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IITJEE CHEMISTRY: Stereoisomerism, Newman Projections & Conformations

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Ashish Kumar Bahekar

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APEX CLASS • IITJEE Organic Chemistry Lab

Stereoisomerism, Newman Projections & Conformations

Master Staggered vs Eclipsed, Gauche Effect Traps, Chirality, and Meso Compounds!

🔬 Free Rotation about C-C single bonds. Select Conformation:
1. Staggered Ethane (Most Stable) H H H H (Front) H H θ = 60° Min. Torsional Strain 2. Eclipsed Ethane (Least Stable) H H H H H H θ = 0° Max. Torsional Strain ⚡ Torsional Strain: The repulsion between the bonding electrons of the front and back carbons when they align! 1. Butane Anti Form (θ = 180°) CH₃ (Back) H H CH₃ (Front) H H Usually Most Stable! No Steric Strain 2. Butane Gauche Form (θ = 60°) CH₃ H H CH₃ H H Steric Repulsion Between bulky CH₃’s makes it less stable. Order of Stability: Anti (180°) > Gauche (60°) > Eclipsed (120°) > Fully Eclipsed (0°). The Gauche Effect: When Gauche > Anti (Ethylene Glycol) OH H H OH / F H H Intramolecular H-Bonding! Normally, Anti is most stable. BUT if adjacent groups can H-bond (like -OH, -F, -NH₂, -COOH), Gauche becomes MORE stable! Cyclohexane Conformations: Chair Flip & 1,3-Diaxial Interactions t-Bu Axial (Unstable) Ring Flip t-Bu Equatorial (Stable!) A Ring Flip converts ALL Axial bonds to Equatorial (and vice versa)! 🔒 Tert-Butyl Locking Group: The massive t-butyl group strongly prefers the Equatorial position to avoid 1,3-diaxial steric strain!
Stability of Cycloalkanes
Chair > Twist-Boat > Boat
Boat suffers from Flagpole interactions.
Dihedral Angle (θ)
Angle between two planes
Anti = 180°, Gauche = 60°, Eclipsed = 0°
Baeyer Strain Theory
Strain ∝ ½(109.5° – α)
Explains instability of Cyclopropane.
Equatorial Preference
Bulky groups go Equatorial
To minimize 1,3-diaxial repulsions.
🪞 Restricted rotation & Chirality. Select a Stereoisomerism Concept:
1. cis-2-Butene C=C CH₃ CH₃ H H μ > 0 Higher Dipole Moment & Boiling Point 2. trans-2-Butene C=C CH₃ CH₃ H H μ = 0 (Dipoles Cancel) Packs better in solid lattice, so Higher Melting Point! E/Z Priority Rule (CIP): Higher atomic number gets higher priority. If High priorities are on Zame side = Z, Epposite = E. Chirality (Asymmetric Center) & Enantiomers C* Grp A Grp B Grp C Grp D Mirror Plane C* Grp A Grp B Grp C Grp D Non-superimposable mirror images are Enantiomers. A 50:50 mixture is a Racemic Mixture (optically inactive due to EXTERNAL compensation). Meso Compounds (Optical Inactivity despite Chiral Centers) COOH COOH H OH H OH Internal Plane of Symmetry (POS) Why is it Optically Inactive? The upper half rotates plane-polarized light in one direction, and the lower mirror-image half rotates it equally in the opposite direction. This cancellation is called: INTERNAL COMPENSATION 🚨 Stereocenter Trap: A molecule with chiral centers can be achiral overall if it has a Plane of Symmetry (POS) or Center of Symmetry (COS)! Calculating Total Number of Stereoisomers (n = number of chiral centers) 1. Unsymmetrical Molecule (Ends of the chain are different) Total Optical Isomers = 2ⁿ Meso Forms = 0 2. Symmetrical Molecule (Ends are identical, can have a POS) If n is EVEN: Active = 2n-1 Meso = 2(n/2)-1 If n is ODD: Active = 2n-1 – 2(n-1)/2 Meso = 2(n-1)/2 Total Stereoisomers = (Number of Optically Active Isomers) + (Number of Meso Forms)
Enantiomeric Excess (ee)
ee = |%d – %l|
Optical Purity = [α]obs / [α]pure
Diastereomers
Not Mirror Images!
Have different physical properties.
Specific Rotation [α]
[α] = αobs / (l × c)
l in dm, c in g/mL.
Racemic Resolution
Separating Enantiomers
Often done via diastereomeric salts.
🔥 High-Yield JEE Stereochem Trap (Allenes & Biphenyls):
Molecules do NOT need a chiral carbon to be optically active! If a molecule lacks a Plane of Symmetry (POS) and Center of Symmetry (COS), it is chiral. For example, unsymmetrically substituted Allenes (C=C=C) and sterically hindered Biphenyls have perpendicular planes that prevent superimposability, making them optically active!
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