Polar molecules cannot have a dipole moment perpendicular to any mirror plane or axis of symmetry!

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1 CHEM 2060 Lecture 5: Chirality L5-1 Polar Molecules Polar molecules cannot have a dipole moment perpendicular to any mirror plane or axis of symmetry! A polar molecule is a molecule with a permanent electric dipole moment. Crudely put, it has more electron density at one end than the other. _ δ+ δ e.g. H Cl The dipole is drawn pointing in the direction of greatest electron density.

2 CHEM 2060 Lecture 5: Chirality L5-2 Any molecule with a C n axis and C 2 axis (or σ h plane) cannot have a dipole in any direction. THEREFORE All D point groups are non-polar (e.g. BF 3 ) All cubic groups (I h, O h, T d ) are non-polar. It is harder to define which point group must be polar because the nature of the atoms themselves also comes under consideration. Molecules belonging to C nv point groups can be polar, but the dipole might be so small owing to the nature of the atoms that the molecule is considered non-polar. Example: Both NH 3 and NF 3 are C 3v, but only NH 3 has a significant dipole moment (toward the N atom). The F atoms and the lone pair on the N atom in NF 3 effectively cancel one another out. Dipole Moments are important in bonding, solvation, reactivity, spectroscopy NB: Even though the molecule as a whole is non-polar, the bonds can be polar.

3 CHEM 2060 Lecture 5: Chirality L5-3 Carbon tetrachloride CCl 4 is non-polar, but chloromethane CH 3 Cl is polar. Point group of CH 3 Cl is C 3v The dipole points toward the Cl atom because it is the most electronegative atom AND it has multiple lone pairs. Dipole Moments (µ) are measured in Debyes (D) C 3v CH 3 Cl µ = 1.87 D C 2v H 2 O D 2h C 2 H 4 µ = 1.85 D µ = D HOMEWORK: Draw methylene chloride CH 2 Cl 2 and chloroform CHCl 3. Assign the point groups. Determine the direction of the dipole, if there is one.

4 CHEM 2060 Lecture 5: Chirality L5-4 Chirality from the Greek word χειρ (kheir) meaning hand CHIRAL MOLECULES are distinguishable from their mirror image. ( as are left and right hands) A chiral molecule and its mirror image are called enantiomers of one another. This is an extremely important property, particularly in Biochemistry. Example: Only one of the two possible enantiomers of alanine (the simplest chiral amino acid) exists naturally in multicellular organisms. HOMEWORK: Prove to yourself that the mirror image of L-alanine is different (not superimposable). H H 2 N C CH 3 C O OH

5 CHEM 2060 Lecture 5: Chirality L5-5 Similarly the carbohydrate glyceraldehyde is chiral. Derivatives of it are produced in living organisms only with the chirality shown. In general, L amino aids OH H C O C H CH 2 OH D nucleotides in nucleic acids Every molecule has a mirror image, however some are identical to the original image (superimposable) and some are not (non-superimposable mirror images are enantiomers). There are various ways to judge if a molecule is chiral. In the case of a tetrahedral centre, there must be 4 different groups attached to the central atom.

6 CHEM 2060 Lecture 5: Chirality L5-6 In other cases we can use symmetry. If molecule has S n it is NOT chiral. RECALL: S 1 is a mirror plane σ S 2 is a center of inversion i Molecules with either a mirror plane or a centre of inversion have improperrotation axes and cannot be chiral. Molecules without i and σ are usually chiral, unless they belong to an S 2n point group (which is relatively rare).

7 CHEM 2060 Lecture 5: Chirality L5-7 HOMEWORK (attempt before next tutorial): 1) For both molecules below, determine the point group of the conformation in which it is drawn. Is that conformation chiral? If yes, would you expect the molecule to remain a single enantiomer in solution (why, or why not)? 2) For each of the following species, draw the molecule in a conformation consistent with the specified point group. Which are chiral? O 2 F 2 (in C 2 ) boric acid (C 3h ) boric acid (C 3v ) boric acid (C 3 )

8 CHEM 2060 Lecture 5: Chirality L5-8 Chiral Pharmaceuticals: Thalidomide Thalidomide was sold as a racemate in the late 1950 s early 1960 s for the treatment of morning sickness. One enantiomer is a sedative, however the other causes birth defects. Today, both enantiomers of any pharmaceutical must be tested for efficacy and side-effects. As a result, the ability to separate enantiomers is an important topic in organic chemistry (translation worth $$$ to pharmaceutical industry!) Thalidomide is still used in the treatment of Kahler s disease (a type of white blood cell cancer) and Erythema nodosum leprosum (an inflammatory complication due to leprosy). Obviously, extreme caution is now taken with respect to its use.

9 CHEM 2060 Lecture 5: Chirality L5-9 Examples of symmetry in the real world identify symmetry elements?

10 CHEM 2060 Lecture 5: Chirality L5-10

11 CHEM 2060 Lecture 5: Chirality L5-11

Health Science Chemistry I CHEM-1180 Experiment No. 15 Molecular Models (Revised 05/22/2015)

Health Science Chemistry I CHEM-1180 Experiment No. 15 Molecular Models (Revised 05/22/2015) (Revised 05/22/2015) Introduction In the early 1900s, the chemist G. N. Lewis proposed that bonds between atoms consist of two electrons apiece and that most atoms are able to accommodate eight electrons