Chapter 4: Stereochemistry

Question 1

isomers

Answer 1

• isomers are different molecules having the same molecular formular
• molecules are not collected the same formation.

Question 2

constitutional isomers

Answer 2

• molecules have the same chemical formula bu their atoms are connected in a different sequance

Question 3

stereoisomers

Answer 3

• 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

Question 4

configuration

Answer 4

• the three dimensional arrangement of bonds around a central atom that connect to the other atom is the central atoms configuration

Question 5

enantiomers

Answer 5

• a pair of stereisomers that are non- superposable mirror images of each other
• non superposable means you cannot perfectly over lap them

Question 6

chirality

Answer 6

• when there is a mirror image but if you overlap the mirror image and it is different
• non superposable

Question 7

how to determine is something is chiral or achiral

Answer 7

• rotation without breaking bonds
• sometimes there is a short cut = a plane of symmetry = achiral

Question 8

chirality vs stereogenicity

Answer 8

• 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

Question 9

sterogenic centre

Answer 9

• the interchange of two of its substituents can produce a new stereoisomer

Question 10

CIP nomenclature and absolute configuration

Answer 10

• the absolute configurations describes the exact 3 dimensional arrangement of atoms around the sterocentres
• it is a small descriptor that indicates the arrangement

Question 11

how to assign R/S confirgurations

Answer 11

1. determine which atoms are sterocentres (the central C is a stereocentre )
2. draw in implied hydrogen for stereocentres
3. assign priority to each group (priority is based on atomic number) priorities are assigned 1 through 4
4. reorient the molecules so that the priority 4 group is facing away from the viewer
5. draw a circular arrow from priority 1 to priority 2 to priority 3

Question 12

if the arrow proceeds clockwise

Answer 12

absolute configuration is R

Question 13

if the arrow proceeds counter clockwise

Answer 13

absolute configuration is S

Question 14

what happens if there’s a double or triple bond

Answer 14

• list the atom it connects to once per bond ( 2x for double and 3x for triple)

Question 15

how to draw a compounds enatiomer

Answer 15

1. directly generate a mirror image
2. invert all stereocentres

Question 16

diasteromers

Answer 16

• a pair of stereoisomers that are not enantiomers (not mirror images with one another

Question 17

how to determine the relationship between two compounds

Answer 17

1. are the two compounds completely different
2. are the two constitutional isomers
3. are the two identical
4. are the two enantiomers
   if the answer to all the previous questions is no they are diastereomers

Question 18

stereocentres of heteroatoms

Answer 18

• nitrogen with lone pairs are not usually stereogenic
• it is possible to make one stereogenic by making the inversion geometrically impossible

Question 19

unusual chirality

Answer 19

• some molecules are chiral without any stereogenic centres
• molecules may be chiral due to hindered/ impossible rotations around bonds

Question 20

meso

Answer 20

Molecules that are achiral with multiple stereogenic centres
- description of a molecule not a relationship between two molecules

Question 21

absolute configuration

Answer 21

• decribes the exact 3 dimensional arrangement of atoms around the double bond

Question 22

how to assign E/Z configurations

Answer 22

1. draw the implied hydrogens for the alkenes
2. assign priority to each group on each alkene carbon separately = based on atomic number
3. assigne based on the relavitely positions of priority 1

Question 23

Z

Answer 23

• two high priority groups on the same side

Question 24

E

Answer 24

• two high priority groups on opposite sides

Question 25

cis and trans system

Answer 25

• 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

Question 26

physical properties of enatiomers and diastereomers

Answer 26

• exactly the same boiling/ melting point
• same colour
• ## same chemical reactions

Question 27

enantiomers physical properties

Answer 27

• do not interact with other chiral molecules identically
• do not interact with plane polarized light identically

Question 28

diastomers physical properties

Answer 28

• physically and chemically different
• different melting and boiling points
• different colours
• undergo different chemical reacts

Question 29

how optical rotation works

Answer 29

• put a sample in the path of light
• known concentration in a solvent
• the plane will be rotated clockwise or counter clockwise

Question 30

classifying enantiomers by optical rotations

Answer 30

• for all compounds one enantiomer will rotate the plane clockwise and the other will rotate the plane counterclockwise

Question 31

dextrorotary

Answer 31

• light clockwise and is indicated using a + or d in brackets

Question 32

levorotary

Answer 32

• counter clckwise and is indicated by using - or l

Question 33

racemic or racemate

Answer 33

• 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

Question 34

enantiopure

Answer 34

• if the mixture is made up of 100% of one enantiomer the sample is

Question 35

scalemic or scalemate

Answer 35

• 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