talks about body in white nomenclature and materials. useful for young engineers
CHEM1602/3 Chemistry for Biologists Section B: Shapes, dynamics and stereochemistry
Chirality ⇒ ch chiro iros s (ha (hand nd in Gre Greek ek))
mirror image of left hand is not a left hand, but a right hand
a ball is a ball is a ball
An object (molecule) is chiral when it cannot be superimposed on its mirror image
CHEM1602/3 Chemistry for Biologists Section B: Shapes, dynamics and stereochemistry
Chirality and Carbon
Carbon centres with four different substituents lead to chiral molecules that are non-superimposable with their mirror image. Molecules whose image and mirror image are non-superimposable are called enantiomers .
CHEM1602/3 Chemistry for Biologists Section B: Shapes, dynamics and stereochemistry
Properties of Enantiomers Enantiomers rotate plane polarised light by the same degree but in opposite directions number of degrees = achiral property;
CHEM1602/3 Chemistry for Biologists Section B: Shapes, dynamics and stereochemistry
R,S-Convention Two enantiomers differ in their configuration at the stereogenic centre How can we tell which one is which? ⇒ The Cahn-Ingold-Prelog (CIP) Rules
a b c
a d
I
b c
d II
Step 1:
Assign a priority to the four groups at the stereogenic centre: a > b> c > d
Step 2:
look along axis from Carbon (Mercedes Star)
⇒
lowest priority substituent d
a b
a c
I
Step 3: if Mercedes Star a → b → c turns clockwise = R -configuration if Mercedes Star a → b → c turns counterclockwise = S - configuration
c
b II
CHEM1602/3 Chemistry for Biologists Section B: Shapes, dynamics and stereochemistry
Priority Rules in the R,S-Convention Rule 1:
atoms directly attached to stereogenic centre are ranked by atomic number e.g.: Cl > O > C > H (Hint: if one atom is H, it necessarily has lowest priority)
Rule 2: if decision cannot be reached under Rule 1, consider next the next atom in the chain e.g.: H3CCH2 > CH3 Rule 3: Multiple bonds are treated as the equal number of single bonds (attach “dummy” atoms) C
C
H
C
C
C
C
C
C
O C H
C O
C
O
CHEM1602/3 Chemistry for Biologists Section B: Shapes, dynamics and stereochemistry
Examples for applying the R,S-Convention H
H3C
H
H
CH3
HO
Br Cl
F
H3C
H O
Br
CHEM1602/3 Chemistry for Biologists Section B: Shapes, dynamics and stereochemistry
Fischer - Projections (used only for amino acids and sugars)
CO2H
CO2H H
H
OH
lactic acid
CO2H
OH H3C
CH3
CH3
what it is
what it means
OH H
how to generate it
Rules •longest carbon chain vertical •most oxidized carbon on top CH=O HO
H CH2OH
L R -(+)-glyceraldehyde
CH=O H
OH CH2OH
D S -(-)-glyceraldehyde
In a given Fischer Projection, look at the stereocentre farthest away from the top carbon if the substituent of highest priority is to the right: configuration D if the substituent of highest priority is to the left: configuration L
CHEM1602/3 Chemistry for Biologists Section B: Shapes, dynamics and stereochemistry
D,L - Nomenclature
CH=O
(used for sugars and amino acids only!) H CH=O HO
H CH2 OH
R -(+)-glyceraldehyde
HO
CH=O H
OH
OH CH2 OH
H
H
OH
H
OH CH2 OH
S -(-)-glyceraldehyde
glucose
In a given Fischer Projection, look at the stereocentre farthest away from the top carbon if the substituent of highest priority is to the right: if the substituent of highest priority is to the left:
configuration D configuration L
Note that the DL nomenclature only assigns the configuration at one centre - it assumes that you know the rest.
CHEM1602/3 Chemistry for Biologists Section B: Shapes, dynamics and stereochemistry
The Configuration of Amino Acids COOH H2 N
H R
COOH H2N
H CH3
L-(+)-alanine
COOH H2N
H CH2SH
L-(+)-cysteine
Most naturally occurring α-amino acids in living systems have L configuration Home revision exercise: Assign the configurations of these amino acids using the R & S system? What about the other amino acids? Are they the same or different? An unsolved mystery: Why did Nature choose L amino acids? Why not D? Was it just chance or is there a reason? See Martin Gardner - “The New Ambidextrous Universe ” and Chris McManus “Left Hand, Right Hand” .
