Module 3

Question 1

What is chirality?

Answer 1

Chirality is the property of “handedness”—an object that cannot be superimposed on its mirror image.

Question 2

What does “chiral” mean?

Answer 2

An object or molecule that cannot be superimposed on its mirror image.

Question 3

What does “achiral” mean?

Answer 3

An object or molecule that can be superimposed on its mirror image.

Question 4

Give an example of a chiral object.

Answer 4

A human hand or a screw (righty-tighty, lefty-loosey).

Question 5

Give an example of an achiral object.

Answer 5

A sock, pencil, or unmarked flask.

Question 6

What is the superimposability test?

Answer 6

A method where you compare an object and its mirror image. If they can’t align perfectly, the object is chiral.

Question 7

What are enantiomers?

Answer 7

Two molecules that are non-superimposable mirror images of each other. They are both chiral.

Question 8

What type of geometry do chiral molecules often have around the central carbon?

Answer 8

Tetrahedral geometry.

Question 9

What structural feature usually indicates a molecule is chiral?

Answer 9

A stereocenter—a carbon atom bonded to four different groups.

Question 10

Is lactic acid chiral or achiral?

Answer 10

Chiral. It has a stereocenter and two non-superimposable mirror images (enantiomers).

Question 11

Is 2-chloropropane chiral or achiral?

Answer 11

Achiral. It has two identical methyl groups, so its mirror image can be superimposed.

Question 12

What is the relationship between chiral molecules and symmetry?

Answer 12

Chiral molecules are typically asymmetric; achiral molecules often have planes of symmetry.

Question 13

What is a stereocenter?

Answer 13

A carbon atom with tetrahedral geometry bonded to four different atoms or groups.

Question 14

What other terms are synonymous with stereocenter?

Answer 14

Stereogenic center and chiral center.

Question 15

What property of a molecule arises due to the presence of a stereocenter?

Answer 15

Chirality

Question 16

What is chirality?

Answer 16

A property where a molecule is not superimposable on its mirror image.

Question 17

What is an enantiomer?

Answer 17

One of a pair of molecules that are mirror images of each other but not superimposable.

Question 18

What geometry must a carbon have to be a stereocenter?

Answer 18

Tetrahedral geometry (sp³ hybridized).

Question 19

What is the key structural requirement for a carbon to be a stereocenter?

Answer 19

It must be bonded to four different groups.

Question 20

Can a carbon in a CH₂ group be a stereocenter? Why or why not?

Answer 20

No, because it is bonded to two identical hydrogen atoms.

Question 21

Can a carbon in a double bond be a stereocenter?

Answer 21

No, because it cannot achieve tetrahedral geometry.

Question 22

Why is 2-chloropropane not chiral?

Answer 22

It has only three different groups attached to the central carbon—two methyl groups are identical.

Question 23

In lactic acid, which carbon is the stereocenter?

Answer 23

The second carbon (carbon #2), bonded to OH, H, CH₃, and COOH.

Question 24

In 1-bromo-2-iodopropane, why is carbon #2 a stereocenter?

Answer 24

Because it is bonded to Br, I, CH₃, and H—four different groups.

Question 25

In 3-methylhexane, what makes carbon #3 a stereocenter?

Answer 25

It is bonded to a methyl, ethyl, propyl, and hydrogen—all distinct groups.

Question 26

Why is it useful to quickly eliminate CH₃, CH₂, or multiple bonded carbons when looking for stereocenters?

Answer 26

Because they cannot have four different groups or the required geometry.

Question 27

What notation is commonly used to mark stereocenters in a molecule?

Answer 27

An asterisk (*) next to the stereogenic carbon.

Question 28

True or False: Wedge and dash bonds always indicate a stereocenter.

Answer 28

False. You must confirm four unique groups are present.

Question 29

What is a racemic mixture?

Answer 29

A racemic mixture contains equal amounts of two enantiomers of a chiral molecule.

Question 30

Which enantiomer of ibuprofen is biologically active?

Answer 30

Only one enantiomer is active at relieving pain; the other is inactive but can be converted to the active form by enzymes.

Question 31

Why was the drug thalidomide withdrawn from use in pregnant women?

Answer 31

One enantiomer treated nausea, but the other caused severe birth defects (teratogenic effects).

Question 32

What is the significance of chirality in olfactory receptors?

Answer 32

Olfactory receptors are chiral, so they can distinguish between enantiomers, which may have very different smells.

Question 33

What are the two enantiomers of carvone associated with in terms of smell?

Answer 33

One smells like spearmint; the other smells like caraway.

Question 34

Do enantiomers have different physical properties like melting point or boiling point?

Answer 34

No, enantiomers have identical physical properties such as melting point, boiling point, and density.

Question 35

Why is separating enantiomers challenging?

Answer 35

Because they have the same physical properties, making traditional separation methods ineffective.

Question 36

What device is used to measure the optical activity of chiral compounds?

Answer 36

A polarimeter.

Question 37

What does a polarimeter measure?

Answer 37

The rotation of plane-polarized light by a chiral compound, which helps determine the enantiomeric composition.

Question 38

What is specific rotation?

Answer 38

A unique value for each enantiomer that describes how much it rotates polarized light under standard conditions.

Question 39

How do you name S-lactic acid using IUPAC rules?

Answer 39

(S)-2-hydroxypropanoic acid or 2S-2-hydroxypropanoic acid.

Question 40

How do you name R-lactic acid using IUPAC rules?

Answer 40

(R)-2-hydroxypropanoic acid or 2R-2-hydroxypropanoic acid.

Question 41

What are the priorities of groups in lactic acid’s chiral center?

Answer 41

1) OH, 2) COOH, 3) CH₃, 4) H

Question 42

What does a counterclockwise rotation from priority 1 → 2 → 3 indicate?

Answer 42

An S (sinister) configuration.

Question 43

What does a clockwise rotation from priority 1 → 2 → 3 indicate?

Answer 43

An R (rectus) configuration.

Question 44

What must you do before determining the R/S configuration?

Answer 44

Orient the molecule so the lowest priority group (usually H) is pointing away from you.

Question 45

How are multiple bonds treated in CIP rules?

Answer 45

Treat double/triple bonds as if bonded to an equivalent number of single atoms. (e.g., C=O is like C–O and C–O)

Question 46

How many stereoisomers does 2-bromo-3-chlorobutane have?

Answer 46

Four stereoisomers (2² = 4), due to two stereocenters.

Question 47

What are the four stereoisomer configurations of 2-bromo-3-chlorobutane?

Answer 47

R,R; S,S; R,S; and S,R.

Question 48

Which pairs of 2-bromo-3-chlorobutane stereoisomers are enantiomers?

Answer 48

1 and 2 (R,R vs. S,S); 3 and 4 (R,S vs. S,R).

Question 49

Which 2-bromo-3-chlorobutane stereoisomer pairings are diastereomers?

Answer 49

1 and 3, 1 and 4, 2 and 3, 2 and 4.

Question 50

Do enantiomers have the same physical and chemical properties?

Answer 50

Yes, except for how they rotate plane-polarized light and how they interact with other chiral substances.

Question 51

Do diastereomers have the same physical and chemical properties?

Answer 51

No, diastereomers often have very different properties (e.g., melting points, boiling points, solubility).

Question 52

Why is it easier to separate diastereomers than enantiomers?

Answer 52

Diastereomers have different physical properties, so standard lab techniques can separate them. Enantiomers require chiral reagents or conditions.

Question 53

What is the relationship between cis- and trans- isomers of 1,2-dichloroethene?

Answer 53

They are diastereomers (not mirror images, but same connectivity and formula).

Question 54

How many stereoisomers does cholesterol have?

Answer 54

Up to 256 stereoisomers (2⁸ = 256), since cholesterol has 8 stereocenters.

Question 55

What does the 2ⁿ rule predict in stereochemistry?

Answer 55

The maximum number of stereoisomers a molecule with n stereocenters can have.

Question 56

How many stereoisomers are predicted for 2,3-dichlorobutane using the 2ⁿ rule?

Answer 56

4 stereoisomers (2² = 4)

Question 57

How many actual distinct stereoisomers exist for 2,3-dichlorobutane?

Answer 57

3 distinct stereoisomers (due to one being a meso compound)

Question 58

What are the configurations of the four stereoisomers of 2,3-dichlorobutane?

Answer 58

R,R S,S R,S S,R

Question 59

Which stereoisomers of 2,3-dichlorobutane are enantiomers?

Answer 59

R,R and S,S

Question 60

What is a meso compound?

Answer 60

A molecule with stereocenters that is superimposable on its mirror image due to internal symmetry.

Question 61

Which stereoisomers of 2,3-dichlorobutane are meso?

Answer 61

R,S and S,R (they are the same compound)

Question 62

Are meso compounds chiral or achiral?

Answer 62

Achiral

Question 63

What feature allows a meso compound to be achiral?

Answer 63

An internal plane of symmetry

Question 64

What is the relationship between a meso compound and its mirror image?

Answer 64

They are superimposable

Question 65

What is the relationship between a meso compound and an enantiomer?

Answer 65

They are diastereomers (not mirror images)

Question 66

Do meso compounds and their enantiomers have the same physical and chemical properties?

Answer 66

No, diastereomers (like meso and enantiomers) have different properties.