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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. Steric effects complement
electronic effects An electronic effect influences the structure, reactivity, or properties of molecule but is neither a traditional bond nor a steric effect. In organic chemistry, the term stereoelectronic effect is also used to emphasize the relation between t ...
, which dictate the shape and reactivity of molecules. Steric repulsive forces between overlapping electron clouds result in structured groupings of molecules stabilized by the way that opposites attract and like charges repel.


Steric hindrance

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 affect torsional
bond angle Bond or bonds may refer to: Common meanings * Bond (finance), a type of debt security * Bail bond, a commercial third-party guarantor of surety bonds in the United States * Chemical bond, the attraction of atoms, ions or molecules to form chemical ...
s. Steric hindrance is responsible for the observed shape of
rotaxane In chemistry, a rotaxane () is a mechanically interlocked molecular architecture consisting of a dumbbell-shaped molecule which is threaded through a macrocycle (see graphical representation). The two components of a rotaxane are kinetically t ...
s and the low rates of racemization of 2,2'-disubstituted
biphenyl Biphenyl (also known as diphenyl, phenylbenzene, 1,1′-biphenyl, lemonene or BP) is an organic compound that forms colorless crystals. Particularly in older literature, compounds containing the functional group consisting of biphenyl less one ...
and
binaphthyl 1,1'-Binaphthyl is an organic compound with the formula (CH). It is one of the dimers of naphthalene (or literally: dimers of naphthyl). A colorless solid, it has attracted some attention because the atropisomers can be isolated due to hindered r ...
derivatives.


Measures of steric properties

Because steric effects have profound impact on properties, the steric properties of substituents have been assessed by numerous methods.


Rate data

Relative rates of chemical reactions provide useful insights into the effects of the steric bulk of substituents. Under standard conditions methyl bromide solvolyzes 107 faster than does
neopentyl bromide Pentyl is a five-carbon alkyl group or substituent with chemical formula -C5H11. It is the substituent form of the alkane pentane. In older literature, the common non-systematic name amyl was often used for the pentyl group. Conversely, the name ...
. The difference reflects the inhibition of attack on the compound with the sterically bulky (CH3)3C group.


A-values

A value 400px, The A-value for a methyl group is 1.74 as derived from the chemical equilibrium above. This means it costs of energy to have a methyl group in the axial position compared to the equatorial position. A-values are numerical values used in ...
s provide another measure of the bulk of substituents. A values are derived from equilibrium measurements of monosubstituted cyclohexanes.E.L. Eliel, S.H. Wilen and L.N. Mander, Stereochemistry of Organic Compounds, Wiley, New York (1994). The extent that a substituent favors the equatorial position gives a measure of its bulk.


Ceiling temperatures

Ceiling temperature Ceiling temperature (T_c) is a measure of the tendency of a polymer to revert to its constituent monomers. When a polymer is at its ceiling temperature, the rate of polymerization and depolymerization of the polymer are equal. Generally, the ceil ...
(T_c) is a measure of the steric properties of the monomers that comprise a polymer. T_c is the temperature where the rate of polymerization and
depolymerization Depolymerization (or depolymerisation) is the process of converting a polymer into a monomer or a mixture of monomers. This process is driven by an increase in entropy. Ceiling temperature The tendency of polymers to depolymerize is indicated by ...
are equal. Sterically hindered monomers give polymers with low T_c's, which are usually not useful.


Cone angles

Ligand cone angles are measures of the size of ligands in
coordination chemistry A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
. It is defined as the solid angle formed with the metal at the vertex and the hydrogen atoms at the perimeter of the cone (see figure).


Significance and applications

Steric effects are critical to chemistry,
biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
, and pharmacology. In organic chemistry, steric effects are nearly universal and affect the rates and activation energies of most
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breakin ...
s 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 protein structure. In pharmacology, steric effects determine how and at what rate a drug will interact with its target bio-molecules. File:BigPhosphite31570-04-4.png,
Tris(2,4-di-tert-butylphenyl)phosphite Tris(2,4-di-tert-butylphenyl)phosphite is an organophosphorus compound with the formula C4H9)2C6H3Osub>3P. This white solid is a widely used stabilizer in polymers where it functions as an antioxidant as well as other roles. The compound is a ...
, a widely used stabilizer in polymers. File:Tricyclohexylphosphine-2D-skeletal.png,
Tricyclohexylphosphine Tricyclohexylphosphine is the tertiary phosphine with the formula P( C6H11)3. Commonly used as a ligand in organometallic chemistry, it is often abbreviated to PCy3, where Cy stands for cyclohexyl. It is characterized by both high basicity (p''K' ...
, a bulky
phosphine ligand A metal-phosphine complex is a In coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis. ...
used in
homogeneous catalysis In chemistry, homogeneous catalysis is catalysis by a soluble catalyst in a solution. Homogeneous catalysis refers to reactions where the catalyst is in the same phase as the reactants, principally in solution. In contrast, heterogeneous catalysis ...
and, with B(C6F5)3, comprises the classic
frustrated Lewis pair A frustrated Lewis pair (FLP) is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form a classical adduct. Many kinds of FLPs have been devised, and many simple substrates exhib ...
. File:2,6-di-tert-butylphenol.svg,
2,6-Di-tert-butylphenol 2,6-Di-''tert''-butylphenol is an organic compound with the structural formula 2,6-((CH3)3C)2C6H3OH. This colorless solid alkylated phenol and its derivatives are used industrially as UV stabilizers and antioxidants for hydrocarbon-based pro ...
is used industrially as UV stabilizers and
antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricants, ...
s for hydrocarbon-based products ranging from petrochemicals to plastics. File:LMW-HA(L)S-1 100.svg,
Hindered amine light stabilizers Hindered amine light stabilizers (HALS) are chemical compounds containing an amine functional group that are used as stabilizers in plastics and polymers. These compounds are typically derivatives of tetramethylpiperidine and are primarily used ...
are widely used in polymers. File:Ti(OiPr)4.png,
Titanium isopropoxide Titanium isopropoxide, also commonly referred to as titanium tetraisopropoxide or TTIP, is a chemical compound with the formula . This alkoxide of titanium(IV) is used in organic synthesis and materials science. It is a diamagnetic tetrahedral mo ...
is a monomer, the corresponding titanium ethoxide is a tetramer. File:OkazakiRSeOH.png, An isolable selenenic acid owing to steric protection.


See also

*
Collision theory Collision theory is a principle of chemistry used to predict the rates of chemical reactions. It states that when suitable particles of the reactant hit each other with correct orientation, only a certain amount of collisions result in a percept ...
* Reaction rate accelerate as result of steric hindrance in the
Thorpe–Ingold effect The Thorpe–Ingold effect, gem-dimethyl effect, or angle compression is an effect observed in chemistry where increasing steric hindrance favours ring closure and intramolecular reactions. The effect was first reported by Beesley, Thorpe, and Ing ...
*
Sterically induced reduction In chemistry, a sterically induced reduction happens when an oxidized metal behaves as, and exhibits similar reducing properties to, the more reduced form of the metal. This effect is mainly caused by the surrounding ligands that are complexed to ...
*
Intramolecular force An intramolecular force (or primary forces) is any force that binds together the atoms making up a molecule or compound, not to be confused with intermolecular forces, which are the forces present between molecules. The subtle difference in the nam ...
*
Van der Waals strain Van der Waals strain is strain resulting from Van der Waals repulsion when two substituents in a molecule approach each other with a distance less than the sum of their Van der Waals radii. Van der Waals strain is also called Van der Waals rep ...
, also known as steric strain


References


External links

* * {{Webarchive , url=https://web.archive.org/web/20171222052426/http://www.gh.wits.ac.za/craig/steric/ , date=December 22, 2017 , title=Steric: A Program to Calculate the Steric Size of Molecules (''gh.wits.ac.za'') Stereochemistry Physical organic chemistry