ERT 102 Organic Chemistry ERT 102/4 ORGANIC CHEMISTRY Alina Rahayu Mohamed School of Bioprocess Engineering, UniMAP

ERT 102 Organic Chemistry Lecture 1 4th August 2008

ERT 102 Organic Chemistry 4.0 Stereochemistry of alkanes and cycloalkanes: 4.1 Conformation in acyclic alkanes A. Ethane B. Propane C. Butane 4.2 Conformation in cyclohexane A. Boat Conformation B. Chair Conformation(Axial and equatorial bonds in cyclohexane) B. Ring-Flipping

ERT 102 Organic Chemistry 4.1 Conformation in acyclic alkanes A. Ethane What is stereochemistry? Stereochemistry is the study of 3- dimensional structures. Consider an ethane molecule. Rotation occurs around C-C σ bond. This allows H atoms on one C atom adopt different orientations relative to H on 2 nd C atom. These arrangements are called conformations.

ERT 102 Organic Chemistry Therefore, different arrangements of atoms that are interconverted by rotation about single bonds are called conformations. So, how does conformations related with conformer? Conformer is a particular conformation.

ERT 102 Organic Chemistry How do you obtain eclipsed conformation? If you rotate the C-H bond about the C-C bond, and that bond is aligned directly with the C-H bond on the adjacent carbon atom. How do you obtain staggered conformation? If you rotate the C-H bond on one C atom, and that bond bisect the H-C-H bond angle on the adjacent carbon atom.

ERT 102 Organic Chemistry These are the Newman projections for eclipsed and staggered conformations of ethane. Dihedral angle=0 o Dihedral angle=60 o Search: the meaning of dihedral angle.

ERT 102 Organic Chemistry

Points: The graph shows how the potential energy of ethane changes with dihedral angle as one CH 3 group rotates relative to each other. The staggered conformation is the most stable conformation, so it is at the lowest energy.

ERT 102 Organic Chemistry As the C-H bonds on one C atom rotate, the energy increases as the C-H bonds get closer until a maximum is reached after 60 o rotation to the eclipsed conformation. As rotation continues, the energy decreases until after 60 o rotation, when the staggered conformer is reached again.

ERT 102 Organic Chemistry The staggered conformation is more stable than eclipsed conformation. WHY?

ERT 102 Organic Chemistry

The major contribution to the energy difference between them is a stabilizing interaction between the C — H σ bonding orbital on one carbon and the C — H σ* antibonding orbital on the other carbon: the electrons in the filled bonding orbital move partially into the unoccupied antibonding orbital.

ERT 102 Organic Chemistry This interaction is greatest in the staggered conformation because only in this conformation are the two orbitals parallel. Such a delocalization of electrons by the overlap of a a orbital with an empty orbital is called hyperconjugation.

ERT 102 Organic Chemistry Recall: from page 32 Bruice

ERT 102 Organic Chemistry 2.3 Conformation in acyclic alkanes B. Propane

ERT 102 Organic Chemistry Question 1: Draw energy versus rotation diagram for rotation around a C-C bond in propane.

ERT 102 Organic Chemistry Answer:

ERT 102 Organic Chemistry The staggered conformer has lower energy by 3.4 kcal/mol.

ERT 102 Organic Chemistry Consider rotation at C2-C Conformation in acyclic alkanes C.Butane

ERT 102 Organic Chemistry The techniques is: rotate one carbon atom in 60 o increments either clockwise or anticlockwise, while keeping the other carbon fixed. Continues until you return to the original conformation.

ERT 102 Organic Chemistry

All staggered and eclipsed conformers that are allowed for one complete 360 degree rotation about the C2-C3 bond in butane. Conformation A is totally eclipsed. Conformations C and E are partially eclipsed. Conformations B and F are gauche. Conformation D is anti.

ERT 102 Organic Chemistry A staggered conformation with 2 larger groups 180 o from each other is called anti. A staggered conformation with 2 larger groups 60 o from each other is called gauche.

ERT 102 Organic Chemistry The two gauche conformations have the same energy, and it is higher than the energy of the anti conformation.

ERT 102 Organic Chemistry The relative energies of the individual staggered conformers or the individual eclipsed conformers depend on their steric strain. So, what is steric strain?

ERT 102 Organic Chemistry Steric strain (gauche interaction) is an increase in energy resulting when atoms are forced too close to one another. Now, compare the anti conformer D and gauche conformers B and F (these are the staggered conformers)

ERT 102 Organic Chemistry

Gauche conformation

ERT 102 Organic Chemistry The methyl groups are further apart in anti conformer D. Therefore, anti conformer D energy is lower, meaning that it is more stable than gauche conformer B and F. In other words, gauche conformer B has higher energy. This is because of the steric strains that results from proximity of the methyl groups-repulsion of electron clouds.

ERT 102 Organic Chemistry Conformer A is less stable compare to conformer C and E. Why? A has dihedral angle of 0 o.

ERT 102 Organic Chemistry Conformer 4 is higher in energy because the 2 larger CH3 groups are forced close to each other, introducing considerable steric strain. Steric strain caused by 2 eclipsed methyl groups

ERT 102 Organic Chemistry

Attempt Tutorial 1. Tutorial session for Group - from 3- 5pm on Wednesday (6th August 2008).