Geometric Isomers: Cis-Trans & E/Z Systems

• The focus is on applying nomenclature rules for geometric isomers in organic chemistry.

Configurational Isomers

• Isomers are categorized into constitutional isomers and stereoisomers.
• Stereoisomers are further divided into configurational and conformational isomers.
• Configurational isomers include geometric and optical isomers.
• Geometric isomers include alkenes and disubstituted cycloalkanes.
• Optical isomers are divided into enantiomers and diastereomers.

Geometric Isomers: Cis-Trans Isomers

• Geometric isomers occur in alkenes and disubstituted cycloalkanes.
• Alkenes must have each carbon connected to two different groups to exhibit cis-trans isomerism.
• Cis isomers have similar groups on the same side.
• Trans isomers have similar groups on opposite sides.

E/Z System for Alkenes

• A priority system is needed for alkenes with non-hydrogen groups to define stereoisomers.
• Use the priority system to label alkenes as either E or Z.
• Z is analogous to cis (zusammen).
• E is analogous to trans (entgegen).

Cahn-Ingold-Prelog Sequence Rules

• Rule 1: Look at the first connected atom. A higher atomic number indicates higher priority.
• Bromine (atomic number 35) has more priority than chlorine (atomic number 17).
• -NO₂ has more priority than -COOH because nitrogen (atomic number 7) has more priority than carbon (atomic number 6).
• Rule 2: If the first atoms are identical, proceed to the second, third, or fourth atoms to assign priority.
• -CH₂CH₃ has more priority than -CH₃ (C-C versus C-H).
• -CH₂OH has more priority than -CH₂CH₃ (C-O versus C-C, where oxygen = 8, carbon = 6).
• Rule 3: Multiple-bonded atoms are treated as equivalent to the same number of single-bonded atoms.
• -C=O has more priority than –CH₂OH.
• -COOH has more priority than -CH=O.
• The more oxidized the carbon atom, the higher its priority.

Examples of E/Z Nomenclature

• Z-1-bromo-1-chloro-2-fluoro-2-iodoethene has the higher priority groups (Br and I) on the same side, hence the Z designation.
• E-1,2-dichloropropene has the higher priority groups on opposite sides, hence the E designation. -Z-1,2-dichloropropene has the higher priority groups on the same sides, thus the Z designation.

Cycloalkane Stereoisomers

• Isomerism occurs due to restricted rotation around C-C bonds in cyclic alkanes.
• Methyl groups are fixed above or below the ring plane.
• Cis-1,2-dimethylcyclopropane (boiling point 37°C) allows closer molecular contact from the H atom side, leading to higher London Dispersion Forces and boiling point.
• Trans-1,2-dimethylcyclopropane (boiling point 29°C) has -CH₃ groups on both sides, hindering close approach and lowering the boiling point compared to the cis isomer.
• Cis isomers have substituents on the same side.
• Trans isomers have substituents on opposite sides.

Summary Points

• Configurational isomers, also known as geometric isomers, exhibit distinct three-dimensional atomic arrangements.
• Alkenes need at least one H atom connected to one alkene C atom to potentially have cis or trans isomerism.
• The two H atoms are on the same side in cis isomers.
• The two H atoms are on opposite sides in trans isomers.
• Cycloalkanes: if two substituents are on the same side of the ring, it’s a cis cycloalkane; if on opposite sides, it’s trans.
• If an alkene doesn't have H atoms attached on both alkene C atoms, assign priorities to connected groups following the Cahn-Ingold-Prelog rules to name the groups as Z or E.
• Assign more priority to atoms with higher atomic numbers.
• Boiling points are impacted by the type of stereoisomer
• Cis isomers typically allow for closer contact, which will have higher boiling points than trans isomers of both alkenes and cycloalkanes