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In chemistry, a molecule is chiral if is not superimposable on its mirror image regardless of how it is contorted. Your hands are also chiral - mirror images of one another and non-superimposable - and chiral molecules are often described as being 'left handed' or 'right-handed'.
The study of chirality falls in the domain of stereochemistry. The two non-superimposable, mirror-image forms of chiral molecules are referred to as enantiomers. Chiral compounds exhibit optical activity so enantiomers also sometimes called optical isomers. A 50/50 mixture of the two enantiomers of a chiral compound is called a racemic mixture and does not exhibit optical activity. Chiral molecules are sometimes referred to as being "dissymmetric"; chirality and dissymmetry being one in the same.
In more technical terms, the symmetry of a molecule (or any other object) determines whether it is chiral or not. A molecule is achiral (that is, not chiral) if and only if it has an axis of improper rotation, that is, an n-fold rotation (rotation by 360°/n) followed by a reflection in the plane perpendicular to this axis which maps the molecule on to itself. Thus a molecule is chiral if and only if it lacks an improper rotation axis. They are not necessarily asymmetric (i.e. without symmetry), because they can have other types of symmetry, for example rotational symmetry. However, all naturally-occurring amino acids (except glycine) and many sugars are indeed asymmetric as well as chiral. Chirality may also be defined in mathematical terms.
Chirality is of central importance in chemistry and unites the traditionally-defined subdisciplines of inorganic chemistry, organic chemistry and physical chemistry. Many biologically active molecules are chiral such as amino acids (the building blocks of proteins), sugars (e.g., deoxy-ribose which, when combined with nucleic acids, form the building blocks of DNA), and vitamins. Interestingly, all these compounds are homochiral, that is all amino acids are left-handed and all sugars are right-handed. The origin of homochirality in the biological world is the subject of vigorous debate. Many coordination compounds are also chiral, for example the well-known [Ru(2,2'-bipyridine)3]2+ complex in which the bipyridine ligands adopt a propeller-like arrangement.
Enzymes, which themselves are always chiral, often distinguish between the two enantiomers of a chiral substrate. This can be visualised in everday terms by imagining the enzymes to have three-dimensional glove shaped cavities which bind these substrates. If this "glove" is right-handed, then right-handed molecules will fit inside snugly and thus be bound tightly. On the other hand, left-handed molecules won't fit well - just like putting your left hand into a right-handed glove. Although this is an oversimplification of the recognition process, i.e., enzyme cavities are not really "glove shaped," this is a good illustration of a more general point: chiral objects have different interactions with the two enantiomers of other chiral objects (whether they be molecules or hands!).
Other biological processes may be triggered by only one of the two possible enantiomers of a chiral molecule, often being unresponsive to the other enantiomer. For example, S-carvone ("left-handed") is the flavor of carraway , while R-carvone ("right-handed") is the flavor of spearmint. Many chiral drugs must be made with extremely high enantiomeric purity due to toxic activity of the 'wrong' enantiomer. An example of this is thalidomide which is racemic - that is, it contains both left and right handed isomers in equal amounts: one enantiomer is effective against morning sickness, and the other is teratogenic. It should be noted that the enantiomers are converted to each other in vivo. That is, if a human is given D-thalidomide or L-thalidomide, both isomers can be found in the serum. Hence, administering only one enantiomer will not prevent the teratogenic effect in humans.
Most commonly chiral molecules have point chirality which centers around a single asymmetric atom (usually a carbon atom). This is the case for chiral amino acids where the alpha carbon atom is the stereogenic centre, having point chirality. A molecule can have multiple chiral centers without being chiral overall if there is a symmetry element (mirror plane or inversion center) which relates those chiral centers. Such compounds are referred to as meso compounds. It is also possible for a molecule to be chiral without any specific chiral centers in the molecule. Examples include 2,2'-binaphthol (BINOL) and 1,3-dichloro-allene which have planar chirality. The [Ru(2,2'-bipyridine)3]2+ complex above — this is an example of a chiral molecule that actually has quite high symmetry. It belongs to the symmetry point group D3., meaning it has one three-fold rotational symmetry axis and three perpendicular two-fold axes. In this case, the Ru atom may be regarded as a stereogenic centre with the complex having point chirality.
One must make a clear distinction between conformation and configuration when discussing chirality in a molecular context. Conformations are temporary positions atoms in a molecule can assume as a result of bond rotation, bending, or stretching as long as a bond is not broken to change the positions of the atoms in a molecule. Configurations are structures of a molecule which are assumed to not be interconvertible under ambient conditions. Enantiomers, and other optically active isomers such as diastereomers, are examples of configurational isomers.
Ernest L. Eliel and Samuel H. Wilen, 1994. The Sterochemistry of Organic Compounds, Wiley-Interscience.
Alex von Zelewsky, 1996. Stereochemistry of Coordination Compounds, Wiley.
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