Organic Chemistry

Absolute Configuration

Describes the spatial position of atoms/groups in chiral centers (does not depend on other atoms or groups in a molecule)

Step 1 of Absolute Configuration

Assign priorities using Cahn-Ingold-Prelog rules: 1. rank by atomic number 2. rank by atomic mass 3. Find the first point of difference (repeat A and B) (multiple bonds = multiple atoms)

Step 2 of Absolute Configuration

Right hand rule: 1. Thumb = #4 2. Fingers = #1 3. Curl fingers

Relative Configuration

Describes the spatial position of atoms/groups relative to other atoms/groups in the molecule (Assign priorities using Cahn-Ingold-Prelog rules, highest priorities on the same side = Z, and highest priorities on the opposite side = E)

Isomers

Molecules with the same molecular formula but different chemical structures

Structural/Constitutional Isomers

Isomers with different atomic connectivity and have different chemical/physical properties

Chiral Molecules

Are nonsuperimposable on their mirror images, usually have chiral centers, and are optically active (can rotate plane polarized light)

Chiral Centers

sp³ hybridized atom bound to four different substituents

Dextrorotatory

d-, or (+)-, clockwise rotation

Levorotatory

l-, (0)-, counterclockwise rotation

Rotation per concentration-path length (α)

= alpha(obs) / c x l

Enantiomeric Excess

alpha(obs) x 100 / rotation per concentration-path length

Stereoisomers

Isomers with the same atomic connectivity but different spatial arrangements of the atoms

Enantiomers/Optical Isomers

Nonsuperimposable mirror images (chiral), no internal plane of symmetry, have identical physical properties, usually have identical chemical properties, have equal but opposite optical activity, and have opposite absolute configuration at every chiral center

Racemic Mixture

A 50/50 mixture of enantiomers and is optically inactive

Diastereomers

Non-superimposable and non-mirror images, have different chemical and physical properties, have different absolute configurations but not all opposite, and have unrelated specific rotations

Types of Diastereomers

Epimers, anomers, meso compounds, conformational isomers, and geometric isomers

Epimers

Diastereomers that differ in absolute configuration at a single chiral center

Anomers

Epimers of carbohydrates at the anomeric carbon

Meso Compounds

An optically inactive member of a set of stereoisomers and must have chiral centers and an internal plane of symmetry

Geometric Isomers

Diastereomers that differ in spatial arrangements of atoms about double bonds or rings

Conformational Isomers / Conformers / Rotamers

Differ in spatial arrangement of atoms by rotation about a single bond and cannot be isolated (identical molecules)

Staggered

Two atoms and/or groups whose dihedral angle is 60^o, or close to 60^o. In other words, their bonds to the axis of rotation are not aligned and the atoms are all equally spaced from each other

Eclipsed

The carbons are aligned so that the hydrogens are lined up with each other. This creates steric hindrance between them

Cyclohexane Rings

All H atoms are eclipsed and wrong bond angles

Chair Conformation

All H atoms are staggered and 109.5 bond angles

1,3 Diaxial Interactions

Are unfavorable and all large substituent must be in the equitorial position