Chirality All three-dimensional objects are either symmetric or asymmetric. A symmetric molecule can be superimposed on its mirror image. An asymmetric molecule cannot be superimposed on its mirror image. The nonsuperimposibility characteristic of an object upon its mirror image, a trait common to all asymmetric objects, is called chirality. The term chiral comes from the Latin word chiro or chir that means “hand” or “handedness.” A molecule possesses chirality if it cannot be superimposed on its mirror image. To be chiral, a molecule should lack/ devoid of all elements of symmetry. There are three elements of symmetry: a) Plane of Symmetry,b) Center of symmetry; c) Axis of symmetry A plane of symmetry is an internal mirror plane

Plane of symmetry Other symmetric molecules have only one or two planes of symmetry. Molecules with a C=C or a C=O double bond have at least one and often two planes of symmetry. Because these functional groups are planar, they impart only symmetry, never asymmetry, to a molecule. . Some stereoisomers lack a plane of symmetry. Without a plane of symmetry, mirror image molecules cannot be superimposed on each other. Molecules that have nonsuperimposible mirror images are called optical isomers Optical isomers are molecules that rotate the plane of plane polarised light in opposite directions.

Center of symmetry Molecules can be symmetric about a point instead of a plane. These points, called centers of symmetry, are usually found in cyclic molecules. A center of symmetry is a point in a molecule from which drawing two lines 180o apart will find the same molecular features. To determine whether or not a molecule has a center of symmetry, draw a line from each substituent on the ring through the center of the ring to the substituent opposite it. If the two substituents on either end of each line are identical, then the molecule has a center of symmetry and is symmetric. . An organic molecule without a plane of symmetry or center of symmetry is asymmetric. A characteristic of all asymmetric molecules is that they possess chirality

Polarization of light beam

Restricted Rotation Giving Rise to Perpendicular Disymmetric Planes Certain compounds that do not contain asymmetric atoms are nevertheless chiral because they contain a structure that can be schematically represented as in fig.( substituted Biphenyls) There is no plane of symmetry and the molecule is chiral; many such compounds have been resolved. Isomers that can be separated only because rotation about single bonds is prevented or greatly slowed are called atropisomers.

Allenes as atropisomers In allenes, the central carbon is sp bonded. The remaining two p orbitals are perpendicular to each other and each overlaps with the p orbital of one adjacent carbon atom, forcing the two remaining bonds of each carbon into perpendicular planes Like biphenyls, allenes are chiral only if both sides are unsymmetrically substituted Spiranes

Chirality due to Helical shape Several compounds have been prepared that are chiral because they have a shape that is actually helical and can therefore be left or right handed in orientation .

Creation of a Chiral (Stereogenic) Center Any structural feature of a molecule that gives rise to optical activity may be called a chiral or stereogenic center. In many reactions, a new chiral center is created,

Enantiomers

Stereo centers other than carbon