Chapter 4: Stereochemistry Flashcards
1
Q
isomers
A
- isomers are different molecules having the same molecular formular
- molecules are not collected the same formation.
2
Q
constitutional isomers
A
- molecules have the same chemical formula bu their atoms are connected in a different sequance
3
Q
stereoisomers
A
- molecules have the same chemical formula
- their atoms are connected in the same sequence
- but they differ in the three dimensional arrangement of those atoms
4
Q
configuration
A
- the three dimensional arrangement of bonds around a central atom that connect to the other atom is the central atoms configuration
5
Q
enantiomers
A
- a pair of stereisomers that are non- superposable mirror images of each other
- non superposable means you cannot perfectly over lap them
6
Q
chirality
A
- when there is a mirror image but if you overlap the mirror image and it is different
- non superposable
7
Q
how to determine is something is chiral or achiral
A
- rotation without breaking bonds
- sometimes there is a short cut = a plane of symmetry = achiral
8
Q
chirality vs stereogenicity
A
- chiral molecules often feature a carbon atom with four different substituents
- many molecules are chiral and have no such carbon atoms
- many carbons are achiral despite having presence of such carbon atoms
9
Q
sterogenic centre
A
- the interchange of two of its substituents can produce a new stereoisomer
10
Q
CIP nomenclature and absolute configuration
A
- the absolute configurations describes the exact 3 dimensional arrangement of atoms around the sterocentres
- it is a small descriptor that indicates the arrangement
11
Q
how to assign R/S confirgurations
A
- determine which atoms are sterocentres (the central C is a stereocentre )
- draw in implied hydrogen for stereocentres
- assign priority to each group (priority is based on atomic number) priorities are assigned 1 through 4
- reorient the molecules so that the priority 4 group is facing away from the viewer
- draw a circular arrow from priority 1 to priority 2 to priority 3
12
Q
if the arrow proceeds clockwise
A
absolute configuration is R
13
Q
if the arrow proceeds counter clockwise
A
absolute configuration is S
14
Q
what happens if there’s a double or triple bond
A
- list the atom it connects to once per bond ( 2x for double and 3x for triple)
15
Q
how to draw a compounds enatiomer
A
- directly generate a mirror image
- invert all stereocentres
16
Q
diasteromers
A
- a pair of stereoisomers that are not enantiomers (not mirror images with one another
17
Q
how to determine the relationship between two compounds
A
- are the two compounds completely different
- are the two constitutional isomers
- are the two identical
- are the two enantiomers
if the answer to all the previous questions is no they are diastereomers
18
Q
stereocentres of heteroatoms
A
- nitrogen with lone pairs are not usually stereogenic
- it is possible to make one stereogenic by making the inversion geometrically impossible
19
Q
unusual chirality
A
- some molecules are chiral without any stereogenic centres
- molecules may be chiral due to hindered/ impossible rotations around bonds
20
Q
meso
A
Molecules that are achiral with multiple stereogenic centres
- description of a molecule not a relationship between two molecules
21
Q
absolute configuration
A
- decribes the exact 3 dimensional arrangement of atoms around the double bond
22
Q
how to assign E/Z configurations
A
- draw the implied hydrogens for the alkenes
- assign priority to each group on each alkene carbon separately = based on atomic number
- assigne based on the relavitely positions of priority 1
23
Q
Z
A
- two high priority groups on the same side
24
Q
E
A
- two high priority groups on opposite sides
25
Q
cis and trans system
A
- some older sources use
- this approach is only valid for some alkenes, generally simple ones
- the cis/trans are limited by relative descriptors
cis= two things are on the same side of something relative to each other
trans = two things are on opposite sides of something relative to each other
26
Q
physical properties of enatiomers and diastereomers
A
- exactly the same boiling/ melting point
- same colour
- ## same chemical reactions
27
Q
enantiomers physical properties
A
- do not interact with other chiral molecules identically
- do not interact with plane polarized light identically
28
Q
diastomers physical properties
A
- physically and chemically different
- different melting and boiling points
- different colours
- undergo different chemical reacts
29
Q
how optical rotation works
A
- put a sample in the path of light
- known concentration in a solvent
- the plane will be rotated clockwise or counter clockwise
30
Q
classifying enantiomers by optical rotations
A
- for all compounds one enantiomer will rotate the plane clockwise and the other will rotate the plane counterclockwise
31
Q
dextrorotary
A
- light clockwise and is indicated using a + or d in brackets
32
Q
levorotary
A
- counter clckwise and is indicated by using - or l
33
Q
racemic or racemate
A
- mixtures made up of 50/50
- will not rotate plane of polarized light
- every molecule that rotates it one way, another rotates it the other
34
Q
enantiopure
A
- if the mixture is made up of 100% of one enantiomer the sample is
35
Q
scalemic or scalemate
A
- if the mixture is made up of any other ratio of the two entiomers
- do rotate plane polarized light
- most chemists dont use the terms scalemic of scalemate they will use = enantioenriched
36
Q
A
