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Steric effects

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(Redirected fromSteric hindrance)

Geometric aspects of ions and molecules affecting their shape and reactivity

The parentcyclobutadiene (R = H) readilydimerizes but the R =*tert*-butyl derivative is robust.^[1]

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 (conformation) andreactivity of ions and molecules. Steric effects complementelectronic effects, which dictate the shape and reactivity of molecules. Steric repulsive forces between overlappingelectron clouds result in structured groupings of molecules stabilized by the way that opposites attract and like charges repel.

Steric hindrance

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Regioselective dimethoxytritylation of the primary 5'-hydroxyl group ofthymidine in the presence of a free secondary 3'-hydroxy group as a result of steric hindrance due to thedimethoxytrityl group and theribose ring (Py =pyridine).^[2]

Steric hindrance is a consequence of steric effects. Steric hindrance is the slowing of chemical reactions due to steric bulk. It is usually manifested inintermolecular reactions, whereas discussion of steric effects often focus onintramolecular interactions. Steric hindrance is often exploited to control selectivity, such as slowing unwanted side-reactions.

Steric hindrance between adjacent groups can also affect torsionalbond angles. Steric hindrance is responsible for the observed shape ofrotaxanes and the low rates of racemization of 2,2'-disubstitutedbiphenyl andbinaphthyl derivatives.

Measures of steric properties

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

Rate data

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Relative rates of chemical reactions provide useful insights into the effects of the steric bulk of substituents. Under standard conditions, methyl bromidesolvolyzes 10⁷ faster than doesneopentyl bromide. The difference reflects the inhibition of attack on the compound with the sterically bulky(CH₃)₃C group.^[3]

A-values

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A-values provide another measure of the bulk of substituents. A-values are derived from equilibrium measurements of monosubstitutedcyclohexanes.^[4]^[5]^[6]^[7] The extent that a substituent favors the equatorial position gives a measure of its bulk.

The A-value for amethyl group is 1.74 as derived from thechemical equilibrium above. It costs 1.74 kcal/mol for the methyl group to adopt to the axial position compared to the equatorial position.

Substituent	A-value
H	0
CH₃	1.74
CH₂CH₃	1.75
CH(CH₃)₂	2.15
C(CH₃)₃	>4

Ceiling temperatures

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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 ofpolymerization anddepolymerization are equal. Sterically hindered monomers give polymers with low $T_{c}$ '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

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Ligand cone angles are measures of the size ofligands incoordination chemistry. It is defined as thesolid angle formed with the metal at the vertex and the hydrogen atoms at the perimeter of the cone (see figure).^[9]

Cone angles of commonphosphine ligands
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

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Steric effects are critical tochemistry,biochemistry, andpharmacology. In organic chemistry, steric effects are nearly universal and affect the rates and activation energies of mostchemical reactions to varying degrees. In some cases, steric effects are necessary to ensure a molecule's stability. In Günther Maier'scorset effect, bulky substituents stabilize a molecular core because decomposition wouldforce the substituents closer together.^[10] Conversely, substituent attraction can stabilize molecules; these molecules are called "dispersion-stabilized".

In biochemistry, steric effects are often exploited in naturally occurring molecules such asenzymes, where thecatalytic site may be buried within a largeprotein structure. In pharmacology, steric effects determine how and at what rate adrug will interact with its target bio-molecules.

Prominent sterically hindered compounds
Tris(2,4-di-tert-butylphenyl)phosphite, a widely usedstabilizer in polymers.
Tricyclohexylphosphine, a bulkyphosphine ligand used inhomogeneous catalysis and, withB(C₆F₅)₃, comprises the classicfrustrated Lewis pair.^[11]
2,6-Di-tert-butylphenol is used industrially asUV stabilizers andantioxidants forhydrocarbon-based products ranging from petrochemicals to plastics.^[12]
Hindered amine light stabilizers are widely used in polymers.^[13]^[14]
Titanium isopropoxide is a monomer, the correspondingtitanium ethoxide is a tetramer.
An isolableselenenic acid owing to steric protection.^[15]

The steric effect of tri-(tert-butyl)amine makeselectrophilic reactions, like forming the tetraalkyl ammonium cation, difficult. It is difficult for electrophiles to get close enough to allow attack by thelone pair of the nitrogen (nitrogen is shown in blue)

References

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^Günther Maier; Stephan Pfriem; Ulrich Schäfer; Rudolf Matusch (1978). "Tetra-tert-butyltetrahedrane".Angew. Chem. Int. Ed. Engl.17 (7):520–1.doi:10.1002/anie.197805201.
^Gait, Michael (1984).Oligonucleotide synthesis: a practical approach. Oxford: IRL Press.ISBN 0-904147-74-6.
^Smith, Michael B.;March, Jerry (2007),Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience,ISBN 978-0-471-72091-1
^E.L. Eliel, S.H. Wilen and L.N. Mander, Stereochemistry of Organic Compounds, Wiley, New York (1994).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.
^Stevens, Malcolm P. (1999). "6".Polymer Chemistry an Introduction (3rd ed.). New York: Oxford University Press. pp. 193–194.ISBN 978-0-19-512444-6.
^Tolman, Chadwick A. (1970-05-01). "Phosphorus ligand exchange equilibriums on zerovalent nickel. Dominant role for steric effects".J. Am. Chem. Soc.92 (10):2956–2965.Bibcode:1970JAChS..92.2956T.doi:10.1021/ja00713a007.
^Maier, G.; Pfriem, S.; Schäfer, U.; Matusch, R. (1978). "Tetra-tert-butyltetrahedrane".Angew. Chem. Int. Ed. Engl.17 (7):520–521.doi:10.1002/anie.197805201.
^Stephan, Douglas W. "Frustrated Lewis pairs": a concept for new reactivity and catalysis. Org. Biomol. Chem. 2008, 6, 1535–1539.doi:10.1039/b802575b
^Helmut Fiege; Heinz-Werner Voges; Toshikazu Hamamoto; Sumio Umemura; Tadao Iwata; Hisaya Miki; Yasuhiro Fujita; Hans-Josef Buysch; Dorothea Garbe; Wilfried Paulus (2002). "Phenol Derivatives".Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. pp. a19_313.doi:10.1002/14356007.a19_313.ISBN 3-527-30673-0.
^Pieter Gijsman (2010). "Photostabilisation of Polymer Materials". In Norman S. Allen (ed.).Photochemistry and Photophysics of Polymer Materials Photochemistry. Hoboken: John Wiley & Sons. pp. 627–679.doi:10.1002/9780470594179.ch17.ISBN 978-0-470-59417-9..
^Klaus Köhler; Peter Simmendinger; Wolfgang Roelle; Wilfried Scholz; Andreas Valet; Mario Slongo (2010). "Paints and Coatings, 4. Pigments, Extenders, and Additives".Ullmann's Encyclopedia Of Industrial Chemistry. pp. o18_o03.doi:10.1002/14356007.o18_o03.ISBN 978-3-527-30673-2.
^Goto, Kei; Nagahama, Michiko; Mizushima, Tadashi; Shimada, Keiichi; Kawashima, Takayuki; Okazaki, Renji (2001). "The First Direct Oxidative Conversion of a Selenol to a Stable Selenenic Acid: Experimental Demonstration of Three Processes Included in the Catalytic Cycle of Glutathione Peroxidase".Organic Letters.3 (22):3569–3572.doi:10.1021/ol016682s.PMID 11678710.