2. The word steric is derived from “stereos” meaning space.
Steric effects arise from the spatial arrangement of atoms.
When atoms come close together there is a rise in the energy of the molecule.
Steric effects are nonbonding interactions that influence the shape (conformation)
and reactivity of ions and molecules.
3. Theory of steric effect:
Steric effects are the effects seen in molecules that come from the fact
that atoms occupy space. When atoms are put close to each other, this
costs energy. The electrons near the atoms want to stay away from each other. This
can change the way molecules want to react. It can also change the shape
(or conformation) of the molecule. The amount of space that a group of atoms
takes is called the "steric bulk.
4. Types of Steric Effect
Following are the different types of stericeffect:
1) Steric Hindrance.
2) Steric shielding.
3) Steric attraction.
5. Steric Hindrance:
Steric hindrance is a phrase used in organic chemistry to describe how a molecule’s
physical structure can affect its ability to react.
When a molecule is bulky, meaning it has multiple bonds to compounds or groups
other than hydrogen, it can slow down or even prevent another molecule from
efficiently finding the desired bond site in a reaction.
6. Example of Steric Hindrance
A simple way to see the effects of steric hindrance is in a reaction between
a nucleophile and an electrophile. We can use the same nucleophile, (HO–), and change the
bulk of the electrophile by adding more methyl, or CH3 groups.
As shown in the diagram below, as more methyl groups are added to the molecule, there is
less space for the covalent bond to the electrophile to form. Therefore, as the steric bulk
increases, a molecule can be hindered from performing different reactions.
7. Steric Shielding:
Steric shielding occurs when a group is so large that it protects nearby reactive groups
from contact with things that would normally react with them.
A classic example is the first stable compound with a silicon-silicon double bond. Since
they are highly unstable, Si=Si bonds undergo 2+2 cycloadditions to form
cyclotetrasilanes.
The compound will still react with reactive small molecules like oxygen or water. But it
cannot react with itself; this can be seen in the two structures below:
8. StericAttraction:
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”
Substituted atoms of the molecules when approach each other often temporary
dipoles are developed. Attraction between these dipoles produces steric attraction
which binds molecules due to their geometry .
9. The Effects of Steric Hindrance
Steric Strain
The lowest energy form of a molecule is usually favored because it is the most
natural structure, meaning there is little to no repulsion within groups and angle
strain between bonds.
But steric strain can occur when there are multiple bulky groups near each other
in a molecule.
Energy is being used to force the bond angles to stay a certain way.
10. A molecule’s steric hindrance can be used to favor a specific reaction.
For example:
There are two types of substitution reactions, SN1 and SN2. Both of these reactions
can be performed with simple molecules;
however, SN1 can occur with bulky molecules, while SN2 cannot because it is too
congested for the mechanism known as a ‘backside attack’.
Because of this, if an SN1 reaction is desired, a bulky molecule may be used to
make sure the SN2 reaction does not occur.
SN2 Reaction
