Stereochemistry: Optical Activity of Chiral Molecules Essay

Optical isomers, or enantiomers, have the same sequence of atoms and bonds but are different in their 3D shape. Two enantiomers are non mirror images of one another (i. e. , chiral), with the most common example being our hands. Our left hand is a mirror image of our right, yet there is no way our left thumb can be over our right thumb if our palms are facing the same way and placed over one another.

Optical isomers also have no axis of symmetry, which means that there is no line that bisects he compound such that the left half is a mirror image of the right half. Most physical properties of enantiomers i. e. , melting point, boiling point, refractive index, etc. are identical. However, they differ in a property called optical activity, in which a sample rotates the plane of polarization of a polarized light beam passing through. In a polarimeter, plane-polarized light is introduced to a tube (typically 10 cm in length) containing a solution with the substance to be measured.

If the substance is optical inactive, the plane of the polarized light will not change in orientation and the bserver will read an angle of 00. If the compound in the polarimetry cell were optical active, the plane of the light would be rotated on its way through the tube. The observed rotation is a result of the different components of the plane polarized light interacting differently with the chiral center. The specific rotation [a] depends on the length of the tube, the wavelength that it is used for the acquisition, the concentration of the optical active compound (enantiomer), and to a certain degree on the temperature as well.

However, the temperature effect is very difficult to pecify since it differs for each compound. Generally the following equation is used to calculate the specific optical rotation from experimental data: Normal monochromatic light contains light that possesses oscillations of the electrical field in all possible planes perpendicular to the direction of propagation. When light is passed through a polarizer only light oscillating in one plane will leave the polarizer. This linear polarized light can be described as a superposition of two counter-rotating components, which propagate with different velocities in an optical active medium.

If one component interacts stronger than the other with a chiral molecule, it will slow down and therefore arrive later at the observer. The result is that the plane of the light appears to be rotated. Key words: Optical Rotation: the rotation of the plane of polarization of plane-polarized light by an optically active substance. chemical analysis) for determining the effect of a substance in rotating the plane of polarization of light. Menthanol: a toxic, colorless, volatile flammable liquid alcohol, originally made by distillation from wood and now chiefly by oxidizing methane. Results:

Unknown #102 was a clear liquid with a strong lemon citrus smell. Table 1: Part 1 Unknown Number 102 [a]D : Optical Rotation -2. 72 Temperature oc 23. 60C A Wavelength 589nrn [a]D Limonene -65. 90 Concentration 4. 127no-5vml Molarity 3. 03XIO-4M Calculations Of Part 1: [COD -65. 90 = c c = 4. 127×10-5g,’t-nL M x X = 3. 03X 10-4 M Unknown #310 was a white powder. 310 Mass of Beaker 27. 836g Mass of Unknown 1. 004g -0. 01 Total Volume 25ml Calculations of Part 2: Therefore the closest specific roation to table 1, to identify the unknown, would be (-)- Mannitol because it’s rotation is -0. 30 Discussion: An enantiomer that rotates plane-polarized light in the positive direction, or clockwise, is called dextrorotary [(+), or R], while the enantiomer that rotates the light in the negative direction, or counterclockwise, is called levorotary [(-), or S]. Due to the observations, the value of a is negative (-), or represented R. Optical activity is measured by a polarimeter, and is dependent on several factors: concentration of the sample, temperature, length of the sample tube or cell, and wavelength of the light passing through the sample.

Rotation is given in +1- degrees, depending on whether he sample has R- (positive) or S- (negative) enantiomers. The standard measurement for rotation for a specific chemical compound is called the specific rotation, defined as an angle measured at a path length of 1 decimeter and a concentration of lg/ml. “The olfactory system sensitively discerns scents from many small molecules as the brain analyses signals from nasal receptors. ” These receptors are selective to some degree, though the mechanism for selectivity is still debatable.

Enantiomers, chiral assessing proposed mechanisms. There is a correlation between molecular structural) flexibility and whether or not the left- and right-handed enantiomers smell the same. In particular, for the fairly extensive class of enantiomers with six- membered ring flexibility, enantiomers do not smell the same. There are, of course, significant experimental uncertainties. This is the structure of limonene, and as you can see it is a 6-ring molecule. This molecule is very flexible due to the carbon bonds. That flexibility is what gave the lemon citrus smell.

We describe the flexible molecules containing six-membered rings for which any one of the following three possible structures. The first group of flexible molecules includes those having cis-trans transitions about the six-membered ring. These correspond to molecules where two functional groups lie across the plane of the ring, and where it is feasible that these groups can isomerize by switching between sides of the plane. Second, there are molecules whose flexibility stems from cyclohexane ring twist transitions from chair to boat conformations.

Third, there are those molecules for which cyclohexene ring twists are possible, similar to cyclohexane chair-boat, but more strained. The molecule is flexible because the bond angles are lose to the geometrical angle of a hexagon as well as the limited amount of torsional strain due to staggered conformations of the hydrogen’s bonded to each carbon in the ring. Therefore we can conclude that the enantiomers smell different because the molecules are flexible and interact with different olfactory receptors.

This is why our unknown’s all smell different because each one has it’s own characteristic of either they way its flexible, the way the molecule is formed or it could be anything, because no enantiomers smell exactly the same. Measurement of optical rotation is performed using an instrument called a olarimeter. Here is how it works. Plane-polarized light is first obtained by passing light through a polarizer. If this polarized light is passed through a second filter (analyzer) oriented parallel to the first, the maximum amount of light reaches our eye.

If the orientation of the analyzer is rotated, the amount of light being transmitted declines (i. e. the light dims). The polarimeter initially has the analyzer oriented perpendicularly to the polarizer so that the transmission of light is at a minimum. When an optically active (chiral) material is placed between the polarizer and nalyzer, the polarization vector of the light is rotated as described above, and the amount of light able to pass through the analyzer increases, i. e. t will appear the angle through which the analyzer must be rotated is a measure of how much rotation was produced by the sample solution. From this optical rotation, we calculated the specific rotation using Biots Law. With the specific rotation, and the table 1 in the lab manual (pg L-1 1), we were able to fgure out our unknown giving for part 2. Conclusion: Optical activity is the ability of a chiral molecule to rotate the plane of plane- olarized light. It is measured using a polarimeter, which consists of a light source, polarizing lens, sample tube and analyzing lens.

When light passes through a sample that can rotate plane-polarized light, the light appears to dim to the eye because it no longer passes straight through the polarizing filters. The amount of rotation is quantified as the number of degrees that the analyzing 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 s affected by path length (l, the distance 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]. Using these we were able to fgure out the unknown’s, the specific rotations, and optical rotation.