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

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