Stereochemistry

Question 1

What is a stereoisomer?

Answer 1

An isomer that has the same constitution but differ in the spatial arrangement of their atoms.

Question 2

Who (Lord Kelvin), and in what year coined a word that describes nonsuperimposable objects?

Answer 2

William Thomson in 1894 coined the word chiral.

Question 3

What’s a chiral molecule?

Answer 3

A molecule whose two mirror-image forms are not superimposable in three dimensions.

Question 4

What is the Greek word for chiral, and what does it mean?

Answer 4

cheir, means “hand”

Question 5

What is the opposite of chiral?

Answer 5

Achiral- a molecule that is superimposable on its mirror image.

Question 6

Name the following compound and decide whether it is chiral or achiral:

BrClFCH

Answer 6

bromochlorofluoromethane is chiral

Question 7

What types of molecules tend two be chiral? (e.g what types of groups should they have)

Answer 7

Molecules in which there is a carbon that is attached to four different groups.

Question 8

What is an enantiomer?

Answer 8

They are stereoisomers that are related as an object and its nonsuperimposable mirror image.

Question 9

How many enantiomers can a chiral molecule have?

Answer 9

One.

Question 10

In C(w,x,y,z), how many groups have to change in order for the molecule to be converted to its enantiomer, and what happens if three groups change.

Answer 10

Two groups have to change because if three groups change the molecule will still be the same but in a different orientation.

Question 11

Is chlorodifluoromethane chiral or achiral?

Answer 11

Achiral.

Question 12

What other terms describe a chirality center?

Answer 12

• asymmetric chenter
• asymmetric carbon
• chiral center
• stereogenic center
• stereocenter

Question 13

Is 2-butanol chiral or achiral?

Answer 13

Chiral.

Question 14

Is 2-propanol chiral or achiral?

Answer 14

Achiral.

Question 15

Can a carbon involved in a double or triple bond be a chirality center?

Answer 15

No.

Question 16

How can a carbon atom in a ring be a chirality center?

Answer 16

If it has two different groups and the two paths traced around the ring are different.

Question 17

A molecule with a plane of symmetry is superimposable/nonsuperimposable and it is chiral/achiral.

Answer 17

Superimposable, achiral.

Question 18

What are the requirements for a center of symmetry?

Answer 18

A line drawn from a point in the center of a molecule to some element, when extended in an opposite direction will ecncounter the same element again.

Question 19

Who is (Jacobus and Joseph Achille) and in what year did they propose what?

Answer 19

Van’t Hoff and Le Bel in 1874 they proposed that four bonds of carbon were directed toward the corners of a tetrahedral.

Question 20

What did van’t Hoff and Le Bel’s proposal mean for molecules?

Answer 20

That molecules with the same constitution could differ in the arrangement of their atoms in space. (stereoisomers are possible)

Question 21

What is optical activity?

Answer 21

The ability of a chiral substance to rotate the plane of

plane-polarized light.

Question 22

What are two properties of the light that measures optical activity?

Answer 22

1. It has a single wavelength
2. It is plane polarized

Question 23

Chiral/achiral substances can rotate the plane of plane-polarized light.

Answer 23

Chiral molecules.

Question 24

What wavelength is used in the light that measures optical activity?

Answer 24

589 nm

(It corresponds to a yellow light produced by a sodium lamp)

Question 25

What french physicist and in what year did he/she discover the phenomenon of optical activity?

Answer 25

Jean-Baptiste Biot in 1815.

Question 26

What is an ideal phase of a substance to be used as a sample in a polarimeter? In the solution form, what are the best solvents that can be used?

Answer 26

The liquid phase should be used.

The best solvents are:

• water
• chloroform
• ethanol

Question 27

What does the analyzer in a polarimeter do?

Answer 27

It measures the magnitude and direction of the observed rotation.

Question 28

To be optically active, the sample must contain a —–substance and one—–must be present in ——-of the other.

Answer 28

Chiral, enantiomer, excess.

Question 29

Optically inactive substances do/do not change the plane of polarized light. All chiral/achiral substances are optically inactive.

Answer 29

Do not/achiral.

Question 30

What is a racemic mixture?

Answer 30

A mixture that contains equal quantities of enantiiomers. They are optically inactive.

Question 31

Describe an optically pure mixture.

Answer 31

All of the molecules are the same handedness. Only one enantiomer is present in the substance.

Question 32

What’s the equation for optical purity? (a.k.a percent enantiomeric excess)

Answer 32

OP=% excess

OP=% enantiomer-% enantiomer

Question 33

What is + and - rotation?

Answer 33

Positive rotation is rotation of the plane of polarized light clockwise and negative rotation is counterclockwise.

Question 34

Give the definition and formula for specific rotation.

Answer 34

Specific rotation accounts for the effects of path length and concentration in different filled polarimeters.

[a]=100a/cl

Question 35

How many cm is one decimeter (dm)?

Answer 35

10 cm.

Question 36

What is absolute configuration?

Answer 36

The exact spatial arrangement of groups in a chirality center in a three dimensional molecule.

Question 37

Compounds with different/same relative configurations have optical rotations of same/opposite signs.

Answer 37

Same,opposite.

Question 39

True or False

In a reaction that does not involve any of the bonds to the chirality center, the starting compound and the product have the same relative configuration.

Answer 39

True.

Question 40

Who (R.S, Sir Christopher, Vladimir) developed a system (what system) to deal with the problem of (what) at a chirality center.

Answer 40

Cahn, Ingold, Prelog.

Sequence rules system.

Absolute configuration.

Question 41

Describe the sequence rules deviced by Cahn, Ingold and Prelog.

Answer 41

1. Identify a chirality center (carbon).
2. Point atom with lowest atomic number away from you.
3. Find atom with highest atomic number.
4. Check from highest to lowest atomic number.

Question 42

Define R and S.

Answer 42

R is clockwise and S is counterclockwise.

Question 43

What is the standard method for stereochemical notation?

Answer 43

The Cahn-Ingold-Prelog system.

Question 44

What German chemist abbreviated stereochemical information?

Answer 44

Emil Fischer.

Question 45

What are Fischer projections?

Answer 45

Molecular drawings in which the vertical bonds are always directed away from you and the horizontal bonds are directed toward you.

Question 46

In Fischer projections the lowest/highest numbered carbon is at the top/bottom.

Answer 46

Lowest/top.

Question 47

How do you verify that a Fischer projection has the R configuration?

Answer 47

Ensure that the lowest-ranked atom is pointing away from you and rank the remaining atoms in the order of decreasing precedence. If the path traced is clockwise then it is in R configuration.

Question 48

Enantiomers have different characteristics for properties that depend on—-but not for the usual physical properties such as —-,——, and —–.

Answer 48

The arrangement of atoms in space. Density, melting point, and boiling point.

Question 49

Read “When Drug Molecules Look In The Mirror.”

Answer 49

Yes.