2. Symmetry – similarity (mirror image of each other).
E.g.. Butterfly.
Structural dualism.
Molecules will be disymmetric when lacking
some symmetry elements like symmetry
plane and rotational axis, etc.
It is important for rotation.
Disymmetry means having only rotational axis of symmetry, where n>1, there
is no second order symmetry.
Eg.H2O2(C2)
It is essential for optical activity not the presence of chiral centre.
3. Assymmetry – carbon atom has attached to four different types of atoms of groups
of atoms(devoid of symmetry).
E.g. bromo chloro methane.
Optically active (cannot
be superimposed).
They are dextro and levo rotatory.
4. TERMS ALTERNATING AXIS OF
SYMMETRY
SIMPLE AXIS OF
SYMMETRY
OPTICAL ACTIVITY
SYMMETRIC PRESENT May of may not be
present
Inactive
DISSYMMETRIC ABSENT May of may not be
present
Usually active
ASSYMETRIC ABSENT Absent Usually active
5. Conformation
It shows different arrangements of
atom in a molecule that can readily
interconvert.
Conversion of one conformation to
another is easy at room temperature.
They are very flexible.
Conformations are inseparable.
Interconversion is done via C-C single
bonds.
It shows different arrangements of
molecule that cannot readily
interconvert.
Converting one configuration of a
molecule into another is difficult.
Configuration are less flexible.
They are separable.
Interconversion done via breaking and
making new chemical bonds
Configuration
6. Conformation
It shows the relative spatial orientation
of a portion of a molecule relative to
another.
Allows free rotation of single bond of
molecule.
It relates the order by which different
substituents linked to the same Central
atom establishes covalent bonds.
No free rotation used in optical
isomerism.
Configuration
7. Free Rotation. If two atoms covalently bonded to each other in an organic molecule can
be rotated around each other in the same or opposite direction up to 360° without
weakening or breaking the bond, there is said to be free rotation around the bond.
8. Rotation around an alkene double bond is restricted.
In alkenes, there is no free rotation about the double bond,
unlike in alkanes.
Consequently, groups attached to a
double bonded carbon cannot exchange places.
The cis and trans isomers are separable
compounds because they cannot interconvert.
Pi bonds which are sp2 hybridised with double
bond have restricted or hindered rotation.
Sigma bonds which are sp3 hybridised with single bond has free rotation.
9. Sigma and Pi Bonds. Sigma and pi bonds are chemical covalent bonds.
Sigma and pi bonds are formed by the overlap of atomic orbitals. Sigma
bonds are formed by end-to-end
overlapping and Pi bonds are
when the lobe of one atomic
orbital overlaps another.
If a bond between two
atoms is broken when one atom
is rotated around the bond axis,
that bond is called a pi bond.
Pi bonds are formed from the overlap of parallel p orbitals on adjacent atoms.
They are not formed from hybrid orbitals.
10. Rotation around a single bond occurs readily, while rotation around a
double bond is restricted. The pi bond prevents rotation because of the
electron overlap both above and below .
Sigma bonds are defined as having their electron density along
the bond axis, while pi bonds have their electron density above and below
the bond axis. It means that pi bonds cannot rotate the same way as sigma
bonds since rotation would break the pi bond interaction the plane of the
atoms.
Electrons can be easily attracted by other atoms in sigma bond so they are
highly reactive.
11. The work done on the body by the external torque equals the change in
the rotational kinetic energy
W=K=I(w)2
W=K2-K1=1/2(w2)2-1/2(w1)2
The work equals the negative of the change in potential energy
12. Is a form of stereoisomer, these isomers interconvert by rotation about single bond.
Conformation corresponds to local minima on energy surface area called
conformation all isomers or conformers.
Conformation corresponds to local maxima on energy surface are transition states
between local minima conformation to another.
Energy barrier decrease to give a free rotation and vice versa.
Molecules exists for long time period as stable called rotational or rotamer. (An
isomer arising from hindered single bond rotation.)
13. Inter conversion involves breaking and reforming of chemical bonds.
I.e./D and R/S configuration.
Rapid inter conversion which makes not isolable at room temperature.
The study of energetic s between different conformation –conformational analysis.
It plays role in rational, structure based drug design.
14. Rotating C-C, the molecule ethane and propane have three local energy minima.
The three slipped conformation, in which the dihedral angles are zero are transition
states connected 2 equivalent energy minima the staggered conformers.
There are two types of non equivalent structures.
Anti-conformers
Gauche-conformers
15. E.g. Butane – three conformers
Two methyl groups ( CH3) groups-two gauche conformers, methyls+60 degree apart
and are enantiomers.
Anti conformers, where four carbon centres are coplanar and substituents are 180
degree apart.
Energy difference between them 0.9kcal/mol.
They are more stable with dihedral angle 0,120 and 240 transition state between
conformers.
