Chiral Molecules

Question 1

What are chiral drugs?

Answer 1

Drugs that exist in enantiomeric forms, where one enantiomer is active and the other may be inactive or have different effects.

Question 2

What are the four scenarios regarding enantiomers in chiral drugs?

Answer 2

• Only one enantiomer is active
• Both enantiomers show essentially identical activities
• Both enantiomers have similar activity, but one is more active
• Enantiomers show different pharmacological activities

Question 3

What is enzymatic resolution?

Answer 3

An enzyme selectively converts one enantiomer in a racemic mixture to another compound, after which the unreacted enantiomer and the new compound are separated.

Question 4

What is chromatographic resolution?

Answer 4

Chiral high performance liquid chromatography (HPLC) where diastereomeric interactions cause enantiomers to move at different rates through a stationary phase.

Question 5

How can a racemic mixture be resolved via diastereomer formation?

Answer 5

By reacting a racemic mixture with a single enantiomer of another compound, yielding a mixture of diastereomers that can be separated.

Question 6

What are the properties of diastereomers that allow their separation?

Answer 6

• Different melting points
• Different boiling points
• Different solubilities

Question 7

Why do conventional methods fail to separate enantiomers?

Answer 7

Enantiomers have identical solubilities in ordinary solvents and identical boiling points.

Question 8

Who first separated a racemic form of a salt of tartaric acid?

Answer 8

Pasteur in 1848.

Question 9

Which enantiomer of Naproxen is active?

Answer 9

(S) enantiomer.

Question 10

What happens to the (R) enantiomer of Naproxen in the body?

Answer 10

It is slowly converted to the (S) enantiomer.

Question 11

What is a characteristic of (S)-Penicillamine?

Answer 11

The (S) isomer is a highly potent therapeutic agent for primary chronic arthritis, while the (R) enantiomer is toxic.

Question 12

What are carbohydrates?

Answer 12

A group of polyhydroxy aldehydes, ketones, or acids, or their derivatives, generally in the form CnH2nOn or Cn(H2O)n.

Question 13

How are monosaccharides classified based on carbon atoms?

Answer 13

• Triose - 3 carbon atoms
• Tetrose - 4 carbon atoms
• Pentose - 5 carbon atoms
• Hexose - 6 carbon atoms

Question 14

What is the structure of disaccharides?

Answer 14

Disaccharides contain a glycosidic acetal bond between the anomeric carbon of one sugar and an –OH group of another sugar.

Question 15

What is the difference between enantiomers and diastereomers?

Answer 15

Enantiomers are mirror images of each other, while diastereomers are not.

Question 16

What is a meso compound?

Answer 16

A non-optically active member of a set of stereoisomers that contains stereogenic centers but is superposable on its mirror image.

Question 17

What is enantioselective synthesis?

Answer 17

A reaction that produces one enantiomer predominantly over its mirror image, influenced by a chiral reagent, catalyst, or solvent.

Question 18

What happens when hyoscyamine is heated with dilute base?

Answer 18

It racemizes, leading to a solution that becomes optically inactive.

Question 19

What is the role of enzymes in organic reactions?

Answer 19

Enzymes are biological catalysts that increase reaction rates and assert chiral influence, binding reactants at a chiral active site.

Question 20

What are Fischer projections used for?

Answer 20

They are useful for representing molecules with several chiral centers, such as sugars.

Question 21

What are glycosides?

Answer 21

The products formed when the cyclic hemiacetal or hemiketal forms of monosaccharides react with alcohol.

Question 22

What is a common glycosidic bond in disaccharides?

Answer 22

A bond between C-1 of the first sugar and the –OH at C-4 of the second sugar, called a 1,4’-link.

Question 23

What is the stereochemical relationship between D-galactose and D-glucose?

Answer 23

They are epimers, differing at one stereogenic center.

Question 24

What occurs during the cyclization of monosaccharides?

Answer 24

Intramolecular nucleophilic addition reaction leads to the formation of a cyclic hemiacetal or hemiketal.

Question 25

What are the two forms that sugars can attain through cyclization?

Answer 25

• Pyranose
• Furanose

Question 26

What is the structural characteristic of monosaccharides?

Answer 26

They contain a –OH and a C=O group.

Monosaccharides exist in an equilibrium between open chain and cyclic forms.

Question 27

What forms do sugars attain through cyclization?

Answer 27

Pyranose and/or furanose forms.

Cyclization produces a new chiral centre at C-1.

Question 28

What is the anomeric carbon in cyclic monosaccharides?

Answer 28

The carbon produced at C-1 during cyclization.

It is drawn with the ring oxygen to the rear and the anomeric carbon furthest right.

Question 29

How are the two anomers designated in cyclic sugars?

Answer 29

α or β based on the chiralities at the anomeric centre and the highest numbered chiral centre.

If chiralities are the same (RS convention), it is β; if different, it is α.

Question 30

What is mutarotation?

Answer 30

The process by which the anomeric hydroxyl group orientation can change.

This affects the designation of β-D-sugars and α-L-sugars.

Question 31

Provide examples of disaccharides.

Answer 31

Sucrose, Lactose.

Students are encouraged to research their structural forms and predict reactivities.

Question 32

What is the mechanism of the pinacol rearrangement?

Answer 32

Protonation of a hydroxyl group, elimination of water, rearrangement of a carbenium ion, and deprotonation to form a product carbonyl.

It is regioselective with the major product from the more stable carbocation.

Question 33

What are the factors that determine the course of a pinacol rearrangement?

Answer 33

Carbocation stability, inherent shifting tendency, steric hindrance, reaction conditions.

These factors influence regioselectivity.

Question 34

Define diastereomers.

Answer 34

Stereoisomers that are not mirror images of each other.

They occur in compounds with more than one stereocenter.

Question 35

What is a chiral molecule?

Answer 35

A molecule that is not identical to its mirror image.

A tetrahedral carbon atom with four different groups is an asymmetric carbon.

Question 36

What is optical activity?

Answer 36

The ability of chiral compounds to rotate the plane of polarized light.

Achiral compounds do not show optical activity due to statistical encounters with mirror-image orientations.

Question 37

What is the (R-S) system in stereochemistry?

Answer 37

A nomenclature system for designating stereocenters.

R configuration is clockwise, S configuration is counterclockwise.

Question 38

How is a racemic mixture defined?

Answer 38

A 50:50 mixture of two chiral enantiomers.

M+ and M- represent the mole fractions of the dextrorotatory and levorotatory enantiomers, respectively.

Question 39

What is the biological relevance of chirality?

Answer 39

Most molecules in plants and animals are chiral.

Naturally occurring amino acids are left-handed (S configuration) and natural sugars are right-handed (R configuration).

Question 40

What is the role of acid in the preparation of glycols?

Answer 40

Catalyzes the reaction of water with epoxides.

This is one method for preparing glycols.

Question 41

True or False: Optical purity can be determined by the specific rotation of a pure enantiomer.

Answer 41

True.

Detection limits are high, requiring a large sample for small rotation compounds.

Question 42

Fill in the blank: A _______ is defined as an atom bearing groups such that an interchange of any two groups will produce a stereoisomer.

Answer 42

stereocenter.