إعاقة فراغية

The parent cyclobutadiene (R = H) readily dimerizes but the R = tert-butyl derivative is robust.^[1]

الإعاقة الفراغية Steric effects هي التفاعل الناتج بين الجزيئات نتيجة لشكلها و/أو العلاقة الفراغية بينها. وتوجد أنوع عديدة للإعاقة الفراغية, وغالبا ما تسمي طبقا للنتيجة الحادثة من الإعاقة الفراغية.

تقع الإعاقة الفراغية أو المقاومة الفراغية عندما لا تتفاعل المجموعات الجزيئية على جزيء (أو جزيئات ولكن كل منهم على حدى) مع بعضها البعض, مع المفترض حدوث إنجذاب طبيعي لها ولكنه لا يحدث بسبب الإعاقة الفراغية. ومثال لذلك, عدم حدوث الإنجذاب الطبيعي بين ذرات جزيء معين لبعضها البعض, نظرا لأن هذه الذرات قد لا تكون على البعد المناسب الذى يساعد على حدوث هذا الإنجذاب, أو وجود ذرات أخرى تمنع حدوث هذا التجاذب. ويوجد للإهعاقة الفراغية تشكلات كيميائية.

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Steric hindrance

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).^[2]

الإعاقة الغطائية وتحدث عند وجود مجموعة مشحونة على الجزيء تبدو كما لو كان يحدث لها إضعاف أو مغطاة في الفراغ بذرات ذات شحنة أقل (أو بشحنة معاكسة). وفى بعض الحالات حتى يمكن للذرات التفاعل مع هذه الذرات المغطاة فراغيا يجب أن تقترب منها فراغيا من المكان الذى يكون فيها الغطاء أقل, وهذا يحدد أين وكيف يحدث التفاعل الجزيئي.

الإنجذاب الفراغي ويحدث عند وجود جزيء له شكل معدل نتيجة لتفاعله مع جزيء أخر. وفى هذه الحالات سوف تتفاعل الجزيئات مع بعضها في ترتيب خاص معين.

وفهم ظاهرة الإعاقة الفراغية في غاية الأهمية لكل من الكيمياء, الكيمياء الحيوية, الصيدلة. وتؤثر الإعاقة الفراغية في الكيمياء على معدلات وطاقات التفاعلات بدرجات متفاوتة. بينما في الكيمياء الحيوية تحدد الإعاقة الفراغية شكل وإنجذابات البروتينات والأحماض الأمينية, كما تؤثر في معدلات تفاعلات الإنزيمات. وفى علوم الصيدلة الإعاقة الفراغية تحدد إلى أى درجة, وكيف يتفاعل الدواء مع الجزيئات الحيوية.

Measures of steric properties

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

بيانات المعدلات

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.^[3]

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.

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.

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

درجات الحرارة السقفية

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.

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

الزوايا القمعية

Ligand cone angle.

Cone angles of common phosphine 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

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).^[9]

الأهمية والتطبيقات

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.

Prominent Sterically Hindered Compounds
Tris(2,4-di-tert-butylphenyl)phosphite, a widely used stabilizer in polymers.
Tricyclohexylphosphine, a bulky phosphine ligand used in homogeneous catalysis and, with B(C₆F₅)₃, comprises the classic frustrated Lewis pair.^[10]
2,6-Di-tert-butylphenol is used industrially as UV stabilizers and antioxidants for hydrocarbon-based products ranging from petrochemicals to plastics.^[11]
Hindered amine light stabilizers are widely used in polymers.^[12]^[13]
Titanium isopropoxide is a monomer, the corresponding titanium ethoxide is a tetramer.
An isolable selenenic acid owing to steric protection.^[14]

The steric effect of tri-(tert-butyl)amine makes electrophilic reactions, like forming the tetraalkyl ammonium 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)

انظر أيضاً

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

المصادر

^ "Tetra-tert-butyltetrahedrane". Angew. Chem. Int. Ed. Engl. 17 (7): 520–1. 1978. doi:10.1002/anie.197805201. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
^ Gait, Michael (1984). Oligonucleotide synthesis: a practical approach. Oxford: IRL Press. ISBN 0-904147-74-6.
^ قالب:March6th
^ 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. doi:10.1021/ja00713a007.
^ Stephan, Douglas W. "Frustrated Lewis pairs": a concept for new reactivity and catalysis. Org. Biomol. Chem. 2008, 6, 1535-1539. DOI:10.1039/b802575b
^ "Phenol Derivatives". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. 2002. doi:10.1002/14356007.a19_313. ISBN 3527306730. {{cite encyclopedia}}: Cite uses deprecated parameter |authors= (help)
^ Norman S. Allen, ed. (2010). "Photostabilisation of Polymer Materials". Photochemistry and Photophysics of Polymer Materials Photochemistry. Hoboken: John Wiley & Sons. pp. 627–679. doi:10.1002/9780470594179.ch17. ISBN 9780470594179. {{cite book}}: Cite uses deprecated parameter |authors= (help).
^ 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. doi:10.1002/14356007.o18_o03. ISBN 978-3527306732.
^ 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.

وصلات خارجية

Steric Effects (chem.swin.edu.au) at the Wayback Machine (archived يوليو 25, 2008)
Steric: A Program to Calculate the Steric Size of Molecules (gh.wits.ac.za) at the Wayback Machine (archived ديسمبر 22, 2017)

[1] "Tetra-tert-butyltetrahedrane". Angew. Chem. Int. Ed. Engl. 17 (7): 520–1. 1978. doi:10.1002/anie.197805201. {{cite journal}}: Cite uses deprecated parameter |authors= (help)

[2] Gait, Michael (1984). Oligonucleotide synthesis: a practical approach. Oxford: IRL Press. ISBN 0-904147-74-6.

[3] قالب:March6th

[ElielWilen-4] E.L. Eliel, S.H. Wilen and L.N. Mander, Stereochemistry of Organic Compounds, Wiley, New York (1994). ISBN 81-224-0570-3

[ElielAllinger-5] Eliel, E.L.; Allinger, N.L.; Angyal, S.J.; G.A., Morrison (1965). Conformational Analysis. New York: Interscience Publishers.

[Hirsch-6] Hirsch, J.A. (1967). Topics in Stereochemistry (first ed.). New York: John Wiley & Sons,Inc. p. 199.

[RomersAltona-7] Romers, C.; Altona, C.; Buys, H.R.; Havinga, E. (1969). Topics in Stereochemistry (fourth ed.). New York: John Wiley & Sons,Inc. p. 40.

[8] 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.

[tolman1970-9] 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. doi:10.1021/ja00713a007.

[10] Stephan, Douglas W. "Frustrated Lewis pairs": a concept for new reactivity and catalysis. Org. Biomol. Chem. 2008, 6, 1535-1539. DOI:10.1039/b802575b

[11] "Phenol Derivatives". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. 2002. doi:10.1002/14356007.a19_313. ISBN 3527306730. {{cite encyclopedia}}: Cite uses deprecated parameter |authors= (help)

[12] Norman S. Allen, ed. (2010). "Photostabilisation of Polymer Materials". Photochemistry and Photophysics of Polymer Materials Photochemistry. Hoboken: John Wiley & Sons. pp. 627–679. doi:10.1002/9780470594179.ch17. ISBN 9780470594179. {{cite book}}: Cite uses deprecated parameter |authors= (help).

[13] 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. doi:10.1002/14356007.o18_o03. ISBN 978-3527306732.

[14] 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.

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