Chapter 3

Question 1

objects that are not superposable on their mirror images are said to be … That is, they show …

Answer 1

chiral; handedness

Question 2

in contrast, when an object and its mirror image are superposable, the object is …, that is, it lacks chirality

Answer 2

achiral

Question 3

we look for symmetries in a molecule to see if it is … if these symmetries are missing, we can conclude that the molecule is …

Answer 3

achiral; chiral

Question 4

the most common elements of symmetry in organic compounds are the … and the … of symmetry

Answer 4

plane; center

Question 5

a plane of symmetry is an imaginary plane passing through an object or a molecule dividing it such that one half is the … of the other half

Answer 5

reflection

Question 6

a center of symmetry is a point so situated that identical components of the object or molecule are located … and on … sides from the point along any axis passing through that point

Answer 6

equidistant; opposite

Question 7

configurational isomers differ by the configuration of … on an atom

Answer 7

substituents

Question 8

chirality can arise from the ability of a molecule to exist as

Answer 8

configurational isomers

Question 9

… are nonsuperposable mirror images

Answer 9

enantiomers

Question 10

a carbon atom with four different groups bonded to it lacks the two key symmetry elements and is called a

Answer 10

chiral center

Question 11

the term stereocenter is also used to describe a carbon bonded to four different groups but is … A stereocenter is an atom about which exchange of two groups produces a ..

Answer 11

broader; stereoisomer

Question 12

…. are stereoisomers that are not mirror images of each other

Answer 12

diastereomers

Question 13

the term diastereomers refers to a relationship between a pair of objects. it is important to note that diastereomers can be … or …. objects but that enantiomers must be … objects

Answer 13

chiral; achiral; chiral

Question 14

chiral centers are not limited to

Answer 14

C

Question 15

stereoisomers are either diastereomers or enantiomers; therefore, we also can apply these terms to … isomers

Answer 15

conformational

Question 16

chirality can be present in molecules without … This condition occurs via …, but it is less common than chirality arising from … isomerism

Answer 16

chiral centers; conformational isomerism; configurational

Question 17

when the barrier to interconversion is large and the enantiomers cannot interconvert at ambient temperature through a planar form, the molecule will be … and the enantiomers can be …

Answer 17

chiral; separated

Question 18

isomers of this sort, which lack a chiral center but do not interconvert because of hindered rotation, are called

Answer 18

atropisomers

Question 19

R,S system: a system for designating the … of a …

Answer 19

absolute configuration; chiral center

Question 20

(assigning priority for R,S) priority is based on …; the higher it is, the … the priority

Answer 20

atomic number; higher

Question 21

(assigning priority for R,S) priority is assigned at the

Answer 21

first point of difference

Question 22

(assigning priority for R,S) if two carbons have substituents of the same priority, priority is assigned to the carbon that has …. of the higher priority substituents

Answer 22

more

Question 23

(assigning priority for R,S) atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar … by … bonds; that is, atoms of the double bond are … and atoms of a triple bond are ..

Answer 23

phantom atoms; single; duplicated; triplicated

Question 24

(assigning R or S)

1. locate the …, identify its four substituents, and assign a priority from 1 to 4 to each substituent
2. orient the molecule in space so that the group of … is directed away from you
3. read the three groups projecting toward you in order from … priority to … priority
4. if it’s read in a clockwise direction, the configuration is designated as …; if they are read in a counterclockwise direction, the configuration is …

Answer 24

chiral center; lowest priority; highest; lowest; R; S

Question 25

when two or more stereocenters exist in a molecule, multiple .. are possible. More double bonds lead to more possible ...

Answer 25

stereoisomers; stereoisomers

Question 26

to generalize, for a molecule with n chiral centers, the maximum number of stereoisomers possible is

Answer 26

2^n

Question 27

certain molecules containing two or more chiral centers have special symmetry properties that reduce the number of stereoisomers to fewer than the ... predicted by the 2^n rule

Answer 27

maximum number

Question 28

if a molecule has chiral isomers and has stereoisomers that are superposable, those stereoisomers are the same compound and are considered

Answer 28

meso

Question 29

a meso compound contains two or more chiral centers but is ... To be a meso compound, a molecule must also have ...

Answer 29

achiral; chiral isomers

Question 30

we can make this generalization about meso compounds: They have an ... (or ...). Commonly, there are .... chiral centers, each with the same four groups; one is ...; the other, ...

Answer 30

internal mirror plane; center of symmetry; R; S

Question 31

because conformational isomers in acyclic systems interconvert extremely rapidly, we need consider only the most ... conformers. Other conformers may be rotated to give the most ... one for determination of symmetry properties

Answer 31

symmetric; symmetric

Question 32

two stereoisomers will be enantiomers if all the chiral centers present are ..., and the stereoisomers will be diastereomers if only some of the chiral centers are ...

Answer 32

reversed; reversed

Question 33

enantiomers have identical ... and ... properties in an achiral

Answer 33

physical; chemical

Question 34

examples of achiral environments are solvents that have no

Answer 34

chiral centers

Question 35

diastereomers have different ... and ... properties, even in ... environments

Answer 35

physical; chemical; achiral

Question 36

Fischer projections show the configuration of molecules with multiple ...

Answer 36

chiral centers

Question 37

to write a Fischer projection, draw a three-dimensional representation of the molecule oriented so that the vertical bonds from the chiral center are ... and the horizontal bonds from the chiral center are ... Then write the molecule as a ... with the chiral center indicated by the point at which the lines cross

Answer 37

directed away from you; directed toward you; two-dimensional figure

Question 38

because a Fischer projection implies that the groups to each side are in front and those at top and bottom are behind the plane of the paper, rotations of these drawings by ... are not permissible

Answer 38

90 degrees

Question 39

when there is only one chiral center, swapping any two groups creates an ... However, when more than one chiral center exists, you need to swap any two groups on ... centers to create an enantiomer; that is, all ...-centers become ... and vice versa

Answer 39

enantiomer; all; R; S

Question 40

conformational isomers are a form of stereoisomers that interconvert by ... along ..

Answer 40

rotations; single bonds

Question 41

if there is restricted rotation such that the enantiomers or diastereomers cannot interconvert at ambient temperatures, then they are caleld

Answer 41

atropisomers

Question 42

one property that differs between enantiomers is their effect on the rotation of the plane of

Answer 42

polarized light

Question 43

each member of a pair of enantiomers rotates the plane of polarized light in ... directions, and for this reason, enantiomers are said to be ...

Answer 43

opposite; optically active

Question 44

certain materials selectively transmit light waves vibrating only in parallel planes. Such radiation is said to be ...

Answer 44

plane polarized

Question 45

A polarimeter consists of a ... source, a ... and an ... and a ...

Answer 45

monochromatic light source; polarizing filter; analyzing filter; sample tube

Question 46

if the sample tube is empty, the intensity of the light reaching the eye is at its ... when the polarizing axes of the two filters are at ... angles. If the analyzing filter is turned either clockwise or counterclockwise, more light is transmitted

Answer 46

minimum; right

Question 47

when a solution of an optically active compound is placed in the sample tube, some light passes through the analyzing filter; the optically active compound has ... the plane of polarized light from the polarizing filter so that it is now no longer at an angle of ... to the analyzing filter. the analyzing filter is then rotated to ... in the field of view. the number of degrees, alpha, through which the analyzing filter must be rotated to restore darkness to the field of view is called the ..

Answer 47

rotated; op degrees; restore darkness; observed rotation

Question 48

if the analyzing filter must be turned to the right (clockwise) to restore darkness, we say that the compound is ... If the analyzing filter must be turned to the left (counterclockwise) we say that the compound is ...

Answer 48

dextrorotatory; levorotatory

Question 49

Specific rotation =

Answer 49

observed rotation (degrees)/ (length (dm) x concentration)

Question 50

a dextrorotatory compound is indicated with a ... in parentheses, and levorotatory compound is indicated with a ... in parentheses. For any pair of enantiomers, one enantiomer is ... and the other is ...

Answer 50

plus sign; minus sign; dextrorotatory; levorotatory

Question 51

no relationship between ... and ... and ... and ... notation

Answer 51

R; S; +; -

Question 52

an equimolar mixture of two enantiomers is called a

Answer 52

racemic mixture

Question 53

because a racemic mixture contains equal numbers of dextrorotatory and levorotatory molecules, its specific rottaion is

Answer 53

zero

Question 54

a racemic mixture is indicated by adding the prefix

Answer 54

(+-) (minus is underneath the plus)

Question 55

optical rotation depends on the ... of a molecule with respect to the direction of the beam of incident light

Answer 55

orientation

Question 56

for chiral molecules, the rotations from opposite directions do not cancel each other, so in solution, we observe an overall ...

Answer 56

net rotation

Question 57

both ... and ... molecules can rotate plane polarized light

Answer 57

achiral; chiral

Question 58

the important thing to remember is that in solution, the net optical activity for achiral molecules or racemic mixtures of chiral molecules will be

Answer 58

zero

Question 59

optical purity is the specific rotation of a mixture of enantiomers divided by the specific rotation of the ... substance when they are at the same concentration

Answer 59

enantiomerically pure

Question 60

percent optical purity =

Answer 60

specific rotation of sample/ specific rotation of pure enantiomers x 100

Question 61

enantiomeric excess: the difference in the number of moles of each enantiomer in a mixture compared to the

Answer 61

total number of moles of both

Question 62

enantiomeric excess =

Answer 62

%R - %S

Question 63

because enzymes are chiral substances and are present as single stereoisomers, most either produce or react only with substances that are

Answer 63

single stereoisomers (if chiral)

Question 64

an enzyme with a specific binding site for a molecule with a chiral center can distinguish between a molecule and its ... or one of its ...

Answer 64

enantiomer; diastereomers

Question 65

the separation of a racemic mixture into its enantiomers is called

Answer 65

resolution

Question 66

one general scheme for separating enantiomers requires chemical conversion of a pair of enantiomers into two ... with the aid of an enantiomerically pure chiral ...

Answer 66

diastereomers; resolving agent

Question 67

the diastereomers thus formed are different compounds and often can be separated by ... means and purified. the final step in this scheme is chemical conversion of the separated diastereomers back to the individual enantiomers and recovery of the chiral resolving agent

Answer 67

physical

Question 68

diastereomers ahve different ... and when the solution cools the less soluble diastereomeric salt ...., it is collected and purified by further recrystallization. the filtrates, richer in the more soluble salt, are concentrated to give this salt, which is also purified by further re crystallization. the purified salts are treated with aqueous ... to precipitate the nearly pure enantiomers

Answer 68

solubities; crystallizes; HCl