1. Nat. Sci. 104 – Organic Chem. Rosel I. Labrador Enantiomer and Diastereomers

2. Enantiomers In chemistry, an enantiomer, from Greek, Modern (enantios) meaning ‘opposite’and (meros), meaning ‘part’, also known as an optical isomer (and archaically termed antipode or optical antipode), is one of two stereoisomers that are non-superimposable (not identical), much as one’s left and right hands have the same shape except for being reversed along one axis (the hands cannot be made appear identical simply reorientation).

3. Enantiomers are stereoisomers that are non- superimposable mirror images. A molecule with 1 chiral carbon atom exists as 2 stereoisomers termed enantiomers (see the example below). Enantiomers differ in their configuration (R or S) at the stereogenic center. Configuration is assigned by the Cahn-Ingold- Prelog (CIP) rules..

7. Sequence rules to assign priorities to substituents Before applying the R and S nomenclature to a stereocenter, the substituents must be prioritized according to the following rules: Rule 1 First, examine at the atoms directly attached to the stereocenter of the compound. A substituent with a higher atomic number takes precedence over a substituent with a lower atomic number. Hydrogen is the lowest possible priority substituent, because it has the lowest atomic number. 1.When dealing with isotopes, the atom with the higher atomic mass receives higher priority. 2.When visualizing the molecule, the lowest priority substituent should always point away from the viewer (a dashed line indicates this). To understand how this works or looks, imagine that a clock and a pole. Attach the pole to the back of the clock, so that when when looking at the face of the clock the pole points away from the viewer in the same way the lowest priority substituent should point away. 3.Then, draw an arrow from the highest priority atom to the 2nd highest priority atom to the 3rd highest priority atom. Because the 4th highest priority atom is placed in the back, the arrow should appear like it is going across the face of a clock. If it is going clockwise, then it is an R-enantiomer; If it is going counter clockwise, it is an S-enantiomer.

8. Rule 2 If there are two substituents with equal rank, proceed along the two substituent chains until there is a point of difference. First, determine which of the chains has the first connection to an atom with the highest priority (the highest atomic number). That chain has the higher priority. I f the chains are similar, proceed down the chain, until a point of difference. For example: an ethyl substituent takes priority over a methyl substituent. At the connectivity of the stereo center, both have a carbon atom, which are equal in rank. Going down the chains, a methyl has only has hydrogen atoms attached to it, whereas the ethyl has another carbon atom. The carbon atom on the ethyl is the first point of difference and has a higher atomic number than hydrogen; therefore the ethyl takes priority over the methyl.

9. Rule 3 If a chain is connected to the same kind of atom twice or three times, check to see if the atom it is connected to has a greater atomic number than any of the atoms that the competing chain is connected to. If none of the atoms connected to the competing chain(s) at the same point has a greater atomic number: the chain bonded to the same atom multiple times has the greater priority If however, one of the atoms connected to the competing chain has a higher atomic number: that chain has the higher priority. A 1-methylethyl substituent takes precedence over an ethyl substituent. Connected to the first carbon atom, ethyl only has one other carbon, whereas the 1-methylethyl has two carbon atoms attached to the first; this is the first point of difference. Therefore, 1-methylethyl ranks higher in priority than ethyl, as shown below: However:

10. Properties of Enantiomers Achiral Properties Any pair of enantiomers are physically and chemically indistinguishable by most techniques in achiral environments. Enantiomers have identical achiral properties such as:  melting point,  boiling point,  density,  solubility in water,  spectroscopic properties (NMR, IR, UV)  same rate of reaction with achiral reagents

11. Some Chiral Organic Molecules There are a number of important biomolecules that could occur as enantiomers, including amino acids and sugars. In most cases, only one enantiomer occurs (although some fungi, for example, are able to produce mirror-image forms of these compounds). We will look later at some of these biomolecules, but first we will look at a compound that occurs naturally in both enantiomer forms. Carvone is a secondary metabolite. That means it is a naturally-occurring compound that is not directly connected to the very basic functions of a cell, such as self-replication or the production of energy. The role of secondary metabolites in nature is often difficult to determine. However, these compounds often play roles in self-defense, acting as deterrents against competitor species in a sort of small-scale chemical warfare scenario. They are also frequently used in communications; this role has been studied most extensively among insects, which use lots of compounds to send information to each other.

12. Problem SC5.1. 1.Draw the two enantiomeric forms of 2- butanol, CH 3 CH(OH)CH 2 CH 3. Label their configurations. 2. Draw the two enantiomeric forms of (R)-2-Bromobutane (S)-2,3- Dihydroxypropanal

13. Diastereomers Diastereomers are stereoisomers that are not related as object and mirror image and are not enantiomers. Unlike enatiomers which are mirror images of each other and non- sumperimposable, diastereomers are not mirror images of each other and non-superimposable. Diastereomers can have different physical properties and reactivity. They have different melting points and boiling points and different densities. They have two or more stereocenters.enantiomers

14. It is easy to mistake between diasteromers and enantiomers. For example, we have four steroisomers of 3- bromo-2-butanol. The four possible combination are SS, RR, SR and RS (Figure 1). One of the molecule is the enantiomer of its mirror image molecule and diasteromer of each of the other two molecule (SS is enantiomer of RR and diasteromer of RS and SR). SS's mirror image is RR and they are not superimposable, so they are enantiomers. RS and SR are not mirror image of SS and are not superimposable to each other, so they are diasteromers.

15. Diastereomers vs. Enantiomers vs. Meso Compounds Tartaric acid, C 4 H 6 O 6, is an organic compound that can be found in grape, bananas, and in wine. The structures of tartaric acid itself is really interesting. Naturally, it is in the form of (R,R) stereocenters. Artificially, it can be in the meso form (R,S), which is achiral. R,R tartaric acid is enantiomer to is mirror image which is S,S tartaric acid and diasteromers to meso-tartaric acid (figure 2). (R,R) and (S,S) tartaric acid have similar physical properties and reactivity. However, meso-tartaric acid have different physical properties and reactivity. For example, melting point of (R,R) & (S,S) tartaric is about 170 degree Celsius, and melting point of meso-tartaric acid is about 145 degree Celsius.

16. Problems Identify which of the following pair is enantiomers, diastereomers or meso compounds.

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