Steric effects
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape (
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
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
A-values
A-values provide another measure of the bulk of substituents. A-values are derived from equilibrium measurements of monosubstituted cyclohexanes.[4][5][6][7] The extent that a substituent favors the equatorial position gives a measure of its bulk.
Substituent | A-Value |
---|---|
H | 0 |
CH3 | 1.74 |
CH2CH3 | 1.75 |
CH(CH3)2 | 2.15 |
C(CH3)3 | >4 |
Ceiling temperatures
Ceiling temperature () is a measure of the steric properties of the monomers that comprise a polymer. is the temperature where the rate of polymerization and depolymerization are equal. Sterically hindered monomers give polymers with low 's, which are usually not useful.
Monomer | Ceiling temperature (°C)[8] | Structure |
---|---|---|
ethylene | 610 | CH2=CH2 |
isobutylene | 175 | CH2=CMe2 |
1,3-butadiene |
585 | CH2=CHCH=CH2 |
isoprene | 466 | CH2=C(Me)CH=CH2 |
styrene | 395 | PhCH=CH2 |
α-methylstyrene |
66 | PhC(Me)=CH2 |
Cone angles
Ligand | Angle (°) |
---|---|
PH3 | 87 |
P(OCH3)3 | 107 |
P(CH3)3 | 118 |
P(CH2CH3)3 | 132 |
P(C6H5)3 | 145 |
P(cyclo-C6H11)3 | 179 |
P(t-Bu)3 | 182 |
P(2,4,6- Me3C6H2 )3 |
212 |
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 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.
-
Tris(2,4-di-tert-butylphenyl)phosphite, a widely used stabilizer in polymers.
-
phosphine ligand used in homogeneous catalysis and, with B(C6F5)3, comprises the classic frustrated Lewis pair.[10]
-
UV stabilizers and antioxidants for hydrocarbon-based products ranging from petrochemicals to plastics.[11]
-
Titanium isopropoxide is a monomer, the corresponding titanium ethoxide is a tetramer.
-
An isolable selenenic acid owing to steric protection.[14]
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
- .
- ISBN 0-904147-74-6.
- ISBN 978-0-471-72091-1
- ISBN 81-224-0570-3
- ^ Eliel, E.L.; Allinger, N.L.; Angyal, S.J.; G.A., Morrison (1965). Conformational Analysis. New York: Interscience Publishers.
- ^ Hirsch, J.A. (1967). Topics in Stereochemistry (first ed.). New York: John Wiley & Sons, Inc. p. 199.
- ^ Romers, C.; Altona, C.; Buys, H.R.; Havinga, E. (1969). Topics in Stereochemistry (fourth ed.). New York: John Wiley & Sons, Inc. p. 40.
- ISBN 978-0-19-512444-6.
- .
- doi:10.1039/b802575b
- ISBN 3-527-30673-0.
- ISBN 978-0-470-59417-9..
- ISBN 978-3-527-30673-2.
- PMID 11678710.
External links
- Steric Effects (chem.swin.edu.au) at the Wayback Machine (archived July 25, 2008)
- Steric: A Program to Calculate the Steric Size of Molecules (gh.wits.ac.za) at the Wayback Machine (archived December 22, 2017)