Anatomy

1. STERIC EFFECT
Presented by:
Nabira Kanwal Nawaz
( M.Phil. Scholar Pharmaceutics)

2.  Steric effect is an influence on a reaction course or the rate
determined by the fact that all the atoms within a molecule occupy
space. Thus, certain collisions pathways are either disfavored or
favored.
 The word steric is derived from “steros” meaning space.
 Steric factor
 It is the ratio between pre-exponential factor and collision frequency
and it is often less than one.

3. Theory of steric effect:
Steric effect arises from a fact that each atom within a
molecule occupies a certain amount of space. If the atoms
are brought closer to each other, there is an associative cost
of energy due to overlapping of electronic clouds. There may
be pauli or exchange interactions or repulsions (born
repulsions) and it will affect the molecules preferred shape
and reactivity.

4.  Types of Steric Effect
 following are the different types of steric effect
 Steric Hindrance:
 Steric shielding:
 Steric attraction:

5. Steric Hindrance:
 it occurs when the large size of group within a molecule prevents the
chemical reactions that are observed in related molecules with smaller
groups. Although steric hindrance sometimes are problem e.g. it prevents
SN2 reactions with tertiary substrates from taking place) but it can be used
as a useful tool and is often exploited by chemists to change the reactivity
pattern of molecules by stepping the unwanted chemical reaction (that are
called as side reactions) or by loading to the preference for one
stereochemical reaction(as in diateroselectivity- in which one disastomer
is formed in preference to another or in which a subset of all possible
diasteromers determines the product mixture. Establishing a preffered
relative sterio chemistry)

6.  This can also be termed as steric protection.
 Steric hindrance between adjacent groups can also restrict torsional bond angles A
dihedral angle is an angle between two intersecting planes on a third plane
perpendicular to the line of intersection
 Torsion angle is a particular example of dihedral angle, used in stereochemistry to
define the geometric relation of two parts of molecule joined by a chemical bond).
However hyperconjugation has been suggested an example for the preference of the
staggered conformation (chemical conformation of an ethene like moiety abc X-Y
def in which substituent a,b,c are at maximum distance from d,e and f. this requires
the torsion angle to be 60) of ethene because the steric hindrance of the small
hydrogen atom is far too small. This is the effect responsive for the observed shape
of rotaxanes mechanically interlocked molecular artitecture consisting of a dumbbell
shaped molecule’s which is threaded through a “macrcycle”.
 When a lewis acid and lewis base cant combine due to steric hindrance, they are said
to form a frustrated lewis base.

7.  Steric Shielding:
 It occurs when a charged group on a molecule is specially shielded or less
shielded like oppositely charged ions e.g.pamper ions in solution or including
counterions in solution-debay shielding). In that case for an atom to interact with
sterically shielded atom. It would have to approach from vicinity from the way
where there is less shielding therefore controlling where end from what direction a
molecular interaction can take place.
 Steric Attraction:
 It often occurs when molecules have shapes or geometries that are optimized for
interaction with one another. In these cases molecule will react with each other
most often in specific arrangements” steric effect can also play an important role in
molecular recognition.

8.  Other phenomenon
1. Steric repulsions
2. Steric inhibition of resonance
3. Steric inhibition of protonation

9.  Applications of steric effect
 Steric effects are critical to chemistry, biochemistry and pharmacology.
 In organic chemistry, steric effects are nearly universal and affects the rate and activation
energy of most of chemical reactions to varying degrees.
 Steric effects often dictate the reaction pathways because there are few configurations in
which the molecule can colloid and react.
 The spatial shielding due to the size of the attached polymer stands.
 In biochemistry, steric effect is often exploited in naturally occurring molecules such as
enzymes, where the catalytic site may be occupied by a large protein molecule.
 In pharmacology, it determines how and at what rate a drug will interact with its target
biomolecules.
 e.g. stereochemistry of ketonization of enols and enolates where diastereoselective
results from steric hindrance.