Question

1. For each of the following methods of determining whether a molecule is chiral or achiral, describe in detail HOW the method works, and illustrate EACH with an example.

a) Locating and counting the number of chiral centers or stereogenic centers in the molecule.

b) Measuring a sample's optical rotation.

ie:
part A) How can you use the method of locating and counting chiral centers in the molecule to determine if it is chiral or achiral?

part B) How can you use the method of measuring a sample's optical rotation (through an experiment utilizing an optical activity test) to determine if a molecule is chiral or achiral?

Answer 1

Part A: Chirality is an important geometric property relating to a molecule's symmetry. Achiral molecule is non-superimposable with its mirror image. Example: OH OH CHICH3 H COCH нсните Но, сн mirror image of 1 Molecule 1 is not superimposable on its mirror image and, therefore, is chiral. An achiral molecule is superimposable with its mirror image.

Example: HTC CHA CHE mirror image of 3 Molecule 3 is superimposable on its mirror image and, therefore, is achiral. Chiral centers: Chiral centers are tetrahedral atoms (usually carbons) that have four different substituents. Each chiral center in a molecule will be either Ror S.

Molecules with a single chiral center are chiral. Molecules with more than one chiral center are usually chiral. The exception are meso-compounds. For examples, Chiral center Not a chiral center Chiral center

Part B Optical activity: Optical activity is the ability of a chiral molecule to rotate the plane of plane-polairsed light, measured using a polarimeter. Polarimetry A simple polarimeter consists of a light source, polarising lens, sample tube and analysing lens. Light source sample tube polarising filter analyser When light passes through a sample that can rotate plane polarised light, the light appears to dim because it no longer passes straight through the polarising filters. The amount of rotation is quantified as the number of degrees that the analysing lens must be rotated by so that it appears as if no dimming of the light has occurred. When rotation is quantified using a polarimeter it is known as an observed rotation, because rotation is affected by path length (l, the time the light travels through a sample) and concentration (c, how much of the sample is present that will rotate the light).

When these effects are eliminated a standard for comparison of all molecules is obtained, the specific rotation, [a]. [a] = 1002 a = cxl Enantiomers will rotate the plane of polarisation in exactly equal amounts (same magnitude) but in opposite directions. Dextrorotary designated as dor (+), clockwise rotation (to the right) Levorotary designated as lor(-), anti-clockwise rotation to the left)

If only one enantiomer is present a sample is considered to be optically pure. When a sample consists of a mixture of enantiomers, the effect of each enantiomer cancels out, molecule for molecule. For example, a 50:50 mixture of two enantiomers or a racemic mixture will not rotate plane polarised light and is optically inactive. Ifa molecule is optically active it is a chiral molecule while it is inactive it is an achiral compound.