| ORGANIC CHEMISTRY I CHEM 2323 |
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| ORGANIC CHEMISTRY I CHEM 2323 |
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CHAPTER 4 - STEREOCHEMISTRY
STEREOCHEMISTRY
stereochemistry - structures in three dimensions (3D).
stereoisomers - how the isomers are arranged in space.
Why was it determined that the tetrahedral structure is correct?
We will use methane CH4 as an example.
In what ways can the 4 hydrogen atoms on carbon be arranged so that the four hydrogen atoms are equivalent?
There are 3 such arrangements:
planar pyramidal tetrahedral dashed lines "----" represent atoms being next to each other
solid lines "___" represent bonds between atoms
In the tetrahedral structure, all atoms are next to each other.
In making the compound CH2Br2, there was found to be only ONE substance produced.
EVIDENCE AGAINST THE PLANAR ARRANGEMENT
If the planar arrangement was possible, we should get TWO compounds when CH2Br2 was made.
Note on the planar arrangement, there can be a compound where the bromo groups are next to each other (structure 1) and where they are directly across from each other (structure 2). But in actuality, only one compound is ever found. Therefore, this type of planar structure is eliminated from the possibilities.
EVIDENCE AGAINST THE PYRAMIDAL ARRANGEMENT
If the pyramidal arrangement was possible, we should get TWO compounds when CH2Br2 was made.
Note on the pyramidal arrangement, there can be a compound where the bromo groups are next to each other (structure 1) and where they are directly across from each other (structure 2). But in actuality, only one compound is ever found. Therefore, this type of pyramidal structure is eliminated from the possibilities.
EVIDENCE FOR TETRAHEDRAL ARRANGEMENT
With the tetrahedral arrangement, the two bromo groups would always be next to each other no matter where they were attached to the carbon. Thus, we would only get ONE compound when made.
Since, in actuality, we only get ONE compound when made, it supports the tetrahedral arrangement.
Optically active substance - polarized light which is rotated when passed through a substance.
An instrument to detect and measure whether a substance is rotating the plane of polarized light is called a polarimeter.
If the substance does not affect the plane of polarization, the substance is said to be optically inactive.
If the substance rotates the plane of polarization, it is said to be optically active.
Rotation of the light to the right (clockwise) is called dextrorotary (+).
Rotation of the light to the left (counter clockwise) is called levorotary (-).
The optical rotation is caused by individual molecules of the active substance, therefore, the amount of rotation depends upon how many molecules the light encounters in passing through the tube.
To keep all factors constant we use a term called specific rotation. This is the number of degrees of rotation observed if a 10 cm sample tube is used and the concentration of the substance is 1 g/ml.
Specific rotation is a physical property of a substance. Specific rotation is written in the following format:
is the angle of specific rotation 20 is the temperature in centigrade
D is the wavelength of light used (in this case the D line of sodium)
- is the direction of rotation
5.90o is the angle of rotation
Enantiomers - mirror images which are not superimposed on each other.
They are not superimposable since no matter how you turn the two structures (without lifting them off the page), they cannot be superimposed on each other.
ENANTIOMERISM AND OPTICAL ACTIVITY
Most compounds do not rotate the plane of polarized light.
When a plane of polarized light passes through an individual molecule; in most cases its plane is rotated a tiny amount by interaction with the charged particles of a molecule, the direction and extent of rotations varies with the orientation of the particle molecule in the beam of light.
But for most compounds, for every molecule the beam of light encounters, there is another identical molecule orientated as the mirror image of the first. The net result is no rotation, since the deflections are canceled out.
But in the special case of optically active compounds, there is not a molecule who is an identical mirror image of the first. Instead we have a molecule of a different isomeric compound.
In a pure sample of a single enantiomer, no molecule can serve as a mirror image of another. There is no canceling out effect. Thus, enantiomers also give rise to optical activity.
PREDICTION OF ENANTIOMERISM: CHIRALITY
Molecules which are not superimposable on their mirror images are chiral. Thus, chiral compounds are also enantiomers.
A carbon to which four different groups are attached is called a chiral center (chiral carbon).
Enantiomers have identical physical properties except for the plane of polarized light (optical activity).
Enantiomers have identical chemical properties except toward optically active reagents.
A mixture of equal parts of enantiomers is called a racemic modification. A racemic modification is optically inactive (cancel each other out).
The prefix ± is used to specify the racemic nature of a particular sample.
The arrangement of atoms that characterizes a particular stereoisomer is called its configuration.
SPECIFICATION OF CONFIGURATION: R and S
Most times we don't want to draw the configuration each time we specify a particular configuration.
Instead, we use the terms R and S to specify which structure we are referring to.
To determine whether it is R a S, follow the following steps.
1. Draw the two mirror images (configurations).
2. Determine the molecular mass of each of the four groups attached to the carbon.
3. Eliminate the group with the lowest molecular mass.
4. Draw a circle in the direction going from the highest molecular mass group to the second highest to the third highest.
5. If the circle goes in a clockwise direction (to the right - rectus), the configuration is labeled R.
If the circle goes in a counter clockwise direction (to the left - sinister), the configuration is labeled S.
Diasteromers have more than one chiral carbon.
Diasteromers are not mirror images of each other.
What is the relationship between configuration I and III and between III and II? There are stereoisomers but not enantiomers. Compound III is a diasteromer of I and also of III.
Diastereomers have similar chemical properties.
Diastereomers have different physical properties.
A meso compound is one whose molecules are superimposable on their mirror image even though they contain chiral centers.
