stereo 1 question

7/30/2019 Stereo 1 Question

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R and S specification of configuration

CIP rules (Cahn, Ingold, Prelog priorities rules)

Step1: Assign a sequence of priority to the 4 atoms attached to the chiral center.

(It depends simply on atomic number. The atom of higher number having higher

priority)

Step2: The molecule is oriented so that the group oflowest priority is directed away from

us. If in proceeding from the group of highest priority to the second and third is in

clockwise direction, the configuration is R (rectus/right). If counterclockwise, the

configuration is S (sinister/left)

-A complete name has both configuration and direction of rotation. Eg: (S)-(+)-sec-butyl

chloride

-A racemic modification can be specified by the prefix RS. Eg: (RS)-sec-butyl chloride


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Conformation isomer

For ethane and propane:

1. Conformations are the different arrangements of atoms that can be converted into one

another by rotation about single bonds. Any molecule spends most of its time in the most

stable conformation.

2. a) Eclipsed conformation

-hydrogens exactly oppose each other

-maximum potential energy (the most unstable form)

b) Staggered conformation

-hydrogen are perfectly staggered or an infinity of intermediate arrangements

-minimum potential energy (the most stable form)

c) skew conformation

-it is the infinity of intermediate conformations between eclipsed and staggered

conformation.

3. There is an energy barrier of 3kcal/mole in ethane. At room temperature, the fraction

of collisions with sufficient energy is large enough for a rapid inter conversion between

staggered arrangements to occur.

4. Torsional energy is the energy required to rotate the ethane molecule about the C-C

bond. The relative instability of the eclipsed conformation or any intermediate skew

conformations is due to torsional strain.

5. As the hydrogen of ethane is replaced by other atoms/groups of atoms, van der Waals

forces, dipole-dipole interactions and hydrogen bonding affects the relative stability of

conformations appear.


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6. However, the tendency for the bond orbitals on adjacent carbons to be staggered

remains. Any rotation away from the staggered conformation is accompanied by torsional

strain.

For n-butane

1. For alkane molecule (with 4 carbon), two other problems arised

a) there are several different staggered conformations. (anti and gauche conformation are

used to replace eclipse and staggered conformations)


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b) steric strain besides torsional strain affects the conformational stabilities.

2. a) Anti conformation

-the methyl groups are 180o

apart.

-it is more stable than gauche

-torsional strain between CH3

and H

b) Gauche conformations

-the methyl groups are only 60o

apart.

-conformations II and III are mirror images of each other with same stability. However,

they are different!!

-Both are free of torsional strain.

- steric strain between 2 CH3 groups

3. The methyl groups are crowded in gauche conformation. Under these condition, the

van der Waals forces are repulsive and raise the energy of the conformation. The steric

repulsion between the methyl groups causes the molecule to be less stable because of van

der Waals strain (or steric strain). No torsional strain!!!


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4. There are several different staggered conformations of n-butane (normal butane) lies at

the bottom of an energy valley (energy minimum).They are separated from the others by

energy hills.

5. Conformational isomers/ conformers are the different conformations corresponding

(similar) to energy minima. They are stereoisomers that differ from each other only in

their special arrangement.

6. a) Gauche conformer II and III are conformational enantiomers (mirror images of each

other)

b) Gauche conformer I and II / I and III are conformational diastereomers (not mirror

images of each other)

c) When equilibrium exists, any given molecule spends the greater part of its time as the

anti-conformer, and divides the smaller part equally between the 2 gauche conformers.

These isomers cant be separated due to rapid inter conversion.

7. Interconvertibility of stereoisomers limits their isolability.

Torsional strain Steric strain


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a) Conformational isomers are easy-to-interconvert isomers where the isomers cannot be

separated.

b) configurational isomers/ inversional isomers are hard-to-interconvert isomers. They

can be separated for resolution of enantiomers.

8. Depending upon the size and nature of substituents, the barrier to rotation about single

bonds can be of any height. Atropisomers are conformational isomers that are readily

isolated, kept and studied. Other conformational isomers can be isolated, not at ordinary

temperatures but at lower temperatures, where the average collision energy is lower.

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