Isomers *** Flashcards
(24 cards)
Structural isomers
share only a molecular formula. They have different physical and chemical properties.
Conformational isomers
differ by rotation around a singe sigma bond.
staggered conformations
have groups 60* apart, as seen in a . Newman projection. In anti staggered molecules, the two largest groups are 180* apart, and strain is minimized. In gauche staggered molecules, the two larges groups are 60* apart.
Eclipsed conformations .
have groups directly in front of each other as seen in Newman projection. In totally eclipsed conformations, the two largest groups are directly in front of each other and strain is maximized.
Angle strain
the strain in cnyclic molecules created by stretching or compressing angles from their normal size.
torsional strain
from eclipsing conformations
nonbonded strains
from interactions between substituents attached to nonadjacent carbons
Cyclic molecules usually adopt
nonplanar shapes to minimize strain
axial
sticking up or down form the plane of the molecule. Create more nonbonded strain.
equatorial
in the plane of the molecule
configurational isomers
can only be interchanged by breaking and reforming bonds
enantiomers
are non-superimposable mirror images and thus have opposite stereochemistry at every chiral carbon. They have the same chemical and physical properties except for rotation of plane-polarized light and reaction in a chiral environment.
Optical activity
refers to the ability of a molecule to rotate plane-polarized light: d- or (+) molecules rotate light to the right; l- or (-) molecules rotate light to the left.
racemic mixtures
with equal concentrations of two enantiomers, will not be optically active because the two enantiomers’ rotations cancel each other out.
meso compound
with an internal plane of symmetry, will also be optically inactive because the two sides of the molecule cancel each other out. They are not optically active.
diasterioisomers
are non-mirror-image stereoisomers. They differ at some, but not all, chiral centers. they have different chemical and physical properties.
cis-trans isomers
are . subtype of diastereomers in which groups differ in position about an immovable bond (such as a double bond or in a cycloalkane.
chiral centers
have four different groups attached to the central carbon
relative configuration
gives the stereochemistry of a compound in comparison to another molecule
absolute configuration
gives the stereochemistry of a compound without having to compare to other molecules.
Cahn-Ingold-Prelog priority rules
in which priority is given by looking at the atoms connected to the chiral carbon or double bonded carbons; whichever has the highest atomic number gets highest priority. If there is a tie, one moves outward from the chiral carbon or double-bonded carbon until the tie is broken.
alkene
is Z if the highest priority substituents are on the same side of the double bond and E if on the opposite sides.
stereocenter’s configuration
determined by putting the lowest priority group in the back and drawing a circle from group 1 to 2 to 3 in descending priority. If this circle is clockwise, the stereocenter is R; if it is counter clock wise, the stereo center is S.
Fischer diagrams
vertical lines go into the plane of the page (dashes); horizontal lines come out of the plane of the page (wedges). switching one pair of substituents in a Fischer diagram inverts the stereochemistry of the chiral center. Switching two pairs retains the stereochemistry. Rotating a Fischer diagram 90* inverts the stereochemistry of the chiral center. Rotating 180* retains the stereochemistry.
