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4 -- Part 4: THE STUDY OF CHEMICAL REACTIONS
We know the ring is in a chair with one bromine atom and one equatorial.
The mirror image of -1,2-dibromocyclohexane is shown below.
There are two mirror-image structures that are nonsuperimposable.
You may be able to see the difference if you make models.
The chair-chair interconversion is rapid at room temperature.
The molecule would quickly undergo chair-chair interconversions if we had a bottle of just the left side of the molecule.
There must be equal amounts of the two mirror images.
Almost all achiral compounds can be found in equilibrium with their mirror-image conformations.
It must be possible to separate a sample containing just one enantiomer for a compound to be considered chiral.
We can't purify a sample of just one enantiomer if the molecule is in rapid equilibrium with its mirror-image conformations.
Imagine that the cyclohexane ring is flat.
Consider the most symmetric part of the molecule to determine whether it is a mobile molecule or not.
Straight-chain compounds can be considered in cyclohexane rings as though they were flat.
There are many organic compounds that are rapidly interconverting chiral confor mations.
ethane has a skew in its conformations.
We intend to focus on observable, persistent properties when we talk about chirality.
Butane can be found in gauche conformations that are different from each other.
They are in equilibrium with both of the anti and totally eclipsed conformations, which implies that butane must be achiral.
It is possible to determine whether it is chiral by drawing it in its most sym.
Two mirror-image conformations are the most stable of 2-methylbutane's conformations.
These conformations can be converted by rotation around images.
They aren't enantiomers.
H changes quickly with their mirror images.
These structures can't be converted by ring-flips.
They are mirror-image isomers.
The molecule's chirality can be seen when drawn in its most symmetric configuration.
Consider the most symmetrical interconvert.
If it can interconvert with its mirror image, you can consider the most stable one.
To determine whether a compound is capable of showing optical activity, you have to make a model of it.
Most organic compounds have at least one carbon atom.
Some compounds have an asymmetric atom that serves as a chiral center.
Some compounds have no asymmetric atoms at all.
Special characteristics of the molecule's shapes lead to chirality in these types of compounds.
Some molecules are so heavy and strained that they can't easily be converted to the mirror-image conformation.
They can't achieve the most symmetric conformation because it has too much strain.
We need to evaluate the individual locked-in conformation of the molecule to determine if it is chiral.
A sterically crowded derivative of biphenyl is shown in Figure 5-18.
The center drawing shows the molecule.
This is a mirror plane of symmetry.
The molecule wouldn't be active if it could pass through it for an instant.
Because the bromine and iodine atoms are too large to be forced close together, this is a very high energy planar conformation.
It can only be seen in one of the two staggered conformations on the left and right.
These are mirror images that do not interconvert.
They can be separated and isolated.
They have the same and opposite specific rotation.
A simple strained molecule can show enantiomerism.
It is strained.
Even for an instant, cyclooctene could not be considered a ring.
You will see that it can't be a ring.
cyclooctene is a type of molecule.
The cyclooctene have been separated and characterized.
I have bromine atoms.
We could imagine that these two structures are enantiomeric.
Allene is achiral.
The mirror image will be the same as the original molecule if you make a model.
The molecule may be asymmetric if we add some substituents.
The planes are not parallel to each other.
There is no asymmetric carbon atom because H is attached to four different atoms.
The allene molecule has two ends.
Which have distinct substituents.
You can use your models for parts that are unclear.
dashed lines and wedges have been used to show perspective in drawing the stereochemistry of asymmetric carbon atoms.
Perspective drawings become time-Consuming and cumbersome when we draw molecule with several asymmetric carbons.
It is difficult to see the similarities and differences in groups of stereoisomers because of the complicated drawings.
The stereochemistry of sugars, which contain as many as seven asymmetric carbon atoms, was studied at the turn of the 20th century.
Minor stereochemical differences in the drawings would be nearly impossible to pick out, and drawing these structures in perspective would be difficult.
A symbolic way of drawing asymmetric carbon atoms was developed by Fischer.
We learn to use projections because they clarify stereochemical differences and allow fast and accurate comparison of structures.
The asymmetric carbon at the point where the lines cross is what makes the projection look like a cross.
The bonds that project out toward the viewer are wedges.
dashed lines are used to project the vertical lines away from the viewer.
Make a model of the first structure and show the relationship of the other structures to it.
You may have noticed that the projections that differ by 180deg are the same.
The vertical bonds end up vertical, and the horizontal lines end up horizontal, when we rotation a Fischer projection by 180 degrees.
The horizontal lines forward and vertical lines back are the same.
The rotation can be done by 180deg.
If we were to change the configuration of the projection, we would confuse the viewer.
The projection has the vertical groups back and the horizontal groups forward.
The horizontal bonds become vertical when we rotation the projection by 90 degrees.
The viewer assumes that the horizontal bonds come back.
The viewer sees a different molecule.
A 90deg rotation is not allowed.
We can't flip them over because we can't rotate them by 90deg.
An incorrect representation of the molecule can be given by either of these operations.
The projection must be kept in the plane of the paper.
The final rule helps make sure that we don't rotate the drawing.
The carbon chain is drawn along the vertical line of the projection, usually with the IUPAC numbering from top to bottom.
Most of the time, this numbering places the most highly oxidation carbon at the top.
C1 is at the top and C3 is at the bottom.
Change any two groups and draw a projection for each compound.
Remember that the cross represents an on a Fischer projection and the carbon chain should be along the vertical with the perspective drawing.
Testing for enantiomerism using projections is very easy.
If the mirror image cannot be made to look the same as the original structure with a 180 degree rotation in the plane of the paper, the two mirror images are enantiomers.
The groups that fail to superimpose are circled in red.
The mirror images are the same.
Propan-2-ol is achiral.
The mirror images are different.
There is a substance called propane-1-2-diol.
The mirror images are different.
The structure is not straight.
It is easy to identify mirror planes of symmetry from the projection.
The mirror images are the same.
The structure is achiral.
The problem was solved using projections.
The Cahn-Ingold-Prelog convention can be used to draw structures.
The lowest priority atom is hydrogen.
The hydrogen atom is usually on the horizontal line in the projection because the carbon chain is along the vertical line.
We can draw an arrow from group 1 to group 2 to group 3 if we have assigned priorities.
We can assign the configuration by turning the arrow around.
Consider the projection formula of one of the enantio mers.
The lowest priority is given to the hydrogen atom.
The projection shows the arrow from group 1 to group 2 to group 3 counterclockwise.
For compounds with two or more asymmetric carbon atoms, they are the most useful.
There are asymmetric carbons at the center of crosses.
The horizontal lines project toward the viewer while the vertical lines project away from the viewer.
The carbon chain is placed along the vertical, with the IUPAC numbering from top to bottom.
This places the carbon with the most bonds to O at the top.
The projection can be changed without changing its stereochemistry.
Interchanging any two groups on an asymmetric carbon causes its stereochemistry to change.
In detail, we have considered enantiomers.
Geometric isomers and compounds with two or more chiral centers are diastereomers.
Stereoisomers are not mirror images of each other.
They are diastereomers.
It is possible when a ring is present.
They are diastereomers and geometric isomers.
Because the cis diastereomer has an internal mirror plane of symmetry, it must be achiral.
Most compounds that show diastereomerism have two or more asymmetric carbon atoms.
There are two asymmetric carbon atoms in 2-bromo-3-chlorobutane.
Stereoisomers are structures that differ in the orientation of their atoms.
The mirror images of the C3 and C2 carbon atoms are different.
If these two compounds were mirror images of each other, asymmetric carbons would have to be mirror images of each other.
The compounds must be diastereomers because they are stereoisomers.
Both of these diastereomers have an enantiomer.
There are two pairs of enantiomers in this picture.
A diastereomer is a member of one pair of enantiomers.
We can see all the types of isomers we need to study, and we can see their relationships and definitions.
Give the relationship between the two compounds for each pair.
Making models will help.
There are four stereoisomers of 2-bromo-3-chlorobutane.
Make models of these structures.
Two of the four structures of 2,3-dibromobutane are identical.
The diastereomer on the right has a mirror plane of symmetry.
It seems like the molecule was a racemic mixture.
Meso compound is a compound with asymmetric carbons.
A compound with two carbon atoms is called a meso compound.
The cis isomer of 1,2-dichlorocyclopentane has two asymmetric car images of each other, yet it is achiral.
It is a compound.
We should look at the molecule in its flat form to see if it is achiral and meso.
The symmetry of the compounds is shown by the projection.
Determine which of the following compounds is different.
Any asymmetric carbon atoms should be drawn in any mirror planes.
They are called meso.
It's an enantiomer.
The isomers of but-2-ene are achiral and they contain stereocenters, so they meet this definition.
They are not called meso because they have no diastereomers.
They were moved to the other side.
Draw each compound in its most symmetrical conformation, star any asymmetric carbon atoms, and draw any mirror planes.
If you prefer, you can use Fischer projections.
COOH groups communicate with each other.
The stereochemical picture of a molecule includes how the atoms are arranged in space.
Since 1951, when X-ray crystallography was first used to find the orientation of atoms in space, chemists have determined the absolute configurations of many chiral compounds.
Before 1951, there was no way to link the stereochemical drawings with the actual enantiomers.
There were no absolute configurations that were known.
To correlate the configuration of one compound with another, it was possible.
Even though we don't know the absolute configuration of either molecule, the relationship between them is determined by the experiment.
2-methylbutan-1-ol reacts with PBr3 to give 1-bromo-2-methylbutane.
The product must have the same configuration at the asymmetric carbon as the starting material because none of the bonds to the asymmetric carbon are broken in this reaction.
The l configuration of most naturally occurring amino acids can be seen in the projection.
Sugars can be degraded to glyc eraldehyde by removing the asymmetric carbons from the aldehyde end.
The sugars are given the d prefix because they degrade to +)@glyceraldehyde.
The bottom asymmetric carbon of the sugar has a group on the right in the projection.
Enantiomers have the same physical properties, except for the direction in which they spin light.
There are different physical properties of diastereomers.
Consider the diastereomers of but-2ene.
The bonds are canceled by-2-ene.
2-ene do not cancel but add together to create a dipole moment.
There are different physical ties for diastereomers that are not geometric isomers.
The mp for -2,3-dibromosuccinic acid is 158 degC.
There are different physical properties of these diastereo mers.
The stereochemistry of one asymmetric carbon atom, C4 is what distinguishes the sugars from each other.
Dilution, recrystallization, and chromatography can be used to separate diastereomers.
The separation of enantiomers is more difficult than we will see in the next section.
Isolation from biological sources is the most common method of obtaining pure enantiomers of optically active compounds.
Only one enantiomer is found in living organisms.
Pure +)@tartaric acid can be isolated from the pre cipitate formed by yeast during wine making.
grapes, sugar beets, sugarcane, and honey are some of the different sugar sources.
The pure + enantiomer is found in Alanine.
A racemic mixture of enantiomers results when a compound is made from achiral reagents.
Louis Pasteur noticed that a salt of racemic ()-tartaric acid turned into mirror-image crystals.
He separated the enantiomeric crystals using a microscope and a pair of tweezers.
He found that the solutions made from the left- and right-handed crystals differed in the direction of the light.
Pasteur accomplished the first artificial resolution of enantiomers.
Other methods of separation are required for racemic compounds to become separate enantiomers.
The traditional method for resolving a racemic mixture into its enantiomers is to use an enantiomerically pure natural product that bonds with the compound.
A pair of diastereomers results when the racemic compound bonds to the pure resolving agent.
Salts are cleaved from the separated enantiomers after the diastereomers are separated.
l-(+)-tartaric acid is readily available for resolving agents that react with alcohol.
An alcohol and carboxylic acid combine to form an ester.
The following equation shows how an acid and an alcohol can combine to form an ester.
We need an active chiral acid to react with butan 2-ol.
Any winery can give large amounts of puretaric acid.
An illustration of Louis Pasteur working can be seen in the physical properties of the diastereomers of 2-butyl tartrate.
No doubt, he is.
We have two flasks, each containing one that contemplates the implications of the diastereomeric esters.
The resolving agent is cleaved from the separated enantiomerism.
A mixture of diastereomers can be produced by the reaction of a pure enantiomer of one compound with a racemic mixture of another compound.
The resolved enantiomers are given by separation of the diastereomers.
The cheap and nontoxic tartaric acid is likely to be thrown away.
Resolving agents that are expensive must be recovered and recycled.
A method for separating compounds is called chorography.
One type of chromatography involves passing a solution through a column of particles.
The compounds that are strongly adsorbed spend more time on the stationary particles than the compounds that are less strongly adsorbed.
In some cases, enantiomers may be resolved by passing the racemic mixture through a column containing particles whose surface is coated with chiral molecules.
Weak complexes are formed when the solution passes through the column.
The dissolved enantiomers are retarded by the time they are complexed with the column packing.
The chiral column packing can be used with diastereomeric complexes.
The diastereomeric com eliminate a stereoisomer.
There will be one and different equilibrium constants for complexation.
One of the two enantiomers will isomer in a racemic mixture and leave more time complexed with the chiral column packing.
The other stereoisomer is untouched.
The atoms are arranged in space in the stereochemical picture.
There are two C double bonds that meet at a single carbon atom.
There are many substituted allenes.
A carbon atom is made up of four different groups.
A carbon atom is made up of four different groups.
An atom holding a set of ligands in a spatial arrangement that is not superimposable on its mirror image is referred to as the IUPAC term.
The most common centers are asymmetric carbon atoms.
A molecule can use its own chirality to differentiate between mirror images.
cis-trans isomers are a subclass of diastereomers.
There are structures that are related to single bonds.
In most cases, they are not different compounds and are not true isomers.
The order in which their atoms are bond together is different.
The plane of light is tilted.
Stereoisomers are not mirror images.
A percentage of the mixture is the excess of one enantiomer.
A method for drawing a carbon atom.
The carbon chain is kept along the vertical, with the IUPAC numbering from top to bottom.
The bonds project away from the viewer.
A plane of symmetry through the middle of a molecule splits it into two mirror image halves.
A molecule with an internal mirror plane of symmetry cannot be different.
There is a store that sells the enantiomers of everyday objects.
The plane of light is counterclockwise.
The achiral compound is usually asymmetric carbon atoms.
An achiral compound has diastereomers.
Capable of rotating the plane of light.
The compounds have the same properties, except for the direction in which the light rotates.
A percentage of the specific rotation of one of the pure enantiomers is expressed.
Similar to excess.
Waves vibrate in only one plane.
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