Optically active compounds can rotate the plane of polarised light. A polarimeter is used to determine, experimentally, if a compound is optically active or not.
1. Light source
2. Unpolarized light
3. Polarizer
4. Polarized light
5. Sample tube containing organic molecules
6. 30° optical rotation
7. Movable analyzer
8. Observer
Light is an electromagnetic wave that vibrates. When light vibrates in all directions it is unpolarised light. A polarizer is used to make the light polarised, i.e., the wave vibrates in a single direction. If a molecule is optically active, it rotates the plane of polarised light. A polarimeter helps to calculate the specific rotation of a molecule. This cannot be calculated on paper but only by observing a molecule using a polarimeter.
If a molecule rotates the direction of incident light to the right, it is dextrorotatory (d, +). However, if it rotates the direction of the incident light to the left, it is laevorotatory (l, -). If a mixture contains both laevorotatory as well as dextrorotatory molecules in equal quantity, the mixture is called a racemic mixture (±).
There are two configurations used to denote optical isomers.
D,L Convention
This should not be confused with d,l which denotes the rotation of the plane of polarised light. D, L denotes the spatial arrangement,i.e., relative configuration of the molecule. It is, usually, used to denote carbohydrates and amino acids.
The Fischer projection of the molecule is drawn.
1. The longest carbon chain is drawn vertically. Based on the IUPAC rules the carbon atoms are numbered. The carbon with the lowest number is on the top.
Example, 2-bromobutane
Based on the IUPAC nomenclature the carbons are numbered. The substituent, Bromine, is attached to the second carbon. According to the rule, the carbon with the least number is placed on top.
2. The vertical bonds represent the groups that are away from the observer (Behind the page) while the horizontal bonds represent the groups that are towards the observer (Coming out of the page).
3. Positions of groups in the horizontal bond of the chiral carbons are observed.
4. If the main substituent of the asymmetric carbon is on the left side it is denoted by L, whereas, when it is on the right it is denoted by D.
The second carbon is chiral (carbon attached to four different substituents). The position of Bromine determines the configuration of the molecule. If Bromine is on the left, the molecule is denoted as L and denoted as D if it is on the right.
In carbohydrates, the penultimate carbon is observed. If the -OH group is on the left it is L, else it is D.
In D-glucose, the hydroxyl group attached to the penultimate carbon is on the right and in the case of L-glucose, the hydroxyl group is attached to the left.
The D, L convention led to a bit of confusion in a few molecules so, the R, S convention was devised.
R, S Convention
The R, S convention is used to assign the configuration of a chiral carbon.
In a Dotted line- Wedge notation, there are three types of lines
- Wedges: Chemical bonds are coming out of the page, i.e., towards the observer.
- Dashes: Chemical bonds are behind the page, i.e., away from the observer.
- Solid lines: Used for the backbone of the structure; on the plane of the paper.
In the molecule,
- The OH group is attached to C using a wedge bond, which means the OH group is towards the observer.
- F is attached to C using a dashed bond, which means the F is away from the observer.
- CH₃ and H are attached to C using straight lines which means these groups are on the plane of the paper.
Assigning R, S convention to a compound that has dotted line-wedge notation, the following steps are followed
- The groups are assigned priority based on Cahn Ingold Prelog rules.
- The molecule is rotated in so that the group with the least priority is away from the observer
- An arrow is drawn starting from the first → second → third.
Note- The direction of the arrow has nothing to do with the rotation of plane polarised light.
4. The configuration assigned- clockwise is R whereas anticlockwise is S.
In the above example, Br gets the highest priority because it has the highest atomic number of the four groups attached. It is followed by Cl and CH₃. H has the least priority. When an arrow is drawn, it is clockwise so the configuration is R.
In the Fischer projection of the molecule, the vertical bonds represent the atoms away from the viewer, whereas, the horizontal bonds are towards the viewer. The following are the rules
1. The groups are assigned priority based on Cahn Ingold Prelog rules.
2. An arrow is drawn starting from the first → second → third.
3a. If the least priority group is on the vertical plane the configuration assigned is based on the normal convention, i.e., clockwise is R whereas anticlockwise is S.
For example, lactic acid is taken. The chiral carbon is attached to four different groups they are, -OH, -H, -COOH, CH₃. The -OH group is given high priority due to its high atomic number. Between -COOH and -CH₃, the carboxyl group is given higher priority because the carbon is attached to oxygen. The Methyl group is third while hydrogen is fourth.
In the above structures the group with the least priority, H, is attached to carbon by a horizontal bond.
So normal convention is followed.
3b. If the least priority group is on the horizontal plane the configuration assigned is flipped, i.e., clockwise is S whereas anticlockwise is R.
Lactic acid is taken as an example,
In the above structures the group with the least priority, H, is attached to carbon by a vertical bond. So the convention is flipped.
When the molecule is rotated anticlockwise by 90°,
Check out my other posts to learn more about isomerism
