Internal compensation

Tartaric acid [526-83-0] (2,3-dihydroxybutanedioic acid, 2,3-dihydroxysuccinic acid), C H O, is a dihydroxy dicarboxyhc acid with two chiral centers. It exists as the dextro- and levorotatory acid the meso form (which is inactive owing to internal compensation), and the racemic mixture (which is commonly known as racemic acid). The commercial product in the United States is the natural, dextrorotatory form, (R-R, R )-tartaric acid (L(+)-tartaric acid) [87-69-4]. This enantiomer occurs in grapes as its acid potassium salt (cream of tartar). In the fermentation of wine (qv), this salt forms deposits in the vats free crystallized tartaric acid was first obtained from such fermentation residues by Scheele in 1769. 

It has been shown that the polarizability of a substance containing no dipoles will indicate the strength o/any dispersive interactions that might take place with another molecule. In comparison, due to self-association or internal compensation that can take place with polar materials, the dipole moment determined from bulk dielectric constant measurements will often not give a true indication of the strength of any polar interaction that might take place with another molecule. An impression of a dipole-dipole interaction is depicted in Figure 11. 

Tropine and -tropine are mutually convertible as already described. Mixtures of tropine and -tropine can be separated by means of the picrates, that of i/i-tropine being the more soluble in water (1-48 per cent, in water at 16°). By the action of sodium amyloxide on tropine, Willstatter has sliown that 4-tropinc is produced and this has been confirmed by Barrowcliff and Tutin, who also support Willstabter s view that both bases are internally compensated, the relation being that of cw-frans-isomerism. The synthesis of 4-tropine has been described already (p. 77). 

Meso compound A symmetrical compound containing two chiral centres configured so that the chirality of one of the centres is equal and opposite to the other. Such internal compensation means that these compounds have no overall effect on polarised light (e.g., meso tartaric acid). 

This, however, is not the only way multilayers of polymers can be formed. If the macromolecule is a compactly folded copolymer, such as a protein, its collapsed state will be determined by internal compensation of energetically favorable interactions. 

The chemical and structural features of the membrane and cell wall are extensively discussed elsewhere in this volume (see Chapters 2, 6, 7 and 10). They usually contain numerous charged groups, which, as far as they are not internally compensated by counterions, give rise to the formation of an electric double layer at the interphase. The net charge of membrane surface plus cell wall is counterbalanced by a diffuse charge with opposite sign. This so-called diffuse... 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

The meso form is inactive although it has two asymmetric carbon atoms. This is because it has a plane of symmetry and the molecule is internally compensated i.e., the rotation of the upper half is balanced by the rotation of the lower half. 

We have already seen that mesotartaric acid is optically inactive because of internal compensation, although, it contains two asymmetric carbon atoms. We have also seen that the molecule as a whole must be asymmetric for being optically active. Therefore, the best criterion to judge optical activity would be whether molecule is superimposable on its mirror image or not. Now superimposability would lead to optical activity and vice-versa and non-symmetrical molecules are non superimposable. To decide whether a molecule is symmetrical or not, we should first try to know whether it has a plane of symmetry, a centre of symmetry or an alternating axis of symmetry. The presence of any one of these would lead the molecule to be symmetrical and hence to optical inactivity. 

Molecules of inheient structural asymmetry aie anisotropic they are optically active and exhibit optical rotation in solution. The typical optically active center is a carbon atom with four different substituents. In addition, any structural dissymmetry that results in a spatial left- and right-handedness will cause optical activity. Compounds of these types of come in a right-hand l R) and left-hand (L) form. When equal amounts of these two forms are mixed (racemic mixtures) there is no optical rotation because the activity of the two forms exactly cancel. Internal compensation of optically active centers m complex molecules is also found. Left- and right-handed optical isomers were first studied by Pasteur well over 100 years ago. and extensive surveys are found in most organic chemical texts. 

Because of the high resistance of the glass membrane (10 to 100 MO), it is not practical to measure the emf directly. Instead, pH meters either use a direct-reading electronic voltmeter or electronically amplify the small current that flows through the cell and detect the voltage drop across a standard resistor potentiometrically. Both battery-operated and ac line-operated pH meters are available connnercially from such firms as Beckman Coulter, Thermo Orion, and Coming. Such pH meters are calibrated to read directly in pH units, have internal compensation for the temperature coefficient of emf, and have provision for scale adjustments. 

M] or [rh] Molecular rotation, defined as [a] x MW/100. Specific rotation corrected for differences in MW. The symbol [M] and the term molecular rotation are now deemed incorrect, and the term molar rotation denoted by [d ] is preferred. meso- Denotes an internally compensated diastereoisomer of a chiral compound having an even number of chiral centres, e.g., me o-tartaric acid. Formally defined as an achiral member of a set of diastereomers that also contains chiral members, mutarotation Phenomenon shown by some substances, especially sugars, in which the optical activity changes with time. A correct presentation is, e.g., [a]n ° + 20.3 -101.2 (2h)(c, 1.2 in HjO). 

Decomp 260-261°. d-Cystine Crystals. Sidy sol in water. dl-Cystine, the synthetic racemic form Crystals. Sltly sol in water. meso-Cystine, the internally compensated form Crystals. Sltly sol in water. 

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