WordNet

of or belonging to neither the right nor the left politically or intellectually
a building dedicated to a particular activity; "they were raising money to build a new center for research" (同)centre
(American football) the position of the player on the line of scrimmage who puts the ball in play; "it is a centers responsibility to get the football to the quarterback"
a position on a basketball team of the player who participates in the jump that starts the game
the position on a hockey team of the player who participates in the face off at the beginning of the game
a place where some particular activity is concentrated; "they received messages from several centers" (同)centre
the sweet central portion of a piece of candy that is enclosed in chocolate or some other covering (同)centre
the object upon which interest and attention focuses; "his stories made him the center of the party" (同)centre, center of attention, centre of attention
an area that is approximately central within some larger region; "it is in the center of town"; "they ran forward into the heart of the struggle"; "they were in the eye of the storm" (同)centre, middle, heart, eye
a point equidistant from the ends of a line or the extremities of a figure (同)centre, midpoint
a cluster of nerve cells governing a specific bodily process; "in most people the speech center is in the left hemisphere" (同)centre, nerve_center, nerve centre
(football) the person who plays center on the line of scrimmage and snaps the ball to the quarterback; "the center fumbled the handoff" (同)snapper
(basketball) the person who plays center on a basketball team
(ice hockey) the person who plays center on a hockey team
politically moderate persons; centrists
the middle of a military or naval formation; "they had to reinforce the center"
move into the center; "That vase in the picture is not centered" (同)centre
equally distant from the extremes (同)halfway, middle, midway
a fractional monetary unit of several countries
(American football) putting the ball in play by passing it (between the legs) to a back; "the quarterback fumbled the snap" (同)snap
being or placed in the center

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〈C〉『セント』(1ドルの1/100;米国・カナダの貨幣単位;《略》￠) / 〈C〉1セント銅貨 / 〈U〉(単位としての)100
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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/07/07 21:28:08」(JST)

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A stereocenter or stereogenic center is an atom bearing groups such that an interchanging of any two groups leads to a stereoisomer.^[1]

A stereocenter consisting of an atom holding a set of ligands (atoms or groups of atoms) in a spatial arrangement which is not superposable on its mirror image. A chiral center is a generalized extension of an asymmetric carbon atom, which is a carbon atom bonded to four different entities, such that an interchanging of any two groups gives rise to an enantiomer. ^[2] In organic chemistry a chiral center usually refers to a carbon, phosphorus, or sulfur atom, though it is also possible for other atoms to be chiral centers in organic and inorganic chemistry.

The term stereocenter was introduced in 1984 by Mislow and Siegel.^[3]

Possible number of stereoisomers

A molecule can have multiple stereocenters, giving it many stereoisomers. In compounds whose stereoisomerism is due to tetrahedral stereogenic centers, the total number of hypothetically possible stereoisomers will not exceed 2ⁿ, where n is the number of tetrahedral stereocenters. Molecules with symmetry frequently have fewer than the maximum possible number of stereoisomers.

Having two chiral centers may give a Rahul compound which is achiral. Certain configurations may not exist due to steric reasons. Cyclic compounds with chiral centers may not exhibit chirality due to the presence of a two-fold rotation axis. Planar chirality may also provide for chirality without having an actual chiral center present.

Chiral carbon

A chiral carbon or asymmetric carbon is a carbon atom which is asymmetric. Most chiral molecules have a chiral carbon, though the presence of a chiral carbon does not necessarily make a molecule chiral (see meso compound). A chiral carbon is often denoted by C*.

If the carbon is chiral, it follows that:

the carbon atom is sp³-hybridized;
there are four different groups attached to the carbon atom.

Almost any other configuration for the carbon would produce a center of symmetry. For example, an sp- or sp²-hybridized molecule would be planar, with a mirror plane. Two identical groups would give a mirror plane bisecting the molecule.

Other chiral centers

Chirality is not limited to carbon atoms, though carbon atoms are often centers of chirality due to their ubiquity in organic chemistry.

Nitrogen and phosphorus atoms are also tetrahedral. Racemization by Walden inversion may be restricted (such as ammonium or phosphonium cations), or slow. This allows the presence of chirality.

Metal atoms with tetrahedral or octahedral geometries may also be chiral due to having different ligands. For the octahedral case, several chiralities are possible. Having three ligands of two types, the ligands may be lined up along the meridian, giving the mer-isomer, or forming a face — the fac isomer. Having three bidentate ligands of only one type gives a propeller-type structure, with two different enantiomers denoted Λ and Δ.

References

^ Solomons & Fryhle. (2004). Organic Chemistry, 8th ed
^ IUPAC-definition of chirality center
^ Stereoisomerism and local chirality Kurt Mislow and Jay Siegel J. Am. Chem. Soc.; 1984; 106(11) pp 3319 - 3328; doi:10.1021/ja00323a043

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English Journal

The generation of stationary π-electron rotations in chiral aromatic ring molecules possessing non-degenerate excited states.

Yamaki M1, Teranishi Y, Nakamura H, Lin SH, Fujimura Y.
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Enantioseparation and determination of the chiral phenylpyrazole insecticide ethiprole in agricultural and environmental samples and its enantioselective degradation in soil.

Zhang Q1, Shi H1, Gao B1, Tian M1, Hua X1, Wang M2.
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An effective method for the enantioselective determination of ethiprole enantiomers in agricultural and environmental samples was developed. The effects of solvent extraction, mobile phase and thermodynamic parameters for chiral recognition were fully investigated. Complete enantioseparation of the
PMID 26556749

Seasonal characteristics and current sources of OCPs and PCBs and enantiomeric signatures of chiral OCPs in the atmosphere of Vietnam.

Wang W1, Wang Y2, Zhang R3, Wang S3, Wei C3, Chaemfa C4, Li J4, Zhang G4, Yu K5.
The Science of the total environment.Sci Total Environ.2016 Jan 15;542(Pt A):777-86. doi: 10.1016/j.scitotenv.2015.10.129. Epub 2015 Nov 7.
Passive air samplers (PAS) were deployed concurrently at 15 locations (nine urban sites and six rural sites) in Vietnam and exposed for approximately 6weeks from June 26, 2012 to August 26, 2012 and from December 8, 2012 to February 8, 2013. The concentration, composition and enantiomeric signatures
PMID 26551278