5.6: Compounds with more than one chiral center (2023)

5.6.1 Diastereomero

this is very commonmore than one chiral center inan organic compound. For the 2-bromo-3-chlorobutane example below, there are 2 chiral centers, C2 and C3. Each chiral center has two possible configurations,RYS, the total number of possibleStereoisomers for this compoundis four, with configurations in C2 and C3 according toRR,SS,RSYRSo.

Usually applies to a connectionnortechirality centersmaximumThe number of stereoisomers for this compound is 2.^norte.

The four stereoisomers of 2-bromo-3-chlorobutane consist of two pairs of enantiomers. stereoisomersAYBare a pair of non-superimposable mirror images, that is, enantiomers. Likewise, the isomersCYD. So what is the relationship between the isomerAYC?

AYCthey are not identical, they are not enantiomers, and they are stereoisomers (they have the same bond but differ in the spatial arrangement of the groups). These stereoisomers are defined asdiastereomero.diastereomeroThey are stereoisomers that are not enantiomers. For the four stereoisomers here, there are four pairs of diastereomers:AYC,AYD,BYC,BYD. The relationship between the four stereoisomers can be summarized as follows:

With the introduction of the diastereomer concept, the isomer categorization and summary can be reviewed inTissue. 5.1acan be replaced byupdated version5.6a. The stereoisomer then has two subtypes, enantiomers and diastereomers, becauseAll stereoisomers that are not enantiomers can always be called diastereomers.. According to such a definition, the geometric isomers that we know earlier also belong to the category of diastereomers.

As already mentioned, enantiomers are very similar to each other and have the same physical properties, except for optical activity (opposite sign for specific rotation). Enantiomers generally also have the same chemical properties, except for reaction with other chiral reagents (fear not in this course).

However, the diastereomers are not as closely related. Diastereoisomers have different physical properties, for example, different Kp, color, density, polarity, solubility, etc. They also have different chemical properties.

Next, we'll look at the examples of cyclic compounds to see how the new concept of a diastereoisomer relates to the knowledge about cyclic compounds we learned earlier.

Getting closer:

There are two chiral centers for the 1-bromo-2-chlorocyclobutane molecule. Therefore, the maximum number of stereoisomers is four. To work with the stereoisomers of cyclic compounds, we can start with the cis/trans isomer and then check if the enantiomer is correct for each case.

Solution:

There's twocis- Leelo,AYB, and are enantiomers of each other; likewise there are also twotrans-Leelo See MoreCYDwhich are also enantiomers of each other.

The relationship betweenanyofcis- isomer aanyoftrans-isomer isDiastereoisomer (AYC, AYre, siYC,BYD). Since these are geometric isomers, remember that geometric isomers can also be called diastereomers.

All geometric isomers are diastereomers (it is always correct to refer to a pair of geometric isomers as diastereomers), but not all diastereomers are geometric isomers!

5.6.2 Meso compound

Next, we see another example of a compound that contains two centers of chirality, 2,3-dichlorobutane, the compound that has the same substituents on carbons C2 and C3.

theoretically existsmaximumfour stereoisomers, the structures are shown here using Fisher projections.

stereoisomerAYBare non-superimposable mirror images, that is, enantiomers.

Let's take a detailed look at the stereoisomer.CYD. Yes, they are mirror images, but are they really non-superimposable? if it is isomericCrotated 180° (a rotation of 180° still gives the same structure to the Fisher projection), so it can overlap the isomerD. also isomerCYDare superimposable mirror images, that is, they are equal,identical!

Then "C"Y"D' are just different designs for the same stereoisomer. The next question is: is this stereoisomer chiral? We verified that this isomer is superimposed on its mirror image, that is, it is asachiral.

This is so weird! How is a compound containing two chiral centers (C2 and C3) achiral?

If that happens! A compound that is achiral but contains centers of chirality is calledmeso compound.A meso compound is achiral and optically inactive.(gesture of disapprovalNOrotate the plane of polarization of plane-polarized light), but have multiple centers of chirality.

Since this stereoisomer is a meso compound, the total number of stereoisomers for 2,3-dichlorobutane isthree.

attention, 2^norteand themaximumnumber of stereoisomers. Some connections may haveany lessthan the maximum, due to the existence of meso compounds.

Answers to Practice Questions Chapter 5

5.6.3 Chiral or achiral looking for a plane of symmetry

The existence of chiral centers does not guarantee the chirality of a molecule, for example the meso compound. Following the definition of chirality always involves comparing the original structure and its mirror image, which requires additional work. Is there an easier way to tell if a molecule is chiral or achiral?

we can checkSymmetrieebene. The plane of symmetry is a plane that cuts the molecule in half and one half is the mirror image of the other.

• If a molecule has a plane of symmetry, then the molecule is achiral.
• The molecule that does thisnotera plane of symmetryis in all conformationschiral.

For the meso isomer of 2,3-dichlorobutane, the plane of symmetry is the plane identified in the following structure.

Solution:

Checking the plane of symmetry provides a quick way to determine the chirality of a molecule. However, sometimes it is necessary to look for the correct conformation to obtain the plane of symmetry. See example below.