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36 terms

Kaplan MCAT OChem Ch. 2: Isomer

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isomer
same molecule formula, diff. structure
Structural Isomerism
least similar of all isomers

can have diff. functional groups and bonding patterns
phys prop
MP, BP, and solubility

determined by intermolecular forces
Chem prop
react w/ other molecules
stereoisomerism
same chem formula, same atomic connectivity, but diff in how atoms are arranged in space
types of stereoisomers
geo isomers

enantiomers

diastereomers

mes compounds

conformational isomers
geo isomers
diff in position of sub attached to DB or cycloalkane

only one sub => cis or trans


differ in many chem and phys. prop

highest priority sub attached to each DB carbon => Z or E
chiral
object not superimposable on mirror image

carbons known as chiral centers

rotating object doesn't change its chirality
enantiomers
nonsuperimposable mirror images of chiral objects

identical phys. and chem prop.

capable of rotating plane of polarized light (this does pass polarizer at perpendicular angle) => opp and same amount for two compounds

dex (+) and lev (-)

direction of rotation obtained exp => not related to absolute config.

are config. isomers
achiral
have plane of symm
chiral centers
carbons on chiral molecule

lack plane of symm
configuration
spatial arrangement of atom or groups of stereoisomer
relative config of chiral molecule
config. in relation to another chiral molecule

often through chem. interconversion

determine whether enan, dias. or same molecule
absolute config of chiral molecule
describes the exact spatial arrangement of atoms or groups, indie of other molecules
fischer projections
horizontal out, vertical in (bowtie)
rotation of plane polarized light
amount depends on number molecules which also depends on conc. and length of tube that light passes
specific rotation
obs. rotation/conc.(g/mL) x length
racemic mixture
+ and - in eq. conc. => rotation cancel each other => no optical activity
diastereomers
non mirror image config. isomers

cis and trans

can have two or more stereogenic centers

required to have multiple chiral centers

differ in chem and phys
meso compounsd
if plane of symm exist, even though have chiral centers => optical activity
conformational isomerism
most simliar

same molecules, only at diff points in their natural rotation

diff. position of atoms around one or more of their single bonds

easily interconverted
2 types of conformational isomerism
straight chain and cyclic conformations
staggered (anti)
180 => min. steric repulsion between atoms' electron clouds => most stable conformation

antiperiplanar
gauche
other type of staggered, 60 apart
eclipsed
groups 120 apart
totally eclipsed
0 degrees apart

highest energy state
straight chain conformation
every molecules wants lowest energy state

higher => less time

barriers overcome at room temp

if low temp => interconversion dramatically slow
may not rotate at all (0 K)
cyclic conformation ring strain 3 factors
angle, torsional, and nonbonded strain
angle strain
bond angles deviate from ideal
torsional strain
cyclic molecules must have conform that have eclipsed interactions
nonbonded strain (van der Waals repulsion), steric
nonadjacent atoms or groups compete for same space

dominant source of energy in flagpole interactions of boat conformation

to alleviate => adopt nonpolar conforms
cyclobutane => V shape
cyclopentane => enevelope
cyclohexane => chair, boat, and twist/skew-boat
cyclohexane (unsub)
most stable = chair

all 3 types of strains

axial and equatorial alternate

boat conformation is intermediate
boat conformation
intermediate of cyclohexane (unsub)

all atoms eclipsed => avoid strain => slightly more stable stable twist/skew boat conformation
ring flip
occurs in cyclohexane (unsub) when one chair to another
cyclohexane (monosub)
bulky group slow interconversion => axial
cyclohexane (disub)
biggest in eq.

cis and trans => doesn't change w/ ring filp