Chapter 3 Flashcards
objects that are not superposable on their mirror images are said to be … That is, they show …
chiral; handedness
in contrast, when an object and its mirror image are superposable, the object is …, that is, it lacks chirality
achiral
we look for symmetries in a molecule to see if it is … if these symmetries are missing, we can conclude that the molecule is …
achiral; chiral
the most common elements of symmetry in organic compounds are the … and the … of symmetry
plane; center
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
reflection
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
equidistant; opposite
configurational isomers differ by the configuration of … on an atom
substituents
chirality can arise from the ability of a molecule to exist as
configurational isomers
… are nonsuperposable mirror images
enantiomers
a carbon atom with four different groups bonded to it lacks the two key symmetry elements and is called a
chiral center
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 ..
broader; stereoisomer
…. are stereoisomers that are not mirror images of each other
diastereomers
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
chiral; achiral; chiral
chiral centers are not limited to
C
stereoisomers are either diastereomers or enantiomers; therefore, we also can apply these terms to … isomers
conformational
chirality can be present in molecules without … This condition occurs via …, but it is less common than chirality arising from … isomerism
chiral centers; conformational isomerism; configurational
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 …
chiral; separated
isomers of this sort, which lack a chiral center but do not interconvert because of hindered rotation, are called
atropisomers
R,S system: a system for designating the … of a …
absolute configuration; chiral center
(assigning priority for R,S) priority is based on …; the higher it is, the … the priority
atomic number; higher
(assigning priority for R,S) priority is assigned at the
first point of difference
(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
more
(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 ..
phantom atoms; single; duplicated; triplicated
(assigning R or S)
- locate the …, identify its four substituents, and assign a priority from 1 to 4 to each substituent
- orient the molecule in space so that the group of … is directed away from you
- read the three groups projecting toward you in order from … priority to … priority
- if it’s read in a clockwise direction, the configuration is designated as …; if they are read in a counterclockwise direction, the configuration is …
chiral center; lowest priority; highest; lowest; R; S
when two or more stereocenters exist in a molecule, multiple .. are possible. More double bonds lead to more possible …
stereoisomers; stereoisomers
to generalize, for a molecule with n chiral centers, the maximum number of stereoisomers possible is
2^n
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
maximum number
if a molecule has chiral isomers and has stereoisomers that are superposable, those stereoisomers are the same compound and are considered
meso
a meso compound contains two or more chiral centers but is … To be a meso compound, a molecule must also have …
achiral; chiral isomers
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, …
internal mirror plane; center of symmetry; R; S
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
symmetric; symmetric
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 …
reversed; reversed
enantiomers have identical … and … properties in an achiral
physical; chemical
examples of achiral environments are solvents that have no
chiral centers
diastereomers have different … and … properties, even in … environments
physical; chemical; achiral
Fischer projections show the configuration of molecules with multiple …
chiral centers
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
directed away from you; directed toward you; two-dimensional figure
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
90 degrees
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
enantiomer; all; R; S
conformational isomers are a form of stereoisomers that interconvert by … along ..
rotations; single bonds
if there is restricted rotation such that the enantiomers or diastereomers cannot interconvert at ambient temperatures, then they are caleld
atropisomers
one property that differs between enantiomers is their effect on the rotation of the plane of
polarized light
each member of a pair of enantiomers rotates the plane of polarized light in … directions, and for this reason, enantiomers are said to be …
opposite; optically active
certain materials selectively transmit light waves vibrating only in parallel planes. Such radiation is said to be …
plane polarized
A polarimeter consists of a … source, a … and an … and a …
monochromatic light source; polarizing filter; analyzing filter; sample tube
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
minimum; right
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 ..
rotated; op degrees; restore darkness; observed rotation
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 …
dextrorotatory; levorotatory
Specific rotation =
observed rotation (degrees)/ (length (dm) x concentration)
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 …
plus sign; minus sign; dextrorotatory; levorotatory
no relationship between … and … and … and … notation
R; S; +; -
an equimolar mixture of two enantiomers is called a
racemic mixture
because a racemic mixture contains equal numbers of dextrorotatory and levorotatory molecules, its specific rottaion is
zero
a racemic mixture is indicated by adding the prefix
(+-) (minus is underneath the plus)
optical rotation depends on the … of a molecule with respect to the direction of the beam of incident light
orientation
for chiral molecules, the rotations from opposite directions do not cancel each other, so in solution, we observe an overall …
net rotation
both … and … molecules can rotate plane polarized light
achiral; chiral
the important thing to remember is that in solution, the net optical activity for achiral molecules or racemic mixtures of chiral molecules will be
zero
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
enantiomerically pure
percent optical purity =
specific rotation of sample/ specific rotation of pure enantiomers x 100
enantiomeric excess: the difference in the number of moles of each enantiomer in a mixture compared to the
total number of moles of both
enantiomeric excess =
%R - %S
because enzymes are chiral substances and are present as single stereoisomers, most either produce or react only with substances that are
single stereoisomers (if chiral)
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 …
enantiomer; diastereomers
the separation of a racemic mixture into its enantiomers is called
resolution
one general scheme for separating enantiomers requires chemical conversion of a pair of enantiomers into two … with the aid of an enantiomerically pure chiral …
diastereomers; resolving agent
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
physical
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
solubities; crystallizes; HCl
