- Steric effects
Steric effects arise from the fact that each
atomwithin a moleculeoccupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energydue to overlapping electron clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation) and reactivity.
Types of steric effects
Steric hindrance or steric resistance occurs when the size of groups within a molecule prevents chemical reactions that are observed in related smaller molecules. 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 (steric protection). Steric hindrance between adjacent groups can also restrict torsional bond angles. However,
hyperconjugationhas been suggested as an explanation for the preference of the staggeredconformation of ethanebecause the steric hindrance of the small hydrogenatom is far too small. ["Hyperconjugation not steric repulsion leads to the staggered structure of ethane " Pophristic, V. & Goodman, L. Nature 411, 565–568 (2001) [http://www.nature.com/nature/journal/v411/n6837/abs/411565a0.html Abstract] DOI|10.1038/35079036] ["Chemistry: A new twist on molecular shape" Frank Weinhold Nature 411, 539-541 (31 May 2001) DOI|10.1038/35079225] .Steric shielding occurs when a charged group on a molecule is seemingly weakened or spatially shielded by less charged (or oppositely charged) atoms, including counterions in solution (Debye shielding). 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
random coilcan't change from one conformation to a closely related shape by a small displacement if it would require one polymerchain to pass through another, or through itself.
Steric effects vs. electronic effects
The structure, properties, and reactivity of a molecule is dependent on straight forward 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 effectsimplying 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.
Understanding steric effects is critical to
chemistry, biochemistryand pharmacology. In chemistry, steric effects are nearly universal and affect the rates and energies of most chemical reactions to varying degrees. In biochemistry, steric effects are often exploited in naturally occurring molecules such as enzymes, where the catalytic site may be buried within a large proteinstructure. In pharmacology, steric effects determine how and at what rate a drug will interact with its target bio-molecules.
* Reaction rate acceleration as result of steric hindrance in the
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