Steric effects

An example of a molecule with bulky groups

Steric effects are the effects seen in molecules that come from the fact that atoms occupy space. When atoms are put close to each other, this costs energy. The electrons near the atoms want to stay away from each other. This can change the way molecules want to react. It can also change the shape (or conformation) of the molecule. The amount of space that a group of atoms takes is called the "steric bulk". An example of steric effects is steric hindrance. This is when a large group in a molecule makes reactions not work. For example, an S_N2 reaction does not happen on carbon atoms that have three substituents. Steric hindrance however can also be a positive thing. Sometimes, chemists would like a reaction to happen in one place rather than another one. A bulky group can block a position. It can also change the shape such that only one diastereomer from a reaction is made.

Steric effects are usually smaller than electronic effects. These affect the molecule's shape and reactivity as well, but they come from the way electrons are placed in bonds.

Steric Effects Media

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  The parent cyclobutadiene (R = H) readily dimerizes but the R = tert-butyl derivative is robust.

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  Regioselective dimethoxytritylation of the primary 5'-hydroxyl 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).

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  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.

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  ConeAng

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  Tris(2,4-di-tert-butylphenyl)phosphite, a widely used stabilizer in polymers.

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  Tricyclohexylphosphine, a bulky phosphine ligand used in homogeneous catalysis and, with B(C6F5)3, comprises the classic frustrated Lewis pair.

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  2,6-Di-tert-butylphenol is used industrially as UV stabilizers and antioxidants for hydrocarbon-based products ranging from petrochemicals to plastics.

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  Hindered amine light stabilizers are widely used in polymers.

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  Titanium isopropoxide is a monomer, the corresponding titanium ethoxide is a tetramer.