Chapter 5

Question 1

Chirality

Answer 1

•Chirality is the property of molecules that allows a compound to exist in 2 non-superimposable mirror image forms

Question 2

Enantiomers

Answer 2

Nonsuperimposable mirror images, different molecules with different properties

Question 3

Asymmetric Carbons

Answer 3

• Carbons with four different groups attached are chiral
• It’s mirror image will be a different compound (enantiomer)

Question 4

Achiral Compounds

Answer 4

When the images can be superposed the compound is achiral

Question 5

Determining a Plane of Symmetry

Answer 5

•Notice the plane can bisect atoms.

Question 6

What does it mean when a molecule has a plane of symmetry?

Answer 6

A molecule that has a plane of symmetry is achiral.

Question 7

Cis Cyclic Compounds (chiral or achiral?)

Answer 7

Cis-1,2-dichlorocyclohexane is achiral because the molecule has an internal plane of symmetry. Both structures above can be superimposed.

Question 8

Trans Cyclic Compounds (chiral or achiral?)

Answer 8

Trans-1,2-dichlorocyclohexane does not have a plane of symmetry so the images are nonsuperimposable and the molecule will have two enantiomers.

Question 9

absolute configuration

Answer 9

(R) or (S) assignment

Question 10

relative configuration

Answer 10

assigned with respect to a reference sample or another stereocenter in the same molecule

•relative configuration is easier to deduce from experimental data than absolute configuration

Question 11

assigning absolute configuration

Answer 11

1) assign priorities to the groups on the asymmetric (chiral) center
2) orient the molecule with the lowest priority group pointing away from you
3) trace a path from group 1 to group 2 to group 3

Question 12

R configuration

Answer 12

clockwise path is the R configuration

Question 13

S configuration

Answer 13

counterclockwise is the S configuration

Question 14

Sequence Rules for Priorities

Answer 14

1) If the atoms attached to the asymmetric center are all different, priority depends on atomic number
2) When relative priority cannot be decided by Rule 1, compare the next set of atoms in the groups working outward from the stereogenic center

(ignore H when atoms within a rank are compared)

3) A double-bonded atom is treated as if there were two such single bonds

(triple-bonded atom treated as three such single bonds)

Question 15

Expanded Notation

Answer 15

• For multiply bonded atoms, expand the bonding by putting the appropriate number of atoms in parentheses
• Real atoms take priority over parenthetical atoms

Question 16

Incorporating Configuration in Nomenclature

Answer 16

• R or S configuration goes in front of the name in parentheses, followed by sign of rotation (if known)
• For molecules with more than one stereocenter, the position number of the stereocenter is added to the R or S

Question 17

Properties of Enantiomers

Answer 17

Enantiomers have identical physical properties except:

–The way they interact with plane polarized light

(equal magnitude/opposite direction)

–The way they interact with other chiral molecules

Question 18

Optically active

Answer 18

compounds rotate the plane of polarized light

Question 19

Optically inactive

Answer 19

compounds do not rotate polarized light

Question 20

Measuring optical rotation

Answer 20

•Optical rotation measured by a polarimeter –

1) monochromatic light passed through a polarizer
2) the plane-polarized light is passed through a sample
3) the analyzer (another polarizing filter) is adjusted to allow the maximum light through, and the angle is noted

Clockwise rotation is (+) dextrorotatory-d

Counter-clockwise rotation is (-) levorotatory-l

Question 21

Racemic Mixtures

Answer 21

• Equal quantities of d- and l- enantiomers.
• Notation: (d,l) or (±)
• No optical activity.
• The mixture may have different boiling point (b. p.) and melting point (m. p.) from the enantiomers!

Question 22

Optical Purity

Answer 22

• optical purity (enantiomeric excess or ee) measures how much more of one enantiomer is present than the other
• optical purity = observed rotation / known rotation (x 100%)
• optical purity = % major enant – % minor enant

Question 23

Chilarity and Equilibriums

Answer 23

• If equilibrium exists between two chiral conformers, the molecule is not chiral.
• Judge chirality by looking at the most symmetrical conformer.
• Cyclohexane can be considered to be planar, on average.

Question 24

Diastereomers

Answer 24

Have more than one stereocenter

• Diastereomers are non-mirror image stereoisomers
• Diastereomers differ in configuration in at least one, but not all, stereogenic centers
• Diastereomers have: different physical properties and can be separated by ordinary physical methods
• Diastereomers have same formula, connectivity, but different 3D arrangement and are not enantiomers

Question 25

Determining the number of possible stereoisomers in molecules with 2 or more stereocenters

Answer 25

• A molecule with n stereogenic centers has 2n POSSIBLE stereoisomers
• Symmetry elements (meso compounds) reduce the number of stereoisomers to be less than the theoretical maximum (2n)

Question 26

Stereogenic Center

Answer 26

any atom at which an interchange of any two groups produces a stereoisomer

Question 27

Chilarity in Cyclic Molecules

Answer 27

Where cyclic molecules have two or more stereocenters, the molecules can exist as cis or trans and optical activity is possible for each

Question 28

cis-diastereomer example (cyclic)

Answer 28

Question 29

trans-diasteromer example (cyclic)

Answer 29

Question 30

meso cis-diasteromer example (cyclic)

Answer 30

Question 31

If the molecule does not contain a chiral center?

Answer 31

Most likely achiral (There are some limited examples of molecules that do not have a chiral center, yet are still chiral.)

Question 32

If a molecule DOES contain a single chiral center

Answer 32

• If a molecule contains a SINGLE chiral center, the molecule WILL be chiral.
• If there is more than one chiral center, the molecule may not be chiral if it has a plane of symmetry.

Question 33

If the molecule does NOT contain a plane of symmetry

Answer 33

likely chiral

Question 34

If a molecule DOES contain a plane of symmetry

Answer 34

Achiral. Molecules with chiral centers and a plane of symmetry are called meso compounds

Question 35

Meso Compounds

Answer 35

• meso compounds have stereocenters, but also have a plane of symmetry
• meso compounds are NOT optically active

Question 36

Identifying Asymmetric (Chiral) Centers

Answer 36

• Fairly easy for acyclic molecules (don’t forget hydrogens that are not shown)
• More difficult in cyclic molecules:
• Trace path around ring in opposite directions simultaneously
• If you find a difference before reconnecting, chiral, if not, achiral

Question 37

Fischer Projections

Answer 37

• Carbon skeleton vertical, substituents off to side
• Vertical bonds project back
• Horizontal bonds project forward

Question 38

Non-carbon Stereocenters:

Answer 38

Any atom that has four different groups attached and is tetrahedral, but does not spontaneously invert into its own mirror image is stereogenic

One of the four different groups can include a lone pair of electrons (lowest possible priority) for groups V and VI, such as P and S

Question 39

Atropisomerism

Answer 39

Question 40

Allenes

Answer 40

• Some allenes are chiral even though they do not have a chiral carbon.
• To be chiral, the groups at the end carbons must have different groups.