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Level 186

Enantiomers & Polarimetry


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Achiral
doesn't have 4 different substituents
jean baptiste biot
grape extract: dextro-tartaric acid, 2 chiral R centers
Louis Pasteur
synthesized tartaric acid in lab, didn't get optically active compound, got mixture instead of dextro and levrotartaric acid (S,S), racemate
enantiopure
100% of one enantiomer
racemate, latin
equal amount of bot enantiomers, a bunch of grapes
crystallized them as ammonium sodium tartrate crystals
how did Pasteur know that he had equal amounts of each enantiomer?
Enantiomers
A substrate with a single chirality center at the carbon attached to the leaving group will give rise to a pair of _______
have identical solubilities
how can both enantiomers make same set of intermolecular attractions with solvent
diastereoisomer
non-superimposable, non mirror image
resolve racemic mixtures
use enantiopure compounds to
Rand S have what identical physical properties?
boiling point, melting point, solubility, Kps, chromatographic mobility on achiral media like silica
use CHIRAL tartaric acid
how to separate 2 enantiomers
polarimetry
technique to know to what extent something is enantiopurified
polarimeter consists of
sodium lamp, passed through optical filter-->single wavelength sodium D-line (589 nm), pass through plane polarizer, put in sample-->rotate, 2nd plane polarize, rotate
if sample is chiral
rotates plane, create new plane of polarization for exiting beam-->measured with 2nd plane polarizer, rotate that, calculate how plane has changed
enantiomeric compounds rotate
exactly to same extent, but in opposite directions, can't predict if R or S is + or -
absorption spectroscopy
monochromatic light beam traverses a sample, absorbed before it reaches a detector, predict how much is absorbed using Beer's Law
Beer
abs=elc extinction coefficient, length, concentration
polarized light
all photons oscillate in same plane
sodium lamp
generates different wavelengths of light
none
achiral sample rotation
R enantiopure sample, polarimetry
L enantiomer is present, will rotate back in opposite direction
[specific rotation]*l*(cdex-clev)
optical rotation: observed rotation=
polarimeters set up
source lamp and plane polarizer
plane polarizer
create plane of polarized light
sample compartment
1 dm, with bubble is solvent trap holds air bubbles, ensures light traverses exactly 1 dm of solution instead of less than 1 dm
enantiomeric excess
how is enantiopurity reported
total abundance
include canceled out, bigger than e.e
goal
isolate enantiopure sample of ibuprofen isomer from racemic mixture
how do physical properties of enantiomers differ?
one way: how they rotate plane of polarized light
% e.e.=
total percentage of most abundant enantiomer, not including what's canceled out