## Types of isomers Isomers are two molecules that have the same molecular formula - **Constitutional Isomers**: Same formula, different connectivity - **Stereoisomers**: Same connectivity, different arrangement in space - **Enantiomers**: stereoisomers which are non-superimposable mirror images - **Diastereomers**: stereoisomers which are not mirror images of each other - **Conformational isomers**: Stereoisomers that differ by rotation about single bonds - **Staggered**: groups are arranged - **Gauche**: dihedral angle between two largest groups is 60° - higher energy due to steric strain - **Anti**: largest groups are clocked 180° from each other - **Eclipsed**: Less stable than staggered due to torsional and steric strain ## Chirality Molecules which are mirror images that aren't superimposable - You can use any plane to mirror a molecule to get it's chiral counterpart, and mirror again to get the original - has an internal mirror plane → achiral - **Axial Chirality**: Stereoisomerism resulting from non-planar arrangement of groups about a stereogenic axis - enantiomers only behave differently when in contact with other chiral objects ## Stereogenic Centers Atoms where swapping any two attached atoms or groups produces a stereoisomer - Must be sp$^{3}$ carbon with four different groups - **Chiral Centers**: tetrahedral carbons that are also stereogenic centers - **Meso**: compounds with stereogenic centers and a mirror plane (achiral) - Relationships: - Invert all stereocenters → enantiomer - Invert some → diastereomer - A chiral molecule doesn't have to have a stereogenic center, i.e. helical molecules - The maximum number of stereoisomers is $2^n$ where $n$ is the number of stereogenic elements (including stereogenic centers, axes, planes, etc) - N with a lone pair isn't an isolable stereocenter because it inverts rapidly ### Naming stereogenic centers: 1. Locate stereogenic centers 2. Prioritize the groups surrounding them from 1 to 4 1. Highest atomic number gets highest priority, implied H is lowest priority 2. Double bonds are counted twice, triple bonds three times 3. Ties are broken by comparing groups bonded to them, stopping at the first point of difference and comparing the set 3. Draw a circular arrow from 1 to 2 to 3 around the center 1. If circular arrow goes clockwise, label it R, and S if counterclockwise 2. If the priority 4 group points toward you, reverse the above ## Sources of strain - **Torsional Strain**: increase in energy due to eclipsing interactions - **Steric Strain**: increase in energy due to atoms being forced close to each other - **Angle strain**: increase in energy due to a bond angle deviating from its ideal value ![[Pasted image 20260929221808.png|220]] ## Newman Projections - eye symbol points at the bond aligned with the newman projection - circle represents the rear carbon atom - dot represents the front carbon atom ## Ring Conformations (cyclohexane chair) - Each ring carbon has one **up** bond and one **down** bond - **Axial**: vertical, parallel to the ring axis. Alternates direction around the ring - **Equatorial**: points outward around the ring's "equator" - **Chair flip**: axials and equatorials swap, but up stays up (same cis/trans) - **cis**: same face of the ring (both wedge or both dash) - same parity carbons: a,a ⇌ e,e - opposite parity carbons: a,e ⇌ e,a - **trans**: opposite sides of the ring (one wedge one dash) - same parity carbons: a,e ⇌ e,a - opposite parity carbons: a,a ⇌ e,e ![[Pasted image 20261002085718.png|172]]![[Pasted image 20261002085729.png|189]] #### Stability: add up the strain in each chair, lowest total wins | Interaction | Cost | | ------------------- | ----------------------------------- | | CH₃/H diaxial | 3.8 kJ/mol each → 7.6 per axial CH₃ | | Gauche CH₃/CH₃ | 3.8 kJ/mol | | 1,3-diaxial CH₃/CH₃ | 15 kJ/mol extra (very bad) | #### Example: 1,2-dimethylcyclohexane - **cis**: both chairs are a,e → each costs 7.6 + 3.8 = 11.4 → equal energy, 50:50 - **trans**: e,e = 3.8 (one gauche) favored over a,a = 15.2 (four CH₃/H diaxial) ![[Pasted image 20261001210844.png|388]]