Notation chiral molecules

Cahn-lngold-Prelog (CIP) system of stereochemical notation chiral molecules, 268—271, 292 priority rules, 173-174, 175 table... 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

Fischer projection A notation used to portray the three-dimensional arrangement of the constituent parts of a chiral molecule. The horizontal lines project forward from the paper, while the vertical lines project backwards. 

The Cahn-Ingold-Prelog R-S notation has been extended to include molecules with a chirality axis. See the article by Mak in the November 2004 issue of the Journal of Chemical Education for a brief discussion of assigning / or Sto chiral molecules that do not contain a chirality center. 

The R and S system is useful in discussing the Fischer projection, which is one of the most important two-dimensional representations of chiral molecules. This type of drawing was initially developed to display the stereochemical relationships among carbohydrates with several chiral centers, but it is also useful in many other applications. In a Fischer projection, molecules are represented by crossing vertical and horizontal lines, with each intersection representing a carbon atom. Horizontal lines represent bonds that would project forward in space if we drew the molecule using perspective notation... 

In all the listed amino acids, with the exception of glycine, the a-carbon is bound to four different substituents hence it is a stereogenic center. From this it follows that every amino acid can appear in the form of two enantiomers. In the following example, both the enantiomers of alanine are represented together with their absolute configurations. However, enantiomers of amino acids can also be represented by the traditional notation of chiral molecules that is called the relative configuration. This nomenclature for configuration was proposed Emil Fischer in the nineteenth century for the representation of the stereochemistry of carbohydrates. 

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