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ORGANIC - MCMURRY 9E

CH.5 - STEREOCHEMISTRY AT TETRAHEDRAL CENTERS

CONCEPT: TYPES OF ISOMERS

Isomers are used to describe relationships between similar molecules.

□ We can order these relationships in order of increasing similarity

● _________________________________

● _________________________________

● _________________________________

● _________________________________



CONCEPT: CHIRALITY

A molecule is chiral when, if placed against a mirror, a different (non-superimposable) image is obtained.

□ The mirror image of any chiral molecule is called an _________________________________

□ If a molecule has an internal line of symmetry, it will have the same mirror image (achiral)

EXAMPLE: Draw the mirror images of the following molecules. Determine if the mirror image is the same or different. If there is an internal line of symmetry, use a dotted line to indicate it on the original molecule.

a.

b.



CONCEPT: CHIRALITY TEST 1 – INTERNAL LINE OF SYMMETRY

As previously stated, if a molecule has an internal line of symmetry, it is ___________________

□ This test is the most useful for _________________

EXAMPLE: Are the following molecules chiral or not?

a.

b.

c.

d.



CONCEPT: CHIRALITY TEST 2 – STEREOCENTERS

Any atom that creates a stereoisomer after swapping groups is called a ______________________________

□ The gold standard chirality test for almost all molecules

EXAMPLE: Which of the following molecules contain a stereogenic center? Of these, how many are chiral?

a b c.

d. e f.

Stereogenic Center (Stereocenter)

● Any atom that creates stereoisomers after swapping groups

Chiral Center

● An atom with 4 different substituents. Chiral

Trigonal Center

● Any double bond capable of forming an E or Z isomer. Achiral



CONCEPT: CAHN-INGOLD-PRELOG NOMENCLATURE

According to IUPAC protocol, each molecule must have a unique, unambiguous name – even stereoisomers.

Step 1. Assign priorities to the four atoms on the chiral center according to their atomic mass on the periodic table.

Step 2. When there is a tie between atomic weights, compare the next set of adjacent atoms (playoffs!).

Step 3. Double bonds count twice. Triple bonds count three times.

EXAMPLE: Determine priorities for the following chiral center:




Step 4. IF the last priority group is in the back, then trace a path from _________ to _________ priority.

- Clockwise = _____, Counterclockwise = _____ □ Always Ignore group 4

Step 5. If the last priority group is NOT in the back, ________ that group with the group that is on the dash.

- Trace path as always, but this time _______ the sign since you ___________ groups.

● ___ ___

● ___ ___



PRACTICE: Provide the full name for the following molecules, taking stereochemistry into account.

a.

b.

c.



CONCEPT: TYPES OF STEREOISOMERS

□ Compounds with no chiral centers are usually achiral

□ Compounds with exactly one chiral center are always _____________, and they can form ___________________

□ Compounds with two or more chiral centers are usually chiral, and can form _____________________

● Follow the 2n (n = stereocenters) rule to predict total number of possible stereoisomers.

EXAMPLE: Draw all the stereoisomers for 2-bromocyclohexan-1-ol, and determine their relationships to each other.

EXAMPLE: Determine the total number of stereoisomers for the following molecule



CONCEPT: ATROPISOMERS

Molecules that contain NO chiral centers yet are chiral due to their inability to freely ______________

1. Allenes

Use TEST 2 to identify trigonal centers (a type of stereocenter):

□ Visualize the allene as a big double bond. If it is able to form ____ or ____ isomers, it is ____________

□ Remember we stated that trigonal centers are achiral if they pass this test. Allenes are different.

EXAMPLE: Which of the following allenes is chiral?

a. b. c.

2. Substituted Biphenyls

These are chiral if all substituents are in the ortho- position, and if none of the rings have two of the same group on them.

EXAMPLE: Which of the following biphenyls is chiral?

a. b. c.



CONCEPT: MESO COMPOUNDS Meso compounds are created when two symmetrical chiral centers cancel out, yielding 2 _______________ enantiomers

□ Meso compounds have an internal line of symmetry (TEST 1), meaning they are actually _____________

□ Meso compounds follow the ____________ rule for total stereoisomers!

□ A compound will be meso if it meets the following 3 criteria:

1. It has ____ or more chiral centers

2. It is atomically ______________________

3. An even number of chiral centers are _________________ to each other

EXAMPLE: Which of the following molecules are meso, and therefore achiral? a. b.



CONCEPT: CHIRALITY TEST 3 - DISUBSTITUTED CYCLOALKANE SHORTCUTS

Some of the most commonly tested molecules fall in this category. Although TEST 1 and/or TEST 2 could be used to

determine chirality, it will be much faster for us to just memorize simple rules for them.

□ TEST 3 only applies when a ring has two identical substituents

● Always achiral ● Cis = Meso, Achiral ● Always achiral

● Trans = Chiral ● Only possible on even-#’d rings

● Except: 1,2-cyclohexane = Chiral

EXAMPLE: Using TEST 3, which of the following molecules is chiral?



CONCEPT: ISOMETRIC RELATIONSHIPS

In a previous chapter, we used the following flowchart to identify constitutional isomers:

Step 1. (Are the atoms all the same?) Count non-_______________ atoms and IHD in both compounds

- If not exactly the same, they are ____________________________

- If the same, then go to step 2

Step 2. (Are the atoms all connected the same?) Look for a _____________ atom, then count bonds from there.

-If not exactly the same, they are _______________________________

-If the same, then _____________________________

Due to the possibility of stereoisomers, now we have to add one more step to the flowchart:

Step 3. Count the number/type of stereogenic centers on the molecule

- If _____ chiral/trigonal centers, the two molecules are ______________

- If _____ chiral center,

● Same: ________________

● Different: ___________________

- If _____ chiral centers,

● All same: ________________

● If one or more different: ___________________

● All different: ___________________

- If _____ chiral centers, symmetrical, opposite: __________________________

- If _____ trigonal center,

● Same: ________________

● Different: ___________________



PRACTICE: Identify the following compounds as identical, constitutional isomers, enantiomers or diastereomers

a.

b.

c.

d.



CONCEPT: FISCHER PROJECTIONS

□ Like Newman projections, there are several common projections used to visualize molecules in different perspectives.

● In all cases, we will often need to convert these structures into ____________________ before analyzing them.

Converting Fischer to Bondline:

□ Make a caterpillar, then rotate every ________ bond

EXAMPLE: Convert the following Fischer Projection into bondline structure.



CONCEPT: FISCHER CONFIGURATIONS

□ We use a slightly different method for determining R and S configurations in Fischer Projections.

● Determine location of lowest priority group:

- If _________________________, chirality is as it looks

- If _________________________, chirality is flipped.

EXAMPLE: Determine the absolute configurations for all chiral centers present.



CONCEPT: CAHN-INGOLD-PRELOG NOMENCLATURE

According to IUPAC protocol, each molecule must have a unique, unambiguous name – even stereoisomers.

Step 1. Assign priorities to the four atoms on the chiral center according to their atomic mass on the periodic table.

Step 2. When there is a tie between atomic weights, compare the next set of adjacent atoms (playoffs!).

Step 3. Double bonds count twice. Triple bonds count three times.





Step 4. IF the last priority group is in the back, then trace a path from _________ to _________ priority.

- Clockwise = _____, Counterclockwise = _____ □ Always Ignore group 4

Step 5. If the last priority group is NOT in the back, ________ that group with the group that is on the dash.

- Trace path as always, but this time _______ the sign since you ___________ groups.

● ___ ___

● ___ ___



PRACTICE: Provide the full name for the following molecules, taking stereochemistry into account.

a.

b.

c.



CONCEPT: OPTICAL ACTIVITY—SPECIFIC AND OBSERVED ROTATION

One of the special features of chiral molecules is that they are able to __________ plane-polarized light.

● Clockwise rotation = dextrorotary (d) or ________ ● Counterclockwise rotation = levororatory (l) or _______

□ These random names have ____________ to do with the chirality of a molecule!



Enantiomeric Excess:

● Specific rotation [α] is the rotation that 100% pure enantiomers produce. Opposite enantiomer = _____________ rotation.

● A perfect 1:1 ratio of enantiomers is called _________________ ● Non-1:1 ratio is called _________________

The enantiomeric excess: Observed Rotation:

EXAMPLE: Calculate the ee and observed rotation for the following chiral mixtures where S-enantiomer

has [α] = +20.



PRACTICE: OPTICAL ACTIVITY

a. When 0.200 g of lactose is dissolved in 10.0 ml of water and placed in a sample cell 10.0 cm in length, the observed rotation is +2 O. Calculate the specific rotation of lactose.

b. Calculate the observed rotation of a chiral mixture that contains 65% (S)-stereoisomer where the [α] of pure (S)-stereoisomer = -118

c. An optically pure (R)-stereoisomer of a molecule has a specific rotation of – 20O. What specific rotation would be observed for a mixture of the (R) and (S) stereoisomer where there is an enantiomeric excess equal to (S) 60%



CONCEPT: NON-CARBON CHIRAL CENTERS

Chirality may exist on atoms other than carbon. Common non-carbon chiral atoms are ( ), ( ), ( ), ( ).

□ The lone pair on a neutral nitrogen is able to _____________________ relatively easily, losing its chirality.

● Chirality is determined using the R/S naming system, where the _________ __________ is always group number _____.



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