Chapter 2.1/2: Structural Isomers & Stereoisomers

the least similar of all of the isomers, they only share the molecular formula

structural (constitutional) isomers

if the isomer does NOT have the same connectivity

structural isomers

results when cyclic molecules must assume conformations that have eclipsed or gauche interactions

torsional strain

has the highest energy state for a Newman projection

totally eclipsed

the least favorable Newman projection conformation

totally eclipsed

when the two methyl groups in a Newman projection directly overlap with each other with 0* separation

totally eclipsed

when two substituents are on opposite sides of the immovable bond

trans

when two substituents on a cyclohexane ring are on opposite sides (one wedge and one dash)

trans

staggered conformation of a newman projection

when largest groups are separated 180* apart

do the dashes and wedges stay the same during a chair flip

yes

gauche * apart

60

molecules that have the same connectivity but opposite configurations at every chiral center in the molecule

enantiomer

if the stereoisomer (configurational) are non superimposable mirror images of each other

enantiomers

two molecules that are nonsuperimposable mirror images of each other

enantiomers

the two categories of configurational isomers

enantiomers and diastereomers

have the same chemical formula and also share the same atomic connectivity- they have the same structural backbone

stereoisomers

if the isomer has the same connectivity

stereoisomers

for substituted rings, the bulkiest group will favor the _____ position to reduce nonbonded strain

bulkiest group

at the molecular level, one enantiomer will rotate plane-polarized light to the same magnitude but ________ of its mirror image

but in the opposite direction

cyclohexane can be in what three different cyclic conformations:

chair boat twist-boat

the most stable conformation of cyclohexane

chair conformation

when a cyclohexane ring chair flips from one chair form to another

chair flip

characteristics that have to do with reactivity of the molecule with other molecules and result in changes in chemical composition Usually determined by the functional groups in the molecule

chemical properties

when its mirror image cannot be superimposed on the original object; there is no internal plane of symmetry (think hands)

chiral

carbon with four different substituents and lack a plane of symmetry

chiral center

When two substituents are on the same side of the immovable bond

cis

when two substituents on a cyclohexane ring are on the same side (both wedges or both dashes)

cis

if the diastereomers differ in arrangement around an immovable bond

cis-trans diastereomers

if the two stereoisomers require bond breaking to interconvert them

configurational isomers

type of isomer that can only change from one form to another by breaking and reforming covalent bonds

configurational isomers

of all of the isomers, ______ isomers are the most similar

conformational

a type of stereoisomer in which they are the same molecule, only at different points in their natural rotation around single bonds

conformational isomers

if the two stereoisomers do not require bond breaking to interconvert the two

conformational isomers

the two categories of stereoisomers

conformational isomers configurational isomers

structural isomers are also called

constitutional isomers

a compound that rotates the plane of polarized light to the RIGHT (clockwise) is labeled

d- and (+)

if the stereoisomer (configurational) are NOT non-superimposable mirror images of each other

diastereomers

molecules that are chiral and share same connectivity; but are not mirror images of each other

diastereomers

For any molecule with n chiral centers, there are ____ possible stereoisomers

2^n

totally eclipsed * apart

0

eclipsed * apart

120

anti * apart=

180

non-mirror image configurational isomers occur when a molecular has two or more stereogenic centers and differs at some, but not all, of these centers

diastereomers

a design in which the molecule is visualized along a line extending through a carbon-carbon bond axis through the middle of the page

Newman projection

a position in a Newman projection in which two methyl groups are 120* apart, and overlap with the hydrogen atoms on the adjacent carbon

eclipsed

_______ have identical physical and chemical properties (which kind of isomer)

enantiomer

objects that have mirror images that can be superimposed (think fork)

achiral

the axial-equatorial orientation ______ around the ring

alternate (if the wedge of C-1 is axial, then the dash on C-2 will be axial, C-3 wedges will be axial, and so on)

results when bond angles deviate from their ideal values by being stretched or compressed

angle strain

the staggered Newman projection conformation is also called

anti

the most energetically favorable position for a Newman projection is

anti (staggered)

when the two largest groups are antiperiplanar (in the same plane, but on opposite sides) to each other in a Newman projection

anti (staggered)

the hydrogen atoms that are perpendicular to the plane of the ring (Sticking up or down) of the cyclohexane chair conformation

axial

after the cyclohexane ring flips the...

axial and equatorial groups flip (all axial groups become equatorial, and all equatorial groups become axial)

interconversion between chair conformations of a cyclohexane ring can be slowed by

bug bulky groups are attached (Ex: tert-butyl)

in a chair formation the larger group will prefer the _____ position

equatorial

the hydrogen atoms that are parallel (sticking out) of the cyclohexane chair conformation

equatorial

the other type of staggered conformation in a Newman projection that occurs when the two largest groups are 60* apart

gauche

cis-trans isomers are also called

geometric isomers

for a molecule to have optical activity it must

have chiral cents lack a plane of symmetry

_____ have the same molecular formula, but different structures

isomers

same molecular formula but different chemical structure

isomers

a compound that rotates the plane of polarized light to the LEFT (counterclockwise) is labeled

l- and (-)

a molecule with chiral centers that has an internal plane of symmetry

meso compound

What property do structural isomers have in common?

molecular formula molecular weight

do the axial and equatorial conformations during a chair flip stay the same?

no (they flip)

results when nonadjacent atoms or groups compete for the same space

nonbonded strain (van der Waals repulsion)

refers to the rotation of plane-polarized light by chiral molecules

optical activity

enantiomers and diastereomers can be considered ______ because the different spatial arrangement of groups in these molecules affects the rotation of plane-polarized light

optical isomers

characteristics of processes that don't change the composition of matter, such as melting point, boiling point, solubility, odor, color, and density

physical properties

When both (+) and (-) enantiomers are present in equal concentrations, they form a ______ (rotations cancel each other out, and no optical activity is observed)

racemic mixture

a ______ displays no optical activity

racemic mixture

differ in how substituents are arranged in space (their wedge-and-dash pattern), and all isomers that are not structural isomers fall into this category

stereoisomers