Steric effects#Steric hindrance

File:Regioselective tritylation.png 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).{{cite book |last=Gait |first=Michael |title=Oligonucleotide synthesis: a practical approach |publisher=IRL Press |location=Oxford |isbn=0-904147-74-6 |year=1984}}]]

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.

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

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.{{March6th}}

A-values provide another measure of the bulk of substituents. A-values are derived from equilibrium measurements of monosubstituted cyclohexanes.E.L. Eliel, S.H. Wilen and L.N. Mander, Stereochemistry of Organic Compounds, Wiley, New York (1994). {{ISBN|81-224-0570-3}}{{cite book|title=Conformational Analysis|year=1965|publisher=Interscience Publishers|location=New York|last1=Eliel|first1=E.L.|last2=Allinger|first2=N.L.|last3=Angyal|first3=S.J.|first4=Morrison|last4=G.A.}}{{cite book|title=Topics in Stereochemistry|year=1967|publisher=John Wiley & Sons, Inc.|location=New York|page=199|edition=first|last1=Hirsch|first1=J.A.}}{{cite book|title=Topics in Stereochemistry|year=1969|publisher=John Wiley & Sons, Inc.|location=New York|page=40|edition=fourth|last1=Romers|first1=C.|last2=Altona|first2=C.|last3=Buys|first3=H.R.|last4=Havinga|first4=E.}} The extent that a substituent favors the equatorial position gives a measure of its bulk.

File:Methyl group cyclohexane.png 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 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.

File:ConeAng.png

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).{{Cite journal

| doi = 10.1021/ja00713a007

| volume = 92

| issue = 10

| pages = 2956–2965

| last = Tolman

| first = Chadwick A.

| title = Phosphorus ligand exchange equilibriums on zerovalent nickel. Dominant role for steric effects

| journal = J. Am. Chem. Soc.

| date = 1970-05-01

| bibcode = 1970JAChS..92.2956T

}}

{{clear}}

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.

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.Stephan, Douglas W. "Frustrated Lewis pairs": a concept for new reactivity and catalysis. Org. Biomol. Chem. 2008, 6, 1535–1539. {{doi|10.1039/b802575b}}

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.{{cite encyclopedia|encyclopedia=Ullmann's Encyclopedia of Industrial Chemistry|author=Helmut Fiege |author2=Heinz-Werner Voges |author3=Toshikazu Hamamoto |author4=Sumio Umemura |author5=Tadao Iwata |author6=Hisaya Miki |author7=Yasuhiro Fujita |author8=Hans-Josef Buysch |author9=Dorothea Garbe |author10=Wilfried Paulus |title=Phenol Derivatives|publisher=Wiley-VCH|place=Weinheim|year=2002|pages=a19_313 |doi=10.1002/14356007.a19_313| isbn=3-527-30673-0 }}

File:LMW-HA(L)S-1 100.svg|Hindered amine light stabilizers are widely used in polymers.{{cite book|chapter=Photostabilisation of Polymer Materials|author=Pieter Gijsman |year=2010|doi=10.1002/9780470594179.ch17 |publisher=John Wiley & Sons|place=Hoboken|title=Photochemistry and Photophysics of Polymer Materials Photochemistry| pages=627–679 | isbn=978-0-470-59417-9 |editor=Norman S. Allen}}.{{cite encyclopedia|title=Paints and Coatings, 4. Pigments, Extenders, and Additives|encyclopedia=Ullmann's Encyclopedia Of Industrial Chemistry|author1=Klaus Köhler|author2=Peter Simmendinger|author3=Wolfgang Roelle|author4=Wilfried Scholz|author5=Andreas Valet|author6=Mario Slongo|year=2010|pages=o18_o03 |doi=10.1002/14356007.o18_o03| isbn=978-3-527-30673-2 }}

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.{{cite journal |doi=10.1021/ol016682s|title=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|year=2001|last1=Goto|first1=Kei|last2=Nagahama|first2=Michiko|last3=Mizushima|first3=Tadashi|last4=Shimada|first4=Keiichi|last5=Kawashima|first5=Takayuki|last6=Okazaki|first6=Renji|journal=Organic Letters|volume=3|issue=22|pages=3569–3572|pmid=11678710}}

File:Triterbutilamin.JPG)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)]]

• Collision theory
• Intramolecular force
• Sterically induced reduction
• Reaction rate accelerate as result of steric hindrance in the Thorpe–Ingold effect
• Van der Waals strain, also known as steric strain

{{reflist}}

• {{Webarchive |url=https://web.archive.org/web/20080725021620/http://www.chem.swin.edu.au/modules/mod4/qsarwebp4.html |date=July 25, 2008 |title=Steric Effects (chem.swin.edu.au)}}
• {{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)}}

Category:Stereochemistry

Category:Physical organic chemistry