Stereochemistry at Tetrahedral Centers

Question 1

What is an enantiomer?

Answer 1

Molecules that are not the same as their mirror.

Question 2

What causes enantiomers?

Answer 2

A tetrahedral bonding to four different substituents.

Question 3

Define chiral.

Answer 3

Molecules that do not have a plane of symmetry and are not superimposable on their mirror image.

Question 4

Define achiral.

Answer 4

Molecules with a plane of symmetry that is thee same as its mirror image.

Question 5

What is the chirality center of a molecule?

Answer 5

Point in a molecule where 4 different groups are attached to crbon

Question 6

Who investigated the nature of plane-polarized light?

Answer 6

Jean-Baptiste Biot

Question 7

Define what it means to be optically-active.

Answer 7

A property of organic compounds to rotate plane-polarized light that passes through it.

Question 8

What is specific rotation?

Answer 8

optical rotation of a chiral compound under standard conditions.

Question 9

What is the formula for specific rotation?

Answer 9

[a]D = observed rotation (degrees)/[pathlength (dm) x concentration (g/cm3) = a/lc

Question 10

Define levorotatory.

Answer 10

Optically active substance that rotates the plane of polarization of plane-polarized light in counterclockwise direction.

Question 11

Define dextrorotatory.

Answer 11

Optically active substance that rotates the plane of polarization of plane-polarized light in clockwise direction.

Question 12

What are the signs given to levorotatory and dextrorotatory?

Answer 12

L = -
R = +

Question 13

What did Louis Pasteur discover about sodium ammonium salts of tartaric acid?

Answer 13

They crystallize into right handed and left handed forms.
Equal concentrations of these forms have opposite optical rotations.

Question 14

What is the first Cahn-Ingold-Prelog rule?

Answer 14

Look at the four atoms attached to the chirality center and rank according to atomic number
Atom with highest atomic number has highest ranking, lowest number = lowest ranking

Question 15

What is the second Cahn-Ingold-Prelog rule?

Answer 15

If a decision cannot be reach by ranking the first atoms in the substituent, look at second, third or fourth atoms until the difference is found.

Question 16

What is the third Cahn-Ingold-Prelog rule?

Answer 16

Multiple-bonded atoms are equivalent to the same number of single-bonded atoms.

Question 17

What is R-configuration?

Answer 17

Curved arrow drawn clockwise.

Question 18

What is S-configuration?

Answer 18

Curved arrow drawn counterclockwise.

Question 19

What is true about stereoisomers of molecules with more than one chirality center?

Answer 19

Usually are not enantiomers.

Question 20

How many stereoisomers can a molecule with n chirality centers have?

Answer 20

up to 2^n

Question 21

What are diastereomers?

Answer 21

Stereoisomers that are not mirror images.

Question 22

What are epimers?

Answer 22

Compounds in which two diastereomers differ at only one chirality center but are the same at all others.

Question 23

What are meso compounds?

Answer 23

Achiral compounds with chirality centers.

Question 24

What are the three stereoisomeric forms of tartaric acid?

Answer 24

two enantiomers and one meso form

Question 25

What is true about meso tartaric acid?

Answer 25

Diasteromeric with + and - forms, it is a different compound with different physical properties.

Question 26

What is a racemate?

Answer 26

A 50:50 mixture of two chiral compounds that are mirror images does not exhibit optical rotation.
Called a racemic mixture

Question 27

What is a common method of resolution of enantiomers?

Answer 27

Uses an acid-base reaction between the racemate of a chiral carboxylic acid and an amine base. Yields an ammonium salt.

Question 28

What does resolution of enantiomers mean?

Answer 28

A process for the separation of a racemic mixture into its enantiomers.

Question 29

Does trivalent nitrogen form a chirality center?

Answer 29

No, it rapidly flips.

Question 30

Can individual enantiomers be isolated in trivalent nitrogen?

Answer 30

No.

Question 31

Does chirality apply to P or S?

Answer 31

Maybe, but they flip slowly.

Question 32

What is a prochiral molecule?

Answer 32

A molecule that is achiral but can become chiral by a single alteration.

Question 33

What type of atoms are prochiral?

Answer 33

sp2 hybridized

Question 34

What is true about planar faces that can become tetrahedral?

Answer 34

Different from the top or bottom

Question 35

What is the Re face?

Answer 35

Face on which the arrows curve clockwise

Question 36

What is the Si face?

Answer 36

Face on which the arrows curve counterclockwise.

Question 37

What type of reactions are prochiral compounds typically involved in?

Answer 37

Biological.

Question 38

What is a prochirality center?

Answer 38

An sp3 hybridized carbon with two groups that are the came.

Question 39

How are two identical groups at a prochirality center distinguished?

Answer 39

By considering one and seeing if it were increased in priority in comparison with the other.
If the center becomes R, the group is pro-R and if it becomes S, the group is pro-S.

Question 40

What is an example of prochirality in nature and chiral environments?

Answer 40

Reaction of ethanol with NAD+ catalyzed by alcohol dehydrogenase.
Deuterium labelled substrates show exclusive removal of pro-R hydrogen from ethanol
Addition occurs only at the Re face of the NAD+ carbon