This document discusses conformational isomerism, which results from different three-dimensional arrangements of atoms that can form due to the rotation of single bonds. It focuses on the conformations of ethane, butane, cycloalkanes like cyclopropane and cyclohexane, and amine compounds. The key conformations discussed are staggered, eclipsed, chair, boat, and twist-boat, with the document explaining their relative stabilities and energies. It also discusses angle strain, torsional strain, and steric strain that can result from different conformations.

1. STEREOCHEMISTRY
Dr. Ashwani Dhingra
Associate Professor
GGSCOP, Yamunanagar
Conformational Isomerism

2. Fischer projection
Sawhorse and Newmann projection for the 2 enantiomers of
2-Butanol
flying wedge projection

3. Conformational Isomers
Conformation: any three-dimensional arrangement of atoms in
a molecule that results from rotation about a single bond
• Conformer: a specific conformation
1. Rotation about single bonds
2. Amine inversion
• Molecules constantly rotate through all the
possible conformations.

4. Conformational Isomers: Rotation about Single Bonds

5. H
H H
H H
H
Conformational Isomers: Rotation about Single Bonds
• Staggered conformation: a conformation about a carbon-
carbon single bond in which the atoms or groups on one
carbon are as far apart as possible from the atoms or
groups on an adjacent carbon
Example: Ethane

6. H H
H
H
H
H
Conformational Isomers: Rotation about Single Bonds
• Eclipsed conformation: a conformation about a carbon-
carbon single bond in which the atoms or groups of atoms
on one carbon are as close as possible to the atoms or
groups of atoms on an adjacent carbon
Example: Ethane

7. Conformational Isomers: Rotation about Single Bonds

8. +12.6 kJ/mol
Conformational Isomers: Rotation about Single Bonds

9. Conformational Isomers: Rotation about Single Bonds
• Dihedral angle (θ) the angle created by two intersecting
planes

10. Conformational Isomers: Rotation about Single Bonds

11. Conformational Isomers: Rotation about Single Bonds
Example: Butane (C2-C3 bond)

12. Conformational Isomers: Rotation about Single Bonds
Eclipsed Butane
– calculated energy difference between (a) the non-
energy- minimized and (b) the energy-minimized
eclipsed conformations is 5.6 kJ (0.86 kcal)/mol

13. CH3
H H
H H
CH3
Conformational Isomers: Rotation about Single Bonds

14. Conformational Isomers: Rotation about Single Bonds
Angle strain
the strain induced in a molecule when the bond angles are
different from the ideal tetrahedral bond angle of 109.5°.
Torsional strain
the strain caused by repulsion between the bonding
electrons of one substituent and the bonding electrons of a
nearby substituent.
Steric strain
strain caused by atoms or groups of atoms approaching each
other too closely.

15. Conformational Isomers: Rotation about Single Bonds

16. 27
Conformational Isomers: Cycloalkanes
2.Cyclopropane
– Large ring strain due to angle compression
– Very reactive, weak bonds
– Torsional strain because of eclipsed hydrogen

17. Conformational Isomers: Cycloalkanes
2.Cyclobutane
– puckering from planar cyclobutane reduces torsional
strain but increases angle strain
– the conformation of minimum energy is a puckered
“butterfly” conformation
– strain energy is about 110 kJ (26.3 kcal)/mol

18. Conformational Isomers: Cycloalkanes
3.Cyclopentane
– puckering from planar cyclopentane reduces torsional
strain, but increases angle stain
– the conformation of minimum energy is a puckered
“envelope” conformation
– strain energy is about 42 kJ (6.5 kcal)/mol

19. Conformational Isomers: Cycloalkanes
4. Cyclohexane
• Chair conformation: the most stable puckered
conformation of a cyclohexane ring
– all bond C-C-C bond angles are 110.9°
– all bonds on adjacent carbons are staggered

20. Conformational Isomers: Cycloalkanes
4. Cyclohexane
Axial and equatorial
ring flip

21. Conformational Isomers: Cycloalkanes
4. Cyclohexane
• Boat conformation: carbons 1 and 4 are bent toward
each other
– there are four sets of eclipsed C-H interactions and
one flagpole interaction
– a boat conformation is less stable than a chair
conformation by 27 kJ (6.5 kcal)/mol

22. Conformational Isomers: Cycloalkanes
4. Cyclohexane
• Twist-boat conformation
– approximately 41.8 kJ (5.5 kcal)/mol less stable than
a chair conformation
– approximately 6.3 kJ (1.5 kcal)/mol more stable than
a boat conformation

23. Conformational Isomers: Cycloalkanes
4. Cyclohexane
• Half-chair conformation

24. Conformational Energy

25. Conformational Isomers
II. Amine Inversion
The lone-pair electrons on nitrogen allow an amine to turn
“inside out” rapidly at room temperature.

26. Amine Inversion
The lone pair is required for inversion: Quaternary
ammonium ions—ions with four bonds to nitrogen and hence
no lone pair—do not invert.
•amine inversion takes place through a transition state in
which the sp3 nitrogen becomes an sp2 nitrogen.

27. Thank
You