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[CHEM 0310] - Midterm Exam Guide - Comprehensive Notes for the exam (10 pages long!)


Department
Chemistry
Course Code
CHEM 0310
Professor
Horne Seth
Study Guide
Midterm

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Pitt
CHEM 0310
MIDTERM EXAM
STUDY GUIDE

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Chapter 5 Stereoisomers
Chiral Molecules
-A molecule that is not superimposable on its mirror image
-Superimposable: The ability for an object to be placed over another object, usually in such a way that
both will be visible
-Non-superimposable: molecules cannot be placed on top of one another and give the same molecule
-Each isomer of the image-mirror pair is called an enantiomer
-Chiral means handedness like the mirror images of hands
-Chiral compounds can contain an atom that is connected to four different substituent groups such a
nucleus is called an asymmetric atom or a sterocenter
-Molecules with one sterocenter are always chiral (not all chiral molecules have stereocenters)
Achiral Molecules
-Compounds having structures that are superimposable on their mirror image
-Examples include: balls, water glass, and hammers
Symmetry in Molecules
-The symmetry in molecules helps to distinguish chiral structures from achiral ones
-The only criterion for chirality is the nonsuperomposable nature of object and mirror image
-For most organic molecules, we have to consider only one test for chirality: the presence or absence of
a plane of symmetry
-A plane of symmetry (mirror plane) is one that bisects the molecule so that the part of the structure
lying on one side of the plane mirrors part on the other side
-Chiral molecules cannot have a plane of symmetry
Optical Activity
-Two enantiomers have the same physical properties identical bonds and energy contents
-However, when a special kind of light, called plane-polarized light, is passed through a sample of one of
the enantiomers, the plane of polarization of the incoming light is rotated in one direction
-When the same experiment is repeated with the other enantiomer, the plane of polarized light is
rotated by exactly the same amount but in the opposite direction
-An enantiomer that rotates the plane of light in a clockwise sense of the viewer faces the light is
dextrorotatory and the compound is referred as the (+) enantiomer
-The other enantiomer which will effect counterclockwise rotation, is levorotatory and called the (-)
enantiomer
-This special interaction with light is called optical activity and enantiomers are called optical isomers
Polarimeter
-When ordinary light is passed through a material called a polarized, all but one type of light waves are
filtered away, and the resulting mean oscillates in only one plane: plane-polarized light
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-If a beam of plane-polarized light is passed through a chiral substance, the electric field interacts
differently with, say, the left and right halves of the molecule
-This interaction results in a rotation of the plane of polarization, called optical rotation
-In polarimeter, light is first plane polarized traverses a cell containing the sample angle of rotation
of the plane of polarization is measured by aligning another polarizer (the analyzer)
-The measured rotation is the observed optical rotation (α) of the sample
-The rotation depends on concentration, structure, length, wavelength of light, solvent, and
temperature
Racemic Mixture
-If there is a 1:1 micture of (+) and (-) enantiomers no rotation and optically inactive
-If one enanatiomer equilibrates with its mirror image, it is said to undergo racemization
-There is no correlation between the sign of rotation and the structure of the particular enantiomer
R/S Stereocenters
-Ranking of four substituents: a>b>c>d
-The lowest priority substituent (d) is placed as far away from us as possible
-Placing substituents from a c counterclockwise, the sterocenter is called S
-Placing substituents form a c clockwise, the center is called R
-The symbol R or S is added as a prefix in parentheses to the name of the chiral compound
-ex: (R)-2-bromobutane
-A racemic mixture can be designated as both R,S if necessary
-The sing of the rotation of plane-polarized light may be added if it is known, as it (S)-(+)-2bromobutane,
but it is important to remember that the symbols R and S are not necessarily correlated with α sign
Ranking of Substituents
1. A substituent atom of higher atomic number takes precedence over one of the lower
2. Rank substituents: ethyl > methyl
3. Double and triple bonds are treated as if they were single, and atoms in them are duplicated or
triplicated at each end by the respective atoms at the other end of the multiple bond
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