WordNet

any obstruction that impedes or is burdensome (同)hinderance, hitch, preventive, preventative, encumbrance, incumbrance, interference
the act of hindering or obstructing or impeding (同)hinderance, interference
something immaterial that interferes with or delays action or progress (同)hinderance, deterrent, impediment, balk, baulk, check, handicap
bring disorder to (同)disarray
a physical condition in which there is a disturbance of normal functioning; "the doctor prescribed some medicine for the disorder"; "everyone gets stomach upsets from time to time" (同)upset
a disturbance of the peace or of public order
destroy the peace or tranquility of; "Dont interrupt me when Im reading" (同)disturb
terminate; "She interrupted her pregnancy"; "break a lucky streak"; "break the cycle of poverty" (同)break
a signal that temporarily stops the execution of a program so that another procedure can be carried out
interfere in someone elses activity; "Please dont interrupt me while Im on the phone" (同)disrupt
make a break in; "We interrupt the program for the following messages" (同)disrupt, break_up, cut_off
a disorderly outburst or tumult; "they were amazed by the furious disturbance they had caused" (同)disruption, commotion, flutter, hurly burly, to-do, hoo-ha, hoo-hah, kerfuffle
an unhappy and worried mental state; "there was too much anger and disturbance"; "she didnt realize the upset she caused me" (同)perturbation, upset
the act of disturbing something or someone; setting something in motion
a condition or state of affairs almost beyond ones ability to deal with and requiring great effort to bear or overcome; "grappling with financial difficulties"
the quality of being difficult; "they agreed about the difficulty of the climb" (同)difficultness
a factor causing trouble in achieving a positive result or tending to produce a negative result; "serious difficulties were encountered in obtaining a pure reagent"
suffer or be susceptible to damage; "These fine china cups damage easily"
inflict damage upon; "The snow damaged the roof"; "She damaged the car when she hit the tree"
the act of damaging something or someone (同)harm, hurt, scathe
loss of military equipment (同)equipment casualty
the occurrence of a change for the worse (同)harm, impairment
move deeply; "This book upset me"; "A troubling thought" (同)upset, trouble
damage as if by shaking or jarring; "Dont disturb the patients wounds by moving him too rapidly!"

PrepTutorEJDIC

〈C〉(…を)じゃまする人(物),(…の)妨害物《+『to』+『名』》 / 〈U〉妨害,じゃま
〈U〉『無秩序』,混乱,乱雑(confusion) / 《しばしば複数形で》(社会的・政治的な)粉争,騒動 / 〈C〉(肉体的・精神的な)不調,異常,障害 / …‘の'秩序を乱す / 〈心身〉‘に'異常を起こさせる
(行動や話の途中で)〈人〉‘を'『さえぎる』,妨げる / 〈行為・話など〉‘を'『中断する』 / 〈物事〉‘を'さえぎる / 『じゃまする』,話の腰を折る
〈U〉乱す(騒がす,妨害する)こと;乱れた状態 / 〈C〉騒ぎ,騒動,動乱 / 〈C〉乱す(騒がす,妨害する)もの
〈U〉『難しさ』,困難 / 〈C〉『困難なこと』,難事,障害 / 〈C〉《通例複数形で》『困難な状況』,(特に)財政困難 / 〈C〉〈U〉(…との)不和,仲たがい《+『with』(『between』)+『名』》 / 〈C〉苦情,反対
〈U〉『損害』,損傷,被害 / 《複数形で》損害賠障[金] / 《the~》《単数形で》《話》飛用 / …‘に'損傷する
…‘を'『かき乱す』,騒がしくする / 〈人〉‘を'『不安にする』,心配させる / 〈人・行動・安眠など〉‘を'『邪魔をする』,妨害する(bother)

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/08/21 17:32:13」(JST)

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See also: intramolecular forces

The steric effect of tri-(tert-butyl)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)

Steric effects arise from a fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation) and reactivity.

Types of steric effects

Steric hindrance

Steric hindrance occurs when the large size of groups within a molecule prevents chemical reactions that are observed in related molecules with smaller groups. Although steric hindrance is sometimes a problem (it prevents S_N2 reactions with tertiary substrates from taking place), it can also be a very useful tool, and is often exploited by chemists to change the reactivity pattern of a molecule by stopping unwanted side-reactions (steric protection) or by leading to a preference for one stereochemical reaction course as in diastereoselectivity. Steric hindrance between adjacent groups can also restrict torsional bond angles. However, hyperconjugation has been suggested as an explanation for the preference of the staggered conformation of ethane because the steric hindrance of the small hydrogen atom is far too small.^[1]^[2] This is the effect responsible for the observed shape of rotaxanes.

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).^[3]

When a Lewis acid and Lewis base cannot combine due to steric hindrance, they are said to form a frustrated Lewis pair.^[4]

Other types of steric effects

Steric shielding occurs when a charged group on a molecule is seemingly weakened or spatially shielded by less charged (or oppositely charged) atoms, including counterions in solution (Debye shielding). In some cases, for an atom to interact with sterically shielded atoms, it would have to approach from a vicinity where there is less shielding, thus controlling where and from what direction a molecular interaction can take place.

Steric attraction occurs when molecules have shapes or geometries that are optimized for interaction with one another. In these cases molecules will react with each other most often in specific arrangements.

Chain crossing: A chain, ring, or a set of rings cannot change from one conformation to another if it would require a chain (or ring - a ring is a cyclic chain) to pass through itself or another chain. This effect, or lack of, is responsible for the shape of catenanes and molecular knots.

Steric repulsions between different parts of a molecular system were found to be of key importance in governing the direction of transition-metal-mediated transformations and catalysis. Steric effects can even induce a mechanism switch in the catalytic reaction.^[5] Steric repulsion is also largely responsible for the stabilizing of colloids by coating the surface with a polymer, and can also cause bond length shortening, and compressional frequency enhancement in the IR spectrum.^[6]

Steric inhibition of resonance is present only in benzene rings. The presence of any group at the ortho position in benzoic acid will throw the carboxylic acid group out of the plane, and thus its mesomeric connection with the benzene ring vanishes. This means that ortho-substituted benzoic acids are stronger than meta- and para-substituted benzoic acids.

Steric inhibition of protonation

Consider 2,2,6,6-Tetramethylpiperidine and in,in-diphosphine.^[7]

Steric effects vs. electronic effects

The structure, properties, and reactivity of a molecule is dependent on straightforward bonding interactions including covalent bonds, ionic bonds, hydrogen bonds and lesser forms of bonding. This bonding supplies a basic molecular skeleton that is modified by repulsive forces. These repulsive forces include the steric interactions described above. Basic bonding and steric are at times insufficient to explain many structures, properties, and reactivity. Thus steric effects are often contrasted and complemented by electronic effects implying the influence of effects such as induction, conjunction, orbital symmetry, electrostatic interactions, and spin state. There are more esoteric electronic effects but these are among the most important when considering structure and chemical reactivity.

A special computational procedure was developed to separate electronic and steric effects of an arbitrary group in the molecule and to reveal their influence on structure and reactivity.^[8]

Significance

Understanding steric effects is 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. Steric effects often dictate reaction pathways in organic synthesis because there are fewer configurations in which molecules can collide and successfully react. In surface chemistry, steric effects manifest in an entropic contribution to the overall Gibbs energy. Hence, colloidal stability can be obtained by steric shielding from polymers, often made possible via block co-polymer adsorption or grafting polymer onto charged nanoparticles. This spatial shielding effect is large-scale in comparison to Debye shielding due to the size of the attached polymer strands. 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.

Note the organic chemical example of Stereochemistry of ketonization of enols and enolates where diastereoselectivity results from steric hindrance.

References

^ Pophristic, Vojislava; Goodman, Lionel (2001). "Hyperconjugation not steric repulsion leads to the staggered structure of ethane". Nature 411 (6837): 565–8. doi:10.1038/35079036. PMID 11385566.
^ Weinhold, Frank (2001). "Chemistry. A new twist on molecular shape". Nature 411 (6837): 539–41. doi:10.1038/35079225. PMID 11385553.
^ Gait, Michael (1984). Oligonucleotide synthesis: a practical approach. Oxford: IRL Press. ISBN 0-904147-74-6.
^ Stephan, Douglas W. "Frustrated Lewis pairs": a concept for new reactivity and catalysis. Org. Biomol. Chem. 2008, 6, 1535-1539. doi:10.1039/b802575b
^ Ananikov, Valentin P.; Szilagyi, Robert; Morokuma, Keiji; Musaev, Djamaladdin G. (2005). "Can Steric Effects Induce the Mechanism Switch in the Rhodium-Catalyzed Imine Boration Reaction? A Density Functional and ONIOM Study". Organometallics 24 (8): 1938. doi:10.1021/om049156o.
^ Zong, J., J. T. Mague, and R. A. Pascal, Jr., Exceptional Steric Congestion in an in,in-Bis(hydrosilane), J. Am. Chem. Soc. 2013, 135, 13235-13237.http://DOI: 10.1021/ja407398w
^ Zong, J., Mague, J. T., Kraml, C. M., & Pascal Jr, R. A. (2013). "A Congested in, in-Diphosphine." Organic letters, 15(9), 2179-2181. DOI: 10.1021/ol400728m
^ Ananikov, Valentine P.; Musaev, Djamaladdin G.; Morokuma, Keiji (2007). "Critical Effect of Phosphane Ligands on the Mechanism of Carbon–Carbon Bond Formation Involving Palladium(II) Complexes: A Theoretical Investigation of Reductive Elimination from Square-Planar and T-Shaped Species". European Journal of Inorganic Chemistry 2007 (34): 5390. doi:10.1002/ejic.200700850.

External links

www.chem.swin.edu.au/modules/mod4/qsarwebp4.html
Steric A Program to Calculate the Steric Size of Molecules

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