Steric effects

group of thymidine in the presence of a free secondary 3'-hydroxy group as a result of steric hindrance due to the dimethoxytrityl group and the ribose ring (Py = pyridine). 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 rotaxanes and the low rates of racemization of 2,2'-disubstituted biphenyl and 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 group. A-values A-values provide another measure of the bulk of substituents. A-values are derived from equilibrium measurements of monosubstituted cyclohexanes. The extent that a substituent favors the equatorial position gives a measure of its bulk. 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 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 some cases, steric effects are necessary to ensure a molecule's stability. In Günther Maier's , bulky substituents stabilize a molecular core because decomposition would force the substituents closer together. Conversely, substituent attraction can stabilize molecules; these molecules are called "dispersion-stabilized". 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, a widely used stabilizer in polymers. File:Tricyclohexylphosphine-2D-skeletal.png|Tricyclohexylphosphine, a bulky phosphine ligand used in homogeneous catalysis and, with 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. )amine makes electrophilic reactions, like forming the tetraalkylammonium cation, difficult. It is difficult for electrophiles to get close enough to allow attack by the lone pair of the nitrogen (nitrogen is shown in blue) == See also ==