Steric Effect:
Steric effects refer to the
impact of nonbonded interactions on the kinetics and thermodynamics of chemical
reactions.
Steric effects are
nonbonding interactions that influence the shape (conformation)
and reactivity of ions and molecules. Steric effects complement
electronic effects, which usually dictate shape and reactivity. Steric effects
result from repulsive forces between overlapping electron clouds. Steric
effects are widely exploited in applied and academic chemistry.
Steric hindrance is
a consequence of steric effects.
Steric hindrance is the slowing of chemical reactions due to steric bulk. It is
usually manifested in intermolecular reactions, whereas discussion of
steric effects often focus on intramolecular interactions. Steric hindrance
is often exploited to control selectivity, such as slowing unwanted
side-reactions.
Steric hindrance between
adjacent groups can also affects torsional bond angles. Steric hindrance
is responsible for the observed shape of rotaxanes and the low rates
of racemization of 2,2' disubstituted biphenyland binaphthyl derivatives.
Steric effects
are usually smaller than electronic effects. These affect the molecule's
shape and reactivity as well, but they come from the
way electrons are placed in bonds.
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". An
example of steric effects is steric hindrance. This is when a large group in a
molecule makes reactions not work. For example, an SN2 reaction does
not happen on carbon atoms that have three substituents. Steric
hindrance however can also be a positive thing. Sometimes, chemists would like
a reaction to happen in one place rather than another one. A bulky group can
block a position. It can also change the shape such that only
one diastereomer from a reaction is made.
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