Ch 3- Stereoisomerism And Chirality

Question 0

What does it mean when an object is achiral?

Answer 0

The object lacks chirality; it has no handedness

Question 1

What does it mean when an object is chiral?

Answer 1

The object and its mirror image are not superposeable

Question 2

An object will be achiral if _____.

Answer 2

it has one or more of certain elements of symmetry, the most common being the plane and center of symmetry

Question 3

stereoisomer

Answer 3

Isomer that has same molecular formula and same connectivity of atoms but different orientation of their atoms in space

Question 4

configurational isomer

Answer 4

Isomer that differs by the configuration of substituents on an atom

Question 5

enantiomer

Answer 5

Stereoisomer nonsuperposeable mirror images of each other; refers to a relationship btwn pairs of objects

Question 6

chiral center

Answer 6

a tetrahedral atom, mostly carbon, that is bonded to 4 different groups

Question 7

stereocenter

Answer 7

Atom about which exchange of 2 groups produces a stereoisomer; chiral centers are one type of stereocenter

Question 8

diastereomer

Answer 8

Stereoisomers that are not mirror images of each other; refers to relationships among 2 or more objects

Question 9

conformational isomer

Answer 9

Type of stereoisomer; not a configurational isomer because bonds do not need to be broken to interchange atoms; the bonds only need to be rotated

Question 10

atropisomer

Answer 10

Enantiomer that lacks a chiral center and differs because of hindered rotation

Question 11

R, S System

Answer 11

A set of rules for specifying absolute configurations about a chiral center

Question 12

absolute configuration

Answer 12

Which of the two possible isomers an enantiomer is (left or right handed)

Question 13

R

Answer 13

Used in R,S convention to show that the order of priority of groups in a chiral center is clockwise

Question 14

S

Answer 14

Used in R,S convention to show that the order of priority of groups on a chiral center is counterclockwise

Question 15

Priority is based on _____.

Answer 15

atomic number; the higher the atomic number, the higher the priority (the highest priority is assigned a priority of 1)

Question 16

When 2 or more groups on a chiral center are the same, priority assignment is made _____.

Answer 16

at the first point of difference

Question 17

When assigning priorities, atoms in double bond or triple bonds are considered to be bonded to _____.

Answer 17

an equivalent number of “phantom atoms” by single bonds, bonded to no other atoms

Question 18

What are the four steps of assigning an R or S configuration to a chiral center?

Answer 18

1. Locate chiral center, identify its 4 substituents, and assign a priority from 1 to 4 to each substituent
2. Orient molecule so that substituent of priority 4 is pointing away from you
3. Read 3 groups in order from 1 to 3
4. If clockwise, R. If counterclockwise, S.

OR use Harran’s RHR

Question 19

When 2 or more stereoisomers exist in a molecule, _____.

Answer 19

multiple stereoisomers are possible

Question 20

For a molecule with n chiral centers, the max number of stereoisomers possible is _____.

Answer 20

2^n

Question 21

meso compound

Answer 21

An achiral compound possessing 2 or more chiral centers that also have chiral isomers

Question 22

2 stereoisomers will be enantiomers if _____ and the stereoisomers will be diastereomers if _____.

Answer 22

all of the chiral centers present are reversed; only some of the chiral centers are reversed

Question 23

Enantiomers have _____ (identical/different) physical and chemical properties in an achiral environment.

Answer 23

identical

Question 24

achiral environment

Answer 24

solvents that have no chiral centers

Question 25

Diastereomers have _____(identical/different) physical and chemical properties.

Answer 25

different

Question 26

Fischer projection

Answer 26

a 2D projection of a molecule; groups on the left and right are in front while those at the top and bottom are to the rear

Question 27

If 2 compounds have the same molecular formula, they are _____ (isomers/not isomers).

Answer 27

isomers

Question 28

If 2 compounds have the same connectivity of atoms, they are _____ (constitutional isomers/stereoisomers).

Answer 28

stereoisomers

Question 29

If 2 compounds do not have the same connectivity of atoms, they are _____ (constitutional isomers/stereoisomers).

Answer 29

constitutional isomers

Question 30

If 2 compounds can interconvert by rotations about single bonds they are _____ (configurational/conformational) isomers.

Answer 30

configurational

Question 31

If 2 stereoisomers cannot interconvert by rotations about single bonds they are _____ (configurational/conformational) isomers.

Answer 31

conformational

Question 32

If 2 stereoisomers are nonsuperposeable mirror images of each other, they are _____ (diastereomers/enantiomers).

Answer 32

enantiomers

Question 33

If 2 stereoisomers are not nonsuperposeable mirror images of each other, they are _____ (diastereomers/enantiomers).

Answer 33

diastereomers

Question 34

If conformational isomers that are also enantiomers readily interconvert at ambient temps, they are _____.

Answer 34

atropisomers

Question 35

optically active

Answer 35

refers to a compound that rotates the plane of polarized light

Question 36

plane-polarized light

Answer 36

light oscillating in only a single plane; the vector sum of left and right circularly polarized light, which are enantiomers- each component interacts in an opposite way with chiral molecules

Question 37

observed rotation

Answer 37

the number of degrees through which a compound rotates the plane of polarized light

Question 38

dextrorotatory

Answer 38

refers to a substance that rotates the plane of polarized light to the right

Question 39

levorotatory

Answer 39

refers to a substance that rotates the plane of polarized light to the left

Question 40

specified rotation [α]

Answer 40

observed rotation in degrees/(length in dm • concentration in g/mL)

Question 41

For any pair of enantiomers, one is _____ and the other is _____.

Answer 41

dextrorotatory; levorotatory

Question 42

For enantiomers, the absolute value of the _____ is the same but the sign is opposite.

Answer 42

specific rotation

Question 43

racemic mixture

Answer 43

a mixture of equal amounts of 2 enantiomers

Question 44

The specific rotation of a racemic mixture is _____, because _____. Alternatively, we say that a racemic mixture is _____.

Answer 44

zero, because there are equal numbers of dextrorotatory and levorotatory molecules; optically inactive