer (L-glyceraldehyde).

Optical Isomers

Enantiomers are said to be optically active because of the way they interact with plane-polarized light. An optically active compound is a compound that rotates the plane of polarized light.

Optical Isomers

• Achiral molecules are optically inactive.

• Chiral molecules are optically active.

Optical Isomers

Because of their ability to rotate the plane of polarized light, enantiomers are sometimes referred to as optical isomers.

Polarimeter

Measure the ability of a compound to change the angle of the plane of plane-polarized light.

Polarimeter

The polarimeter allows us to measure the specific rotation of a compound: that is, the degree to which it rotates plane-polarized light

Optical Isomers

An enantiomer that rotates plane-polarized light in a CLOCKWISE DIRECTION (to the right) is said to be DEXTROROTATORY

Optical Isomers

An enantiomer that rotates plane-polarized light in a COUNTERCLOCKWISE DIRECTION (to the left) is said to be LEVOROTATORY

Optical Isomers

A PLUS OR MINUS SIGN inside parentheses is used to denote the direction of rotation of plane-polarized light by a chiral compound.

Optical Isomers

• Clockwise Direction (RIght) +

• Counterclockwise (Left) -

Diastereomers

• Non-mirror images of each other

• Non-superimposable neither enantiomers of each other

Diastereomers

stereoisomers that are not mirror images of each other.

Diastereomers

The members of each of these four pairs are epimers.