Lecture 2

Importance of chirality: Many drug molecules are chiral. However they are sold as racemates. In most cases only one enantiomer is the active agent. The other enantiomer is usually inactive while in some cases they may be toxic. Example: anti inflammatory agent R isomer inactive S isomer is active R isomer inactive (non antiinflammatory) as well as nephrotoxic

Methyldopa (antihypertensive) (S – isomer is active) These drugs are what are called synthetic drugs If the drug is a natural product, it is available in chiral form. Chiral Drugs Drugs need to be sold in the form which is pharmacologically active. Thalidomide Two forms : (+) & (-) Or R/S One form used as a drug Other form is a teratogen It was sold in the racemic form. Teratogen molecules cause birth defect. The drug was withdrawn. Importance of chirality was demonstrated through this painful experience

Q. Which molecule is chiral? Ans. Molecule having a nonsuperimposable mirror image. Q. How do I know whether or not a molecule has a nonsuperimposable mirror image? Ans. One can check the symmetry elements of the molecule and then come out with a conclusion. Q. What are SYMMETRY ELEMENTS? Ans. These are elements that may be present in a molecule and give an idea how symmetrical a molecule is. Q. How many symmetry elements a molecule can possess? Ans. Four a) simple axis [C n ] b) plane [ σ]. c) centre [I] d) alternating axis [S n ].

Simple axis of Symmetry (C n ): A molecule has a n fold simple axis of symmetry if it is rotated by an angle of /n around the axis, you obtain a same looking molecule. Plane of symmetry (σ ): An imaginary mirror plane dividing the molecule into two halves such that one half is the mirror image of the other half. Centre of symmetry (i): An imaginary point ….if any substituent is joined by a line and if the line is extended in opposite direction, you obtain the same substituent at equal distance. This is true for every substituent present in the molecule.

Alternating axis of symmetry (S n ) A molecule rotated around an axis by /n an image taken through a mirror placed perpendicular to the axis the mirror image looks identical with the original.

When a molecule is devoid of all the symmetry elements, it is called “asymmetric”. When a molecule is devoid of , I and S n, it is called “dissymmetric” If a molecule is devoid of S n, it is dissymmetric and hence “chiral” Dissymmetry is the minimum condition for optical activity Since  = S 1 and i = S 2 THUS

Newman projection formula: In writing such a formula we imagine ourselves viewing the molecule from one end directly along the carbon – carbon bond axis. The bonds of the front carbon atom are represented as (a) and those of the back carbon as (b) For n-butane: Energy profile:

Conformational Analysis What are conformations? Conformations of straight chain alkanes Conformations of cycloalkanes. The study of energy changes that occur in a molecule when a group or atoms rotate around single bonds is called conformational analysis. Conformations: The infinite no. of arrangements obtained by rotation around single bond e.g. C - C Conformers: The conformations lying in the energy minima are called conformers. Each point represents a conformations but X,Y, Z are the only conformers

Q. Why is gauche from less stable than anti/ staggered from? Ans: Mainly because of torsional strain Q. What is torsional strain? Ans: the strain imposed in a molecule when it is rotated from its stable state. In n-butane Torsional strain highest in the fully eclipsed form, followed by partially eclipsed and then gauche form. In the eclipsed forms, there is also van der Walls repulsive force (steric strain). Difference in energy between anti and gauche form = ~ 0.9 Kcal / mole