Stereochemistry Flashcards
Describe the two different types of isomers
Constitutional isomers: isomers with different atomic connectivity
Stereoisomers: isomers with the same atomic connectivity but with differed 3D arrangement
define an isomer
different compounds with the same molecular formula
define chirality
the ability of objects to exist as non-superposable mirror images of each other.
- an object that contains a mirror plane and can be superimposed with its mirror image is achiral
- an objects that does not have a mirror plane and cannot be superimposed with its mirror image
define enantiomers
chiral molecules and their non-superimposable mirror images
what does chirality often originate from?
an atom that is connected to four different substituents - called a stereogenic centre, chirality centre, or stereo centre
do enantiomers have identical physical and chemical properties?
yes; melting points, boiling points, and density are identical. The IR and NMR spectra of enantiomers are also identical
how can enantiomers be differentiated from each other?
- interactions with other chiral molecules
- optical activity (rotation of plane-polarised light by same angle but in opposite directions, + or -)
optical rotation α
the angle that a sample rotates plane-polarised light
- clockwise rotation (α > 0): dextrorotary (+ or d)
- counterclockwise rotation (α < 0) levorotary (- or l)
define a racemic mixture
a 1:1 mixture of two enantiomers
does a racemic mixture rotate polarised light?
no; the single enantiomers rotate light in opposite directions and cancel each other out
how do you determine the absolute configuration of a chirality centre?
- identify the chirality centre
- determine the priorities of attached groups
- rotate the molecule to put the lowest priority group (number 4) at the back
- determine the direction of priority group numbers 1,2, and 3
if clockwise -> R (right)
if counter-clockwise -> S (left)
define diastereomers
stereoisomers that are non-superimposable, non-mirror images
how do diastereomers differ from enantiomers?
enantiomers: changing configuration at all stereocenters
diastereomers: changing configuration at some stereocenters
do diastereomers differ in their physical and chemical properties?
yes; boiling point, melting point, IR and NMR spectra, solubility, etc
maximum number of stereoisomers =
2^n, where n = number of chirality centres
define a meso compound
- a molecule that has chirality centres but is a chiral due to an internal mirror plane
- the molecule and its mirror image are superimposable = achiral
double bond isomerism
E = highest priority groups are trans
Z = highest priority groups are cis
chirality depends on
whether the molecule lacks a plane of symmetry and has a non-superimposable mirror image.
do diastereomers have optical activity?
yes - diastereomers have different physical properties including optical activity
