steric
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Steric effects are nonbonding interactions that influence the shape (
conformation Conformation generally means structural arrangement and may refer to: * Conformational isomerism, a form of stereoisomerism in chemistry ** Carbohydrate conformation ** Cyclohexane conformation ** Protein conformation ** Conformation activity relat ...
) and
reactivity
reactivity
of ions and molecules. Steric effects complement
electronic effects An electronic effect influences the chemical structure, structure, reactivity (chemistry), reactivity, or chemical properties, properties of molecule but is neither a traditional Chemical bond, bond nor a steric effect. In organic chemistry, the te ...
, which dictate the shape and reactivity of molecules. Steric repulsive forces between overlapping
electron cloud In atomic theory Atomic theory is the scientific theory A scientific theory is an explanation of an aspect of the natural world and universe that has been repeatedly tested and verified in accordance with the scientific method The sc ...
s 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 angles. Steric hindrance is responsible for the observed shape of
rotaxane macrocycle. A rotaxane is a mechanically interlocked molecular architectures, mechanically interlocked molecular architecture consisting of a "dumbbell shaped molecule" which is threaded through a " macrocycle" (see graphical representation). T ...

rotaxane
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 , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hy ...

biphenyl
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 atropisomer 300px, Atropisomers of 6,6'-dinitro- ...

binaphthyl
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. The difference reflects the inhibition of attack on the compound with the sterically bulky
(CH3)3C
(CH<sub>3</sub>)<sub>3</sub>C
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 Bulk can refer to: Industry * Bulk cargo * Bulk liquids * Bulk mail * Bulk material handling * Bulk pack, packaged bulk materials/products * Bulk purchasing * Baking * Bulk fermentation, the period after mixing when dough is left alone to fermen ...
of substituents. A values are derived from equilibrium measurements of monosubstituted cyclohexane conformation, 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. image:Methyl group cyclohexane.png, 400px, center, The A-value for a methyl group is 1.74 as derived from the chemical equilibrium above. It costs 1.74 kcal/mol for the methyl group to adopt to the axial position compared to the equatorial position.


Ceiling temperatures

Ceiling temperature (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 are equal. Sterically hindered monomers give polymers with low T_c's, which are usually not useful.


Cone angles

image:ConeAng.png, 230px, left, Ligand cone angle. Ligand cone angles are measures of the size of ligands in coordination chemistry. 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, and pharmacology. In organic chemistry, steric effects are nearly universal and affect the rates and activation energies of most chemical reactions to varying degrees. In biochemistry, steric effects are often exploited in naturally occurring molecules such as enzymes, where the catalysis, catalytic site may be buried within a large protein structure. In pharmacology, steric effects determine how and at what rate a medication, drug will interact with its target bio-molecules. File:BigPhosphite31570-04-4.png , Tris(2,4-di-tert-butylphenyl)phosphite, a widely used Stabilizer (chemistry), stabilizer in polymers. File:Tricyclohexylphosphine-2D-skeletal.png, Tricyclohexylphosphine, a bulky phosphine ligand used in homogeneous catalysis and, with tris(pentafluorophenyl)borane, B(C6F5)3, comprises the classic frustrated Lewis pair. File:2,6-di-tert-butylphenol.svg, 2,6-Di-tert-butylphenol is used industrially as UV stabilizers and antioxidants for hydrocarbon-based products ranging from petrochemicals to plastics. File:LMW-HA(L)S-1 100.svg, Hindered amine light stabilizers are widely used in polymers. File:Ti(OiPr)4.png, Titanium isopropoxide 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 * Reaction rate accelerate as result of steric hindrance in the Thorpe–Ingold effect * Sterically induced reduction * Intramolecular force * Van der Waals strain, 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