Le Bel

Chirality (handedness) is older than life on tliis planet. Still it was not until 1848 when Pasteur manually separated enantiomeric crystals that chirality in chemistry was first appreciated ". The independent work of Van t Hoff and Le Bel revealed the molecirlar origin behind this phenomenon. 

Stereochemistry refers to chemistry in three dimensions Its foundations were laid by Jacobus van t Hoff and Joseph Achille Le Bel m 1874 Van t Hoff and Le Bel mde pendently proposed that the four bonds to carbon were directed toward the corners of a tetrahedron One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms m space IS different Isomers that have the same constitution but differ m the spatial arrangement of their atoms are called stereoisomers We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns m alkenes and m cycloalkanes... 

The experimental facts that led van t Hoff and Le Bel to propose that molecules having the same constitution could differ m the arrangement of their atoms m space concerned the physical property of optical activity Optical activity is the ability of a chiral sub stance to rotate the plane of plane polarized light and is measured using an instrument called a polarimeter (Figure 7 5)... 

Although Pasteur was unable to provide a structural explanation—that had to wait for van t Hoff and Le Bel a quarter of a century later—he correctly deduced that the enantiomeric quality of the crystals was the result of enantiomeric molecules The rare form of tartanc acid was optically inactive because it contained equal amounts of (+) tartaric acid and (—) tartaric acid It had earlier been called racemic acid (from Latin racemus meaning a bunch of grapes ) a name that subsequently gave rise to our pres ent term for an equal mixture of enantiomers... 

J. A. Le Bel, whose name is often also associated with this concept, did indeed independently suggest a 3-diitiensional modet for the 4-coordinate C atom, but vigorously opposed the tetrahedral stereochemistry of van t Hoff for many years and favoured an alternative square pyramidal arrangement of the bonds. 

The polarimeter is an instrument with which the essential oil chemist cannot possibly dispense. The hypothesis, first seriously enunciated by Le Bel and van t Hoff, that substances which contained an asymmetric carbon atom i.e. a carbon atom directly united to four different atoms or radicles) were capable of rotating the plane of polarisation of a beam of polarised light, has now become a fundamental theory of organic chemistry-. The majority of essential oils contain one or more components containing such a carbon atom, and so possess the power of effecting this rotation. In general, the extent to which a given oil can produce this effect is fairly constant, so that it can be used, within limits, as a criterion of the purity or otherwise of the oil. 

Although Kekule and Couper were correct in describing the tetravalent nature of carbon, chemistry was still viewed in a two-dimensional way until 1874. In that year, Jacobus van t Hoff and Joseph Le Bel added a third dimension to our ideas about organic compounds when they proposed that the four bonds of carbon are not oriented randomly but have specific spatial directions. Van t Hoff went even further and suggested that the four atoms to... 

Joseph Achille Le Bel (IB47-1930I was born in Pechelbronn, France, and studied at the Ecole Polytechnique and the Sorbonne in Paris. Freed by his family s wealth from the need to earn a living, he established his own private laboratory. 

LDA, see Lithium diisopropylamide LDL, heart disease and, 1090-1091 Le Bel, Joseph Achille, 7-8 Leading strand, DNA replication and, 1107... 

The results shown in the table provide further evidence of the extraordinary extent to which the tetrahedral carbon atom of van t Hoff and Le Bel determines the structure of organic molecules. 

Methyl parathion has been reported in groundwater in Idaho at a median level of 0.01 ppb with contamination due to a point source (EPA 1988c). A study of tap water in Ontario showed no detectable methyl parathion at a detection limit of 1 ng/L (Le Bel et al. 1979). 

Le Bel GL, Williams DT, Griffith G, et al. 1979. Isolation and concentration of organophosphorus pesticides from drinking water at the ng/L level, using macroreticular resin. J AOAC 62 241-249. 

The method of choice for the determination of a- and P-endosulfan in blood, urine, liver, kidney, brain, and adipose tissue is gas chromatography equipped with an electron capture detector (GC/ECD) (Coutselinis et al. 1976 Demeter and Heyndrickx 1979 Demeter et al. 1977 Le Bel and Williams 1986). This is because GC/ECD is relatively inexpensive, simple to operate, and offers a high sensitivity for halogens (Griffith and Blanke 1974). After fractionation of adipose tissue extracts using gel permeation chromatography, detection limits of low-ppb (1.2 ng/g) were achieved for endosulfan and other chlorinated pesticides using GC/ECD (Le Bel and Williams 1986). 

GC/MS has been employed by Demeter et al. (1978) to quantitatively detect low-ppb levels of a- and P-endosulfan in human serum, urine, and liver. This technique could not separate a- and P-isomers, and limited sensitivity confined its use to toxicological analysis following exposures to high levels of endosulfan. More recently, Le Bel and Williams (1986) and Williams et al. (1988) employed GC/MS to confirm qualitatively the presence of a-endosulfan in adipose tissue previously analyzed quantitatively by GC/ECD. These studies indicate that GC/MS is not as sensitive as GC/ECD. Mariani et al. (1995) have used GC in conjunction with negative ion chemical ionization mass spectrometry to determine alpha- and beta-endosulfan in plasma and brain samples with limits of detection reported to be 5 ppb in each matrix. Details of commonly used analytical methods for several types of biological media are presented in Table 6-1. 

Le Bel GL, Williams DT. 1986. Determination of halogenated contaminants in human adipose tissue. 

Williams DT, Le Bel GL, Junkins E. 1988. Organohalogen residues in human adipose autopsy samples from six Ontario municipalities. J Assoc Off Anal Chem 71 410-414. 

H. van t Hoff (Dutch scientist) proposed a tetrahedral structure for carbon atom in September of 1874. J. A. Le Bel (French scientist) published the same idea independently in November of 1874. 

Within this historical setting, the actual birth of stereochemistry can be dated to independent publications by J. H. van t Hoff and J. A. Le Bel within a few months of each other in 1874. Both scientists suggested a three-dimensional orientation of atoms based on two central assumptions. They assumed that the four bonds attached to a carbon atom were oriented tetrahedrally and that there was a correlation between the spatial arrangement of the four bonds and the properties of molecules, van t Hoff and Le Bell proposed that the tetrahedral model for carbon was the cause of molecular dissymmetry and optical rotation. By arguing that optical activity in a substance was an indication of molecular chirality, they laid the foundation for the study of intramolecular and intermolecular chirality. 

In 1874, varft Hoff [26] and Le Bel [27] independently surmised that tetracoordinate carbon is surrounded by substituents in a tetrahedral geometry. This perception marked the very beginning of modern organic chemistry, which is increasingly being determined by stereochemical argumentation. Some time ago, attempts were made to synthesize stable planar tetracoordinate carbon compounds [28—30]. 

The reaction of the trimethylphosphane-stabilized bis(trimethyl)silyl zirconocene 66 (Rj = R2 = SiMe3) with (HBEt2)2 not only gives the anti van t Hoff/Le Bel compound 69, but also the dimeric species Cp[g-(r 1 T 5-C5H4)]ZrC(SiMe3)=C(H)(SiMe3) 2, which... 

Institut Le Bel, Laboratoire de Cristallochimie et de Chimie Structurale, 4, rue Blaise Pascal, F-67070 Strasbourg Cedex... 

Hoff- Le Bel theory ofthe asymmetric carbon that the use ofthis theory seems justifiable. 

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