Chirality and Fischer Projections: Key Concepts

Axial Chirality

• Occurs when molecules are nonsuperimposable on their mirror images due to restricted rotation around a single bond.
• Differs from chirality due to a chiral carbon.
• Restricted rotation prevents interconversion between conformations, resulting in nonsuperimposable mirror images.
• Planar conformation of bivalent derivatives can be sterically crowded, leading to locked conformations.
• Staggered conformations can be chiral.
• Bulky groups (e.g., t-butyl groups, large atoms like iodine, benzene rings) cause restricted rotation.
• Substituted biphenyls (two benzene rings connected) are examples of axial chirality.

Allenes

• Allenes are chiral molecules despite lacking a chiral carbon.
• Allene structure: a carbon atom is sp hybridized with two double bonds directly attached.
• For chirality, the groups at the ends of the carbons must have different attachments.
• If the carbon is sp hybridized then one group is coming out and the other is going in.
• If groups at each end are different (e.g., a, b, c), the molecule is chiral.
• If two groups are identical, the molecule is not chiral.
• Example: Penta-2,3-diene is a chiral molecule because it is nonsuperimposable on its mirror image.
• Restricted rotation prevents interconversion of substituents.

Fischer Projections

• Used to represent three-dimensional structures in a two-dimensional format.
• Chiral carbon is located at the intersection of horizontal and vertical lines.
• Horizontal lines represent bonds coming towards the viewer.
• Vertical lines represent bonds going behind the plane.
• Useful for representing sugars and amino acids.
• Sugars typically have names ending in "-ose" (e.g., glucose, lactose, mannose, cellulose).
• To interpret a Fischer projection of tetrahedral carbon, tilt the molecule slightly.
• Convert wedge and dash formulas by determining R/S configurations.

Rules for Fischer Projections

• Highest oxidized carbon is placed at the top.
• The most oxidized carbon has the greatest number of bonds to oxygen.
• Example: Aldehyde (CHO) is more oxidized than alcohol (CH2OH).

Carbohydrates

• General formula: $(CH<em>2O)</em>n$ (carbon and water).
• Pentose: a five-carbon sugar.
• Aldose: a sugar with an aldehyde functional group.
• Ketose: a sugar with a ketone functional group.

Determining R/S configuration

• Essential for drawing Fischer projections accurately.
• Requires assigning priorities to substituents around each chiral center, working out the orientation and using the right hand rule.
• This is typically performed on each chiral center of the molecule to orientate the molecule and place important functional groups appropriately.