Week 5 Textbook Flashcards
(24 cards)
isomers
different molecules formed from the SAME set of atoms
constitutional isomers
the atoms are connected in different sequences or by different bond types
stereoisomers
have the same atoms connected in the same sequence using the same bond types, but they differ in the 3D arrangement of those atoms
-the configuration of atoms in 3D is usually represented on paper with solid wedge bonds (project OUT page) and hashed bonds (project INTO page)
enatiomers
non-superposable mirror images
-rotating enantiomers does NOT make their structures match
-chiral molecules
chirality
the ability to exist as non-superposable mirror images
-always possible to draw a mirror image of a molecule, but it is only chiral if the mirror images are non-superposable
achiral
compound that does not have an enantiomer
if a plane of symmetry exists…
that molecule can’t be chiral
stereogenic center aka chiral center
atoms that are connected to different groups in such a way that interchanging any 2 groups produces a stereoisomer
-these atoms are also called stereocenters
-atoms that are connected to 4 different* groups are a very common form of a stereogenic center
how to know when a molecule with a stereogenic center is chiral
has ONE sp3 molecule
atropisomers
stereoisomers that result from hindered rotation around a single bond
CIP system
used to describe the absolute configuration of a stereogenic center
-exact 3D arrangements of the atoms around it
-lowest priority gets projected to the back (hashed bond)
-cw=R and ccw=S
differentiating between atoms of the SAME element
This is accomplished by examining the atoms immediately adjacent to the ones directly attached to the stereogenic center
The adjacent atoms are compared by starting with the ones with the highest atomic numbers and then working down the chain until the first difference is encountered
Stop at the first point of difference
Since carbon has a higher atomic number than hydrogen, the group on the right (ethyl) has a higher priority than the group on the left (methyl).
=This resolves the tie, makes it possible to assign priorities, and determines that the overall configuration is R.
naming molecules with stereocenters
If a molecule contains only one stereocenter, the configuration is indicated by an italic R or S in parentheses at the start of the name of the compound
When the molecule has more than one stereogenic center, the atom number indicating location precedes each configuration symbol
drawing enantiomers
-The fundamental way to draw the enantiomer of a molecule is to use a mirror plane to reflect the molecule
-Note that the configuration of each stereocenter in the mirror image is always the opposite of that for the corresponding stereocenter in the original molecule
-A more common method for drawing the opposite enantiomer is to invert the configuration of the center by changing a hashed bond to a wedge or a wedge to a hashed bond
–When using this method, it is important to preserve the orientation of the backbone of the structure
diastereomer
-stereoisomers that are NOT enantiomers
-only the config. of some of the stereocenters
changes = diastereomer
-contain more than one stereogenic center
-each center can have either R or S config, molecules with multiple centers have many stereoisomers, some of which are NOT mirror images of each other
estimating the # of stereoisomers
Since a stereogenic center can exist in one of two configurations (R or S), each stereocenter in a molecule contributes a factor of TWO to the number of possible stereoisomers for that molecule
Consequently, a compound that contains n stereocenters can exist in as many as 2n stereoisomers
-this value is the MAX #of stereoisomers that are possible for a given compound
For some compounds, certain combinations of configurations produce IDENTICAL structures, thus reducing the actual number of stereoisomers
meso compounds
Molecules that contain MORE than one stereocenter and have superposable mirror images are called meso compounds
Meso compounds contain a plane of symmetry, sometimes called a mirror plane
-this plane bisects the molecule, so the 2 halves are mirror images of each other
double-bond stereoisomers
If the two highest priority groups are OPPOSITE to one another (trans), the double bond is designated E (entgegen, German for “opposite”)
If the two highest priority groups are on the SAME side of the double bond (cis), the double bond has a Z configuration (zusammen, German for “together”).
properties of diastereomer
The diastereomers of a compound have distinct molecular structures, so they differ from one another in their physical and chemical properties, such as melting points, boiling points, physical states, and solubility.
Diastereomers also differ in terms of their chemical reactivity
properties of enantiomers
By contrast, pure enantiomers of a compound have exactly the same melting point, boiling point, and all other physical properties except one. Enantiomers are completely indistinguishable from each other except under two conditions: (1) when they interact with another chiral material (described in this section); or (2) when they interact with polarized light
optical rotation
The only physical measurement that differentiates enantiomers is optical rotation, which involves the interaction of enantiomers with plane-polarized light
When plane-polarized light is passed through a sample of a chiral substance, the plane of polarization ROTATES as the light passes through the sample
-the light emerges with the oscillation plane at a different angle
Materials that interact with plane-polarized light in this way are called OPTICALLY ACTIVE, and only CHIRAL materials have this property
optical rotation
The two enantiomers of a substance rotate plane-polarized light in opposite directions
The angle that a given sample rotates plane-polarized light is called the optical ROTATION, α, of the sample
CW rotation is defined as +, while CCW rotation is -
Compounds that rotate plane-polarized light CW(α > 0) are called dextrorotary (d or +), and those that rotate light CCW (α < 0) are called levorotary (l or −)
true or false:
The sign of the optical rotation for a sample does not indicate the absolute configuration of stereogenic centers in the compound.
true
specific rotation
The magnitude of optical rotation depends on the number of molecules the light encounters, so samples are compared by calculating the specific rotation, [α], of each sample
