Faculty : Science Department : Chemistry Course : BSC SEM : IV Unit : IV (Conformational isomerism) Paper : II Teacher : Dr M Karunakar Dr M Karunakar Both can called configurational
Conformational isomerism is a form of stereoisomerism in which the isomers can be interconvert just by rotations about formally single bonds
While any two arrangements of atoms in a molecule that differ by rotation about single bonds can be referred to as different conformations The study of the energetics between different conformations is referred to as conformational analysis
Q: Explain the three different angle strain, ecplised strain steric strain? Q: write possible conformational structure of ethane, butane explain their conformational stability along with potential energy diagram? Q: what are axial and equatorial bonds in cyclohexane? explain the stability order in cyclohexane(chair,twist boat,boat, half chair) conformation Q: explaine stability order mono substituted cyclohexan conformations and their stabilty?
Newman projection formula of chair, boat A dihedral angle or torsional angle is the angle between two intersecting planes
Eclipsed strain/tortianal strain/Pitzer strain: eclipsing strain due to repulsion between the bonded electrons. This effect decrease the stability
Steric strain : can also be referred to as steric hindrance, which is related to. Van Der Waals repulsion. This strain is the increase in potential energy(decrese stability) of a molecule due to repulsion between groups.
H- Bonding: conformation at which( dihydral angle= 60’)(Gauche ) where EN atom oxygen form hydrogen bonding with adjecent Hydrogen. This H-bonding increase the stabilty
Ring Strain occurs because the carbons in cycloalkanes are sp3 hybridized, which means that they do not have the expected ideal bond angle of 109.5o ; this causes an increase in the potential energy because of the desire for the carbons to be at an ideal 109.5o
ideal angle for sp3 Carbon 109.5o
Highly strain Ethane NEWMAN projection Back/ rare Carbon
H H Back/ rare Carbon Front Carbon represented as represented Front Carbon circle with just centre point H H H H
Just separated by 120’ bond C-C angle
Distance between two hydrogen's
Dihydral angle 2.9 ~ 3 k cal/mole
eclipsed anti eclipsed eclipsed Conformations of butane
Front carbon
Rare/back carbon
4 3 2 1 butane
Sawhorse projection n- Butane all possible conformations
Fully Partially
Fully eclipsed VII (Just like I) Partially
Fully eclipsed(total=2), Gauche(2), Anti/Staggered(1), Partially eclipsed(2) I , VII II, VI IV III,V Stability Order of n-butane conformations
Anti > Gauche > Partially eclipsed > Fully Eclipsed
Anti/staggered conformation
two CH3 groups are anti to each other , free from steric strain and eclipsed strain Gauche conformation two CH3 groups are separated by 60’ , which can suffer with steric strain due to the interaction of groups/atoms(restricted space), posses steric strain and but no eclipsed strain
Partially eclipsed conformation
one CH3 group and one hydrogen atom are close to each other , suffer with eclipsed strain and little steric strain between H and CH3
Fully eclipsed conformation
Two CH3 groups are close to each other(dihydral angle zero) , suffer with highly eclipsed strain and steric strain
Steric strain can also be referred to as steric hindrance, which is related to. Van Der Waals repulsion. This strain is the increase in potential energy(decrese stability) of a molecule due to repulsion between groups Potential energy Conformations of butane Potential energy Conformations of butane
Dihydral angle
Fully eclipsed Partially Gauche Partially eclipsed Anti gauche Fully eclipsed eclipsed H- bonding
Gauche: strong Hydrogen bonding more stable Anti : free from steric and eclipsed strain
Stability order Gauche > Anti > Partially Eclipsed > Fully Ecplised Propane, ethylene Glycol not in syllabus but some time come in exams
Back Carbon Write all possible confirmations propane structure
Front Carbon CH3
Propane conformation on rotation about c-c bond forms two main eclipsed, staggared/anti forms (due to all H in Trans even CH3 in trans with H)
eclipsed: strong having eclipsed strain , less stable Anti : free from steric and eclipsed strain , more stable Write all possible confirmations Conformational analysis of cyclic compounds: Cyclo hexane
angle
A planar structure for cyclohexane is bond angle is 120º, deviate from actual ideal sp3 carbon bond angle i.e 109.5º
Also, every carbon-carbon bond in such a structure would be eclipsed. The resulting angle and eclipsing strains would severely destabilize this structure.
If two carbon atoms on opposite sides of the six-membered ring are lifted out of the plane of the ring, much of the angle strain can be eliminated. (1,3,5on one plane,2,4,6 C are in another plane) Therefore, the cyclohexane ring tends to assume certain non-planar (warped/Puckered) conformations, which have all angles closer to 109.5° and therefore a lower strain energy than the flat planar shape.
The most important shapes are chair, half-chair, boat, and twist-boat. Their relative stabilities are: chair > twist boat > boat > half-chair. All relative conformational energies are shown below
10kcal/mol 6.5kcal/mol
5.5kcal/mol Conformational analysis of cyclic compounds: Cyclo hexane
The chair conformation is staggered (from newman projection) about all bonds and therefore there is no eclpised strain. Also it doesn't have any angle strain as compared to all other conformations, thus most stable.
The half chair form is least stable due to maximum strain. It is 10kcal/mol less stable than the chair form and is least stable.
The twist boat conformation has less steric and eclipsed strain as compared to half chair conformation and It is 5.5kcal/mol less stable than the chair form.
Due to interaction between two flagpole hydrogens, there is steric strain in boat form and also torsional strain is present. It is 6.5kcal/mol less stable than the chair form. axial Rare Carbon Rare Carbon front Carbon both equitorial
axial front Carbon
axial axial
equitorial equitorial H 2 H H 3 H H H Fully staggered H 4 like ethane 1 H H H H6 5 H
Eclipsed hydrogens
3 4 5 6
2 1
Cyclohexane what are axial and equatorial bonds?
Axial bonds are parallel to the axis of the ring, while equatorial bonds are perpendicular to the axis of the ring and lie along the equator of the chair. ... Without any other substituents, the final chair conformation would have hydrogen atoms at the ends of all the axial and equatorial bonds.
1,3-di axial interaction
1,3-di axial interaction destabilize the conformations by increasing potentia energy Anti
Anti
Equatorial methyl substitution: free from all interaction/strain Gauche
Axial methyl substitution in cyclohexane conformation: multiple gauche interactions between methyl and cyclohexane ring makes this less stable