See also: intramolecular forces Steric effects arise from a fact that
each atom within a molecule occupies a certain amount of space. If atoms are
brought too close together, there is an associated cost in energy due to
overlapping electron clouds, and this may affect the molecule's preferred
shape and reactivity. Types of steric effects
= Steric hindrance= Steric hindrance occurs when the large
size of groups within a molecule prevents chemical reactions that are
observed in related molecules with smaller groups. Although steric
hindrance is sometimes a problem, it can also be a very useful tool, and is often
exploited by chemists to change the reactivity pattern of a molecule by
stopping unwanted side-reactions or by leading to a preference for one
stereochemical reaction course as in diastereoselectivity. Steric hindrance
between adjacent groups can also restrict torsional bond angles. However,
hyperconjugation has been suggested as an explanation for the preference of the
staggered conformation of ethane because the steric hindrance of the small
hydrogen atom is far too small. This is the effect responsible for the observed
shape of rotaxanes. When a Lewis acid and Lewis base cannot
combine due to steric hindrance, they are said to form a frustrated Lewis
pair. = Other types of steric effects=
Steric shielding occurs when a charged group on a molecule is seemingly
weakened or spatially shielded by less charged atoms, including counterions in
solution. In some cases, for an atom to interact with sterically shielded atoms,
it would have to approach from a vicinity where there is less shielding,
thus controlling where and from what direction a molecular interaction can
take place. Steric attraction occurs when molecules
have shapes or geometries that are optimized for interaction with one
another. In these cases molecules will react with each other most often in
specific arrangements. Chain crossing: A chain, ring, or a set
of rings cannot change from one conformation to another if it would
require a chain to pass through itself or another chain. This effect, or lack
of, is responsible for the shape of catenanes and molecular knots.
Steric repulsions between different parts of a molecular system were found
to be of key importance in governing the direction of transition-metal-mediated
transformations and catalysis. Steric effects can even induce a mechanism
switch in the catalytic reaction. Steric repulsion is also largely responsible
for the stabilizing of colloids by coating the surface with a polymer, and
can also cause bond length shortening, and compressional frequency enhancement
in the IR spectrum. Steric inhibition of resonance is
present only in benzene rings. The presence of any group at the ortho
position in benzoic acid will throw the carboxylic acid group out of the plane,
and thus its mesomeric connection with the benzene ring vanishes. This means
that ortho-substituted benzoic acids are stronger than meta- and para-substituted
benzoic acids. Steric inhibition of protonation
Consider 2,2,6,6-Tetramethylpiperidine and in,in-diphosphine.
Steric effects vs. electronic effects The structure, properties, and
reactivity of a molecule is dependent on straightforward bonding interactions
including covalent bonds, ionic bonds, hydrogen bonds and lesser forms of
bonding. This bonding supplies a basic molecular skeleton that is modified by
repulsive forces. These repulsive forces include the steric interactions
described above. Basic bonding and steric are at times insufficient to
explain many structures, properties, and reactivity. Thus steric effects are
often contrasted and complemented by electronic effects implying the
influence of effects such as induction, conjunction, orbital symmetry,
electrostatic interactions, and spin state. There are more esoteric
electronic effects but these are among the most important when considering
structure and chemical reactivity. A special computational procedure was
developed to separate electronic and steric effects of an arbitrary group in
the molecule and to reveal their influence on structure and reactivity.
Significance Understanding steric effects is critical
to chemistry, biochemistry and pharmacology. In organic chemistry,
steric effects are nearly universal and affect the rates and activation energies
of most chemical reactions to varying degrees. Steric effects often dictate
reaction pathways in organic synthesis because there are fewer configurations
in which molecules can collide and successfully react. In surface
chemistry, steric effects manifest in an entropic contribution to the overall
Gibbs energy. Hence, colloidal stability can be obtained by steric shielding from
polymers, often made possible via block co-polymer adsorption or grafting
polymer onto charged nanoparticles. This spatial shielding effect is large-scale
in comparison to Debye shielding due to the size of the attached polymer
strands. In biochemistry, steric effects are often exploited in naturally
occurring molecules such as enzymes, where the catalytic site may be buried
within a large protein structure. In pharmacology, steric effects determine
how and at what rate a drug will interact with its target bio-molecules.
Note the organic chemical example of Stereochemistry of ketonization of enols
and enolates where diastereoselectivity results from steric hindrance.
See also Collision theory
Reaction rate accelerate as result of steric hindrance in the Thorpe–Ingold
effect Sterically induced reduction
References External links
www.chem.swin.edu.aumod4/qsarwebp4.html Steric A Program to Calculate the Steric
Size of Molecules